Embryogenesis
Encyclopedia
Embryogenesis is the process by which the embryo
is formed and develops, until it develops into a fetus
.
Embryogenesis starts with the fertilization of the ovum
(or egg) by sperm
. The fertilized ovum is referred to as a zygote
. The zygote undergoes rapid mitotic
division
s with no significant growth (a process known as cleavage
) and cellular differentiation
, leading to development of an embryo.
Although embryogenesis occurs in both animal
and plant development
, this article addresses the common features among different animals, with some emphasis on the embryonic development of vertebrates and mammals
.
, having an "animal pole
" (future ectoderm
and mesoderm
) and a "vegetal pole" (future endoderm
). It is also covered with different protective envelopes, with different layers. The first envelope - the one in contact with the membrane
of the egg - is made of glycoproteins and is known as the vitelline membrane
(zona pellucida
in mammals). Different taxa show different cellular and acellular envelopes englobing the vitelline membrane.
Fertilisation
(also known as 'conception', 'fecundation' and 'syngamy'), is the fusion of gametes to produce a new organism. In animals, the process involves a sperm
fusing with an ovum
, which eventually leads to the development of an embryo
. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external fertilisation. The fertilized egg cell is known as the zygote
.
Cell division with no significant growth, producing a cluster of cells that is the same size as the original zygote, is called cleavage. At least four initial cell divisions occur, resulting in a dense ball of at least sixteen cells called the morula
. The different cells derived from cleavage, up to the blastula stage
, are called blastomere
s. Depending mostly on the amount of yolk in the egg, the cleavage
can be holoblastic (total) or meroblastic (partial).
Holoblastic cleavage occurs in animals with little yolk in their eggs, such as humans and other mammals who receive nourishment as embryos from the mother, via the placenta
or milk
, such as might be secreted from a marsupium
. On the other hand, meroblastic cleavage occurs in animals whose eggs have more yolk; i.e. birds and reptiles. Because cleavage is impeded in the vegetal pole, there is a very uneven distribution and size of cells, being more numerous and smaller at the animal pole of the zygote.
In holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, and the second cleavage is perpendicular to the first. From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms:
The end of cleavage is known as midblastula transition
and coincides with the onset of zygotic
transcription.
In amniotes, the cells of the morula
are at first closely aggregated, but soon they become arranged into an outer or peripheral layer, the trophoblast
, which does not contribute to the formation of the embryo proper, and an inner cell mass
, from which the embryo is developed. Fluid collects between the trophoblast and the greater part of the inner cell-mass, and thus the morula is converted into a vesicle
, called the blastodermic vesicle. The inner cell mass remains in contact, however, with the trophoblast at one pole of the ovum; this is named the embryonic pole, since it indicates the location where the future embryo will be developed.
s, the embryo is called a blastula
. The blastula is usually a spherical layer of cells (the blastoderm
) surrounding a fluid-filled or yolk-filled cavity (the blastocoel).
Mammals at this stage form a structure called the blastocyst
, characterized by an inner cell mass
that is distinct from the surrounding blastula. The blastocyst must not be confused with the blastula; even though they are similar in structure, their cells have different fates.
Before gastrulation
, the cells of the trophoblast become differentiated into two strata: the outer stratum forms a syncytium
(i.e. a layer of protoplasm studded with nuclei, but showing no evidence of subdivision into cells), termed the syncytiotrophoblast
; while the inner layer, the cytotrophoblast
or "Layer of Langhans," consists of well-defined cells. As already stated, the cells of the trophoblast do not contribute to the formation of the embryo proper; they form the ectoderm
of the chorion
and play an important part in the development of the placenta
. On the deep surface of the inner cell mass
, a layer of flattened cells, called the entoderm, is differentiated and quickly assumes the form of a small sac, called the yolk sac
. Spaces appear between the remaining cells of the mass and, by the enlargement and coalescence of these spaces, a cavity called the amniotic cavity
is gradually developed. The floor of this cavity is formed by the embryonic disk
, which is composed of a layer of prismatic cells, the embryonic ectoderm
, derived from the inner cell mass and lying in apposition with the entoderm.
becomes oval and then pear-shaped, the wider end being directed forward. Near the narrow, posterior end, an opaque streak, called the primitive streak
, makes its appearance and extends along the middle of the disk for about one-half of its length; at the anterior end of the streak there is a knob-like thickening termed Hensen's knot. A shallow groove, the primitive groove
, appears on the surface of the streak, and the anterior end of this groove communicates by means of an aperture, the blastopore
, with the yolk sac
. The primitive streak is produced by a thickening of the axial part of the ectoderm
, the cells of which multiply, grow downward, and blend with those of the subjacent entoderm. From the sides of the primitive streak a third layer of cells, the mesoderm
, extends lateralward between the ectoderm and entoderm; the caudal end of the primitive streak forms the cloacal membrane
. The blastoderm now consists of three layers, named from without inward: ectoderm, mesoderm, and entoderm; each has distinctive characteristics and gives rise to certain tissues of the body. For many mammals, it is sometime during formation of the germ layers that implantation of the embryo in the uterus
of the mother occurs.
s. The embryo during this process is called a gastrula. The germ layers are referred to as the ectoderm
, mesoderm
and endoderm
. In diploblastic animals only the ectoderm and the endoderm are present .
In most animals, a blastopore
is formed at the point where cells are entering the embryo. Two major groups of animals can be distinguished according to the blastopore's fate. In deuterostome
s the anus forms from the blastopore, while in protostome
s it develops into the mouth. See Embryological origins of the mouth and anus
for more information.
, two longitudinal ridges, caused by a folding up of the ectoderm
, make their appearance, one on either side of the middle line formed by the streak. These are named the neural folds
; they commence some little distance behind the anterior end of the embryonic disk
, where they are continuous with each other, and from there gradually extend backward, one on either side of the anterior end of the primitive streak. Between these folds is a shallow median
groove, the neural groove
. The groove gradually deepens as the neural folds become elevated, and ultimately the folds meet and coalesce in the middle line and convert the groove into a closed tube, the neural tube
or canal, the ectodermal wall of which forms the rudiment of the nervous system. After the coalescence of the neural folds over the anterior end of the primitive streak, the blastopore
no longer opens on the surface but into the closed canal of the neural tube, and thus a transitory communication, the neurenteric canal, is established between the neural tube and the primitive digestive tube. The coalescence of the neural folds occurs first in the region of the hind brain, and from there extends forward and backward; toward the end of the third week, the front opening (anterior neuropore
) of the tube finally closes at the anterior end of the future brain
, and forms a recess that is in contact, for a time, with the overlying ectoderm; the hinder part of the neural groove presents for a time a rhomboidal shape
, and to this expanded portion the term sinus rhomboidalis has been applied. Before the neural groove is closed, a ridge of ectodermal cells appears along the prominent margin of each neural fold; this is termed the neural crest
or ganglion ridge, and from it the spinal and cranial nerve ganglia and the ganglia of the sympathetic nervous system
are developed. By the upward growth of the mesoderm, the neural tube is ultimately separated from the overlying ectoderm.
The cephalic end of the neural groove exhibits several dilatations which, when the tube is closed, assume the form of three vesicles; these constitute the three primary cerebral vesicles, and correspond respectively to the future 'fore-brain' (prosencephalon
), 'midbrain' (mesencephalon), and 'hind-brain' (rhombencephalon
) (Fig. 18). The walls of the vesicles are developed into the nervous tissue and neuroglia of the brain, and their cavities are modified to form its ventricles. The remainder of the tube forms the medulla spinalis or spinal cord; from its ectodermal wall the nervous and neuroglial elements of the medulla spinalis are developed, while the cavity persists as the central canal
.
takes place throughout the whole of the embryonic and extra-embryonic areas of the ovum, except in certain regions. One of these is seen immediately in front of the neural tube
. Here the mesoderm extends forward in the form of two crescentic masses, which meet in the middle line so as to enclose behind them an area that is devoid of mesoderm. Over this area, the ectoderm and endoderm come into direct contact with each other and constitute a thin membrane, the buccopharyngeal membrane
, which forms a septum between the primitive mouth and pharynx.
is afterward developed, and this region is therefore designated the pericardial area. A second region where the mesoderm is absent, at least for a time, is that immediately in front of the pericardial area. This is termed the proamniotic area, and is the region where the proamnion is developed; in humans, however, a proamnion is apparently never formed. A third region is at the hind end of the embryo where the ectoderm and entoderm come into apposition and form the cloacal membrane
.
s (primitive segments) are produced. These segmented tissue blocks differentiate into skeletal muscle, vertebrae, and dermis of all vertebrates.
Somitogenesis begins with the formation of somitomeres (whorls of concentric mesoderm) marking the future somites in the presomitic mesoderm
(unsegmented paraxial). The presomitic mesoderm gives rise to successive pairs of somites, identical in appearance that differentiate into the same cell types but the structures formed by the cells vary depending upon the anteroposterior (e.g., the thoracic vertebrates have ribs, the lumbar
vertebrates do not). Somites have unique positional values along this axis and it is thought that these are specified by the Hox (homeotic) genes.
Toward the end of the second week after fertilization, transverse
segmentation of the paraxial mesoderm
begins, and it is converted into a series of well-defined, more or less cubical masses, also known as the somites, which occupy the entire length of the trunk on either side of the middle line from the occipital
region of the head. Each segment contains a central cavity (known as a myocoel), which, however, is soon filled with angular and spindle-shaped cells. The somites lie immediately under the ectoderm
on the lateral aspect of the neural tube
and notochord
, and are connected to the lateral
mesoderm
by the intermediate cell mass. Those of the trunk may be arranged in the following groups, viz.: cervical
8, thoracic 12, lumbar
5, sacral
5, and coccygeal from 5 to 8. Those of the occipital region of the head are usually described as being four in number. In mammals, somites of the head can be recognized only in the occipital region, but a study of the lower vertebrates leads to the belief that they are present also in the anterior part of the head and that, altogether, nine segments are represented in the cephalic region.
begins. The first stage in vertebrate
s is called neurulation
, where the neural plate
folds forming the neural tube
(see above). Other common organs or structures that arise at this time include the heart
and somites (also above), but from now on embryogenesis follows no common pattern among the different taxa of the animal kingdom
.
In most animals organogenesis along with morphogenesis
will result in a larva
. The hatching of the larva, which must then undergo metamorphosis, marks the end of embryonic development.
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
is formed and develops, until it develops into a fetus
Fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...
.
Embryogenesis starts with the fertilization of the ovum
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
(or egg) by sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
. The fertilized ovum is referred to as a zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
. The zygote undergoes rapid mitotic
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
division
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
s with no significant growth (a process known as cleavage
Cleavage (embryo)
In embryology, cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a...
) and cellular differentiation
Cellular differentiation
In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...
, leading to development of an embryo.
Although embryogenesis occurs in both animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
and plant development
Plant embryogenesis
Plant embryogenesis is the process that produces a plant embryo from a fertilised ovule by asymmetric cell division and the differentiation of undifferentiated cells into tissues and organs. It occurs during seed development, when the single-celled zygote undergoes a programmed pattern of cell...
, this article addresses the common features among different animals, with some emphasis on the embryonic development of vertebrates and mammals
Mammalian embryogenesis
Mammalian embryogenesis is the process of cell division and cellular differentiation during early prenatal development which leads to the development of a mammalian embryo.-Difference from human embryogenesis:...
.
Fertilization and the zygote
The egg cell is always asymmetricAsymmetric
Something which is asymmetric displays asymmetry. Specific uses of the term may include:*Asymmetric relation for information on such relations in mathematics and set theory*Asymmetric warfare for information and theories of modern war...
, having an "animal pole
Animal pole
In developmental biology, a blastula embryo is divided into two hemispheres: the animal pole and the vegetal pole.The animal pole consists of small cells that divide rapidly, in contrast with the vegetal pole below it. The animal pole draws its name from its liveliness relative to the...
" (future ectoderm
Ectoderm
The "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...
and mesoderm
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
) and a "vegetal pole" (future endoderm
Endoderm
Endoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and mesoderm , with the endoderm as the intermost layer...
). It is also covered with different protective envelopes, with different layers. The first envelope - the one in contact with the membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
of the egg - is made of glycoproteins and is known as the vitelline membrane
Vitelline membrane
The vitelline membrane is a structure directly adjacent to the outer surface of the plasma membrane of an ovum. It is composed mostly of protein fibers, with protein receptors needed for sperm binding which, in turn, are bound to sperm plasma membrane receptors...
(zona pellucida
Zona pellucida
The zona pellucida is a glycoprotein membrane surrounding the plasma membrane of an oocyte. It is a vital constitutive part of the latter, external but of essential importance to it...
in mammals). Different taxa show different cellular and acellular envelopes englobing the vitelline membrane.
Fertilisation
Fertilisation
Fertilisation is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo...
(also known as 'conception', 'fecundation' and 'syngamy'), is the fusion of gametes to produce a new organism. In animals, the process involves a sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
fusing with an ovum
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
, which eventually leads to the development of an embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external fertilisation. The fertilized egg cell is known as the zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
.
Cleavage and morula
Morula
A morula is an embryo at an early stage of embryonic development, consisting of cells in a solid ball contained within the zona pellucida....
. The different cells derived from cleavage, up to the blastula stage
Blastula
The blastula is a hollow sphere of cells formed during an early stage of embryonic development in animals . The blastula is created when the zygote undergoes the cell division process known as cleavage. The blastula is preceded by the morula and is followed by the gastrula in the developmental...
, are called blastomere
Blastomere
A blastomere is a type of cell produced by division of the egg after fertilization.- References :* "Blastomere." Stedman's Medical Dictionary, 27th ed. . ISBN 0-683-40007-X...
s. Depending mostly on the amount of yolk in the egg, the cleavage
Cleavage (embryo)
In embryology, cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a...
can be holoblastic (total) or meroblastic (partial).
Holoblastic cleavage occurs in animals with little yolk in their eggs, such as humans and other mammals who receive nourishment as embryos from the mother, via the placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
or milk
Milk
Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...
, such as might be secreted from a marsupium
Marsupium
Marsupium is the Latin word for a pouch in several animal groups:* Pouch , in marsupials* Brood pouch , in peracarid crustaceans* Brood pouch , in syngnathids such as sea horses...
. On the other hand, meroblastic cleavage occurs in animals whose eggs have more yolk; i.e. birds and reptiles. Because cleavage is impeded in the vegetal pole, there is a very uneven distribution and size of cells, being more numerous and smaller at the animal pole of the zygote.
In holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, and the second cleavage is perpendicular to the first. From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms:
| Holoblastic | Meroblastic |
|---|---|
|
Fish Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups... , bird Bird Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from... s, reptile Reptile Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors... s) Insect Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae... s) |
The end of cleavage is known as midblastula transition
Midblastula
In developmental biology, midblastula or midblastula transition is a stage during embryonic development in which zygotic gene transcription is activated. There are three major characteristics of pre-MBT embryos. Firstly, all of the embryonic cells undergo cell division at the same time...
and coincides with the onset of zygotic
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
transcription.
In amniotes, the cells of the morula
Morula
A morula is an embryo at an early stage of embryonic development, consisting of cells in a solid ball contained within the zona pellucida....
are at first closely aggregated, but soon they become arranged into an outer or peripheral layer, the trophoblast
Trophoblast
Trophoblasts are cells forming the outer layer of a blastocyst, which provide nutrients to the embryo and develop into a large part of the placenta. They are formed during the first stage of pregnancy and are the first cells to differentiate from the fertilized egg...
, which does not contribute to the formation of the embryo proper, and an inner cell mass
Inner cell mass
In early embryogenesis of most eutherian mammals, the inner cell mass is the mass of cells inside the primordial embryo that will eventually give rise to the definitive structures of the fetus...
, from which the embryo is developed. Fluid collects between the trophoblast and the greater part of the inner cell-mass, and thus the morula is converted into a vesicle
Vesicle (biology)
A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
, called the blastodermic vesicle. The inner cell mass remains in contact, however, with the trophoblast at one pole of the ovum; this is named the embryonic pole, since it indicates the location where the future embryo will be developed.
Formation of the blastula
After the cleavage has produced over 100 cellCell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s, the embryo is called a blastula
Blastula
The blastula is a hollow sphere of cells formed during an early stage of embryonic development in animals . The blastula is created when the zygote undergoes the cell division process known as cleavage. The blastula is preceded by the morula and is followed by the gastrula in the developmental...
. The blastula is usually a spherical layer of cells (the blastoderm
Blastoderm
-In amniotes:A blastoderm is the layer of cells formed at one pole of macrolecithal eggs such as the yolky egg of birds. The yolk prevents the division from taking place through the egg, resulting in meroblastic cleavage during the many cleavage divisions...
) surrounding a fluid-filled or yolk-filled cavity (the blastocoel).
Mammals at this stage form a structure called the blastocyst
Blastocyst
The blastocyst is a structure formed in the early embryogenesis of mammals, after the formation of the morula. It is a specifically mammalian example of a blastula. It possesses an inner cell mass , or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast,...
, characterized by an inner cell mass
Inner cell mass
In early embryogenesis of most eutherian mammals, the inner cell mass is the mass of cells inside the primordial embryo that will eventually give rise to the definitive structures of the fetus...
that is distinct from the surrounding blastula. The blastocyst must not be confused with the blastula; even though they are similar in structure, their cells have different fates.
Before gastrulation
Gastrulation
Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm.Gastrulation takes place after cleavage...
, the cells of the trophoblast become differentiated into two strata: the outer stratum forms a syncytium
Syncytium
In biology, a syncytium is a large cell-like structure; filled with cytoplasm and containing many nuclei. Most cells in eukaryotic organisms have a single nucleus; syncytia are specialized forms used by various organisms.The term may also refer to cells that are connected by specialized membrane...
(i.e. a layer of protoplasm studded with nuclei, but showing no evidence of subdivision into cells), termed the syncytiotrophoblast
Syncytiotrophoblast
Syncytiotrophoblast is the epithelial covering of the placenta villous tree. It is a unique tissue in that it is a multi-nucleated, terminally differentiated syncytium, extending to 13m^2...
; while the inner layer, the cytotrophoblast
Cytotrophoblast
The cytotrophoblast is the inner layer of the trophoblast, interior to the syncytiotrophoblast in an embryo.Cytotrophoblast is considered to be the trophoblastic stem cell; it differentiates into the other forms of trophoblastic tissue .-External links: - "Female Reproductive System: placental...
or "Layer of Langhans," consists of well-defined cells. As already stated, the cells of the trophoblast do not contribute to the formation of the embryo proper; they form the ectoderm
Ectoderm
The "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...
of the chorion
Chorion
The chorion is one of the membranes that exist during pregnancy between the developing fetus and mother. It is formed by extraembryonic mesoderm and the two layers of trophoblast and surrounds the embryo and other membranes...
and play an important part in the development of the placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
. On the deep surface of the inner cell mass
Inner cell mass
In early embryogenesis of most eutherian mammals, the inner cell mass is the mass of cells inside the primordial embryo that will eventually give rise to the definitive structures of the fetus...
, a layer of flattened cells, called the entoderm, is differentiated and quickly assumes the form of a small sac, called the yolk sac
Yolk sac
The yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals...
. Spaces appear between the remaining cells of the mass and, by the enlargement and coalescence of these spaces, a cavity called the amniotic cavity
Amniotic cavity
The amniotic cavity is the closed sac between the embryo and the amnion, containing the amniotic fluid. The amniotic cavity is formed by the fusion of the parts of the amniotic fold, which first makes its appearance at the cephalic extremity, and subsequently at the caudal end and sides of the embryo...
is gradually developed. The floor of this cavity is formed by the embryonic disk
Embryonic disk
The floor of the amniotic cavity is formed by the embryonic disk composed of a layer of prismatic cells, the embryonic ectoderm, derived from the inner cell-mass and lying in apposition with the endoderm....
, which is composed of a layer of prismatic cells, the embryonic ectoderm
Ectoderm
The "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...
, derived from the inner cell mass and lying in apposition with the entoderm.
Formation of the germ layers
The embryonic diskEmbryonic disk
The floor of the amniotic cavity is formed by the embryonic disk composed of a layer of prismatic cells, the embryonic ectoderm, derived from the inner cell-mass and lying in apposition with the endoderm....
becomes oval and then pear-shaped, the wider end being directed forward. Near the narrow, posterior end, an opaque streak, called the primitive streak
Primitive streak
The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Introduction:...
, makes its appearance and extends along the middle of the disk for about one-half of its length; at the anterior end of the streak there is a knob-like thickening termed Hensen's knot. A shallow groove, the primitive groove
Primitive groove
A shallow groove, the primitive groove, appears on the surface of the primitive streak, and the anterior end of this groove communicates by means of an aperture, the blastophore, with the yolk-sac....
, appears on the surface of the streak, and the anterior end of this groove communicates by means of an aperture, the blastopore
Blastopore
A blastopore is an opening into the archenteron during the embryonic stages of an organism. The distinction between protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore...
, with the yolk sac
Yolk sac
The yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals...
. The primitive streak is produced by a thickening of the axial part of the ectoderm
Ectoderm
The "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...
, the cells of which multiply, grow downward, and blend with those of the subjacent entoderm. From the sides of the primitive streak a third layer of cells, the mesoderm
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
, extends lateralward between the ectoderm and entoderm; the caudal end of the primitive streak forms the cloacal membrane
Cloacal membrane
The cloacal membrane is the membrane that covers the embryonic cloaca when still in the development of the urinary and reproductive organs.It is formed by ectoderm and endoderm coming into contact with each other...
. The blastoderm now consists of three layers, named from without inward: ectoderm, mesoderm, and entoderm; each has distinctive characteristics and gives rise to certain tissues of the body. For many mammals, it is sometime during formation of the germ layers that implantation of the embryo in the uterus
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
of the mother occurs.
Formation of the gastrula
During gastrulation cells migrate to the interior of the blastula, consequently forming two (in diploblastic animals) or three (triploblastic) germ layerGerm layer
A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges produce two or three primary tissue layers...
s. The embryo during this process is called a gastrula. The germ layers are referred to as the ectoderm
Ectoderm
The "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...
, mesoderm
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
and endoderm
Endoderm
Endoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and mesoderm , with the endoderm as the intermost layer...
. In diploblastic animals only the ectoderm and the endoderm are present .
- Among different animals, different combinations of the following processes occur to place the cells in the interior of the embryo:
- EpibolyEpibolyEpiboly is a cell movement that occurs in the early embryo, at the same time as gastrulation. It is one of many movements in the early embryo that allow for dramatic physical restructuring . The movement is generally characterized as being a thinning and spreading of cell layers...
- expansion of one cell sheet over other cells - Ingression - cells move with pseudopods
- InvaginationInvaginationInvagination means to fold inward or to sheath. In biology, this can refer to a number of processes.* Invagination is the morphogenetic processes by which an embryo takes form, and is the initial step of gastrulation, the massive reorganization of the embryo from a simple spherical ball of cells,...
- forming the mouthMouthThe mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....
, anusAnusThe anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
, and archenteronArchenteronThe primitive gut that forms during gastrulation in the developing blastula is known as the archenteron. It develops into the digestive tract of an animal.... - Delamination - the external cells divide, leaving the daughter cells in the cavity
- Polar proliferation - Cells at the polar ends of the blastula/gastrula proliferate, mostly at the animal pole.
- Epiboly
- Other major changes during gastrulation:
- Heavy RNA transcription using embryonic genes; up to this point the RNARNARibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
s used were maternal (stored in the unfertilized egg). - Cells start major differentiationCellular differentiationIn developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...
processes, losing their totipotentiality.
- Heavy RNA transcription using embryonic genes; up to this point the RNA
In most animals, a blastopore
Blastopore
A blastopore is an opening into the archenteron during the embryonic stages of an organism. The distinction between protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore...
is formed at the point where cells are entering the embryo. Two major groups of animals can be distinguished according to the blastopore's fate. In deuterostome
Deuterostome
Deuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...
s the anus forms from the blastopore, while in protostome
Protostome
Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
s it develops into the mouth. See Embryological origins of the mouth and anus
Embryological origins of the mouth and anus
This article needs to be clarified and may contain errors.The embryological origin of the mouth and anus is an important characteristic, and forms the morphological basis for separating bilaterian animals into two natural groupings: the protostomes and deuterostomes.In animals at least as complex...
for more information.
Formation of the early nervous system - neural groove, tube and notochord
In front of the primitive streakPrimitive streak
The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Introduction:...
, two longitudinal ridges, caused by a folding up of the ectoderm
Ectoderm
The "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...
, make their appearance, one on either side of the middle line formed by the streak. These are named the neural folds
Neural folds
In front of the primitive streak two longitudinal ridges, caused by a folding up of the ectoderm, make their appearance, one on either side of the middle line...
; they commence some little distance behind the anterior end of the embryonic disk
Embryonic disk
The floor of the amniotic cavity is formed by the embryonic disk composed of a layer of prismatic cells, the embryonic ectoderm, derived from the inner cell-mass and lying in apposition with the endoderm....
, where they are continuous with each other, and from there gradually extend backward, one on either side of the anterior end of the primitive streak. Between these folds is a shallow median
Median
In probability theory and statistics, a median is described as the numerical value separating the higher half of a sample, a population, or a probability distribution, from the lower half. The median of a finite list of numbers can be found by arranging all the observations from lowest value to...
groove, the neural groove
Neural groove
The neural groove is a shallow median groove between the neural folds of an embryo. The neural folds are two longitudinal ridges that are caused by a folding up of the ectoderm in front of the primitive streak of the developing embryo...
. The groove gradually deepens as the neural folds become elevated, and ultimately the folds meet and coalesce in the middle line and convert the groove into a closed tube, the neural tube
Neural tube
In the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord...
or canal, the ectodermal wall of which forms the rudiment of the nervous system. After the coalescence of the neural folds over the anterior end of the primitive streak, the blastopore
Blastopore
A blastopore is an opening into the archenteron during the embryonic stages of an organism. The distinction between protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore...
no longer opens on the surface but into the closed canal of the neural tube, and thus a transitory communication, the neurenteric canal, is established between the neural tube and the primitive digestive tube. The coalescence of the neural folds occurs first in the region of the hind brain, and from there extends forward and backward; toward the end of the third week, the front opening (anterior neuropore
Anterior neuropore
The rostral neuropore or anterior neuropore is a region corresponding to the opening of the embryonic neural tube in the anterior portion of the prosencephalon...
) of the tube finally closes at the anterior end of the future brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
, and forms a recess that is in contact, for a time, with the overlying ectoderm; the hinder part of the neural groove presents for a time a rhomboidal shape
Rhombus
In Euclidean geometry, a rhombus or rhomb is a convex quadrilateral whose four sides all have the same length. The rhombus is often called a diamond, after the diamonds suit in playing cards, or a lozenge, though the latter sometimes refers specifically to a rhombus with a 45° angle.Every...
, and to this expanded portion the term sinus rhomboidalis has been applied. Before the neural groove is closed, a ridge of ectodermal cells appears along the prominent margin of each neural fold; this is termed the neural crest
Neural crest
Neural crest cells are a transient, multipotent, migratory cell population unique to vertebrates that gives rise to a diverse cell lineage including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia....
or ganglion ridge, and from it the spinal and cranial nerve ganglia and the ganglia of the sympathetic nervous system
Sympathetic nervous system
The sympathetic nervous system is one of the three parts of the autonomic nervous system, along with the enteric and parasympathetic systems. Its general action is to mobilize the body's nervous system fight-or-flight response...
are developed. By the upward growth of the mesoderm, the neural tube is ultimately separated from the overlying ectoderm.
The cephalic end of the neural groove exhibits several dilatations which, when the tube is closed, assume the form of three vesicles; these constitute the three primary cerebral vesicles, and correspond respectively to the future 'fore-brain' (prosencephalon
Prosencephalon
In the anatomy of the brain of vertebrates, the prosencephalon is the rostral-most portion of the brain. The prosencephalon, the mesencephalon , and rhombencephalon are the three primary portions of the brain during early development of the central nervous system...
), 'midbrain' (mesencephalon), and 'hind-brain' (rhombencephalon
Rhombencephalon
The rhombencephalon is a developmental categorization of portions of the central nervous system in vertebrates.The rhombencephalon can be subdivided in a variable number of transversal swellings called rhombomeres...
) (Fig. 18). The walls of the vesicles are developed into the nervous tissue and neuroglia of the brain, and their cavities are modified to form its ventricles. The remainder of the tube forms the medulla spinalis or spinal cord; from its ectodermal wall the nervous and neuroglial elements of the medulla spinalis are developed, while the cavity persists as the central canal
Central canal
For the engineering project, see Indiana Central Canal.The central canal is the cerebrospinal fluid-filled space that runs longitudinally through the length of the entire spinal cord. The central canal is contiguous with the ventricular system of the brain...
.
Formation of the early septum
The extension of the mesodermMesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
takes place throughout the whole of the embryonic and extra-embryonic areas of the ovum, except in certain regions. One of these is seen immediately in front of the neural tube
Neural tube
In the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord...
. Here the mesoderm extends forward in the form of two crescentic masses, which meet in the middle line so as to enclose behind them an area that is devoid of mesoderm. Over this area, the ectoderm and endoderm come into direct contact with each other and constitute a thin membrane, the buccopharyngeal membrane
Buccopharyngeal membrane
The region where the crescentic masses of the ectoderm and endoderm come into direct contact with each other constitutes a thin membrane, the buccopharyngeal membrane , which forms a septum between the primitive mouth and pharynx...
, which forms a septum between the primitive mouth and pharynx.
Early formation of the heart and other primitive structures
In front of the buccopharyngeal area, where the lateral crescents of mesoderm fuse in the middle line, the pericardiumPericardium
The pericardium is a double-walled sac that contains the heart and the roots of the great vessels.-Layers:...
is afterward developed, and this region is therefore designated the pericardial area. A second region where the mesoderm is absent, at least for a time, is that immediately in front of the pericardial area. This is termed the proamniotic area, and is the region where the proamnion is developed; in humans, however, a proamnion is apparently never formed. A third region is at the hind end of the embryo where the ectoderm and entoderm come into apposition and form the cloacal membrane
Cloacal membrane
The cloacal membrane is the membrane that covers the embryonic cloaca when still in the development of the urinary and reproductive organs.It is formed by ectoderm and endoderm coming into contact with each other...
.
Somitogenesis
Somitogenesis is the process by which somiteSomite
A somite is a division of the body of an animal. In vertebrates this is mainly discernible in the embryo stage; in arthropods it is a characteristic of a hypothetical ancestor.- In vertebrates :...
s (primitive segments) are produced. These segmented tissue blocks differentiate into skeletal muscle, vertebrae, and dermis of all vertebrates.
Somitogenesis begins with the formation of somitomeres (whorls of concentric mesoderm) marking the future somites in the presomitic mesoderm
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
(unsegmented paraxial). The presomitic mesoderm gives rise to successive pairs of somites, identical in appearance that differentiate into the same cell types but the structures formed by the cells vary depending upon the anteroposterior (e.g., the thoracic vertebrates have ribs, the lumbar
Lumbar
In tetrapod anatomy, lumbar is an adjective that means of or pertaining to the abdominal segment of the torso, between the diaphragm and the sacrum ...
vertebrates do not). Somites have unique positional values along this axis and it is thought that these are specified by the Hox (homeotic) genes.
Toward the end of the second week after fertilization, transverse
Transverse plane
The transverse plane is an imaginary plane that divides the body into superior and inferior parts. It is perpendicular to the coronal and sagittal planes....
segmentation of the paraxial mesoderm
Paraxial mesoderm
Paraxial mesoderm is the area of mesoderm that forms just lateral to the neural tube on both sides.It differentiates rostrally into somatomeres and caudally into somites.It gives rise to the somitomeres/somites and mesoderm of the branchial arches....
begins, and it is converted into a series of well-defined, more or less cubical masses, also known as the somites, which occupy the entire length of the trunk on either side of the middle line from the occipital
Occipital
The word occipital, in zoology, pertains to the occiput .Occipital is a descriptor for several areas of animal & human anatomy.*External occipital protuberance* Internal occipital crest* Greater occipital nerve...
region of the head. Each segment contains a central cavity (known as a myocoel), which, however, is soon filled with angular and spindle-shaped cells. The somites lie immediately under the ectoderm
Ectoderm
The "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...
on the lateral aspect of the neural tube
Neural tube
In the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord...
and notochord
Notochord
The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...
, and are connected to the lateral
Lateral
Lateral may refer to:*Lateral , an anatomical direction*Lateral canal, a canal built beside another stream*Lateral consonant, an ℓ-like consonant in which air flows along the sides of the tongue...
mesoderm
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
by the intermediate cell mass. Those of the trunk may be arranged in the following groups, viz.: cervical
Cervical
In anatomy, cervical is an adjective that has two meanings:# of or pertaining to any neck.# of or pertaining to the female cervix: i.e., the neck of the uterus.*Commonly used medical phrases involving the neck are**cervical collar...
8, thoracic 12, lumbar
Lumbar
In tetrapod anatomy, lumbar is an adjective that means of or pertaining to the abdominal segment of the torso, between the diaphragm and the sacrum ...
5, sacral
Sacral
Sacral may refer to:*sacred*sacrum...
5, and coccygeal from 5 to 8. Those of the occipital region of the head are usually described as being four in number. In mammals, somites of the head can be recognized only in the occipital region, but a study of the lower vertebrates leads to the belief that they are present also in the anterior part of the head and that, altogether, nine segments are represented in the cephalic region.
Organogenesis
At some point after the different germ layers are defined, organogenesisOrganogenesis
In animal development, organogenesis is the process by which the ectoderm, endoderm, and mesoderm develop into the internal organs of the organism. Internal organs initiate development in humans within the 3rd to 8th weeks in utero...
begins. The first stage in vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s is called neurulation
Neurulation
Neurulation is the stage of organogenesis in vertebrate embryos, during which the neural tube is transformed into the primitive structures that will later develop into the central nervous system....
, where the neural plate
Neural plate
In human embryology, formation of neural plate is the first step of neurulation. It is created by a flat thickening opposite to the primitive streak of the ectoderm.-Development:...
folds forming the neural tube
Neural tube
In the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord...
(see above). Other common organs or structures that arise at this time include the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
and somites (also above), but from now on embryogenesis follows no common pattern among the different taxa of the animal kingdom
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
.
In most animals organogenesis along with morphogenesis
Morphogenesis
Morphogenesis , is the biological process that causes an organism to develop its shape...
will result in a larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
. The hatching of the larva, which must then undergo metamorphosis, marks the end of embryonic development.
See also
- Cdx2CDX2Homeobox protein CDX-2 is a protein that in humans is encoded by the CDX2 gene. The protein encoded by this gene is a homeobox transcription factor.-Function:...
gene - Drosophila embryogenesisDrosophila embryogenesisDrosophila embryogenesis, the process by which Drosophila embryos form, is a favorite model system for geneticists and developmental biologists studying embryogenesis. The small size, short generation time, and large brood size make it ideal for genetic studies. Transparent embryos facilitate...
- EnterocoelyEnterocoelyEnterocoely is a process by which some animal embryos develop. In enterocoely, a mesoderm is formed in a developing embryo, in which the coelom forms from pouches "pinched" off of the digestive tract...
- HomeoboxHomeoboxA homeobox is a DNA sequence found within genes that are involved in the regulation of patterns of anatomical development in animals, fungi and plants.- Discovery :...
genes - Mammalian embryogenesisMammalian embryogenesisMammalian embryogenesis is the process of cell division and cellular differentiation during early prenatal development which leads to the development of a mammalian embryo.-Difference from human embryogenesis:...
- Plant embryogenesisPlant embryogenesisPlant embryogenesis is the process that produces a plant embryo from a fertilised ovule by asymmetric cell division and the differentiation of undifferentiated cells into tissues and organs. It occurs during seed development, when the single-celled zygote undergoes a programmed pattern of cell...
- Schizocoely
External links
- Cellular Darwinism
- Embryogenesis & MMPs, PMAP The Proteolysis MapThe Proteolysis MapThe Proteolysis MAP is an integrated web resource focused on proteases.-Rationale:PMAP is to aid the protease researchers in reasoning about proteolytic networks and metabolic pathways.-History and funding:...
-animation - Development of the embryo (retrieved November 20, 2007)
- Human Embryo (retrieved November 20, 2007)
- Video of embryogenesis of the frog Xenopus laevis from shortly after fertilization until the hatching of the tadpole; acquired by MRI (DOI of paper)