Human embryogenesis - AbsoluteAstronomy.com

Human embryology is the study of human development during the first eight weeks from gametogenesis

Gametogenesis

Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. Depending on the biological life cycle of the organism, gametogenesis occurs by meiotic division of diploid gametocytes into various gametes,...

(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...

and oocyte

Oocyte

An oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell which undergoes a mitotic...

development) pre-conception through fertilization

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...

(conception of sperm and oocyte in the ampulla arm where the fallopian tube and ovary adjoin) up to and including the 8th week after implantation of 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...

(fertilized egg) in the uterus. After the 8th week of implantation, the developing embryo becomes a fetus. Usually, most individuals spend the first nine months (38 weeks or 266 days) of life within the uterus of the mother.

Human embryogenesis is the process of cell 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...

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...

of the human 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...

that occurs during early stages of this development. To be exact, it spans from the moment when a spermatozoon

Spermatozoon

A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...

meets and fuses 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 is called fertilization

Human fertilization

Human fertilization is the union of a humanoid egg and sperm, usually occurring in the ampulla of the uterine tube. The result of this union is the production of a zygote, or fertilized egg, initiating prenatal development...

to the end of the 8th week of development. This process largely involves coordinated spatial and temporal changes in gene expression. There are many similarities in embryonic development between humans and other species and especially among Chordates.

Fertilization

Human fertilization is the union of a female oocyte (egg) and male sperm, usually occurring in the ampulla hand of the fallopian tube to form the zygote

Zygote

(single diploid cell). It constitutes the penetration of the oocyte (egg) by a sperm succeeded by fusion of their genetic material. This genetic material consists of the 23 chromosomes contained in the nucleus of the ovum and 23 chromosomes from the nucleus of the sperm. The 46 chromosomes undergo changes prior to the mitotic division

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...

which leads to the formation of an embryo having two cells. The fertilized ovum thus begins to divide into several cells, i.e. it starts to undergo 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...

. The two daughter cells are still surrounded by 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...

Cleavage

A human develops from a single diploid cell called a zygote

Zygote

, which results from the fusion of two reproductive cells

Gamete

A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...

; an ovum

Ovum

(egg) being fertilized

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...

by a single spermatozoon

Spermatozoon

A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...

(sperm

Sperm

). The cell is surrounded by a strong membrane of glycoprotein

Glycoprotein

Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...

s called the 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...

(membrane derived from the ovum) which the successful sperm has managed to penetrate.

The zygote undergoes cleavage

Cleavage (embryo)

, increasing the number of cells within the zona pellucida. After the 8-cell stage, embryos undergo what is called compactation, where the cells bind

Cell junction

A cell junction is a type of structure that exists within the tissue of a some multicellular organism . Cell junctions consist of protein complexes and provide contact between neighbouring cells or between a cell and the extracellular matrix...

tightly to each other, forming a compact sphere. After compactation, the embryo is in 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....

stage (32 cells). Cavitation occurs next, where the outermost layer of cells - 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...

- secrete fluid into the morula. As a consequence of this, when the number of cells reaches 40 to 150, a central, fluid-filled cavity (blastocoel

Blastocoele

A blastocoel or blastocele is the fluid-filled central region of a blastocyst...

) has been formed. The 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...

begins to degenerate, allowing the embryo to increase its volume. This stage in the developing embryo, reached after four to six days, is 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,...

(akin to the 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...

stage), and lasts approximately until implantation in the uterus, and is referred to as the preimplantation phase of development.

Each cell of the preimplantation embryo is totipotent

Cell potency

The potency of a cell specifies its differentiation potential, or potential to differentiate into different cell types.-Totipotency:Totipotency is the ability of a single cell to divide and produce all the differentiated cells in an organism, including extraembryonic tissues.Totipotent cells...

. That is, each cell has the potential to form all of the different cell types in the developing embryo. This totipotency

Cell potency

means that some cells can be removed from the preimplantation embryo and the remaining cells will compensate for their absence. This has allowed the development of a technique known as preimplantation genetic diagnosis

Preimplantation genetic diagnosis

In medicine and genetics pre-implantation genetic diagnosis refers to procedures that are performed on embryos prior to implantation, sometimes even on oocytes prior to fertilization. PGD is considered another way to prenatal diagnosis...

(PGD), whereby a small number of cells from the preimplantation embryo created by IVF

In vitro fertilisation

In vitro fertilisation is a process by which egg cells are fertilised by sperm outside the body: in vitro. IVF is a major treatment in infertility when other methods of assisted reproductive technology have failed...

, can be removed by biopsy

Biopsy

A biopsy is a medical test involving sampling of cells or tissues for examination. It is the medical removal of tissue from a living subject to determine the presence or extent of a disease. The tissue is generally examined under a microscope by a pathologist, and can also be analyzed chemically...

and subjected to genetic diagnosis. This allows embryos that are not affected by defined genetic diseases to be selected and then transferred to the mother's 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...

Blastocyst differentiation

The blastocyst

Blastocyst

is characterized by a group of cells, called 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...

(also called embryoblast

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...

) and the mentioned trophoblast (the outer cells), and a blastocyst cavity (blastocoel).

The inner cell mass gives rise to the embryo proper, the amnion

Amnion

The amnion is a membrane building the amniotic sac that surrounds and protects an embryo. It is developed in reptiles, birds, and mammals, which are hence called “Amniota”; but not in amphibians and fish , which are consequently termed “Anamniota”. The primary role of this is the protection of 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...

and allantois

Allantois

Allantois is a part of a developing animal conceptus . It helps the embryo exchange gases and handle liquid waste....

, while the trophoblast will eventually form 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...

. The blastocyst can be thought of as a ball of a (mostly single) layer of trophoblast

Trophoblast

cells, with the inner cell mass attached to this ball's inner wall. The embryo plus its 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...

s is called the conceptus

Conceptus

Conceptus denotes the embryo and its adnexa or associated membranes . The conceptus includes all structures that develop from the zygote, both embryonic and extraembryonic...

. By this stage the conceptus is in the uterus

Uterus

. The zona pellucida ultimately disappears completely, allowing the blastocyst to invade the endometrium, performing implantation.

Implantation

The trophoblast

Trophoblast

then differentiates into two distinct layers: the inner is 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...

(containing cell boundaries) consisting of cuboidal cells that are the source of dividing cells

Cell (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....

, and the outer is 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...

(no cell boundaries).

The syncytiotrophoblast implants the blastocyst in the endometrium

Endometrium

-Function:The endometrium is the innermost glandular layer and functions as a lining for the uterus, preventing adhesions between the opposed walls of the myometrium, thereby maintaining the patency of the uterine cavity. During the menstrual cycle or estrous cycle, the endometrium grows to a...

(innermost epithelial

Epithelium

Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

lining) of the uterus by forming finger-like projections called chorionic villi

Chorionic villi

Chorionic villi are villi that sprout from the chorion in order to give a maximum area of contact with the maternal blood.Embryonic blood is carried to the villi by the branches of the umbilical arteries, and after circulating through the capillaries of the villi, is returned to the embryo by the...

that make their way into the uterus, and spaces called lacunae that fill up with the mother's blood. This is assisted by hydrolytic enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

s that erode the epithelium

Epithelium

. 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...

also produces human chorionic gonadotropin

Human chorionic gonadotropin

Human chorionic gonadotropin or human chorionic gonadotrophin is a glycoprotein hormone produced during pregnancy that is made by the developing embryo after conception and later by the syncytiotrophoblast .. Some tumors make this hormone; measured elevated levels when the patient is not...

(hCG), a hormone

Hormone

A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...

that "notifies" the mother's body that she is pregnant, preventing menstruation by sustaining the function of the corpus luteum

Corpus luteum

The corpus luteum is a temporary endocrine structure in mammals, involved in production of relatively high levels of progesterone and moderate levels of estradiol and inhibin A...

. The villi begin to branch, and contain blood vessels of the 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...

that allow gas exchange

Respiration (physiology)

'In physiology, respiration is defined as the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...

between mother and child.

Inner cell mass differentiation

While the syncytiotrophoblast starts to penetrate into the wall of the uterus, the inner cell mass (embryoblast) also develops.

The embryoblast forms a bilaminar (two layered) embryo, composed of the epiblast
Epiblast
In amniote animal embryology, the epiblast is a tissue type derived either from the inner cell mass in mammals or the blastodisc in birds and reptiles. It lies above the hypoblast.-Mammals:...

and the hypoblast. The epiblast is adjacent to the trophoblast and made of columnar cells; the hypoblast is closest to the blastocyst cavity, and made of cuboidal cells. The epiblast, now called primitive ectoderm will perform 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...

, approximately at day 16 (week 3) after fertilization. In this process, it gives rise to all three germ layer

Germ 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 of the embryo: ectoderm, mesoderm, and endoderm. The hypoblast, or primitive endoderm, will give rise to extraembryonic structures only, such as the lining of the primary (primitive) yolk sac (exocoelomic cavity).

Cavity formation

By separating from the trophoblast, the epiblast forms a new cavity, the amniotic cavity. This is lined by the amnionic membrane, with cells that come from the epiblast (called amnioblasts). Some hypoblast cells migrate along the inner cytotrophoblast lining of the blastocoel, secreting an extracellular matrix

Extracellular matrix

In biology, the extracellular matrix is the extracellular part of animal tissue that usually provides structural support to the animal cells in addition to performing various other important functions. The extracellular matrix is the defining feature of connective tissue in animals.Extracellular...

along the way. These hypoblast cells and extracellular matrix are called Heuser's membrane
Heuser's membrane
Heuser's membrane is a short lived combination of hypoblast cells and extracellular matrix.At day 9-10, at the abembryonic pole, cells from the cytotrophoblast flatten to create a thin exocoelomic membrane known as Heuser’s Membrane. It surrounds the exocoelomic cavity.i.e...

(or exocoelomic membrane), and the blastocoel is now called the primary yolk sac (or exocoelomic cavity).

Cytotrophoblast cells and cells of Heuser's membrane continue secreting extracellular matrix between them. This matrix is called the extraembryonic reticulum. Cells of the epiblast migrate along the outer edges of this reticulum and form the extraembryonic mesoderm, which makes it difficult to maintain the extraembryonic reticulum. Soon pockets form in the reticulum, which ultimately coalesce to form the chorionic cavity or extraembryonic coelom
Extraembryonic coelom
The extraembryonic coelom is a portion of the conceptus consisting of a cavity between Heuser's membrane and the Trophoblast....

.

Another layer of cells leaves the hypoblast and migrates along the inside of the primary yolk sac. The primary yolk sac is pushed to the opposite side of the embryo (the abembryonic pole), while a new cavity forms, the secondary or definitive yolk sac. The remnants of the primary yolk sac are called exocoelomic vesicles (exocoelomic cysts).

Susceptibility

Toxic exposures during the first two weeks following fertilization (second and third weeks of gestational age) may cause prenatal death but do not cause developmental defects. Instead, the body performs a miscarriage

Miscarriage

Miscarriage or spontaneous abortion is the spontaneous end of a pregnancy at a stage where the embryo or fetus is incapable of surviving independently, generally defined in humans at prior to 20 weeks of gestation...

. On the other hand, subsequent toxic exposures in the embryonic period often cause major congenital malformations, since the precursors of the major organ systems are developing.

External links

Photo of blastocyst in utero
Slideshow: In the Womb

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Fertilization

Cleavage

Blastocyst differentiation

Implantation

Inner cell mass differentiation

Cavity formation

Susceptibility

See also

External links