Allantois
Encyclopedia
Allantois is a part of a developing animal conceptus
(which consists of all embryo
nic and extra-embryonic tissues). It helps the embryo exchange gases and handle liquid waste.
The allantois, along with the amnion
and chorion
(other embryonic membranes), identify humans, and other mammal
s, as amniote
s. Other amniots include reptile
s, dinosaur
s, and bird
s. Of the vertebrates, only Ichthyopsidas (fish
and amphibian
s) lack this structure.
and excretion
, and is webbed with blood vessel
s.
The function of the allantois is to collect liquid waste from the embryo, as well as to exchange gases used by the embryo.
Oxygen
is absorbed by the allantois through the egg shell. The allantois functions similarly in monotremes, which are egg-laying mammals.
Also, most marsupial allantoises do not fuse with the chorion
. An exception is the allantois of the bandicoot
, which has a vasculature, and fuses with the chorion.
.
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...
(which consists of all 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...
nic and extra-embryonic tissues). It helps the embryo exchange gases and handle liquid waste.
The allantois, along with 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...
and 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...
(other embryonic membranes), identify humans, and other mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s, as amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...
s. Other amniots include 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, dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s, and 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. Of the vertebrates, only Ichthyopsidas (fish
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...
and amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
s) lack this structure.
Function
This sac-like structure is primarily involved in nutritionNutrition
Nutrition is the provision, to cells and organisms, of the materials necessary to support life. Many common health problems can be prevented or alleviated with a healthy diet....
and excretion
Excretion
Excretion is the process by which waste products of metabolism and other non-useful materials are eliminated from an organism. This is primarily carried out by the lungs, kidneys and skin. This is in contrast with secretion, where the substance may have specific tasks after leaving the cell...
, and is webbed with blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...
s.
The function of the allantois is to collect liquid waste from the embryo, as well as to exchange gases used by the embryo.
In reptiles, birds, and monotremes
The structure first evolved in reptiles and birds as a reservoir for nitrogenous waste, but also as a means for oxygenation of the embryo.Oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
is absorbed by the allantois through the egg shell. The allantois functions similarly in monotremes, which are egg-laying mammals.
In most marsupials
In most marsupials, the allantois is avascular, having no blood vessels, but still serves the purpose of storing nitrogenous (NH3) waste.Also, most marsupial allantoises do not fuse with 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...
. An exception is the allantois of the bandicoot
Bandicoot
Bandicoots are a group of about 20 species of small to medium-sized, terrestrial marsupial omnivores in the order Peramelemorphia.- Etymology :...
, which has a vasculature, and fuses with the chorion.
In placental mammals (Eutheria)
In placental mammals, the allantois is part of and forms an axis for the development of the umbilical cordUmbilical cord
In placental mammals, the umbilical cord is the connecting cord from the developing embryo or fetus to the placenta...
.
- The mouseMouseA mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...
allantois consists of mesodermMesodermIn 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...
al tissue, which undergoes vasculogenesis to form the mature umbilical artery and vein.
- The humanHumanHumans are the only living species in the Homo genus...
allantois is an endodermEndodermEndoderm 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...
al evagination of the developing hindgutHindgutThe hindgut is the posterior part of the alimentary canal. In mammals, it includes the distal third of the transverse colon and the splenic flexure, the descending colon, sigmoid colon and rectum.-Blood flow:...
which becomes surrounded by the mesodermal connecting stalk. The connecting stalk forms the umbilical vasculature. The fetal bladder is connected to the allantois via the urachusUrachusThe urachus is a fibrous remnant of the allantois, a canal that drains the urinary bladder of the fetus that joins and runs within the umbilical cord...
, which removes nitrogenous waste from the fetal bladder. The allantois is vestigial and may regress, yet the homologous blood vessels persist as the umbilical arteries and veins connecting the embryo with the placenta.
