Temporary organs formed in a certain period of individual development in the larvae of multicellular animals and embryos are called provisional organs. In humans and mammals, they function only at the stage of the embryo and perform both the basic functions of the body and specific ones. After reaching the maturity of adult organs in the process of metamorphosis, temporary ones disappear. These formations accompanying the development of many animals are of interest for evolutionary morphology, physiology, and embryology.
The following temporary organs are characteristic of humans and mammals: amnion, chorion, allantois, yolk sac and placenta.
Amnion
An amnion, an aqueous membrane, an amniotic bladder, or a sac is one of the germinal membranes characteristic of mammals, birds, and reptiles. It arose in the process of evolution when animals adapted to life on land. The main function of the amnion is to protect the embryo from environmental factors and create favorable conditions for its development. It arises from an ectoblastic vesicle and forms a cavity filled with liquid. In close relationship with amnion, serosa develops.
During the birth of mammals, the water membrane bursts, the liquid flows out, and the remains of the bladder remain on the body of the newborn.
Division of Anamnias and Amniotes
The presence or absence of such a provisional organ as amnion served as the main principle for dividing all vertebrate organisms into two groups: amniotes and anamnias. From the point of view of evolution, the most ancient are those animals that developed in the aquatic environment (cyclostomes, fish, amphibians). They do not need an additional water shell for the embryo. They belong to anamnias.
Mammals, birds and reptiles are the highest vertebrate organisms that have highly efficient and coordinated organ systems that allow them to exist in a wide variety of land and water conditions. In fact, they have mastered all habitats. This would not have been possible without complex and specific embryonic development.
The common provisional organ anamnius and amniot is the yolk sac. Besides him, the first group of animals has nothing more. In amniotes, the temporary organs are also represented by chorion, allantoin, amnion and placenta. In the photo below, the primate embryo scheme.
Allantois
Translated from the Greek language, allantois means "sausage-shaped", which quite accurately reflects its appearance. It is formed as a result of protrusion of the wall of the primary intestine into the space between the yolk sac and the amnion. In a human embryo, this happens by 16 days after fertilization.
Allantois is a provisional organ consisting of two leaves: extra-fetal ectoderm and mesoderm. It is most pronounced in animals whose development occurs in the egg. In them, it serves as a reservoir for the accumulation of metabolic products, mainly urea. In mammals, this need is completely absent, therefore, allantois is poorly developed. It performs another function. In its walls, the formation of umbilical vessels branching in the placenta. Thanks to them, the placental circle of blood circulation is further formed.
Yolk sac
The yolk sac is a provisional organ (birds, amphibians, reptiles, mammals) of endodermal origin. As a rule, it is an outgrowth of the intestine, inside which there is a supply of yolk. The latter is used by the embryo or larva for nutrition. From the point of view of evolution, the primary role of the yolk sac was the digestion of the yolk and the digestion of digestion products, followed by their transportation to the bloodstream of the embryo. To do this, he has a branched network of blood vessels. However, the supply of yolk during embryonic development of mammals and humans is absent. Preservation of the yolk sac is associated with an important secondary function - hematopoiesis. In the photo, it is indicated by a black circle (6th week of embryonic development).
The role of the yolk sac in human development
The formation of the yolk sac from the endoblastic vesicle occurs on the 29-30th day of pregnancy. During the period of human embryonic development, the provisional organ plays an important role. The size of the yolk sac in the early stages of pregnancy (up to six weeks) is significantly larger compared to the amnion along with the germinal disc. On the 18-19th day after fertilization, foci of erythropoiesis are formed in its walls, which subsequently form the capillary network. After another ten days, the yolk sac becomes a source of primary germ cells. They migrate from it to the gonad bookmarks.
Until the sixth week after fertilization, the yolk sac continues to produce many proteins (including transferrin, alpha-fetoprotein, alpha-2-microglobulin), acting as the "primary liver".
Like all other provisional organs of mammals, the yolk sac at some point becomes unnecessary. Its tissues perform a wide variety of functions, including excretory, hematopoietic, immunoregulatory, synthetic, metabolic. However, this happens even until the corresponding organs begin to work in the fetus. In humans, the yolk sac ceases to function at the end of the first trimester of pregnancy. It is reduced and remains only in the form of a small cystic type formation located at the base of the umbilical cord.
Exclusively in the yolk sac are provisional organs in anamnias.
Embryo implantation
A characteristic feature of the development of higher mammals is the relatively tight connection of the embryo with the wall of the uterus, which is established a few days after the start of development. For example, in the mouse this happens on the 6th day, and in humans - on the 7th. The process is called implantation, it is based on immersion in the wall of the uterus of the secondary chorionic villi. As a result, a special provisional organ is formed - the placenta. It consists of the embryonic part - the chorionic villus and the maternal - a relatively altered uterine wall. The first also includes the allantoid leg, which plays an important role in the blood supply to the fetus in lower (marsupial) mammals. Their maternal part of the placenta is not developed.
Chorion
Chorion or, as it is often called, serosa is the outermost shell of the embryo; it adjoins the shell or maternal tissues. It forms like an amnion from somatopleura and ectoderm in humans at 7-12 days after fertilization, and its transformation into part of the placenta occurs at the end of the first trimester of pregnancy.
Chorion consists of two parts: smooth and branched. The first does not contain villi and surrounds the fetal egg almost completely. The branching chorion forms at the site of contact of the walls of the uterus with the embryo. It has numerous outgrowths (villi) that penetrate the mucous and submucous layer of the uterus. It is the branching chorion in the future that becomes the fetal part of the placenta.
This temporary organ performs functions similar to those for which a functionally mature placenta serves: fetal respiration and nutrition, isolation of metabolic products, protection against adverse external factors, including infections.
Placenta
The placenta is an embryonic organ that is formed in all placental mammals from the germinal membranes (chorion, villous, allantois), which are closely adjacent to the uterine wall. It is connected to the embryo through the umbilical cord (umbilical cord).
The placenta forms the so-called hematoplacental barrier. The vessels of the fetus branch in it to the smallest capillaries and together with the supporting tissues form chorionic villi. In primates (including humans), they are immersed in lacunae filled with maternal blood. This determines the following functions of the provisional authority:
- gas exchange - oxygen enters the fetal blood from the mother’s blood according to the laws of diffusion, and carbon dioxide moves in the opposite direction;
- excretory and trophic: removal of metabolites (creatine, creatinine, urea) and the intake of water, mineral and nutrients, electrolytes, vitamins;
- hormonal;
- protective, because the placenta has immune properties and passes the mother’s antibodies to the fetus.
Types of the placenta
Depending on how deeply the chorionic villi of the embryo are immersed in the mucous membrane of the uterus, the following types of placenta are distinguished.
- Half placenta. It is found in horses, lemurs, cetaceans, hippos, pigs, camels. The half-placenta is characterized by the fact that the chorionic villi are simply immersed in the folds of the uterine mucosa, like fingers in a glove, while penetration into the epithelial layer is not observed.
- Desmochorial placenta. It is characteristic of ruminants. With this type of placenta, chorionic villi destroy the uterine mucosa at the point of contact and penetrate into its connective layer, but they do not reach the walls of its blood vessels.
- Endotheliochorial placenta. It is characteristic of higher predatory amniotes. The provisional organ establishes even closer contact between the vessels of the mother and the fetus. Chorionic villi penetrate through the entire layer of the uterine connective tissue. Only the endothelial wall separates them from its vessels.
- Hemochorial placenta. It provides the closest connection between the vessels of the mother and the fetus, characteristic of primates. Chorionic villi penetrate the endothelium of maternal blood vessels located in the mucous membrane of the uterus and are immersed in blood gaps filled with mother's blood. In fact, only the thin outer shell of the chorion and the walls of the capillary vessels of the embryo share the blood of the fetus and mother.