The 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 basic and specific functions of the body. After reaching the maturity of the organs of the adult type in the process of metamorphosis, the 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
Amnion, aquatic membrane, amniotic bladder or sac is one of the embryonic membranes characteristic of mammals, birds and reptiles. It arose in the process of evolution during the adaptation of animals 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 fromectoblastic vesicle and forms a cavity filled with fluid. In close relationship with the amnion, serosa develops.
During the birth of mammals, the water shell bursts, the fluid flows out, and the remnants of the bubble remain on the body of the newborn.
Division into anamnia and amniotes
The presence or absence of such a provisional organ as the amnion served as the main principle for dividing all vertebrate organisms into two groups: amniotes and anamnia. 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 the anamnia.
Mammals, birds and reptiles are higher vertebrate organisms with 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 the complex and specific embryonic development.
The common provisional organ of anamnia and amniotes is the yolk sac. In addition to him, the first group of animals has nothing else. In amniotes, temporary organs are also represented by the chorion, allantoin, amnion and placenta. The photo below is a diagram of a primate embryo.
Allantois
In translation from Greek, allantois means "sausage", which quite accurately reflects its appearance. It is formed as a result of protrusion of the wall of the primaryintestines 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 sheets: extra-embryonic ectoderm and mesoderm. It is most pronounced in animals whose development occurs in the egg. In them, it acts as a reservoir for the accumulation of metabolic products, mainly urea. In mammals, this need is completely absent, so the allantois is poorly developed. It performs a different function. In its walls, the formation of umbilical vessels, branching in the placenta, occurs. Thanks to them, the placental circle of blood circulation is further formed.
Yolk sac
The yolk sac is a provisional organ (of birds, amphibians, reptiles, mammals) of endodermal origin. As a rule, it is an outgrowth of the intestine, inside of 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 to digest the yolk and assimilate the products of digestion with their subsequent transportation to the circulatory system of the embryo. To do this, it has a branched network of blood vessels. However, the supply of yolk during the embryonic development of mammals and humans is absent. The 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
Formationyolk 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 much larger compared to the amnion together with the germinal disc. On the 18-19th day after fertilization, erythropoiesis foci form in its walls, which later form a capillary network. After another ten days, the yolk sac becomes the source of primary germ cells. They migrate from it to the anlages of the gonads.
Up to the sixth week after fertilization, the yolk sac continues to produce many proteins (including transferrins, alpha-fetoprotein, alpha-2-microglobulin), acting as the "primary liver".
Like all other provisional organs of mammals, the yolk sac becomes unnecessary at some point. Its tissues perform a wide variety of functions, including excretory, hematopoietic, immunoregulatory, synthetic, and metabolic. However, this happens evenly 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 formation of a cystic type, located at the base of the umbilical cord.
The yolk sac exclusively represents provisional organs in anamnia.
Fetal 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 a mouse, this occurs on the 6th day, and in humans, on the 7th. The process is called implantation, it is based on the immersion of secondary chorionic villi into the wall of the uterus. As a result, a special provisional organ is formed - the placenta. It consists of the germinal part - the villi of the chorion and the maternal part - a relatively altered wall of the uterus. The first also includes the allantoid stalk, 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 is adjacent to the shell or maternal tissues. It is formed like an amnion from the somatopleura and ectoderm in humans 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. A branched chorion forms at the point of contact of the walls of the uterus with the embryo. It has numerous outgrowths (villi) that penetrate the mucous and submucosal layer of the uterus. It is the branched chorion that later 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, excretion of metabolic products, protection from adverse externalfactors, including infections.
Placenta
The placenta is an embryonic organ that is formed in all placental mammals from embryonic membranes (chorion, villous, allantois) that are tightly adjacent to the wall of the uterus. It is connected to the embryo through the umbilical cord (umbilical cord).
The placenta forms the so-called hematoplacental barrier. Vessels of the fetus branch in it to the smallest capillaries and, together with 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 body:
- gas exchange - oxygen penetrates into the blood of the fetus 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 intake of water, minerals and nutrients, electrolytes, vitamins;
- hormonal;
- protective, because the placenta has immune properties and passes the mother's antibodies to the fetus.
Types of placenta
Depending on how deep into the uterine mucosa the villi of the chorion of the embryo are immersed, the following types of placenta are distinguished.
- Semi-placenta. It is found in horses, lemurs, cetaceans, hippos, pigs, camels. The semi-placenta is characterized by the fact that the chorionic villi simply sink into the folds of the uterine mucosa, like fingers in a glove, while penetrating intoepithelial layer is not observed.
- Desmochorial placenta. It is characteristic of ruminants. With this type of placenta, the chorionic villi destroy the uterine mucosa at the point of contact and penetrate into its connective layer, but do not reach the walls of its blood vessels.
- Endotheliochorionic 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 the entire layer of connective tissue of the uterus. Only the endothelial wall separates them from her vessels.
- Hemochorionic placenta. It provides the closest connection between the vessels of the mother and the fetus, which is typical for primates. The chorionic villi penetrate the endothelium of the maternal blood vessels located in the uterine mucosa and sink into the blood lacunae filled with the mother's blood. In fact, the blood of the fetus and mother is separated only by the thin outer shell of the chorion and the walls of the capillary vessels of the embryo itself.
