This and the next chapters return again to ‘organospecificity’, i.e. similarity of the same cell type in the animal kingdom. The series of facts in italics will lead you through.
Organisms are divided into two major groups, their classification depending on whether their cells possess a nuclear envelope. Prokaryotes lack a true nucleus, e.g. bacteria, blue-green algae. Eukaryotes have a well formed nuclear envelope surrounding their chromosomes, e.g. protists, fungi, plants, animals.
Multicellular organisms that pass through embryonic stages of development are called Metazoans.
Birds and mammals are descendants of reptilian species. Mammalian development parallels that of reptiles and birds. Gastrulation movements of reptilian and avian embryos, which evolved around yolky eggs, are retained even in mammalian embryo despite the lack of large amount of yolk. The mammalian inner mass is sitting atop of an imaginary ball of yolk, following instructions typical for its ancestors.
Mammalian embryonic epiblast, that is believed to contain all the cells that generate the actual embryo, is similar in many ways to the avian epiblast.
While the mammalian embryonic epiblast is undergoing cell movements reminiscent of those seen in reptilian or avian gastrulation, the extraembryonic cells are making the distinctly mammalian tissue that enable the fetus to survive within the maternal uterus, i.e. placenta.
Although the initial trophoblastic cells of mice and man appear like regular cells, i.e. cytotrophoblast, they give rise to a population of cells wherein nuclear division occurs in the absence of cytokinesis: syncytiotrophoblast. The cytotrophoblast adheres to the endometrium through a series of adhesion molecules, and in humans contains proteolytic enzymes that enable these cells to enter uterine wall and remodel uterine blood vessels so that the maternal blood bathes fetal blood vessels. Syncytiotrophoblast furthers the penetration of the embryo into the uterine wall. This proteolytic activity ceases after 12 weeks of pregnancy.
Syncytiotrophoblast produces three hormones essential for mammalian development: chorionic gonadotropin, that causes i.a. other placental cells to produce progesterone. Placental progesterone is used by the fetal adrenal gland as a substrate for corticosteroid hormones. Chorionic somatomammotropin is responsible for promoting maternal breast development, enabling milk production later.
The new extra-embryonic organ, that consists of trophoblast and blood vessel-containing mesoderm, is called chorion. Chorion acts to exchange gases and nutrients between mother and fetus but is also an endocrine gland. Chorion protects the fetus from the immune response of the mother as well. Human fetus expresses major histocompatibility antigens from both parents, and mother’s body ought to reject it as ‘non-self’ because it contains also paternally-derived antigens. Chorion has evolved several mechanisms that inhibit the immune response of pregnant female against the fetus.
Early vertebrate development
There is a famous quote from the best embryologist of his time, Karl Ernst von Baer, according to which he forgot to label two small embryos preserved in alcohol, and was unable to determine the genus to which they belonged, because they could be lizards, small birds, or mammals. In other words, different groups of animals share certain common features during early embryonic development, and their features become more and more characteristic of their species as development proceeds.
From his study of comparative embryology v.Baer derived four laws:
1/ The general features of animals of a certain genus appear earlier in embryo than do specialized features. All developing vertebrates appear very similar shortly after gastrulation. For example, all vertebrate embryos have gill arches, notochords, spinal cords, and pronephric kidneys.
2/ Less general characteristics are developed from the more general, until finally the most specialized appear. As examples, all vertebrates have initially the same type of skin, or early development of the limb is essentially the same in all vertebrates.
3/ Each embryo of a given species, instead of passing through the adult stages of other animals, departs more and more from them. For example, the visceral clefts of embryonic birds and mammals resemble visceral clefts of embryonic fishes and other embryonic vertebrates rather than gill slits of adult fishes.
4/ Early embryo of higher animal is never like the adult of lower animal, but is like its early embryo.
Von Baer recognized that there is a common pattern to all vertebrate development: three germ layers give rise to different organs, and this derivation of the organs is constant whether the organism is a fish, a frog, or a chick. Ectoderm forms skin and nerves, endoderm forms respiratory and digestive tubes, and mesoderm forms connective tissue, blood cells, heart, urogenital system, and parts of most internal organs.
In vertebrates a dorsal mesoderm signals ectodermal cells on the top of it to develop into the columnar neural plate cells, and as a result of this neural induction, the cells of the prospective neural plate are distinguished from the surrounding ectoderm, which is destined to become epidermis. This interaction between dorsal mesoderm and its overlying ectoderm is one of the most important interactions during the development of all vertebrates, for it initiates organogenesis, the creation of specific tissues and organs. In this interaction, the chordamesoderm directs the ectoderm to form the hollow neural tube, i.e. neurulation.
The mechanisms of neural tube formation is similar in amphibians, reptiles, birds, and mammals.
Differentiation of the neural tube into the various regions of CNS occurs simultaneously in three different ways. Grossly anatomically, the neural tube and its lumen, bulge and constrict, to form the chambers of brain and spinal cord. On the tissue level, the cell populations within the wall of the neural tube rearrange themselves in various ways to form the different functional regions of CNS. On the cellular level, neuroepithelial cells differentiate into the numerous types of neurons and supportive (glial) cells present in the body.