Chapter Summary

Embryos of mammals, birds, and reptiles develop within enclosed extraembryonic membranes, the innermost of which is called the amnion. These animals are therefore called amniotes. Amniotes such as chickens and mice form an excellent system to study to understand human embryogenesis mainly due to their readily accessible embryos which are amenable to manipulations. Furthermore, the wealth of genetic information available for the mouse renders it suitable for study of molecular details of embryonic development.

During gametogenesis in birds a large amount of yolk is deposited in the oocyte within the ovary. On release from the ovarian follicle, the egg is fertilized; while it travels through the oviduct, it accumulates proteins such as ovalubumin and lysozyme secreted by a specialized portion of the oviduct called the magnum. Early development of avian embryos entails the formation of the blastodisc; it forms by rapid mitotic cleavage of a small disc (~ 1 mm) of cytoplasm perched on one side of the yolk. A thin fluid-filled cavity, called the subgerminal cavity, forms between the central portion of the blastodisc and the yolk. Cells in the blastodisc give rise to the epiblast. A few cells from the epiblast migrate into the underlying subgerminal cavity and form the hypoblast.

The sequence of cleavage events during early chick development occurs later during the passage of the egg through the reproductive tract than is shown in fig 5.1. Though fertilization does occur in the infundibulum, the egg passes quickly through the magnum and isthmus (about 4 hours total), and cleavage only begins as the egg moves from the isthmus to the uterus. Cleavage occurs during the next 20 to 24 hours as the egg rotates in the uterus, and cells are delaminated, or shed, from the blastodisc during this time. Hypoblast formation, resulting from delamination and outgrowth from Koller's sickle, takes place during the 12 hours after egg laying.

The net result of the intrauterine cleavage and hypoblast formation occurring after egg laying is a one-cell-layer thick epiblast (area pellucida) surrounded by a multicell layer area opaca, all of it underlain by a hypoblast. Recent evidence suggests that the first formed hypoblast layer is shoved anteriorly by subsequent contributions of cells from the marginal zone. Thus, just prior to primitive streak formation, the hypoblast is a result of an early layer being pushed anteriorly by another layer (called endoblast) advancing from the posterior marginal zone.

The three germ layers in amniotes are all derived from the epiblast. Cells in the posterior half of the epiblast move toward the midline, eventually resulting in a marked thickening along this area. The thickening gradually lengthens along the anterior-posterior axis and becomes an area of active invagination and ingression and is called the primitive streak. Some cells in this area move inward into the underlying hypoblast, thus displacing the previously existing cells of the hypoblast laterally, and become the endoderm. Other cells move in through the primitive streak and come to lie loosely in between the hypoblast and the epiblast; these cells form the mesoderm of the embryo. [fig 5.2]

Cell marking and transplantation experiments have highlighted the inductive capacity of the Hensen's node region. The Hensen's node in amniotes can be considered analogous the Spemann organizer in amphibians and is also known to express genes similar to those expressed in the amphibian organizer.

Embryonic development of mammalian embryos takes place in the uterus which is a specialised region of the oviduct designed to provide nutrition to the growing embryo by supplying nutrients from maternal circulation. These functions are satisfied by the placenta which provides a humid environment for the growing embryo as well as means for respiration and excretion of metabolic wastes.

Oogenesis in mammals is characterised by the growth of the oocyte, meiosis, and formation of membranes by the follicle cells surrounding the egg. Oogenesis is regulated by hormonal signals initiated in the pituitary. Fertilization of the egg takes place in the Fallopian tubes (the mammalian oviducts). This is followed by mitosis; early mitotic divisions in mammalian embryos are relatively slow as compared to that in invertebrates. Also, mammalian embryos begin zygotic transcription much earlier in development (as early as the two-cell stage).

The Inner Cell Mass (ICM) is unique to mammalian embryos. After the fourth cleavage cells adopt different lineages-trophoblastic or ICM based on their position. It has been recently shown that such differences in cellular fate can be recognizable as early as the two-cell stage. One of the cells after the first cleavage always gives rise to the upper portion of the ICM, and the other cell contributes to mural trophoectoderm and primitive endoderm. The ICM later generates the epiblast which undergoes continuous cell division and forms an anterior node region similar to the Hensen's node in the chick. Transplantation experiments in the mouse suggest that mammals may posses a second head organizer in the anterior visceral endoderm that induces the formation of a head [fig. 5.13a].

While the ICM divides and gives rise to the embryo proper, cells belonging to the trophoblast undergo rapid proliferation and invade the mother's richly vascularized mesenchymal tissue which underlies the uterine epithelium. The developing Trophoblast remains connected to the embryo by a mesodermal strand which later becomes the umbilicus [fig. 5.14b]. The placenta is an important organ during mammalian embryonic development as it serves to supply nutrients to the growing embryo as well as facilitates respiration and excretion. The placenta also forms a barrier between the mother and the growing fetus, so that the mother's immune system may not mount an immunological attack against the tissues of the growing embryo.

Further Reading

The following Web site is a particularly good resource for movies on embryonic development in different animals. One can see how gastrulation and organization of germs layers takes place.

Stern Lab at Kings College, London.


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