Gastrulation, formation of axial organ rudiments, and extraembryonic membranes
12 topics with brief reviews: definitions, key facts, and seminar questions.
Gastrulation is a turning point in embryogenesis when the single-layered embryo transforms into a three-layered one. In this section, we analyze the mechanisms of gastrulation from an evolutionary perspective: from the primitive amphioxus and amphibians to birds and mammals. This is the foundation without which further tissue development cannot be understood.
The second part of the section is dedicated to notogenesis—the formation of the axial organ rudiments—as well as the development of trunk folds that separate the embryo's body from extraembryonic material. Special emphasis is placed on provisional (temporary) organs: the yolk sac, amnion, allantois, and chorion. They provide nutrition, respiration, and protection for the fetus, and are discarded after birth.
What is most frequently asked in class? Instructors love to demand a comparative analysis of bird and mammalian gastrulation, as well as precise knowledge of the tissue sources for the development of each provisional organ. Make sure you understand how the chorion forms and what role the allantois plays—these are classic "killer" questions during colloquia.
What is the main criterion for the completion of gastrulation?
The process is considered fully complete at the moment when all three germ layers are definitively formed in the embryo: ectoderm, mesoderm, and endoderm.
How many lips does the blastopore form in amphioxus?
The margins of the blastopore (primitive mouth) in amphioxus consist of four lips: one dorsal, one ventral, and two lateral.
What type of cleavage precedes gastrulation in amphibians?
Cleavage is complete, but unequal and asynchronous. Due to the abundance of yolk at the vegetal pole, division there proceeds significantly slower.
From which layer do all three germ layers form in birds?
All three germ layers (ectoderm, mesoderm, endoderm) form exclusively from the upper layer—the epiblast, which forms after delamination.
Why does the mammalian egg cell contain little yolk?
The mammalian egg cell is secondarily oligo- and isolecithal. The yolk regressed during evolution since the primary nutrition of the developing embryo is provided by the maternal organism's resources.
What is the neurula stage and what characterizes it?
The neurula is a stage of embryogenesis during which the neural tube (neurulation) and other axial organs are established. The process is induced by the notochord.
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