Gastrulation in mammals

Preparation for gastrulation and blastocyst structure

Before gastrulation begins, the mammalian zygote undergoes total, unequal, and asynchronous cleavage. The outcome of this process is the formation of a specific type of blastula — the blastocyst.

Structurally, the blastocyst is a vesicle consisting of three main components:

• Trophoblast — the outer wall of the vesicle.
• Embryoblast (inner cell mass) — a compact group of embryonic cells adjacent to the inner surface of the trophoblast.
• Blastocoele — the internal cavity.

An important morphological event prior to the onset of gastrulation is the shape change of the embryoblast cells. They flatten, arranging themselves into a structure that is visually and functionally analogous to the avian blastoderm. It is this similarity that dictates that subsequent cell mass movements will follow the "avian" scenario.

Stage 1: Delamination

The initial phase of mammalian gastrulation occurs via delamination. This process takes place entirely within the embryoblast and involves two sequential stages of cellular cleavage.

1. Primary cleavage. The single embryoblast cell mass divides into two layers. The inner layer, facing the blastocyst cavity (blastocoele), forms the hypoblast. The outer layer, which remains adjacent to the trophoblast, is called the epiblast.
2. Secondary cleavage. This stage is a distinctive feature of higher animals and humans. The resulting epiblast undergoes a second split into two new layers:
3. Amniotic ectoderm — the upper layer of cells maintaining contact with the trophoblast.
4. Embryonic epiblast — the lower cell layer.

A slit-like space forms between these two layers — the amniotic cavity. The main result of the entire delamination phase is the formation of the embryonic shield from the embryonic epiblast cells. Similar to the avian epiblast, this shield acts as a universal source for the development of all three germ layers.

Stage 2: Immigration (cell migration)

The second phase of gastrulation — immigration — unfolds within the embryonic epiblast. The process begins with active cell migration across the epiblast surface from the periphery toward the center.

Meeting at the midline, the migrating cells form thickenings known as axial structures:

• Primitive streak, along the axis of which a depression forms — the primitive groove.
• At the anterior end of the streak, the primitive (Hensen's) node forms with a central primitive pit.

Following the formation of these structures, invagination begins: cells move beneath the embryonic shield through the groove and pit. Subsequent differentiation depends on the timing and depth of migration:

• The first batch of cells sinks deeply, displacing the primary hypoblast to the periphery. These cells completely replace it, forming the embryonic endoderm. The displaced original hypoblast thereby becomes the extraembryonic endoderm.
• The second batch of cells distributes in the space above the newly formed endoderm. Proliferating, they form the mesoderm and the centrally located notochordal rudiment.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Features of asynchronous cleavage in the mammalian zygote
• Functions of the trophoblast during implantation and nutrition
• Mechanisms of extraembryonic organ formation (amnion, chorion)
• Histogenesis and mesodermal derivatives at late stages of development
• Comparative analysis of mammalian and avian gastrulation

Other topics in "Gastrulation, formation of axial organ rudiments, and extraembryonic membranes"

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