Initial state and first stage
By the onset of gastrulation, the avian embryo is a discoblastula—a plate of cells located on the surface of the uncleaved yolk.
The first stage of gastrulation is called delamination. The essence of this process is the splitting of the initial single-cell layer of the blastodisc into two independent sheets via cell divisions:
- Upper layer (epiblast) consists of tall columnar cells. In this layer, nuclei are located at different heights, creating a pseudostratified appearance. It is the epiblast that serves as the source for the formation of the embryonic shield. Later on, all three classic germ layers—ectoderm, mesoderm, and endoderm—will develop from it.
- Lower layer (hypoblast) is formed by flattened cells and directly covers the yolk mass. According to modern concepts, the hypoblast does not give rise to the tissues of the embryo proper.
Second stage: immigration and formation of axial structures
The second stage is called immigration. Due to continuous active cell division in the epiblast, an excess of cells arises, and cell masses begin complex movements.
At the first stage of migration, epiblast cells move across the surface from the edges of the blastodisc toward its central axis (midline) and then plunge downward.
As a result of the accumulation of these migrating cells in the central part of the disc, characteristic elevations appear:
- Primitive streak (primitive streak).
- Primitive node (Hensen's node)—a thickening at the anterior margin of the primitive streak.
In the center of these structures, migrating cells literally "flow" inward, forming depressions: along the primitive streak, the primitive groove appears, and in the center of the node, the primitive pit. Evolutionarily, this entire complex (streak, node, groove, and pit) is an analogue (homologue) of the amphibian blastopore.
Differentiation of germ layers
Having plunged into the embryo, the cells change their direction of movement, spreading from the center to the periphery. This is how the germ layers form:
- Endoderm: The first wave of migrating cells pushes the hypoblast aside and forms the embryonic endoderm. The displaced hypoblast cells become the extraembryonic endoderm.
- Mesoderm and notochord: A second batch of cells spreads as a loose layer between the epiblast and the newly formed endoderm. In the center, they lay down the axial structure—the notochordal primordium—while "mesodermal wings"—the mesoderm—form on either side of it.
- Ectoderm: Cells that did not participate in migration at all and remained on the surface (material of the former epiblast) become the ectoderm.
Microscopic structure
In a cross-section of a chicken embryo at the primitive streak stage, the following structures can be observed:
- The embryo still retains the flat shape of a discoblastula.
- In the center of the section, a thickening with a depression—the primitive groove—is identified.
- Ectoderm appears as the most massive, thick layer with a pseudostratified arrangement of nuclei.
- Mesoderm lies as a loose cluster of individual cells, resembling mesenchyme.
- Endoderm is represented by the thinnest layer of flat (flattened) cells.
Briefly: answers to topical questions
Which organs and tissues develop from the embryonic ectoderm of birds subsequently?
The embryonic ectoderm gives rise to the neural tube, neural crest cells (ganglionic plates), and surface ectoderm. The differentiation of this germ layer proceeds in three main directions:
- Neural tube—forms the brain and spinal cord.
- Neural crest cells (ganglionic plates)—form peripheral nerve ganglia.
- Surface ectoderm—gives rise to the epidermis of the skin and its derivatives (feathers/hair, nails, sebaceous and sweat glands), the epithelium of the initial (oral cavity) and terminal (anal part of the rectum) segments of the digestive tract, the vaginal epithelium, as well as the lens and anterior corneal epithelium.
To remember the stages: first we divide (DELamination - two layers), then we move (IMMIgration - inward cell movement).
Class and exam questions
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.
What is the primitive streak and what is its significance?
The primitive streak is an accumulation of epiblast cells along the midline through which cells migrate into the embryo. This complex is homologous to the amphibian blastopore.
What is the function of the hypoblast?
The hypoblast covers the yolk and, according to modern data, does not participate in forming the tissues of the embryo proper, instead forming the extraembryonic endoderm.
What does the mesoderm look like at the primitive streak stage?
On a histological specimen, the mesoderm is represented by loosely arranged individual cells (mesenchymal type) lying between the ectoderm and endoderm.
What else to study on this topic
- Morphology of the avian ovum
- Features of discoidal cleavage
- Structure of the discoblastula
- Homology of the primitive streak and blastopore
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