Embryonic disc and its differentiation

The embryonic disc is a flat, two-layered structure that forms from the inner cell mass of the blastocyst around Day 8 after fertilization. It represents the earliest organisation of the embryo before the formation of organs and complex tissues.

Bilaminar Germ Disc (Week 2)

By the second week of development, implantation is completed, and the inner cell mass differentiates into the bilaminar germ disc. It consists of two cellular layers:

Day 9 of human embryo

Layers:

1. Epiblast - a layer of columnar cells that forms the roof of the amniotic cavity.
2. Hypoblast - a layer of cuboidal cells that lines the blastocoel and contributes to the formation of the primary yolk sac.

Day 8

• The amniotic cavity forms within the epiblast
• The primary yolk sac forms from migrating hypoblast cells.
• The trophoblast differentiates into:
  • Syncytiotrophoblast - an invasive outer layer that erodes maternal tissue.
  • Cytotrophoblast - a mitotically active inner cell layer.

Days 9-12

• Extraembryonic mesoderm forms between cytotrophoblast and the yolk sac/amniotic cavity.

Day 13

• Formation of chorionic cavity (extraembryonic coelom).
• The definitive yolk sac forms as the primary yolk sac is remodeled.
• The epiblast continues forming the roof of the amniotic cavity.

Relative Positioning of the Layers

• The epiblast faces the amniotic cavity.
• The hypoblast faces the yolk sac.

Trilaminar Germ Disc (Week 3): Gastrulation

During the third week, the embryo undergoes gastrulation, a process that transforms the bilaminar disc into a trilaminar structure composed of the ectoderm, mesoderm and the endoderm.

Primitive Streak

• It appears on the dorsal epiblast surface
• It establishes the cranial-caudal axis and defines the body plan.

Formation of the Germ Layers

Epiblast cells migrate through the primitive streak to form:

• Endoderm - the cells that replace the hypoblast.
• Mesoderm - the cells that migrate between the endoderm and the ectoderm.
• Ectoderm - the cells that remain on the surface.

Notogenesis

• Notogenesis is the process by which the mesodermal cells migrate cranially from the primitive node and form the notochord.
• The notochord induces the formation of the neural plate and initiates neurulation.

Embryonic Axes

By the end of gastrulation, the embryo has established:

• Cranial-caudal axis
• Dorsal-ventral axis
• Left-right axis

Germ Layer Derivatives (Weeks 3-8)

Gastrulation and derivatives

- 3rd to 8th week of development

• Ectoderm derivatives:

CNS & PNS, epidermis, hair, nails, lens of eye, enamel

• Mesoderm derivatives:

Bones, cartilage, muscles, cardiovascular system, kidneys, gonads, connective tissues

• Endoderm derivatives:

Epithelial lining of GI tract, respiratory tract, liver, pancreas, thyroid

• Patterning & Axis Formation:

Homeobox (HOX) genes regulate anteroposterior patterning.

Morphogen gradients determine cell fate & organ positioning.

References

1. Sadler, T. W. Langman’s Medical Embryology, 14th edition
2. Junqueira, L. C., Carneiro, J. Basic Histology: Text and Atlas, 15th edition
3. Moore, K. L., Persaud, T. V. N., & Torchia, M. G. The Developing Human: Clinically Oriented Embryology.
4. Moore, K. L. The Developing Human.