Embryonic disc and its differentiation

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Introduction[edit | edit source]

The embryonic disc is a flat, two-layered structure formed from the inner cell mass of the blastocyst. It is the early developing embryo before organs form.

  • Appears around Day 8 after fertilization.

Bilaminar Germ Disc[edit | edit source]

- 2nd week of development

Bilaminar Germ Disc

Day 8: Formation of amniotic cavity (epiblast cells) and primary yolk sac (hypoblast cells)

  • Implantation is complete and inner cell mass forms a bilaminar disc.

Layers:

  1. Epiblast → columnar cells, forms amniotic cavity above.
  2. Hypoblast → cuboidal cells, lines blastocoel, contributes to yolk sac.


Day 9–12: Extraembryonic mesoderm forms between cytotrophoblast and yolk sac/amniotic cavity.

Trophoblast differentiation:

  • Syncytiotrophoblast (invades endometrium)
  • Cytotrophoblast (proliferative)

Day 13:

  • Formation of chorionic cavity (extraembryonic coelom).
  • Epiblast still maintains the amniotic cavity.

Position:

  • Epiblast faces the amniotic cavity.
  • Hypoblast faces the yolk sac.

Trilaminar Germ Disc[edit | edit source]

- 3rd week of development: gastrulation

  • Primitive streak forms and starts cranial-caudal axis formation
  • Gastrulation: when epiblast cells migrate through the primitive streak and form three layers:
  1. Ectoderm: epiblast cells on remaining on the surface
  2. Mesoderm: epiblast cells between epiblast and hypoblast
  3. Endoderm: epiblast cells replacing hypoblast
  • Notochord formation: from mesodermal cells migrating cranially, it induces neural plate formation
  • Cranial-caudal, dorsal-ventral, left-right body axes become defined

Embryonic period: Germ Layer Derivatives[edit | edit source]

- 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[edit | edit source]

  1. Sadler, T. W. Langman’s Medical Embryology, 14th edition
  2. Junqueira, L. C., Carneiro, J. Basic Histology: Text and Atlas, 15th edition
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