DEV2011 Lecture Notes - Lecture 7: Yolk Sac, Epiblast, Hypoblast
25 May 2018
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Lecture 7 – Embryo, Gastrulation and Germ Layers
Differentiation
• Lineage specification and restriction of developmental potential
• Totipotent cells of epiblast differentiate and become more restricted in their
developmental potential as gastrulation and embryogenesis proceeds
o Restrictions: 3 germ layers
Fertilisation and Implantation in Humans
• Oocytes ovulated in ovary → moves down fallopian tubes (where sperm finds
oocytes)
• Pre-implantation development – cleavage
o 1 cleavage division per day for first 3-4 days
▪ Driven by mRNA and proteins in eggs
o Human foetal genome switches on at 4-8 cell stage
• Early implantation: epiblast and hypoblast
o Cells of inner cell mass → epithelial like → delaminate to form two
layers (bilaminar germ disk)
▪ Upper (dorsal) layer – epiblast
• Epiblast becomes embryonic tissue
▪ Lower (ventral) layer – hypoblast
• Hypoblast forms primitive/parietal endoderm that lines
yolk sac
• Gives rise to extra-embryonic
Establishment of Asymmetry: anterior-posterior and dorso-ventral axis
• Second polar body marks anterior – determines axis of first cleavage
• Dorsal ventral axis forms at right angles to A-P axis
• Gastrulation → third L/R axis is defined
Gastrulation
• Phase in embryonic development of most animals
• Consists of complex and coordinated series of cellular movements – occurs at
end of cleavage
• Refers to phase in early embryonic development – embryonic morphology is
dramatically restructured
• Process of formation of 3 embryonic germ layers
o Ectoderm
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Document Summary
Lecture 7 embryo, gastrulation and germ layers. Differentiation: lineage specification and restriction of developmental potential, totipotent cells of epiblast differentiate and become more restricted in their developmental potential as gastrulation and embryogenesis proceeds, restrictions: 3 germ layers. Establishment of asymmetry: anterior-posterior and dorso-ventral axis: second polar body marks anterior determines axis of first cleavage, dorsal ventral axis forms at right angles to a-p axis, gastrulation third l/r axis is defined. Timing: human: around 14-16 days after fertilisation, after implantation, gastrulation may occur prior to implantation in some species. Primitive streak: first sign of gastrulation, formal process of gastrulation begins with formation of primitive streak on surface of epiblast, streak is initially vaguely defined but 15-16 day embryo. Clearly visible as narrow groove: cephalic head end of streak primitive node: consists of slightly elevated area. Cell movement during gastrulation: actual mechanisms that drive and regulate cell movement during gastrulation remain unclear, as gastrulation proceeds, primitive groove extends anteriorly.
