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#hypoblast

3 public questions tagged with this topic.

chick development-l

The hypoblast in chick embryos contributes to:

Chick hypoblast assembles in two phases: primary islands polyingressing from epiblast into subgerminal cavity, then secondary sheet spreading from posterior Koller's sickle region. It forms lower layer beneath epiblast covering yolk initially. Chimera experiments prove hypoblast cells become extraembryonic endoderm lining yolk sac and contribute to stalk of membranes, secreting inhibitors Cerberus and Dickkopf1 that antagonize Wnt and Nodal anteriorly, permitting head formation and limiting primitive streak to posterior. Eventually ingressing definitive endoderm from streak displaces hypoblast to periphery. It never forms embryonic foregut, confirming extraembryonic role.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Hypoblast contribution to extraembryonic membranes and patterning.

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Chick development-ll

Hypoblast formation in chick embryos completes at:

Eyal-Giladi and Kochav staging scheme describes chick prestreak stages X to XIV. Hypoblast islands start appearing stage X-XI, spreading anteriorly, continuous layer completed by stage XIII just before appearance of primitive streak stage XIV. Before stage XIII hypoblast incomplete, after completion it covers entire subgerminal cavity and secretes Dkk1 and Cerberus inhibiting streak anteriorly, restricting streak to posterior. Therefore hypoblast formation completion at stage XIII marks transition from area pellucida to gastrulation competence and prerequisite for primitive streak initiation. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.

Ref: Eyal-Giladi and Kochav, Avian staging - Hypoblast completion at stage XIII, Developmental Biology handbook.

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Chick development-ll

Development of the chick primary hypoblast involves:

Primary hypoblast development in chick originates from polyinvagination of centrally located epiblast cells during EGK stages X-XII that individually delaminate down toward yolk forming islands of cuboidal hypoblast. Subsequently posterior marginal zone contributes secondary hypoblast cells migrating anteriorly under epiblast to complete continuous sheet. Live imaging shows localized cell migration rather than rapid proliferation or apoptosis, with cells moving via filopodia on vitelline membrane. This dual origin hypoblast transiently inhibits streak formation anteriorly before being displaced by definitive endoderm ingressing through streak. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.

Ref: Eyal-Giladi and Kochav, From cleavage to primitive streak formation in chick, Hypoblast migration origin.

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