Chapter 32-5. Avian Embryology
Recommended post: 【Biology】 Chapter 32. Embryology
1. (Cleavage stage) Axis determination
2. (Gastrulation stage) Organizer 1. Hensen’s node
3. (Gastrulation stage) Germ layer formation
4. (Organogenesis) Organizer 2. Limb bud
5. (Organogenesis) Neural tube development
1. (Cleavage stage) Axis determination
⑴ Anterior-posterior axis formation by shh (sonic hedgehog homolog)
⑵ Low shh concentration: Anterior formation
⑶ High shh concentration: Posterior formation
2. (Gastrulation stage) Organizer 1. Hensen’s node
⑴ Ingression site: the primitive streak, which forms along the embryo’s anterior–posterior axis.
⑵ Hensen’s node
① Cells that ingress through Hensen’s node become the notochordal mesoderm.
② Evolutionarily conserved: transplantation of the amphibian organizer—the dorsal lip of the blastopore—still results in normal development.
3. (Gastrulation stage) Germ layer formation
⑴ High yolk content in the center makes cell movement difficult → Different pattern of germ layer formation
⑵ Epiblast: Forms ectoderm, mesoderm, endoderm
⑶ Hypoblast: Assists germ layer formation
**4. (Organogenesis) Organizer 2. Limb bud **
⑴ Limb bud
Figure 1. Wing bud of a chick embryo on day 3 of development
① Composed of two organizers: Apical ectodermal ridge (AER), zone of polarizing activity (ZPA)
② Depending on the region of action, it is divided into leg bud (forearm) and wing bud (wing)
⑵ Limb buds develop in a proximal → distal sequence.
① In the leg (hindlimb) bud / wing (forelimb) bud, the basal (proximal) region is prespecified for the zeugopod (forearm in the forelimb; shank in the hindlimb), while the distal region is prespecified for the digits (phalanges).
② Experiment: graft a basal piece of the leg bud onto the distal region of the wing bud.
○ If grafted before leg/wing identity is determined: wing digits form.
○ If grafted after leg/wing identity is determined but before proximal–distal positional information is set: leg digits form.
○ If grafted after both leg/wing identity and proximal–distal positional information are set: a forearm bone forms.
⑶ Organizer 2-1. Apical ectodermal ridge (AER): secretes growth factors.
① Development shows little difference even when AER is replaced with fibroblast growth factor 8 (FGF8).
⑷ Organizer 2-2. Zone of polarizing activity (ZPA): secretes Shh.
① Lower Shh → anterior structures form → associated with the 4th digit.
② Higher Shh → posterior structures form → associated with the 1st digit.
③ Shh antagonizes BMP and can induce motor neurons.
5. (Organogenesis) Neural tube development
⑴ shh protein secreted from the notochord determines the ventral-dorsal axis of the neural tube
⑵ Depending on the shh concentration gradient, different types of neurons differentiate in the neural tube
① From closest to the notochord: floor plate, motor neurons, V2 interneurons, V1 interneurons
② Additional notochord transplanted into the neural plate of the embryo induces extra floor plate and motor neurons
⑶ Anti-shh antibodies inhibit the function of shh
6. Homeoboxgenes
Input: 2019.03.17 14:11