Which layer gives rise to secondary mesenchyme cells?
At 60-cell stage vegetal hemisphere resolves into Veg1 and Veg2 tiers plus micromere quartets, each with distinct gene regulatory states. Veg2 comprises eight cells just above macromeres and micromeres, expressing endomesodermal programs including FoxA, GataE and Gcm. During gastrulation Veg2 descendants undergo epithelial-mesenchymal transition and ingress as secondary mesenchyme cells, forming pigment cells, blastocoelar immune cells, circumesophageal muscle and coelomic pouch components. Veg1 mainly becomes endoderm and ectoderm border, large micromeres primary mesenchyme, mesomeres ectoder
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 8: Secondary mesenchyme origin from Veg2 tier.
