#embryonic cells

Sea Urchin -ll

Autonomous specification in sea urchins is observed in:

Large micromeres inherit concentrated maternal determinants such as nuclear β-catenin, Disheveled protein, and localized mRNAs that activate Pmar1 transcriptional repressor. Even when isolated and cultured in calcium-free seawater, large micromeres autonomously activate skeletogenic program and produce birefringent spicules without neighboring signals, demonstrating autonomous specification. Mesomeres, Veg1 and Veg2 remain conditional, requiring Wnt and Delta paracrine signals for endomesoderm induction and archenteron formation. This autonomy underlies organizer activity of micromeres, which can induce secondary axis when transplanted to animal pole. Hence large micromere lineage exemplifies inherited determinant-driven specification in otherwise regulative embryo.

Ref: Oliveri et al., Development 2008, Double-negative gate; Gilbert Chapter 8: Autonomous specification of micromeres.

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Sea Urchin -ll

Which cells in the sea urchin embryo form the larval skeleton?

Large micromeres derived from vegetal pole during unequal fourth cleavage autonomously specify as primary mesenchyme cells after ingression. Upon entering blastocoel they express conserved transcription factors Alx1, Ets1, Tbr and downstream skeletogenic gene battery including MSP130, Sm50 and collagen. Guided by ectodermal patterning cues such as VEGF and FGF, these cells migrate and fuse into syncytial cables depositing calcium carbonate to form triradiate spicules that elongate into pluteus larval skeleton. Small micromeres contribute to germline, while Veg2 becomes secondary mesenchyme and Veg1 forms endoderm, not skeleton.

Ref: Davidson et al., Gene regulatory network for sea urchin skeletogenesis, Science; Gilbert Ch 8: Micromere specification.

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amphibian Development-l

The primary germ layer derived from vegetal hemisphere cells is:

Maternal mRNAs and proteins localized vegetally during oogenesis bias vegetal hemisphere toward endodermal fate, establishing first germ layer map. VegT, Vg1 localized there activate transcription factors Sox17, Mixer, GATA factors after fertilization, driving gut differentiation. Cells inheriting high VegT and Nodal signaling differentiate as endoderm forming archenteron lining, while equatorial cells receiving intermediate Nodal become mesoderm and animal cap lacking VegT defaults to ectoderm. Lineage tracing confirms large yolky vegetal blastomeres predominantly contribute to archenteron lining, while mesoderm, ectoderm, neural tissues originate more animally, reflecting conserved anamniote pattern.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 8: Vegetal hemisphere endoderm origin maternal determinants.

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chick development-l

Hensen’s node contains cells destined to become:

Lineage analysis of Hensen's node using quail-chick chimeras reveals median pit cells ingress after streak regression to form notochordal rod extending caudally, expressing Brachyury, FoxA2 and Sonic hedgehog providing signals for ventral neural tube and somite patterning. Lateral and rostral node cells contribute prechordal mesendoderm underlying forebrain, expressing Goosecoid, Otx2 and Hex, crucial for head induction and forebrain specification by inhibiting Wnt and BMP. Together axial progenitors provide structural scaffold and signaling center for body plan, not limb or epidermal derivatives.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Hensen's node progenitors - notochord and prechordal mesoderm formation.

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chick development-l

Cells ingressing through posterior primitive streak in chick form:

Fate mapping using DiI labeling of chick primitive streak reveals anterior-posterior regionalization: Hensen's node contributes notochord and prechordal plate head mesoderm expressing Goosecoid, anterior streak forms paraxial somitic mesoderm, middle forms intermediate mesoderm, posterior streak contributes lateral plate and extraembryonic mesoderm. Posterior epiblast exposed to high BMP4 and Wnt8 signals activates lateral plate markers FoxF1 and promotes vascularization. Posterior ingressing cells split into somatic and splanchnic layers generating body wall, limbs and yolk sac vasculature guided by FGF and BMP gradients.

Ref: Wolpert, Principles of Development, 5th ed., Chapter 5: Posterior primitive streak forming lateral plate mesoderm fate.

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