dev unit 2 species development

order of axes dev in zebrafish and when do they occur

D/V before gastrulation through cortical rotation (nodal and lefty then make a belt to form mesoderm)

A/P during gastrulation through convergent extension

L/R at end of gastrulation through Ca2+ → nodal → Pitx2

gastrulation in zebrafish

begins with epiboly, blastoderm spreads vegetally over yolk cell

marginal zone cells undergo involution AND ingression at germ ring and embryonic shield to form inner hypoblast, leaving outer epiblast

neurulation zebrafish

weird hybrid of primary and secondary

neural plate thickens into neural keel

neural keel then cavitates front to back to open a central lumen

xenopus order and method of axes development

D/V axis before gastrulation using cortical rotation

A/P axis during and after gastrulation by posterior → anterior Wnt, FGF, BMP gradient set up by cerberus, DKK, and FrzB. Later, retinoic acid gradient patterns the trunk and hindbrain driving Hox expression

L/R axis after gastrulation by L/R organizer. ciliary fluid flow triggers Ca2+ influx, asymmetric gene expression of nodal → Pitx2 in left lateral plate mesoderm

mesoderm induction and organizer specification in xenopus

vegetal cells express VegT and VgI, produce a high dorsal low ventral nodal morphogen gradient

Spemman-mangold organizer forms at dorsal marginal zone/dorsal blastopore lip where there is high beta catenin and nodal

organizer secretes BMP and Wnt antagonist (chordin, noggin, cerberus, FrzB) and expresses TF like goosecoid

gastrulation xenopus

initiated on dorsal side at dorsal lip of blastopore

morphogenic movements- driven by bottle cells undergoing apical constriction and primary invagination. prospective mesendoderm cells internalize via involution and undergo convergent extension, crawling along ECM (fibronectin) using lamellipodia and filipodia

simultaneously, ectodermal precursors undergo epiboly to enclose embryo

neurulation xenopus

primary- begins at mid trunk and extends anteriorly and posteriorly

driven by BMP inhibition from notochord and organizer, localized apical constriction at 3 hinge points, and cell adhesion changes epithelial to neural cadherin

opposing neural folds make contact and fuse to form neural tube

cortical rotation

fertilization at the animal pole triggers 30 degree cortical rotation

driven by sperm delivered centrioles that develop a microtubule array complete with kinesins and dyneins

moves dorsalizing factors disheveled and GBP to the future dorsal side

stabilizing nuclear beta catenin on dorsal side will form nieuwkoop center which will later develop the S-M organizer (xenopus)

mesoderm induction and organizer specification xenopus

vegetal cells express VegT and Vg1, produce nodal morphogen gradient high dorsal low ventral

S-M organizer forms at DMZ/DBL where there is high beta catenin and high nodal

organizer secretes BMP and Wnt antagonists (chordin, noggin, cerberus, FrzB) and expresses TF like goosecoid

chick axes development

A/P established before laying/gastrulation. rotation through oviduct, lighter yolk components tilt against one side of the blastoderm, establishing posterior marginal zone and kollers sickle. this makes P → A Wnt gradient

D/V patterned during and after gastrulation. hensen’s node acts simultaneously as D/V and L/R organizer. L/R symmetry breaking involves asymmetric polonaise cell movements in epiblast and biased Shh (left) vs.. FGF8 (right) expression in HN, leading to left nodal and Pitx2. more pitx2 regulated by TGFbeta signaling and tissue stretch drives gut tilting

chick gastrulation

morphology: at laying, chick embryo is flat bilaminar disk composed of an upper epiblast and a lower hypoblast, resting on massive yolk

cell movement: KS and endoblast interactions trigger nodal, wnt, and FGF signaling, inducing the primitive streak to form at the posterior edge. epiblast cells undergo EMT and ingress through primitive streak to form mesoderm. HN forma at anterior tip of primitive streak and regresses posteriorly, laying down the notochord and somite precursors. ventral body wall folding closes the primitive gut tube over the yolk

chick neurulation

primary: initiates at a single closure site in the future mesencephalon region and zips rapidly both anteriorly and posteriorly

involves neural plate thickening, MHP and DLHP formation via antagonistic BMP/Shh/Noggin signaling, apical constriction via actomyosin, E and N cadherin redistrubution, and filopodial gripping between apposing folds

human axes development

AP, DV, LR axes patterned around week 3 via high WNT, BMP4, and nodal signaling from amniotic ectoderm and posterior epiblast regions, highly analogous to avian and murine systems. LR asymmetry establishes normal anatomical situs solitus

human gastrulation

morphology: following implantation, embryo forms a flat bilaminar germ disk consisting of epi and hypoblast, situated between the amniotic cavity and the yolk sac

cell movements: initiates around days 14-19 post fert with the appearance of primitive streak and primitive node on epiblast surface. epiblast cells ingress through primitive streak via EMT to form the embryonic mesoderm and definitive endoderm, leaving remaining surface cells ectoderm

human neurulation

primary: two closure sites, hindbrain/cervical, rostral extremity

defects: anencephaly, spina bifida, need folic acid for methylation