Embryo Development

Zygote → 2-cell → 4-cell → morula (8 blastomeres)

via repeated cleavage over ~2 days

Compaction

morula outer surface smooths; cells acquire apical-basal polarity

Blastocyst cavity forms ~E3.0 (TS4)

by coalescence of micro-lumens ("string of pearls")

Trophectoderm (TE) = outer layer of morula

placenta only

Inner cell mass (ICM)

pluripotent and can contribute both to the fetus as well as extraembryonic tissues.

Hippo pathway mediates TE vs ICM fate: in outer cells,

YAP→nucleus, activates Cdx2 (TE); in inner cells

YAP phosphorylated/degraded

Sox2 expressed (ICM). Conserved across mammals

ICM segregation: Gata6+

primitive endoderm;

Nanog+

epiblast (mutually repressive)

By ~E4.5 (TS6): primitive endoderm

visceral endoderm (stays with epiblast; expresses Sox17, Gata4, Gata6, Afp, Hnf4a) + parietal endoderm (EMT, lines mural TE; expresses Snai1, Sox17, Gata4, Gata6)

~E3.5: zona pellucida thins, ruptures

embryo "hatches" → ready for implantation

Polar TE proliferates, pushes epiblast into blastocyst cavity

extraembryonic ectoderm (ExE) + ectoplacental cone

Result: elongated egg cylinder (ExE + epiblast)

covered by visceral endoderm (VE)

Proamniotic cavity begins ~E5.0 (TS7a)

extends into ExE by ~E6.0 (TS9a)

Orientation: proximal

ectoplacental cone (implantation site)

distal

epiblast

Human embryo is a flat bilaminar germ disc at equivalent stage (epiblast + hypoblast = mouse VE)`

not a cylinder — morphologically different but topologically equivalent

~E5.0–5.5 (TS7a–c): DVE emerges at distal tip of VE; columnar morphology, distinct from

surrounding cuboidal/squamous VE

DVE markers: Hex, Cer1, Lefty1

Induced by NODAL (from epiblast), restricted by BMP (from ExE)

~E5.5 (TS7d): DVE migrates to one side

becomes anterior visceral endoderm (AVE)

AVE secretes CER1, LEFTY1, DKK1 → inhibits NODAL and WNT

restricts primitive streak to opposite (posterior) side

AVE also induces neural pattern

in underlying epiblast

AVE is a transient anatomical region (E5.5–E6.5), occupied by a changing cell population;

displaced AVE cells revert to surrounding VE transcriptional state

DVE equivalent cells may exist in human bilaminar disc

similar transcriptional signature + thicker columnar hypoblast cells noted in Carnegie collection sections

~E6.5 (TS9b): gastrulation begins

presaged by Nodal, Cripto, Eomes, Wnt3 expression at streak site

Epiblast cells lose epithelial integrity (SNAIL1 represses E-cadherin/Cdh1) → EMT

delaminate to form mesoderm and endoderm

Streak elongates distally E6.5–E7.0 (TS9a–TS10b) until

it spans full epiblast length

Node forms at distal end: classical organizer; transplantation induces secondary axis in host; ciliated cells generate leftward flow

LR asymmetry (Nodal, Lefty1/2 expressed on left)

FGF8 drives mouse mesoderm migration; FGF2/FGF4 fill this role in humans

SNAI2 robust in human but not mouse nascent mesoderm

Cell competition in epiblast eliminates metabolically

impaired cells before gastrulation

Endoderm: definitive endoderm intercalates into VE in a Sox17-dependent manner;

partial EMT-MET (not full EMT) → gut epithelium

Mesoderm subtypes

emerge from different streak positions/times

Axial

from node → notochord (Shh = dorsoventral neural tube morphogen)

Paraxial

from distal streak → head mesoderm + pre-somitic mesoderm → somites → sclerotome (vertebrae) + dermamyotome (dermis + muscle)

Lateral plate

from proximal streak → heart, vasculature, body wall; splits into somatic (+ ectoderm) and splanchnic (+ endoderm) → intraembryonic coelom between them

Intermediate

caudal, between paraxial and lateral → gonads, urinary system

Extraembryonic

from most proximal streak ~E6.5 → visceral yolk sac + blood islands (first hematopoietic cells); Mesp1+ early cardiac progenitors → cardiac crescent ~E7.5

Ectoderm

cells that don't delaminate through streak; epigenetically primed in epiblast; forms neural plate (induced by AVE) → neural tube (CNS) + surface ectoderm (skin epidermis)

~E7.5–E9.0: rostral, caudal, and lateral folding convert flat trilaminar disc

into "tube within a tube" body plan

Rostral folding:

positions heart inferior to brain; forms foregut diverticulum/anterior intestinal portal; moves juxta-cardiac field to become proepicardium; requires Bmp2 in VE

Lateral folding:

brings cardiac crescent halves together → linear heart tube (atria caudal to ventricle); forms midgut tube; encloses coelomic cavity / ventral body wall; failure causes cardiac bifida (Foxa2 null)

Caudal folding

encloses hindgut; accompanied by embryonic turning → fetal appearance

Mouth and butthole form by apoptosis at

ends of blind-ended gut tube