ANHB2212 Study Guide - Final Guide: Neural Crest, Axial Skeleton, Neuron
3 Jun 2018
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Embryo and Foetus Anatomy
Early Embryology – The First Four Weeks:
• Day 1 → fertilization
o Takes 24 hours
o Occurs in the ampulla of the uterine tube
o 3 phases
▪ Phase 1 → passage of sperm through the corona radiata
▪ Phase 2 → penetration of zona pellucida
▪ Phase 3 → fusion of oocyte and sperm cell membranes
o Following fertilization the following has occurred
▪ Restoration of the diploid number of chromosomes
▪ Determination of sex
▪ Initiation of cleavage
• Day 2-4 → cleavage
o Rapid mitotic divisions to increase cell number
o Not accompanied by cell growth → large zygote subdivided into
many smaller daughter cells → blastomeres
o By the end of day 4 → 16-32 cells → referred to as a morula
• Day 4-5 → blastocyst formation
o Blastocyst hatches from zona pellucida prior to implantation
o Fluid penetrates through zona pellucida and into intra-cellular
space of inner cell mass → blastocystic cavity
o Inner cell mass → embryoblast → primodium of the embryo
o Outer layer → trophophast → gives rise to embryonic placenta
• Day 6 → implantation
o Blastocyst attaches to endometrial epithelium
o Trophoblast begins differentiation into 2 layers
▪ Cytotrophoblast → inner layer, divides and migrates to
maintain the syncytiotrophoblast
▪ Syncytiotrophoblast → outer layer, allows blastocyst to
burrow into the endometrium
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• Day 7-9 → differentiation of the inner cell mass
o Embryoblast splits into 2 layers → bilaminar embryonic disc
▪ Epiblast → external (upper) layer of columnar cells
▪ Hypoblast → internal (lower) layer of cuboidal cells,
primitive endoderm
o Cavity forms within epiblast → enlarges and forms amniotic cavity
o Hypoblast and Heusers membrane give rise to primitive yolk sac
→ exocoelomic cavity
• Day 11-12 → development of extraembryonic mesoderm
o Blastocyst is completely embedded in endometrial stroma
o New cells derived from yolk sac appear between the inner surface
of cytotrophoblast and outer surface of yolk sac → extraembryonic
mesoderm
o Cavities form and fuse in mesoderm → chorionic cavity
o Cavity separates embryo from outer wall of chorion
• Day 14 → definitive two-layered embryo
o Bilaminar embryonic disc suspended in chorionic cavity by
connecting stalk
o Second yolk sac forms within its primary counterpart
o Ectodermal vesicle → amniotic cavity
o Endodermal vesicle → secondary yolk sac
o Embryonic disc → point of contact between two vesicles
o Perchordal plate develops as a local thickening of the hypoblast →
gives rise to the cranial region of the embryo
• Day 15 → primitive streak
o Primitive streak → first sign of gastrulation
o Thickening containing a midline (primitive) groove that forms
along the midsaggital plane of embryonic disc
o Cranial end expands into primitive node → contains depression →
primitive pit
o Primitive streak defines cranial-caudal axis, dorsal-ventral
surfaces and right-left sides
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o Epiblast cells enter the primitive streak and migrate as individual
mesoderm and endoderm cells → ingression
• Day 16 → formation of germ layers
o Gastrulation → process by which the three germ layers are
established in an embryo
o Represents the beginning of morphogenesis → development of
body form
o Three germ layers
▪ Endoderm
▪ Mesoderm
▪ Ectoderm
o All arise from the multiplication, migration and differentiation of
epiblast cells
• Day 17 → notochord formation
o Hollow notochord process grows laterally from primitive node to
prechordal plate
o Transforms from a hollow tube to solid notochord between days
17-22
▪ Fusion of ventral wall with endoderm
▪ Fused layer degenerates and instigates communication
with umbilical vesicle
▪ Floor of notochord disappears and a flattened grooved
notochord plate forms
▪ Notochordal cells proliferate cranially and the plate infolds
to form notochord
o Notochord functions as the primary signaling center in early
embyo
• Day 18 → formation of neural plate
o First event in the development of future central nervous system
o Thickened neural plate forms in ectoderm cranial to primitive
node → neural induction
o Plate folds in week 4 to form neural tube → precursor to CNS
o Expanded cranial portions form brain
o Narrower caudal portions form the spinal cord
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