Embryology weeks 1-3
Blastomeres multiply – become a morula of 12-32 blastomeres
Morula > Blastocyst when cavity begins to form (differentiation begins there)
Blastocyst = trophoblast + embryoblast
Embryoblast > embryo
Trophoblast > placenta
Decidual rxn occurs when the egg hits the uterine lining
Cytotrophoblasts mitotically divide, continually adding to the pool of
syncytiotrophoblasts which is Melding with the uterine liningEmbryoblast > hypoblast (yolk sac) these cells begin to migrate and line inner surface of
yolk sac forming the exocoelomic membrane. Epiblast (amnion) (closest to uterine wall)
Epiblast > eventually becomes 3 germ layers ecto meso endo
Amniotic cavity forms as epiblast expands
12 DAYS: Hypoblast> primitive yolk sac lining (exocoelomic membrane)>
WEEK 2
Extraembryonic mesoderm ( placental formation) >1. Splanchnic (visceral)
mesoderm-lines yolk sac 2. Somatic mesoderm-lines trophoblast covers amnion
Epiplast +hypoblast > Embryonic disc
Primary yolk sac squeezed off of the bottom, becoming the exocoelomic cyst, leaving
Behind the secondary (definite) yolk sac
Prechordal plate> future mouth
Prenotochordalplate –occurring at the same time as the germ layers
Week 3
Day 21 Gastrulation marked by the appearance of the primitive streak (from epiblast)
thin, narrow groove. This is when the ectoderm, intraembryonic Mesoderm (organ
formation), and endoderm form.
Intraembryonic Mesoderm:
Paraxial- Somatomiers, somites, sclerotome, myotome, dermatome
Intermediate- urogenital
Lateral Plate- 1. Somatic mesoderm 2. Visceral mesoderm
Primitive node marks the cranial end of the prim. Streak; the primitive pit is w/in the
Primitive node
Cells of the epiblast move to the primitive streak, separate from the epiblast to displace
the hypoblast, becoming embryonic endoderm. Others come between the
epiblast and new endoderm, creating the mesoderm. The remaining
epiblast cells form the ectoderm.
As epiblast invaginate the hypoblast, prenotochordal cells enter as well, forming the notoChordal plate, which slowly detaches from the endoderm(towards the
epiblast), becoming the
Definitive notochord -underlying the neural
tube; basis for axial skeleton, and is important in bilateral symmetry of the
developing embryo)
1. Induces overlying ectoderm to turn into neuroectoderm to form
neural plate
2. Formation of each vertebral body
3. Nucleus pulposus of intervertebral disc
Buccopharyngeal membrane and cloacal membrane do not contain mesoderm!!!!!
Prechordal plate mesoderm>forebrain/ mouth
Notochord and prechordal mesoderm induces the ectoderm to > neural plate, groove and
tube (neurulation)
Day 20 Neurulation: Formation of the neural tube. While forming communication
occurs with the anterior and posterior neuro pores. Anterior ends at 25 days and Posterior
ends at 28 days. Once neurulation complete, and pores close, the CNS and spinal cord
are formed.
Neural Crest Cells. (must know entire list) Leave neuroectoderm (which was induced to
form by the notochord) where they developed and form:
1. Sensory ganglia (dorsal root)
2. Sympathetic neurons and enteric (gut), and preaortic ganglia
3. Schwann cells ( form myelin)
4. Adrenal medulla cells,
5. Craniofacial skeleton,
6. Melanocytes,
7. Cranial nerve ganglia 5, 7, 9, 10.
8. Cells of the thyroid
9. dermis of face and skull
10. anything to face….
11. endocardial cushions **** said this to me in office hours.
*Know that if you see an abnormality in the skull you can also see an abnormality
in the heart.
NCC’s form at the tips of neural folds but don’t migrate until tube closure
is complete. Turn into mesenchyme ( fibrous like cell)
Layers of the early neural tube: Ventricular zone (neuroepithelial layer) gives rise to
cells that line canal
Defects in Neural Tube closure-Acephaly, spina bifida (cystica, occulta-skin over)
Intraembryonic mesoderm > Paraxial, intermediate, and Lateral plate mesoderm
Paraxial> located on sides of neural tube.
1.Somatomiers ( head region)
2.Somites that then form axial skeleton
Somite differentiation: they surround neural tube and notochord
forming: “Sclerotome”>verts and ribs; “Myotome” muscles of back;
“Dermatome” of posterior arms
3. Intermediate mesoderm-urogenital system (connects lateral plate and
paraxial mesoderm) pg 75 langman’s
4.Lateral plate mesoderm develops a cavity, splitting into two layers >
1. somatopleura (parietal)( covers amnion) >intraembryonic
cavity lining, ventral body wall, dermis and limbs And
2. splanchnopleura ( visceral) ( covers yolk sac)>organ lining,
wall of gut tube, peritoneal cavity, thin membrane around organs. The layer in between
parietal and visceral is called Intraembryonic cavity > thorax, abdomen, pelvis
Folding- Endoderm>gut tube; formation of umbilical ring. Amnion pulled ventrally to
surround embryo, together w/ the head and tail folding it closes the body wall around the
umbilical ring.
Diaphragm development: Septum transversarium (will become central tendon of
diaphragm) does not separate thoracic and abdominal cavities completely leaves room
for Pericardioperitoneal canals. Pleuroperitoneal membranes fill in the canals.
Failure to close will cause a hiatal hernia!!!…etc. pg 162 langman’s
Pleuropericardial membranes: (surround heart) grow to join septum t. and contain
phrenic n. and cardial veins.
Body Wall Anomalies: Ectopia Cordis- heart outside body
Omphaalocele-loops of bowel outside body
Diapragmatic hernias- failure of closure pleuroperitoneal canal
MUSCULOSKELETAL SYSTEM dev:
1. Paraxial, lateral plate (somatic mesoderm component.), and neural crest cells
2. Skeletal muscle: paraxial;
3. Smooth muscle: splanchnic
SKULL Develp:
A. Neurocranium: around brain ( paraxial and neural crest)
B. Viscerocranium- skel of face ( neural crest)
Anomalies of skull dev: Craniosynostosis- early suture closure.
Limb Develp: upper limb first then lower. (somatic and lateral plate) End of limb bud
(ectoderm) Apical Ectodermal Ridge (AER)> mesenchymal cells ( rapid proliferation)
Muscles in limbs (somite) migrate into limbs carrying nerves
VERTEBRAE develp:

All from “sclerotome” portion of somites.

Each vert is composed of caudal part of one somite and cranial half
of adjacent somite

Head musculature from somatomiers and first few somites

Dorsal epimeres: muscles of back

Ventral hypomeres: muscles of body wall and limbs
Peripheral Nervous System
Spinal n. develop 6th week.
Dorsal Primary rami- to epimeres ( true deep muscles of back)
Ventral Primary Rami- to hypomeres ( Muscles of anterolateral body
wall and extremities)
Horns: Ventral ( motor), Dorsal ( sensory), Lateral/intermediate-Contains cell body of
sympathetic neurons.
Vertebral column grows faster than spinal nerves.
Sympathetic NS- cell bodies are found in intermediate Horn
T1-l2 outflow
(from basal plate of neural tube) White rami (only in t1-L2) communicates to
sympathetic trunk via Preganglionic fibers – they may also pass trunk to synapse on
preaortic ganglia.
(from Neural Crest cells) postganglionic sympathetic neurons within syn. Chain ganglia,
prevertebral s. ganglia, and chromaffin cells of adrenal medulla.
Parasympathetic NS.-originates from neural tube and neural crest cells
Preganglionic neurons originate in brain stem and spinal cord. CN 3. 7,
9,10 S2-4
Synapse in ganglia located near the viscera or in the visceral walls.
Postganglionic neurons- from ncc’s. and are short. Pass to structures they
innervate.
Hirschsprung’s disease- lack of parasympathetic ganglia in the walls of colon bc neural
crest cells fail to migrate.
Spinal Cord- @ 12 weeks spinal nerves align w/ intervertebral foramina
PNSPNS ganglia-(outside cns) NCC’s
Motor cell body – (inside cns) Neural Tube