Embryology – Lecture 2

I. 

Second week of development 

A. 

Bilaminar embryonic disc

1. 

Trophectoderm cells rapidly proliferate and will form the placenta

2.  Fluid accumulates between cells of inner cell mass – forms amniotic

cavity

a.  Roof of amniotic cavity (Amnion) is derived from inner cell mass

b. 

Reminder of inner cell mass partitioned into two cell layers =

“Bilaminar embryonic disc” 

i. 

Epiblast – cells that face amniotic cavity

ii. 

Hypoblast – cells that face the yolk sac

3.  Embryo – derived almost completely from epiblast layer

a. 

Hypoblast and amnion become extraembryonic tissue

b.  Cells from hypoblast migrate to encircle blastocyst cavity – 

renamed yolk sac cavity

4.  Yolk sac – several functions in metabolism and embryo/maternal

transport 

a.  Ultimately regresses

B. 

Chorionic cavity

1.  Epiblast cells proliferate and some migrate between yolk sac and

trophectoderm extraembryonic mesoderm and won’t contribute to

mature embryo

2. 

Chorionic cavity forms between layers of extraembryonic mesoderm and

almost completely encloses embryonic dis, yolk sac, and amnion

a.  Primary yolk sac is pinched off into a smaller secondary yolk sac

b. 

Connecting stalk - Small amount of tissue which connect 

embryonic disc0amnion-yolk sac to the outer trophectoderm

though the chorionic cavity

i.  Will become umbilical cord

3. 

Midgestation, as fetus becomes larger, chorionic cavity is obliterated and

the amnionic and chorionic membranes become fused

a. 

Fetus then surrounded y amniotic fluid

C. 

Twinning

1.  Fraternal (dizygotic) twins arise when two eggs are released and both

fertilized and implant 

a.  Each twin surrounded by its own amnion and by its own chorion

b. 

If implant near each other, chorions and placentas may fuse

together along their common edge

2. 

Identical arise from single fertilized egg

a.  Majority involves subdivision of inner cell mass

i. 

Common trophectoderm and chorion encloses both twins

at a single implantations site with a single placenta

 

Each twin enclosed in separate amnion

b.  Minority arise from complete splitting of embryo into two

independent blastocysts, which implant independently

3.  Conjoined twins occur when IC splits only partially

II. 

Gastrulation

 

A.  During third week, Bilaminar embryonic disc converted into trilaminar disc by

formation of the three definitive germ layers (ectoderm, mesoderm, an

endoderm)

1. 

All three layers derived by proliferation and migration of cells from

epiblast 

2. 

First wave of migration, cells from epiblast migrate ventrally and

displace hypoblast cells from dorsal surface of the yolk sac – endoderm

3. 

Secondary wave of migration 0 cells from epiblast migrate between the

epiblast and the endoderm to form the mesoderm

4. 

Remainder of epiblast is now called the ectoderm

B. 

Occurs at the embryonic midline

1.  Morphology of midline cells as they ingress is different than those in

more lateral position

2.  PRIMITIVE streak – depression on the surface of the embryo that is the

location of the ingression

C.  Two sides where mesoderm does not penetrate between ectoderm and

endoderm

1.  Oropharyngeal membrane at the anterior end of the embryo – position of 

future mouth

2.  Cloacal membrane at the caudal end marks position of future anus

3. 

Direct transition for ectoderm to endoderm and a corresponding

transition in vascular and nerve supply

III. 

Neurulation

A. 

Notochord – longitudinal midline of the embryo, condensation of mesoderm

1.  Responsible for direction the inductive events which cause the overlying

ectoderm to become the neural plate, which will give rise to CNS

2.  18th day of development, groove forms in he midline of the neural plate

to delimit neural folds

3.  Neural folds end towards each other and fuse to create neural tube

4. 

Neural tube fuses first in the middle of the embryo by the end of the 3

week and progresses cranially and caudally becoming fully closed by end

of the 4th week 

5. 

Once closed, neutral tube becomes stratified into dorsal and ventral

regions which will generate sensory and motor neurons

IV. 

Neural crest 

A.  Transient population of cells which originate from the dorsal most portion of 

the neural folds

B.  Cells migrate away from closing neural tube as single cells and contribute to

variety of structure

1.  Nervous system derivatives

a. 

Peripheral nervous system derives from neural crest including

dorsal root ganglia, portions of cranial nerves and sensory

neurons

b. 

Schwann cells and portions of the meninges of he brain and SC

2. 

Craniofacial derivatives

a. 

Cranial bones, portions of teeth, vascular smooth muscle

3.  Pigment cells throughout body

4. 

Chromaffin cells in the medullary layer of adrenal gland

5.  Smooth muscle in the cardiovascular outflow region

 

C.  Neural crest derived from ectoderm via the neural plate

V. 

Neural tube closure defects

A. 

Spina bifida – failure of the halves of the vertebral arches to develop fully and

fuse along the midline (sacral and lumbar regions)

1.  Most severe forms occur because of failure of neural tube to close in

caudal region of embryo during 4th week of development 

2.  Also results from later failure in vertebral development 

B. 

Anencephaly –failure of cranial neural folds to fuse

1.  Embryonic brain remains exposed to surrounding amniotic fluid and

degenerates

2. 

Face is derived from neural crest and not neural folds themselves, the

faces in such embryos are approximately normal though they lack most 

of the head