7. Third week of development:
Trilaminar germ disc
Gastrulation
• Bilamelar germ disc is
converted into a
trilamelar germ
– Ectoderm
– Mesoderm
– Endoderm
• Primitive streak
– Thickened linear band of
epiblast
– 15- to 16-day embryo:
clearly visible
10. Derivatives of the ectodermal germ layer
1. Central nervous system
2. Peripheral nervous system
3. Skin, including hair and nails
4. Sensory epithelium of ear, nose, and eye
5. Pituitary, mammary, and sweat glands and
enamel of the teeth
11. Derivatives of the mesodermal germ layer
• Connective tissue,
cartilage, bone and
striated and smooth
muscles
• heart, blood and lymph
vessels and cells
• kidney, ovary and testis,
genital ducts, serous
membranes lining, spleen,
and adrenal cortex
12. Derivatives of the endodermal germ layer
1. Epithelial lining of the
gastrointestinal tract,
respiratory tract, and
urinary bladder
2. Parenchyma of the
thyroid, parathyroid,
liver and pancreas
3. Epithelial lining of the
tympanic cavity and
auditory tube
13. Fate of the primitive streak
• intraembryonic mesoderm 을
4주말까지 만들다가 감소
• insignificant structures in the
sacrococcygeal region of the
embryo
• Sacrococcygeal teratoma
14. Teratogenesis associated with
gastrulation
• The beginning of the third week of development,
when gastrulation is initiated, is a highly sensitive
stage for teratogenic insult.
• At this time, fate maps can be made for various organ
systems, such as the eyes and brain anlage, and
these cell populations may be damaged by
teratogens.
• For example, high doses of alcohol at this stage kill
cells in the anterior midline of the germ disc,
producing a deficiency of the midline in craniofacial
structures and resulting in holoprosencephaly.
15. Caudal dysgenesis (sirenomelia)
• Insufficient mesoderm
in the lumbosacral
region of the embryo
− Hypoplasia and
fusion of lower limb
− Vertebral
abnormalities
− Renal agenesis
− Imperforate anus
− Anomalies of the
genital organs
17. Notochord:
1. Primitive axis of the embryo
2. Induction - neural plate (future
nervous system)
Fate of the Notochord:
– Regress in the vertebral bodies
– Intervertebral disc: persist as the
nucleus pulposus
18. 20 days
Neurulation includes
the formation of the neural
plate (day 18-19), neural
folds (day 20-21), and the
neural tube (day 22-26); the
latter will develop into the
future brain and spinal
cord
23. Lissencephaly
• “Smooth brain”
• Severity : the absence
(agyria) to reduction
(pachygyria) of normal
gyral pattern.
• Classic L./cobblestone L.
• Onset : no later than the
12th–16th week of
gestation
• DDx: immature GA (until
27-28W)
•GA 31 wks
•46, XX, del(17)(p13)
•“Miller-Dieker” syndrome
25. Neural crest cells
• Vulnerable cell population
• Easily killed by compounds such as alcohol and retinoic acid.
• Deficient in superoxide dismutase and catalase enzymes that
are responsible for scavenging free radicals.
• Neural crest derivatives
Connective tissue and bones of the face and skull
Dermis in face and neck
☞ Severe craniofacial malformations
☞ Treacher-Collins' Syndrome, DiGeorge anomaly...
Conotruncal septum in the heart
☞ cardiac anomalies including persistent truncus arteriosus,
TOF and TGA
Cranial nerve ganglia, spinal ganglia, sympathetic chain and
preaortic ganglia, parasympathetic ganglia of the
gastrointestinal tract, glial cells, schwann cells, adrenal
medulla, C cells of the thyroid gland, arachnoid and pia
mater, melanocytes, odontoblasts
27. MIGRATION PATHWAYS OF
NEURAL CREST CELLS
from forebrain, midbrain,
and hindbrain regions into
their final locations (shaded
areas) in the pharyngeal
arches and face
Skeletal structures of the
head and face. Mesenchyme
for these structures is
derived from neural crest
(blue), lateral plate
mesoderm (yellow), and
paraxial mesoderm (red).
28. Treacher-Collins' Syndrome /
mandibulofacial dysostosis
• Characterized by malar
hypoplasia due to
underdevelopment of cheek
bones, mandibular
hypoplasia, down-slanting
eyes and malformed
external ears
• Normal development and
intelligence
• AD with variable penetrance
• 60% as new mutation
www.treachercollins.org/main.ht
29. Velocardiofacial Syndrome/DiGeorge anomaly
• 22q11.2 deletion
• “CATCH 22”
– Cardiac defects
– Abnormal face
– Thymic hypoplasia
– Cleft palate
– Hypocalcemia
• Abnormal development of
neural crest cells
• Specific facial features
– low-set ears, wide-set eyes,
a small jaw, and a short
groove in the upper lip
• Etiology
– Genetic causes, exposure to
retinoic acids, alcohol, and
maternal DM
30. Limb growth and development
A. 5-week embryo B. 6-week embryo C. 8-week embryo
The hindlimb buds are less well developed than those of the
forelimbs.
The most sensitive period for teratogen-induced limb
malformations is the fourth and fifth weeks of development
32. A.B. 6.5-week embryo
The palatine shelves
are in the vertical
position on each side
of the tongue.
C.D. 7.5-week embryo
The tongue has
moved downward,
and the palatine
shelves have reached
a horizontal position.
E.F. 10-week embryo
The two palatine
shelves have fused
with each other and
with the nasal septum.
C D
FE
33.
34. A, day 18
B, day 20
C, day 21
D, day 22*
Heart tube
39. Third to eighth week:
The embryonic period
• Period of
organogenesis
• Each of three germ
layers gives rise to
its own tissues and
organs.
• Major features of the
external body form
recognizable by the
end of the second
month
