During the embryonic period from 3-8 weeks, important developmental processes occur. Neurulation involves the formation of the neural tube from the neural plate through the processes of neural folding and closure of the neural tube. Neural crest cells migrate throughout the embryo and give rise to many tissues. Somite formation along the paraxial mesoderm results in the segmentation of the body plan and the formation of tissues including bone, muscle and skin. Organogenesis is also occurring as the heart, brain and other organs begin to form and develop. Certain environmental factors during this critical period can disrupt development and cause birth defects.

1. Embryonic period
(3rd -8th week)
Dr Jyoti Chopra
Professor
Department of Anatomy
KGMU, Lucknow

2. Neurulation
- Process of formation of neural tube from neural plate
- completed by end of 4th week with closure of caudal neuropore
Formation of neural plate- neural groove- neural tube

3. Neurulation

4. Neural crest cells
-cells along lateral border of
neuroectoderm
-dissociate from neighbours
-epithelial to mesenchymal
transition
-active migration from
neuroectoderm
-enters underlying mesoderm

5. Derivatives
-sensory ganglia
of spinal and
cranial nerves
-ganglia of ANS
-nerolemmal
sheath of PN
-adrenal
medulla
-pigment cells
Derivatives-
• Connective tissue and bones of face and skull
• Conotruncal septum of heart
• Odontoblasts
• Schwann cells
• Glial cells
Fourth germ
layer

6. Neural tube defects
Anencephaly
Anterior neuropore doesn’t close
Most of the brain fails to form
Lethal defect

7. Spina bifida
• Closure fails anywhere cervical region
caudally
• Most common site lumbosacral region
• Lose neurological function based on
level and severity of defect
NTDs reduced significantly after folic acid administration
50-70% prevented by 400microgm daily dose from 3months prior
to conception and continue during pregnancy

8. Derivatives of ectoderm
• Central nervous system
• Peripheral nervous system
• Sensory epithelium of eye, nose, ear
• Epidermis including hair and nail
• Subcutaneous glands
• Mammary gland
• Pituitary gland
• Parotid gland
• Enamel of teeth

9. Intraembryonic mesoderm

10. End of third week- paraxial mesoderm condense, differentiate in
cuboidal paired bodies- somites
Appear in craniocaudal sequence

12. Somite period- 20-30 days
Somitomeres- head region-7
Somites- 44 pairs
- occipital-4, cervical-8, thoracic-12, lumbar-5,
sacral-5, coccygeal-8to10

13. Number of somites related with age in days
No of somites Approximate age in days
1-4 20
4-7 21
7-10 22
10-13 23
13-17 24
17-20 25
20-23 26
23-26 27
26-29 28
34-35 30

14. Somite differentiation
Epithelization Formation of sclerotome- 4th wk
Sclerotome
Vertebrae
Ribs
Dermomyotome
Dermis of skin of back
Muscles of back and body wall
Some limb muscles
Each somite
has its own
segmental
nerve
supply
Muscle cells
migrate to
parietal layer
of lateral
plate
mesoderm

15. Intermediate mesoderm
Forms-
Urinary system
Genital system

16. Formation of intra embryonic coelom
Parietal layer
Dermis of skin of
body wall, limbs
Bones and
connective tissue
of limbs
Sternum
mesothelium
Splanchnic layer
Wall of gut tube
During 2nd
month
pericardial,
pleural and
peritoneal
cavity appear

17. •Horse shoe shaped
•Communicates with
extra embryonic coelom
•Forms pericardial,
pleural, peritoneal
cavity

18. Early development of cardiovascular system
At the beginning of 3rd week- vasculogenesis and
angiogenesis- in extraembryonic mesoderm of yolk sac,
connecting stalk and chorion
During 3rd week
utero-placental
circulation
develops

19. Vasulogenesis and Angiogenesis
• Mesenchymal cells-angioblasts- blood islands
• Cavities appear in blood islands
• Angioblasts flatten-endothelial cells
• Fusion- endothelial channels
• Vessels sprout by endothelial budding
• Blood cells- endothelial cells- progenitor blood cells

20. Primordial cardiovascular system
• Endocardial heart tube- 3rd week-
splanchnopleuric mesoderm
• Fuse to form primordial heart tube
• Tubular heart joins with-
– vessels in embryo
– connecting stalk
– chorion
– yolk sac
Primordial cardiovascular system
• Heart begins to beat on 21st-22nd day
• Can be detected by doppler
ultrasonography by 5th week

22. Situs Inversus

23. Teratogenesis associated with gastrulation
Holoprosencephaly
High dose alcohol during beginning of 3rd wk.
Very sensitive period
Kill cells in anterior midline of germ disc
Forebrain small, single lateral ventricle, hypotelorism (eyes close together)

24. Caudal Dysgenesis (Sirenomelia)
• Insufficient mesoderm formed in caudal region
• Hypoplasia or fusion of lower limb, Vertebral
abnormalities, renal agenesis, anomalies of genital
organs, imperforate anus

25. What happens if there is “not enough” gastrulation?
Caudal agenesis (sirenomelia)
Premature regression of the primitive streak leads to widespread loss of trunk
and lower limb mesoderm.
VATeR association:
Vertebral defects
Anal atresia
Tracheo-esophageal fistula
Renal defects
VACTeRL association:
those above plus…
Cardiovascular defects
Limb (lower) defects

26. Embryologists
Fertilization age: moment of fertilization is dO
Division of pregnancy corresponding to
development:
0-3 weeks –early development
3-8 weeks –embryonic period
(organogenesis)
8 wks-term –fetal period
Total gestation time = 38 weeks
Clinicians
Menstrual age: last menses is dO
Division of pregnancy into
trimesters
Total gestation time = 40 weeks
Timing of
pregnancy