This document discusses embryonic and fetal development from 3-8 weeks (embryonic period) and 9 weeks to birth (fetal period). During the embryonic period, the three germ layers give rise to specific tissues and organs as the main organ systems are established. Neurulation occurs as the neural tube forms from the neural plate. Neural crest cells migrate throughout the body. The mesoderm forms somites which differentiate into muscle, bone and skin tissues. Blood islands form and later hematopoietic stem cells arise. The endoderm forms the gastrointestinal tract. During the fetal period, organs mature and the fetus grows rapidly in the third, fourth and fifth months.

1. Embryonic & Fetal Periods
Dr. Prabhakar Yadav
Assistant Professor
Department of Human Anatomy
B.P. Koirala Institute of Health Sciences

3. Embryonic period (3-8 weeks)
• Period of organogenesis
• 3 germ layers gives rise to specific tissues & organs.
• Main organ systems is established & features of external body is recognizable by the end of second month.
Derivatives of the Ectoderm
At beginning of 3rd week, ectodermal layer has shape of a disc
- broader in cephalic than caudal region.

4. Derivatives of the Ectoderm
Appearance of notochord & prechordal mesoderm induces the overlying ectoderm to thicken & form neural plate.
Cells of the neural plate make up the neuroectoderm

5. NEURULATION: Process of formation of neural tube by neural plate.
End of 3rd week: Lateral edges of neural plate become elevated to form neural folds
Depressed midregion forms neural groove.
Neural folds approach each other in midline & fuse
Fusion begins in cervical region (fifth somite) which proceeds cranially & caudally- neural tube.
Until fusion is complete, cephalic & caudal ends of neural tube communicate with amniotic cavity by way of the
anterior (cranial) and posterior (caudal) neuropores.

6. Closure of: Cranial neuropore occurs on ------ 25 day (18- to 20-somite stage)
Posterior neuropore closes on------ 28 day (25-somite stage)
Central nervous system is represented by a closed tubular structure with:
• Narrow caudal portion- spinal cord
• Broader cephalic portion- characterized by a number of dilations- brain vesicles.

7. By the time neural tube is closed, two bilateral ectodermal thickenings: otic placodes & lens placodes, are visible
Otic placodes invaginate & form otic vesicles- develop into structures needed for hearing & maintenance of
equilibrium.
Lens placodes invaginate & form t lenses of the eyes.

8. Neural tube defects (NTDs): Occurs when neural tube fails to close
Anencephaly:
Neural tube fails to close in cranial region & most of the brain fails to form.
Spina bifida:
neural tube fail to close anywhere form cervical region caudally.
Most common site: Lumbosacral region
50-70% NTDs can be prevented by……………………………………………………….

9. As neural folds elevate & fuse, cells at lateral border or crest of neuroectoderm begin to dissociate - Neural crest
cells.
Neural crest cells undergo epithelial-to-mesenchymal transition as it leaves neuroectoderm by active migration &
displacement to enter underlying mesoderm.

10. Neural Crest cells from the trunk region leave neuroectoderm after closure of neural tube & migrate along one of
two pathways:
(1) Dorsal pathway through the dermis, they enter the ectoderm to form melanocytes in the skin & hair follicles
(2) ventral pathway through anterior half of each somite to become sensory ganglia, sympathetic & enteric
neurons, Schwann’s cells & cells of adrenal medulla.

11. Neural Crest cells from cranial neural folds leave neuroectoderm before closure of neural tube.
They contribute to craniofacial skeleton, neurons for cranial ganglia, glial cells, melanocytes, and other cell types.
Neural Crest Derivatives
1. Connective tissue & bones of the face & skull
2. Cranial nerve ganglia
3. C cells of the thyroid gland
4. Conotruncal septum in the heart
5. Odontoblasts
6. Dermis in face and neck
7. Spinal (dorsal root) ganglia
8. Sympathetic chain and preaortic ganglia
9. Parasympathetic ganglia of the gastrointestinal
tract
10.Adrenal medulla
11.Schwann cells
12.Glial cells
13.Arachnoid and pia mater (leptomeninges)
14.Melanocytes

12. Derivatives of the Mesodermal Germ Layer
Paraxial mesoderm; Intermediate mesoderm ;lateral plate mesoderm
(a) somatic or parietal mesoderm layer: layer continuous with mesoderm covering amnion
(b) splanchnic or visceral mesoderm layer: layer continuous with mesoderm covering yolk sac.
Together, parietal & visceral layers line- intraembryonic cavity, which is continuous with the extraembryonic cavity
on each side.

13. PARAXIAL MESODERM
3rd week- paraxial mesoderm -organize into segments -somitomeres
somitomeres – first appear in cephalic region & proceeds cephalocaudally.
Somatomeres 1–7, located from cephalic to otic vesicle, do not condense to form somites but contribute to mesoderm of head &
neck region- forms striated muscles in head & neck region.
somatomeres located caudal to otic vesicle condense to form -somites.
• First pair of somites arises in the occipital region at 20th day of IUL
• New somites appear in craniocaudal sequenceat a rate of three pairs per day, by end of 5th week, 42 to 44 pairs are present

14. • There are 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8 to 10 coccygeal pairs of somites.
• First occipital & last five to seven coccygeal somites later disappear, remaining somites form axial skeleton.
 Age of an embryo can be determined during early gestation by counting somites as somites appear with a
specified periodicity

15. By 4th week- cells forming ventromedial walls of the somite lose their compact organization & shift their position to
surround notochord- sclerotome- surround spinal cord & notochord - form vertebral column

16. • Cells forming dorsomedial & ventrolateral wall of the upper region of somite form precursors for muscle cells
• Cells between dorsomedial & ventrolateral groups form Dermatome- subcutaneous tissue & dermis of skin
• Cells from dorsomedial & ventrolateral groups migrate beneath the dermatome to create the dermomyotome.
• Cells from ventrolateral wall migrate into the parietal layer of lateral plate mesoderm to form most of the
musculature for the body wall (external and internal oblique and transversus abdominis muscles) & limb
muscles

17. Intermediate Mesoderm
Differentiates into urogenital structures.

18. Lateral Plate Mesoderm
• Parietal layer together with overlying ectoderm--form lateral & ventral body wall.
• Visceral layer together with embryonic endoderm- form wall of gut
• cells of parietal layer surrounding the intraembryonic cavity will form mesothelial membranes, or serous
membranes, which will line the peritoneal, pleural, and pericardial cavities and secrete serous fluid
• Cells of visceral layer will form a thin serous membrane around each organ.

19. Blood and Blood Vessels
- arise from mesoderm.
Blood vessels form in two ways:
Vasculogenesis: vessels arise from blood islands .
Angiogenesis: vessels sprouting from existing vessels.
3 weeks - First blood islands appear in mesoderm surrounding
the yolk sac & later in lateral plate mesoderm and other
regions.

20. Blood islands are induced by fibroblast growth factor 2 (FGF-2)
to form hemangioblasts- precursor for vessel & blood cell.
Hemangioblasts- in the center of blood islands- form
hematopoietic stem cells.
Peripheral hemangioblasts differentiate into angioblasts-
angioblasts proliferate and are induced to form endothelial cells
by vascular endothelial growth factor (VEGF)

21. Once vasculogenesis establishes, additional
vasculature is added by angiogenesis.
VEGF, stimulates proliferation of endothelial cells at
points where new vessels are to be formed.
First blood cells arise in the blood islands of yolk sac
Definitive hematopoietic stem cells arise from
mesoderm surrounding the aorta [aorta-gonad-
mesonephros region (AGM)].
Definitive hematopoietic stem cells cells will colonize
the liver- major hematopoietic organ of the fetus.
Stem cells from the liver will colonize bone marrow, --
-definitive blood-forming tissue.

22. Derivatives of the Endodermal Germ Layer:
• Gastrointestinal tract
Endoderm forms ventral surface of embryo &
forms roof of the yolk sac.
With development embryonic disc bulge into
amniotic cavity & fold cephalocaudally.
Larger portion of endoderm-lined cavity is
incorporated into the body of the embryo
proper.
In anterior part, endoderm forms foregut
In tail region, it forms hindgut.
Part between foregut & hindgut is midgut.
Midgut temporarily communicates with the yolk
sac by way of a broad stalk -vitelline duct .

23. vitelline duct :is wide initially, but with growth of
embryo, it becomes narrow and much longer.
At its cephalic end, foregut is temporarily bounded by
an ectodermalendodermal membrane -
buccopharyngeal membrane
4th week: buccopharyngeal membrane ruptures,
establishing connection between amniotic cavity &
primitive gut.
7th week: cloacal membrane ruptures- create
opening for anus

24. Due to rapid growth of somites, the flat embryonic disc folds laterally, and the embryo obtains a round appearance

25. During further development, it gives rise to
(a) Epithelial lining of the respiratory tract
(b) parenchyma of the thyroid, parathyroids, liver, and
pancreas
(c) Reticular stroma of the tonsils and thymus
(d) Epithelial lining of urinary bladder & urethra
(e) Epithelial lining of tympanic cavity & auditory tube

26. Fetal period: ninth week to birth characterized by maturation of tissues & organs & rapid growth of body
Length of the fetus is usually indicated as:
crown-rump length (CRL) -sitting height
crown-heel length (CHL)- standing height- from vertex of skull to heel
These measurements, expressed in centimeters; are correlated with age of the fetus in weeks or months.
Growth in length – marked during 3rd , 4th & 5th months.
Increase in weight – marked during last 2 months.
Length of pregnancy - 280 days, or 40 weeks after the onset of last normal menstrual period (LNMP) / 266 days or
38 weeks after fertilization.

27. During fetal life there is relative slowdown in growth of head compared with rest of the body.
3rd month of IUL : head constitutes approximately 1/2 of the CRL.
5th month of IUL: head is about 1/3 of the CHL,
By birth: 1/4 of the CHL.

28. During 3rd month –
• Face becomes more like human
• Eyes, initially directed laterally, move to ventral aspect of face,
• Ears come to lie at the side of the head .
• Limbs become relative in length in comparison with rest of body
• Primary ossification centers are present in long bones and skull
• External genitalia are develop - can be determined by external examination
• During the 6th week intestinal loops cause a large swelling (herniation) in
the umbilical cord, but by the 12th week the loops withdraw into the
abdominal cavity.

29. 4th and 5th months - fetus lengthens rapidly,
By end of first half of IUL- CRL is approximately 15 cm,
Fetus is covered with fine hair- lanugo hair; eyebrows and head hair are also visible.
During the fifth month movements of the fetus can be felt by the mother.
During second half of intrauterine life, weight increases considerably, particularly during the last 2.5 months
During sixth month, skin of fetus is reddish & has a wrinkled appearance because of lack of underlying connective tissue.

30. 6.5 to 7 months, fetus has a length of about 25 cm and weighs approximately 1100 g.
If born at this time, 90% chance of surviving.

31. 8-9 months, fetus obtains well-rounded contours - deposition of
subcutaneous fat
• By the end of IUL, skin is covered by a whitish, fatty substance
(vernix caseosa) composed of secretory products from
sebaceous glands.
• At end of ninth month: skull has largest circumference.
At the time of birth:
• weight: 3000 - 3400 g
• CRL : 36 cm
• CHL : 50 cm
• Sexual characteristics are pronounced,
• Testes should be in the scrotum.

32. Ectodermal germ layer
a) central nervous system
b) Peripheral nervous system
c) sensory epithelium of ear, nose &
eye
d) skin, including hair & nails
e) pituitary, mammary
f) sweat glands
g) enamel of the teeth.
Endodermal germ layer
a) Gastrointestinal tract,
b) Respiratory tract
c) Urinary bladder
d) tympanic cavity & auditory tube
e) thyroid
f) Parathyroids
g) live
h) pancreas.
Mesodermal germ layer
 vascular system- heart, arteries, veins, lymph vessels & all blood and lymph cells.
 urogenital system: kidneys, gonads, and their ducts
 spleen and cortex of the suprarenal glands
Paraxial mesoderm forms somitomeres,which give rise to
 Myotome -muscle tissue
 Sclerotome - cartilage and bon
 Dermatome -subcutaneous tissue of the skin