third to eighth week: the embryonic period ((organogenesis)) · third to eighth week: the embryonic...
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Third to Eighth Week: The Embryonic Period
((organogenesis I))
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The Embryonic Period
• ectoderm, mesoderm, and endoderm,
gives rise to a number of specific tissues
and organs
• By the end of this period, the main organ
systems established, representing the
major features of the external body form.
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Derivatives of the Ectodermal Germ Layer
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Derivatives of the Ectodermal
Germ Layer
Appearance of the notochord and prechordal mesoderm
induces the overlying ectoderm to thicken and form the neural
plate which will form the neuroectoderm
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Neurulation • Once induction occurred the
neural plate gradually
expands.
• End of 3rd week: the lateral
edges be more elevated,
neural folds,
• Neural folds approach each
other and fuse and neural
tube formed.
• Fusion begins in cervical
region (5th somite))
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Neurulation • Once induction occurred the
neural plate gradually
expands.
• End of 3rd week: the lateral
edges be more elevated,
neural folds,
• Neural folds approach each
other and fuse and neural
tube formed.
• Fusion begins in cervical
region (5th somite))
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Neurulation • Cranial and caudal neuropores:
1. closure of the cranial neuropore: day 25
2. posterior neuropore closes at day 28
• Neurulation completed,
• CNS represented as closed tubular of narrow caudal portion, the spinal cord, and broader cephalic portion having number of dilations, the brain vesicles.
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Clinical correlate
• Neural Tube Defects
1. Anencephaly
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Clinical correlate
• Neural Tube
Defects
2. Spina bifida
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Neural Crest Cells
•At the lateral border of the neuroectoderm
•Begin to dissociate from their neighbors
•Undergo an epithelial -to-mesenchymal
transition.
•Mesenchyme refers to loosely organized
embryonic connective tissue regardless of
origin .
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Neural Crest Cells
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Neural Crest Cells • Crest cells from the TRUNK
1. Ventrally…through the anterior half of each
somite to become:
A. sensory ganglia,
B. sympathetic and enteric neurons,
C. Schwann cells
D. cells of the adrenal medulla
2. Dorsally…through the dermis, to form
melanocytes in the skin and hair follicles
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Neural Crest Cells • Crest cells from the CRANIAL neural folds:
contribute to:
1. craniofacial skeleton
2. neurons for cranial ganglia,
3. glial cells,
4. melanocytes,
5. other cell types
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Neural Crest Cells • Crest cells from the CRANIAL neural folds:
By the time the neural tube is closed two bilateral ectodermal thickenings:
1. Otic placodes :
invaginate and form the otic vesicles(develop into structures needed for hearing and maintenance of equilibrium)
2. Lens placodes :
during the fifth week, form the lenses of the eyes
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Derivatives of the ectoderm • The ectodermal germ layer gives rise to organs and
structures that maintain contact with the outside world:
1. CNS
2. PNS
3. The sensory epithelium of the ear, nose & eye
4. the epidermis including the hair and nail
5. subcutaneous glands and the mammary gland
6. the pituitary gland
7. the enamel of the teeth
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Derivatives of the Mesodermal
Germ Layer During the third week, sheet of
loose mesodermal tissue
differentiate into three parts:
1.Paraxial mesoderm :
thickened plates on the side
of the midline. form the
segmented embryonic
somites.
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Derivatives of the Mesodermal
Germ Layer 2. Lateral Plate Mesoderm; forms
two layers
• somatic or parietal layer
adjacent to the ectoderm
splanchnic or
• visceral layer adjacent to the
endoderm.
3.Iintermediate mesoderm forms
the embryonic urogenital
system.
•
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Derivatives of the Mesodermal Germ
Layer 1.the supporting tissues as : bone, cartilage & C.T
2.muscles.
3.The CVS and the blood.
4.the lymph cells and vessels.
5.the kidneys and their ducts.
6.the gonads and their ducts.
7.the cortex of the suprarenal gland.
8.the spleen.
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THE END