d erivatives of g erm layers dr rania gabr. o bjectives by the end of this lecture,the student...
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DERIVATIVES OF GERM LAYERS
Dr Rania Gabr
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OBJECTIVES
By the end of this lecture ,the student should be able to:• Explain the results of folding• List the derivatives of ectoderm• List the derivatives of endoderm• List the derivatives of mesoderm
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RESULTS OF FOLDING1- Embryo
change into cylinderical embryo.
2-Transposition between septum transversum and cardiogenic plate( S.T lies cranial then ventral and lastly caudal).
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3- Yolk sac is reduced in size ÷d into: a- intraembryonic ( gut). b- extraembryonic ( atrophies). c- yolk stalk (degenerates).
4- Allantois& connecting stalk become dorsal then caudal then ventral.
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AFTER TAIL FOLD The connecting stalk
(primordium of umbilical cord) is attached to the ventral surface of the embryo.
Allantois (a diverticulum of yolk sac) is partially incorporated into the embryo as a part of hindgut.
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5- formation of umbilical cord.
6- The oral membrane was cranially ventral.
7- The cloacal membrane and allantois was caudal ventral.
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RESULTS OF FOLDING
•The amniotic cavity enlarged.•The Yolk sac smaller & divided into (intraembryonic Y.S, Yolk stalk& extra embryonic Y.S).•Allantois& connecting stalk shifted caudally.•S.T Shifted anterior to Cardiogenic plate.
•The amniotic cavity more enlarged.•Allantois& connecting stalk shifted ventrally and form the umbilical cord which contains the extra embryonic Y.S and stalk. •S.T Shifted caudal to Cardiogenic plate.* Placenta will face the umblical cord.
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DERIVATIVES OF THE ECTODERM
Ectoderm is divided into:
Surface ectoderm
Neuroectoderm
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SURFACE ECTODERM DERIVATIVES
Epidermis of the skin Hair Nails Sweat & Sebaceous glands Mammary glands Enamel of the teeth Lens of eye Epithelium of sensory organs in the
inner ear & nose Anterior lobe of the pituitary gland
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NEUROECTODERMNeural TubeNeural Crest Cells
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NEURAL TUBE DERIVATIVES
Central nervous system (Brain and spinal cord) Peripheral nervous system Retina Sensory epithelia of nose & ear Pineal gland Posterior lobe of the pituitary gland
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NEURAL CREST CELLS DERIVATIVES Sensory ganglia of the spinal nerves( dorsal root
ganglia) Sensory ganglia of the cranial nerves Autonomic ganglia Meninges (Pia mater & Arachnoid mater) of the
brain & spinal cord Schwann cells: Neurolemmal sheath of peripheral
nerves Satellite cells Melanoblasts of the skin Suprarenal medulla (chromaffin cells) Several skeletal & muscular components in the
head (derived from pharyngeal arches)
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DERIVATIVES OF ENDODERM
Endoderm gives rise to the epithelial lining of:
Trachea
Bronchi
Lungs
Respiratory
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DERIVATIVES OF ENDODERMEndoderm gives rise to the epithelial lining
of:
Gastrointestinal tract
Liver
Pancreas
Urinary bladder
UrachusGIT
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DERIVATIVES OF ENDODERM
Endoderm gives rise to the epithelial lining of:
Pharynx
Thyroid
Tympanic cavity
Pharyngeotympanic tube
Tonsils
Parathyroid glands
Pharyngeal arches
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DERIVATIVES OF MESODERM Connective tissue Cartilage Bone Striated & smooth muscles Heart Blood & lymphatic vessels Kidneys, ovaries, testes& genital ducts Serous membrane lining the body cavities Spleen & cortex of the supra renal gland
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DEVELOPMENT OF SOMITES
As the notochord and neural tube forms
Embryonic mesoderm on each side of them proliferate:
Form thick longitudinal columns of paraxial mesoderm
Each column is continuous with intermediate mesoderm
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DEVELOPMENT OF SOMITES
Intermediate mesoderm gradually thins into a layer of lateral mesoderm
Lateral mesoderm is continuous with the extraembryonic mesoderm
Extraembryonic mesoderm covers the yolk sac and amnion
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SOMITES Paraxial mesoderm differentiates and begins to
divide into cuboidal bodies called somites by the end of the 3rd week
These blocks of mesoderm are located on each side of the developing neural tube
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SOMITES
About 42-44 pairs of somites are present by the end of 5th week
They are triangular in transverse section
Form distinct surface elevations on the embryo
They are used as one of the criteria to know the age of the embryo at this stage
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SOMITES
First appear in the future occipital region
Soon develop craniocaudally
Gives rise to the axial skeleton and associated musculature
Also forms adjacent dermis of the skin
The first pair of somites appear at the end of 3rd week : day 20
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SOMITES
First appear at a short distance caudal to the cranial end of the notochord
Subsequent pairs form in a craniocaudal sequence
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INTRAEMBRYONIC COELOM
Also known as primordium of embryonic body cavity
Appears as isolated coelomic spaces in the lateral mesoderm and cardiogenic mesoderm
These spaces soon coalesce to form a single horseshoe shaped cavity called intraembryonic coelom
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PARIETAL & VISCERAL LAYERS
Somatic or parietal layer continuous with the extraembryonic mesoderm covering the amnion
Splanchnic or visceral layer continuous with the extraembryonic mesoderm covering the yolk sac
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PARIETAL & VISCERAL LAYERS
Somatic mesoderm with overlying embryonic ectoderm form the embryonic body wall or somatopleure
Splanchnic mesoderm with underlying embryonic endoderm form the embryonic gut or splanchnopleure
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FATE OF INTRAEMBRYONIC COELOM
During the 2nd month, the intraembryonic coelom is divided into 3 body cavities:
Pericardial cavity
Pleural cavity
Peritoneal cavity
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