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Derivatives of Germ layers. Dr Rania Gabr. 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. Results of folding.

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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
results of folding
Results of folding

1- Embryo change into cylinderical embryo.

2-Transposition between septum transversum and cardiogenic plate( S.T lies cranial then ventral and lastly caudal).


3- Yolk sac is reduced in size &divided into:

a- intraembryonic ( gut).

b- extraembryonic ( atrophies).

c- yolk stalk (degenerates).

4- Allantois& connecting stalk become dorsal then caudal then ventral.

after tail fold
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.

5- formation of umbilical cord.

6- The oral membrane was cranially ventral.

7- The cloacal membrane and allantois was caudal ventral.

results of folding1
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.
derivatives of the ectoderm
Derivatives of the Ectoderm
  • Ectoderm is divided into:

Surface ectoderm


surface ectoderm derivatives
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
  • Neural Tube
  • Neural Crest Cells
neural tube derivatives
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
neural crest cells derivatives
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)
derivatives of endoderm
Derivatives of Endoderm

Endoderm gives rise to the epithelial lining of:

  • Trachea
  • Bronchi
  • Lungs


derivatives of endoderm1
Derivatives of Endoderm

Endoderm gives rise to the epithelial lining of:

  • Gastrointestinal tract
  • Liver
  • Pancreas
  • Urinary bladder
  • Urachus


derivatives of endoderm2
Derivatives of Endoderm

Endoderm gives rise to the epithelial lining of:

  • Pharynx
  • Thyroid
  • Tympanic cavity
  • Pharyngeotympanictube
  • Tonsils
  • Parathyroid glands

Pharyngeal arches

derivatives of mesoderm
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
development of somites
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
development of somites1
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
  • Paraxial mesoderm differentiates and begins to divide into cuboidal bodies called somitesby the end of the 3rdweek
  • These blocks of mesoderm are located on each side of the developing neural tube
  • 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
  • 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 3rdweek : day 20
  • First appear at a short distance caudal to the cranial end of the notochord
  • Subsequent pairs form in a craniocaudal sequence
intraembryonic coelom
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
parietal visceral layers
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
parietal visceral layers1
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
fate of intraembryonic coelom
Fate of Intraembryonic Coelom

During the 2nd month, the intraembryonic coelom is divided into 3 body cavities:

  • Pericardial cavity
  • Pleural cavity
  • Peritoneal cavity