Derivatives of germ layers
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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

Thank you