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 FOLDING1- 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 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 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

Ectoderm is divided into:

Surface ectoderm

Neuroectoderm

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

NEUROECTODERMNeural TubeNeural Crest Cells

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 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

Endoderm gives rise to the epithelial lining of:

Trachea

Bronchi

Lungs

Respiratory

DERIVATIVES OF ENDODERMEndoderm gives rise to the epithelial lining

of:

Gastrointestinal tract

Liver

Pancreas

Urinary bladder

UrachusGIT

DERIVATIVES OF ENDODERM

Endoderm gives rise to the epithelial lining of:

Pharynx

Thyroid

Tympanic cavity

Pharyngeotympanic tube

Tonsils

Parathyroid glands

Pharyngeal arches

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

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 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

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

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

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

SOMITES

First appear at a short distance caudal to the cranial end of the notochord

Subsequent pairs form in a craniocaudal sequence

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

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 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

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

Pericardial cavity

Pleural cavity

Peritoneal cavity