D ERIVATIVES OF G ERM LAYERS Dr Rania Gabr

O BJECTIVES 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

R ESULTS 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.

A FTER T AIL F OLD 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.

R ESULTS 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.

D ERIVATIVES OF THE E CTODERM Ectoderm is divided into: Surface ectoderm Neuroectoderm

S URFACE E CTODERM D ERIVATIVES 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

N EUROECTODERM Neural Tube Neural Crest Cells

N EURAL T UBE D ERIVATIVES Central nervous system (Brain and spinal cord) Peripheral nervous system Retina Sensory epithelia of nose & ear Pineal gland Posterior lobe of the pituitary gland

N EURAL C REST C ELLS D ERIVATIVES 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)

D ERIVATIVES OF E NDODERM Endoderm gives rise to the epithelial lining of: Trachea Bronchi Lungs Respiratory

D ERIVATIVES OF E NDODERM Endoderm gives rise to the epithelial lining of: Gastrointestinal tract Liver Pancreas Urinary bladder Urachus GIT

D ERIVATIVES OF E NDODERM Endoderm gives rise to the epithelial lining of: Pharynx Thyroid Tympanic cavity Pharyngeotympanic tube Tonsils Parathyroid glands Pharyngeal arches

D ERIVATIVES 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

D EVELOPMENT OF S OMITES 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

D EVELOPMENT OF S OMITES 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

S OMITES Paraxial mesoderm differentiates and begins to divide into cuboidal bodies called somites by the end of the 3 rd week These blocks of mesoderm are located on each side of the developing neural tube

S OMITES About pairs of somites are present by the end of 5 th 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

S OMITES 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 3 rd week : day 20

S OMITES First appear at a short distance caudal to the cranial end of the notochord Subsequent pairs form in a craniocaudal sequence

I NTRAEMBRYONIC C OELOM 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

P ARIETAL & V ISCERAL L AYERS 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

P ARIETAL & V ISCERAL L AYERS 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

F ATE OF I NTRAEMBRYONIC C OELOM During the 2 nd month, the intraembryonic coelom is divided into 3 body cavities: Pericardial cavity Pleural cavity Peritoneal cavity