EMBRYOLOGY

Highlights…………………. Implantation Types and normal site of implantation Clinical aspects – placenta prevea Formation of endoderm Formation of ectoderm Secondary yolk sac Extra embryonic mesoderm

First week of intra uterine life

SECOND WEEK OF INTRA UTERINE LIFE

Prochordal plate Buccopharyngeal membrane Primitive streak Cloacal membrane

DEVELOPMENT OF NOTOCHORDAL PROCESS

TRANSFORMATION OF NOTOCHORDAL PROCESS

Formation of mesoderm

SUBDIVISIONS OF INTRA EMBRYONIC MESODERM Separates the ectoderm and endoderm except at : Prochordal plate, cloacal membrane and in the midline where the notochord is present. At the edge both intra and extra embryonic mesoderm are continuous. The three divisions of the intra embryonic mesoderm: 1. Paraxial mesoderm. 2. Lateral plate mesoderm and 3. The intermediate mesoderm.

X Y Prochordal plate Paraxial Mesoderm Primitive streak Intermediate

Endoderm Notochord Intermediate mesoderm

Paraxial mesoderm segments into cubical masses - the somitomeres which form the somites. Small cavities appear in the lateral plate mesoderm,which join together to form the intra embryonic coelom. The horse shoe shaped cavity joins in the midline cranial to the prochordal plate. Intra embryonic coelom divides the mesoderm into somatopleuric and splanchnopleuric mesoderm. The coelom gives rise to pericardial, pleural and the peritoneal cavities.

Neural plate Y X

Neural plate Lateral plate mesoderm

Intra embryonic coelom X Y

X Y Intra embryonic coelom opening

Amniotic cavity A B C

Somatopleuric mesoderm Ectoderm

The heart is developed from the splanchnopleuric mesoderm cranial to the prochordal plate and this area is the cardiogenic area Cranial to the cardiogenic area the mesoderm does not split.It forms the future septum transversum

Fate of the somites The paraxial mesoderm forms segments- the somites that lie on either side of the neural tube. Somite period – 20-30 days of development Total number of somites – 44 pairs The somites are divided into three parts: 1. Ventromedial - Sclerotome. 2. Lateral - Dermatome. 3. Intermediate - Myotome.

Surface ectoderm Neural tube Dermatome Somite Myotome

Sclerotome – Cells of the sclerotome migrate medially. They surround the neural tube and form the vertebral column Dermatome – These cells migrate and lie on the deep surface of the ectoderm. These give rise to the dermis of skin and subcutaneous tissue. Myotome –It gives rise to the striated muscle. Each myotome is innervated by a spinal nerve. The number of somites is equal to number of spinal nerves