Derivatives of germ layers

 DERIVATIVE OF GERM LAYERS

INTRODUCTION :

1)Germ layers arise from a process of gastrulation ,an early phase of embryological development in which implanted blastula develops into 3 germ cell layers ,occur during 3rd week of development.

2) Its marked by extensive cell movement where blastula  becomes organised into 3 layers .

 MORULA ( a ball of cells) ,BLASTULA(hallow ball) ,gastrulation ( 3 layers of hollow ball )

3) BLASTOCOEL: fluid filled cavity that forms in the blastula of early amphibian and its functions is to permit cell migration during gastrulation and to prevent cell beneath it from interacting prematurely with other cells .

DERIVATIVE OF ECTODERM :

1) Epidermis of skin ,hair,nail,sweat,sebaceous gland .

2) epidermis lower part of anal canal,terminal part of urinary and genital tract .

3)epidermis of cheek,gum,palate,oral cavity,nasal cavity,para nasal sinus

4) enamel of teeth 

5) central nervous system : formed by neural crest cells 

6) peripheral nervous system : retina,ciliary body 

7) musculature of iris.

DERIVATIVE OF MESODERM :

1) Connective tissue ,cartilage ,bones

2) dentin of teeth 

3)blood cells 

4)lymph vessels ,lymph node,spleen 

5)part of genital system 

6) pericardium ,pleura and peritoneum

DERIVATIVE OF ENDODERM :

1) Primordial germ cells

2)epithelium of gastrointestinal tract 

3)epithelium of female urethra,male prostate

4)parenchyma of liver ,parathyroid gland.

ENDODERM : inner most layer 

ECTODERM ; outermost layer 

MESODERM : layer between ecto and endoderm 

METHOD  OF EPITHELIAL MOVEMENT DURING GASTRULATION :

1) INVAGINATION : In folding of cell sheet into embryo 

example- Endoderm

2) INVOLUTION : Inturing of cell sheet over basal surface of an outer layer 

example - Amphibian 

3) EPIBOLY : Expansion of 1 cell sheet over the other cells 

TROPHOBLAST FUNCTION IN EMBRYO:

The INNER MASS of cells present  in a blastula is called a trophoblast.

Trophoblast attaches blastocyst  to endometrium 

1)CYTOTROPHOBLAST : inner layer of trophoblast

• It can be differentiated 
• the tissue is formed by individual cell 

 2) SYNCYTOTROPHOBLAST : OUTER LAYER OF TROPHOBLAST 

• IT establishes nutritional supply between mother to fetus 
• It terminally differentiated cell
• The tissue is specialized and is multi nucleated with no individual cell.
• Secrets : leptin,human chorionic gondotrophin,human placental lactogen 

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