EMBRYONIC DEVELOPMENT | NEET | MANISH MEVADA

EMBRYONIC DEVELOPMENT

Embryonic development or embryogenesis is the development of embryo from fertilised ovum and its subsequent development into a young organism .

Principal events during development 

( i ) Zygote - 12-14 hours after - Haploid sperm unites with haploid egg , making the latter a diploid zygote with unique ovulation genotype .

( ii ) Cleavage - 30 hours to 3 days - Mitotic divisions increase number of cells without increasing total cytoplasmic mass

( iii ) Morula - Third to fourth day - Solid ball formed , comprising outer layer of small cells around an inner mass of large cells .

( iv ) Blastocyst - Fifth day to end of  second week - Hollow ball formed , comprising trophoblast , embryonic knob and blastocyst cavity , implantation occurs , embryonic disc forms , primary germ layers established .

Blastocyst then gets attached to the endometrium ( trophoblast ) and the inner cell mass gets differentiated as the embryo .

After attachment , the uterine cells divide rapidly and covers the blastocyst . As a result blastocyst become embedded in the endometrium . This is called implantation . It begins about seventh day after fertilisation of ovum .

Implantation leads to pregnancy . In human beings it is approximately 9 months ± 7days . If hCG ( Human chorionic Gonado tropin ) is present in a woman's urine it indicates her pregnancy . After implantation , finger - like projections appear on the trophoblast called chorionic villi . Chorionic villi and uterine tissue becomes interconnected with each other and jointly form placenta between mother and developing fetus .

Placenta connects with fetus by a rope - like structure called umbilical cord .

Transformation of blastocyst into gastrula by formation of primary germ layers by rearrangement of cells is called gastrulation.

In all triploblastic animals , three germ layers -- ectoderm , mesoderm and endoderm are formed .

Many structures are derived from the three embryonic germ layers during organogenesis

Ectoderm

Epidermis of skin and its derivatives (including sweat glands , hair follicles )

Epithelial lining of mouth and rectum

Sensory receptors in epidermis

Cornea and lens of eye

Nervous system

Adrenal medulla

Epithelium of pineal and pituitary glands

Mesoderm

Notochord

Skeletal and muscular systems

Muscular layer of stomach , intestine , etc.

Reproductive systems ( except germ cells )

Excretory , circulatory and lymphatic systems

Dermis of skin and lining of body cavity

Adrenal cortex

Endoderm

Epithelial lining of digestive tract

Epithelial lining of respiratory system

Lining of urethra , urinary bladder and reproductive system

Liver

Pancreas

Thymus

Thyroid and parathyroid glands

The growing embryo / fetus develops four membranes called the extra - embryonic or fetal membranes . These include

( i ) Chorion completely surrounds the embryo and protects it takes part in the formation of placenta .

( ii ) Amnion and the fluid - filled amniotic cavity nearly surround the embryo . Amniotic fluid absorbs shocks , regulates temperature of fetus and prevents desiccation of the embryo .

( iii ) Allantois is small and non - functional except for furnishing blood vessels to the placenta .

( iv ) Yolk sac is non - functional in human beings except that it functions as the site of early blood cell formation .

Manish Mevada

M.Sc, M.Phil, B.Ed