Human Development Before Birth

Human development before birth:

 

  • Human development before birth, also known as prenatal development, is a remarkable and complex process that transforms a single-celled zygote into a fully-formed foetus. 


  • This process spans approximately nine months and can be divided into three main stages: the germinal stage, the embryonic stage, and the foetal stage.

1.   Germinal stage or Pre-embryonic Development (Week 1 to Week 2):

- This stage begins at conception when the sperm fertilises the egg, forming a zygote.

- The zygote starts dividing rapidly as it travels down the fallopian tube toward the uterus.

- It includes cleavage and implementation-

Cleavage:

(i) Mitotic Division: After fertilisation, the zygote undergoes mitotic divisions as it moves through the oviduct towards the uterus.

(ii) Cleavage: These divisions result in the formation of daughter cells called blastomeres, leading to the formation of a solid ball of cells called a morula.

(iii) Morula to Blastocyst: The morula continues to divide and transforms into a blastocyst as it moves further into the uterus.

(iv) Blastocyst Structure: The blastocyst has an outer layer called the trophoblast and an inner group of cells attached to the trophoblast called the inner cell mass.

Implantation:

(i) Attachment and Differentiation: The trophoblast layer attaches to the endometrium (lining of the uterus), while the inner cell mass differentiates into the embryo.

(ii) Rapid Cell Division: The uterine cells divide rapidly, covering the blastocyst.

(iii) Implantation: The blastocyst becomes embedded in the endometrium, establishing a connection between the embryo and the maternal blood supply.

(iv) Implantation leads to pregnancy and marks the beginning of further development.

 

2.           Embryonic stage (Week 3 to Week 8):

Embryonic stage: Week 3 to Week 8 after conception.

-After implantation, chorionic villi with finger-like projections appear on the trophoblast.

- Chorionic villi and uterine tissue interdigitate to form the placenta.- The placenta serves as a structural and functional unit between the embryo (foetus) and the maternal body.

- Placenta facilitates the exchange of oxygen, nutrients, and waste materials between the embryo and maternal blood.

- Connected to the embryo via the umbilical cord, which aids in substance transport.

- The placenta acts as an endocrine tissue, producing hormones like hCG, hPL, estrogens, progestogens, and relaxin.

- Levels of hormones like estrogens, progestogens, cortisol, prolactin, and thyroxine increase significantly in maternal blood during pregnancy.

- Increased hormone production supports foetal growth, metabolic changes in the mother, and maintenance of pregnancy.

- After implantation, the inner cell mass (embryo) differentiates into three layers: ectoderm, mesoderm, and endoderm.

- Ectoderm gives rise to the nervous system, skin, and sensory organs.

- Mesoderm forms muscles, bones, and the circulatory system.

- Endoderm becomes the digestive and respiratory systems.

- The inner cell mass contains stem cells with the potential to develop into all tissues and organs.

 


Fertilisation and growing embryo 

 

3.           Foetal stage (Week 9 to birth):

- The foetal stage is characterised by the growth and refinement of the organs and systems developed during the embryonic stage.

- The foetus undergoes rapid growth and development, and its body proportions change.

- By the end of the first trimester (around week 12), the foetus has developed all essential organs and is considered relatively low-risk.

- During the second trimester, the foetus's movements become more apparent to the mother, and its sex can be determined.

- The third trimester is a period of further growth and preparation for birth. The foetus gains weight and develops more body fat for insulation.

- By the end of the ninth month, the foetus is ready for birth and is typically positioned head-down in the uterus.

 

                              

Graphical representation of foetal development in human