Explain the steps involved from fertilization to foetal development in human.

Explanation

Fertilization in human is internal and occurs in the Oviduct of the female genital tract. It takes place in the ampulla of the fallopian tube. An oocyte is alive for about 24 hours, after it is released from follicle. The sperm enters into the ovum, fuses and forms a zygote, which is a fertilized ovum. This process is called fertilization.
(a) Cleavage and formation of Blastula: The first cleavage takes place about 30 hours after fertilization. Cleavage is a series of rapid mitotic divisions of the zygote, to form many celled blastula (Blastocyst), which comprises outer layer of smaller cells and inner mass of larger cells.
(b) Implantation: The blastocyst (fertilized egg) reaches the uterus and gets implanted. The process of attachment of the blastocyst to the Uterine wall is called Implantation. The fertilized egg, implanted in about 6 to 7 days after fertilization.
(c) Gastrulation: After implantation, the transformation of blastula into Gastrula and the formation of primary germ layers (Ectoderm, Mesoderm and Endoderm) by rearrangemer of the cells takes place, which is called Gastrulation.
(d) Organogenesis: The various organs of the foetus are established from the different germ layers, during Organogenesis.
(e) Formation of Placenta: Placenta is a disc shaped structure, attached to the Uterine wall between the developing embryo and maternal tissues. It allows the exchange of food materials, diffusion of oxygen, excretion of nitrogenous wastes and elimination of carbondi-oxide. The cord which connects the placenta with the foetus is called the umbilical cord.
(f) Pregnancy (Gestation): Embryo attains it.s development in the uterus. Gestation period of human is 280 days. During pregnancy the Uterus expands upto 500 times of its normal size.
(g) Parturition (Child Birth): Oxytocin from the posterior pituitary stimulates the uterine contractions and provides force to expel the baby from the uterus, causing birth. Parturition is the expulsion of young one from the mother.s Uterus at the end of gestation.
(h) Lactation: The process of milk production after child birth from the mammary glands of the mother is called lactation. Milk production from the mammary gland is stimulated by prolactin, a hormone secreted by anterior pituitary. The ejection of milk is stimulated by posterior pituitary hormone oxytocin.