HUMAN REPRODUCTION
(1) Reproduction is the formation of new individuals of their own kind by living organisms.
(2) The Male Reproductive System consists of:
(a) Primary sex organs i.e. a pair of testes suspended in a scrotum.
Secondary sorgans i.e. a pair of ducts each differentiated into an epididymis, a vas deferens and an ejaculatory duct.
(c) A male urethra passing through an erectile penis.
(d) Three types of Glands – a pair of seminal vesicles, a prostate gland and a pair of Cowper’s glands.
(3) The Female Reproductive system consists of:
(a) Primary sex organ i.e. a pair of ovaries
(b) Secondary sex organs i.e. a pair of fallopian tubes (oviducts) , a uterus (womb), a vagina.
(4)Ovaries produce female gametes called ova.
Phases of embryonic development
Embryonic development involves following dynamic changes and identifiable process.
(1) Gametogenesis : It involve the formation of haploid sex cells or gametes called sperms and ova from diploid primary germ cells called gametogonia present in the reproductive organs called gonads (testes and ovary). It is of two types;
(i) Spermatogenesis: Formation of sperm.
(ii) Oogenesis : Formation of ova
(2) Fertilization: It involves the fusion of haploid male and female gametes to form diploid zygote. The fusion of gametic pronuclei is called Karyogamy while the mixing of two sets of chromosomes of two gametes is called amphimixis.
(3) Cleavage: It includes the rapid mitotic division of the zygote to form a single layered hollow spherical larva called blastula and its formation is called blastulation.
(4) Implantation: The process of attachment of the blastocyst (mammalian blastula) on the endometrium of the uterus is called implantation.
(5) Gastrulation: It includes the mass and orderly migration of the organ specific areas from the surface of blastula to their predetermined position which finally produces a 3 layered gastrula larva. It is with 3 primary layers.
(6) Organogenesis: It includes the formation of specific organs system from three primary germ layers of gastrula and also includes the morphogenesis and differentiation.
Fertilization
(1) Definition: Fusion of a haploid male gamete (spermatozoon) and a haploid female gamete (ovum) to form a diploid cell, the zygote, is called fertilization or syngamy.
(2) Site of fertilization: Fertilization in human female is internal as in other mammals. It takes place usually in the ampulla of the fallopian tube.
(3) Steps of fertilization
(i) Approach of sperm to ovum:
(a) Male discharges semen (3.5 ml) in the female’s vagina close to the cervix during coitus. This is called ejaculation or insemination. This ejaculation contains as many as 400 million sperms but only about 100 sperms reach the fallopian tube because many sperms are either killed by the acidity of female genital tract or engulfed by the phagocytes of the vaginal epithelium.
(b) The sperm swim in the seminal fluid at the rate of 1-4 mm per minute by the aspiratory action of the uterus and peristaltic movement of the fallopian tube.
(c) Capacitation is the phenomenon of physiological maturation of sperms by breaking of acrosome membrane inside the female genital tract. It takes about 5-6 hours.
(d) Ovum is released on the 14th day of menestrual cycle trapped by the fimbriae of the ampulla of fallopian tube and move towards the uterus by peristalsis and ciliary action.
(e) At the time of ovulation, egg is at secondary oocyte stage.
(f) Fertilizability of human sperm in the female genital tract is of 12 to 24 hours while its survival value is upto 3 days and of ovum is only 24 hours though it can live for about 72 hours.
(ii) Penetration of sperm:
(a) The ovum secretes a chemical substance called fertilizin, which has a number of spermophillic sites on its surface where the sperm of species specific type can be bound by their antifertilizin site.
(b) This fertilizin-antifertilizin interaction causes agglutination (sticking together) of egg and sperm.
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