Fertilization and Implantation
During copulation (coitus) insemination takes place i.e. semen containing sperm cells is released by the penis into the vagina. The motile sperms swim rapidly, pass through the cervix, enter into the uterus, and finally reach the junction of the isthmus and ampulla (ampullary-isthmic junction) of the fallopian tube. On the other hand, the ovum released by the ovary is also transported to the ampullary-isthmic junction where fertilization takes place. For the process of fertilization to take place the sperm cells first have to undergo capacitation in which the sperm cells get activated by the secretion of the female genital tract for the release of enzymes stored in the acrosome.
Now fertilization i.e. fusion of male and female gamete will take place. It occurs only when the ovum and sperms are transported simultaneously to the ampullary-isthmic junction failing to do will miss fertilization and pregnancy. During fertilization, a sperm comes in contact with the zona pellucida layer of the ovum, secrets enzymes from the acrosome for its entry while inducing changes in the membrane that block the entry of additional sperms (Prevent polyspermy) thus ensuring only one sperm cell fertilizes an ovum. This induces the completion of the meiotic division of the secondary oocyte. The second meiotic division is also unequal and results in the formation of a small second polar body and a haploid egg. Soon the haploid nucleus of the sperm and that of the ovum will fuse to form a diploid zygote.
The mitotic division called the cleavage will start as the zygote moves towards the uterus through the isthmus and forms 2, 4, 8, 16 celled daughter cells called blastomeres. The embryo with 8 to 16 blastomeres is called a morula which will continue to divide and transforms into a blastocyst. In the blastocyst, the blastomeres are arranged into an outer layer called trophoblast (later get attached as finger-like projections called chorionic villi to the endometrium) and an inner group of cells called inner cell mass (give rise to the embryo ) attached to trophoblast. After attachment, the uterine cell divides rapidly and covers the blastocyst embedding it in the endometrium of the uterus. This is called implantation and it leads to pregnancy.
Sex Determination in Humans
Fusion of the male and female nuclei will determine the sex of the baby. As the chromosomal pattern in human females is XX and males is XY, the gametes will be X in the case of females whereas male gametes will either be X or Y. When the sperm cell carrying an X chromosome will fertilize the female ovum the zygote will have XX i.e. it will be a female child and when the Y chromosome carrying sperm will fertilize the ovum the zygote will be XY i.e. the baby will be a male.
Also Read: Sex Determination
NCERT Notes on Human Reproduction Class 12 Chapter 2
NCERT Notes of Class 12 Chapter 2 Human Reproduction: Human reproduction is the biological process by which a new individual offspring is produced from one or two parent organisms. The Human Reproduction process involves the fusion of gametes, which are specialized cells that carry genetic information from each parent, resulting in the formation of a zygote.