Class 12 Biology

Fertilisation and Implantation

During coitus, semen is released by the penis into the vagina. This process is called insemination.

The motile sperms swim rapidly, pass through the cervix, enter into the uterus and finally reach the junction of the isthmus and ampulla of the fallopian tubule.

The ovum (released by the ovary) is also transported to the ampullary-isthmic junction, where fertilization takes place. Fertilization can only occur if the ovum and sperm are transported simultaneously to the ampullary-isthmic junction.


The process of fusion of a sperm with an ovum is called fertilization. During fertilization, a sperm comes in contact with the zona pellucid layer of the ovum. The sperm induces changes in the membrane that block the entry of additional sperms. Thus, it ensures that only one sperm can fertilise and ovum.

The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zona pellucid and the plasma membrane. This induces the completion of the meiotic division of the secondary occyte. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid). Soon the haploid nucleus of the sperm and that of the ovum fuse together; to form a diploid zygote.

Sex Determination: The gender of the baby is decided at the stage of fertilization itself. You may recall, that the chromosome pattern in the human female is XX and that in the male is XY. Hence, all the female gametes have sex chromosome X. On the other hand, the male gametes can have X or Y as sex chromosome. If a sperm with X chromosome fertilizes the ovum, the resulting zygote will lead to the birth of a girl child. If a sperm with Y chromosome fertilizes the ovum, the resulting zygote will lead to the birth of a male child.


When the zygote moves through the isthmus towards the uterus, it undergoes mitosis. The cell divisions in the zygote are called cleavage. Repeated mitotic divisions in the zygote result in formation of 2, 4, 8 to 16 daughter cells. These daughter cells are called blastomeres. The embryo with 8 to 16 blastomeres is called a morula.

The morula continues to divide and changes into blastocyst; while it moves further into the uterus. The blastomeres in the blastocyst are arranged in two layers. The outer layer is called trophoblast and the inner group of cells is called the inner cell mass. The trophoblast gets attached to the endometrium. The inner cell mass gets differentiated as the embryo. After attachment, the uterine cells divide rapidly, and cover the blastocyt. As a result, the blastocyst becomes embedded in the endometrium. This is called implantation, and it leads to pregnancy.

Pregnancy and Embryonic Development

Chorionic Villi: After implantation, finger-like projections appear on the trophoblast. These projections are called chorionic villi. The chorionic villi are surrounded by the uterine tissue and maternal blood. The chorionic villi and uterine tissue become interdigitated with each other. They jointly form a structural and functional unit between developing embryo (foetus) and maternal body. This structure is called placenta.

Placenta: The placenta facilitates the supply of oxygen and nutrients to the embryo and also removal of carbon dioxide and excretory materials produced by the embryo. The placenta is connected to the embryo through an umbilical cord which helps in the transport of substances to and from the embryo. Placenta also acts as an endocrine tissue and produces many hormones; like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens, progestogens, etc. In later phase of pregnancy, a hormone; called relaxin is also secreted by the ovary. It is important to note that these hormones are produced only during pregnancy.

In addition, levels of other hormones; like estrogens, progestogens, cortisol, prolactin, thyroxin, etc. also increase several folds in the maternal blood during pregnancy. Increased production of these hormones is essential for supporting the fetal growth, metabolic changes in the mother and maintenance of pregnancy.

Further Development of Embryo

Immediately after implantation, the inner cell mass (embryo) differentiates into an outer layer and inner layer; respectively called ectoderm and endoderm. After that, the mesoderm appears between these two layers. These three layers give rise to all tissues (organs) in adults. The inner cell mass also contains certain cells; called stem cells. The stem cells have the potency to give rise to all the tissues and organs.

Gestation: The human pregnancy lasts for 8 months or 40 weeks. The phase from implantation to child birth is called gestation period.

Parturition and Lactation

Parturition: The process of delivery of the foetus (childbirth) is called parturition. It is induced by a complex nueroendocrine mechanism. The signals of parturition originate from the fully developed foetus and the placenta. These signals induce mild uterine contractions; called foetal ejection reflex. This triggers the release of oxytocin from the maternal pituitary. Oxytocin acts on the uterine muscle and causes stronger uterine contractions. These contractions stimulate further secretion of oxytocin. The stimulatory reflex between the uterine contraction and oxytocin secretion continues resulting in stronger and stronger contractions. This leads to expulsion of the baby out of the uterus through the birth canal. After the infant is delivered, the placenta is also expelled out of the uterus.

Lactation: The mammary glands of the female undergo differentiation during pregnancy. These glands start producing milk towards the end of pregnancy by the process called lactation. The milk produced during the initial few days of lactation is called colostrum. The colostrum contains various antibodies which are absolutely essential to develop resistance for the new born baby.