Full biology

• Humans are sexually reproducing and viviparous.
• The reproductive events in humans include formation of gametes (gametogenesis), transfer of sperms into the female genital tract (insemination) and fusion of male and female gametes (fertilisation) leading to formation of zygote.
• This is followed by formation and development of blastocyst and its attachment to the uterine wall (implantation), embryonic development (gestation) and delivery of the baby (parturition).
• A human ovary releases an average of 250 eggs per month.
• If a mother gives birth to identical twins, an estimated 400 eggs would have been released by the ovary.
• These reproductive events occur after puberty.
• There are remarkable differences between the reproductive events in the male and in the female, for example, sperm formation continues even in old men, but formation of ovum ceases in women around the age of fifty years.
• The male reproductive system is located in the pelvis region and includes a pair of testes alongwith accessory ducts, glands and the external genitalia.
• During ovulation only one ovum is released per menstrual cycle.
• The reproductive cycle of female primates is called menstrual cycle.
• The zygote undergoes repeated mitotic division to form a blastocyst, which is implanted in the uterus resulting in pregnancy.
• Menstrual cycle starts only after attaining sexual maturation (puberty).
• The process of childbirth is called parturition which is induced by a complex neuroendocrine mechanism involving cortisol, estrogens and oxytocin.
• The new-born baby is fed milk by the mother (lactation) during the initial few months of growth.
• The cyclical changes in the ovary and the uterus during menstrual cycle are induced by changes in the levels of pituitary and ovarian hormones.
• After nine months of pregnancy, the fully developed foetus is ready for delivery.
• Mammary glands differentiate during pregnancy and secrete milk after child-birth.
• A normal human sperm is composed of a head, neck, a middle piece and tail.
• The presence of X or Y chromosome in the sperm determines the sex of the embryo.
• The process of formation of mature female gametes is called oogenesis.
• After coitus, sperms are transported to the ampulla, where the sperm fertilises the ovum leading to formation of a diploid zygote.
• The testes are situated outside the abdominal cavity within a pouch called scrotum which maintains the low temperature of the testes necessary for spermatogenesis.
• In adults, each testis is oval in shape, with a length of about 4 to 5 cm and a width of about 2 to 3 cm.
• Each testis is covered by a dense covering.
• Each testis has about 250 compartments called testicular lobules.
• Each lobule contains one to three highly coiled seminiferous tubules in which sperms are produced.
• Each seminiferous tubule is lined on its inside by two types of cells called male germ cells (spermatogonia) and Sertoli cells.
• The male germ cells undergo meiotic divisions finally leading to sperm formation, while Sertoli cells provide nutrition to the germ cells.
• The regions outside the seminiferous tubules called interstitial spaces, contain small blood vessels and interstitial cells or Leydig cells.
• Leydig cells synthesise and secrete testicular hormones called androgens.
• Other immunologically competent cells are also present in the male sex accessory ducts including rete testis, vasa efferentia, epididymis and vas deferens.
• The placenta facilitates the supply of oxygen and nutrients to the embryo and also removal of carbon dioxide and excretory/waste materials produced by the embryo.
• During pregnancy the levels of other hormones like estrogens, progestogens, cortisol, prolactin, thyroxine, etc., are increased several- folds in the maternal blood.
• These three layers ectoderm, endoderm and mesoderm give rise to all tissues (organs) in adults.
• A mesoderm soon appears between the ectoderm and the endoderm.
• The inner cell mass contains certain cells called stem cells which have the potency to give rise to all the tissues and organs.
• This is called implantation and it leads to pregnancy.
• The trophoblast layer then gets attached to the endometrium and the inner cell mass gets differentiated as the embryo.
• After one month of pregnancy, the embryo’s heart is formed.
• After attachment, the uterine cells divide rapidly and covers the blastocyst.