Reproductive System - Part 3

REPRODUCTIVE SYSTEM

PART 3

MENSTRUAL CYCLE
•    The reproductive cycle in the female primates (e.g. monkeys, apes and human beings) is called menstrual cycle. The first menstruation begins at puberty and is called menarche.
•    In human females, menstruation is repeated at an average interval of about 28/29 days, and the cycle of events starting from one menstruation till the next one is called the menstrual cycle.

MENSTRUAL CYCLE - HARMONES
•    One ovum is released (ovulation) during the middle of each menstrual cycle. The cycle starts with the menstrual phase, when menstrual flow occurs and it lasts for 3-5 days.
•    The menstrual flow results due to breakdown of endometrial lining of the uterus and its blood vessels which forms liquid that comes out through vagina. Menstruation only occurs if the released ovum is not fertilized.
•    Lack of menstruation may be indicative of pregnancy. However, it may also be caused due to some other underlying causes like stress, poor health etc.
•    The menstrual phase is followed by the follicular phase. During this phase, the primary follicles in the ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of uterus regenerates through proliferation.
•    These changes in the ovary and the uterus are induced by changes in the levels of pituitary and ovarian hormones.
•    The secretion of gonadotropins (LH and FSH) increases gradually during the follicular phase, and stimulates follicular development as well as secretion of estrogens by the growing follicles.
•    Both LH and FSH attain a peak level in the middle of cycle (about 14 th day). Rapid secretion of LH leading to its maximum level during the mid-cycle called LH surge induces rupture of Graafian follicle and thereby the release of ovum (ovulation).
•    The ovulation (ovulatory phase) is followed by the luteal phase during which the remaining parts of the Graafian follicle transform as the corpus luteum.
•    The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium. Such an endometrium is necessary for implantation of the fertilised ovum and other events of pregnancy.
•    During pregnancy all events of the menstrual cycle stop and there is no menstruation. In the absence of fertilisation, the corpus luteum degenerates. This causes disintegration of the endometrium leading to menstruation, marking a new cycle.
•    In human beings, menstrual cycles ceases around 50 years of age; that is termed as menopause.
•    Cyclic menstruation is an indicator of normal reproductive phase and extends between menarche and menopause.

FERTILISATION AND IMPLANTATION
•    During copulation (coitus) semen is released by the penis into the vagina (insemination). The motile sperms swim rapidly, pass through the cervix, enter into the uterus and finally reach the ampullary region of the fallopian tube.
•    The ovum released by the ovary is also transported to the ampullary region where fertilisation takes place.
•    Fertilisation can only occur if the ovum and sperms are transported simultaneously to the ampullary region. This is the reason why not all copulations lead to fertilisation and pregnancy.
•    The process of fusion of a sperm with an ovum is called fertilisation. During fertilisation, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. Thus, it ensures that only one sperm can fertilise an ovum.
•    The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zona pellucida and the plasma.
•    Ovum surrounded by few sperm blastomeres is called a morula. The morula continues to divide and transforms into blastocyst as it moves further into the uterus.
•    The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called the inner cell mass. The trophoblast layer then gets attached to the endometrium and the inner cell mass gets differentiated as the embryo.
•    After attachment, the uterine cells divide rapidly and covers the blastocyst. As a result, the blastocyst becomes embedded in the endometrium of the uterus. This is called implantation and it leads to pregnancy.

IN VITRO FERTILIZATION
•    In some women oviducts are blocked. These women are unable to bear babies because sperms cannot reach the egg for fertilization. In such cases, doctors collect freshly released egg and sperms and keep them together for a few hours for IVF or In Vitro Fertilization (fertilization outside the body).
•    In case fertilization occurs, the zygote is allowed to develop for about a week and then it is placed in the mother’s uterus. Complete development takes place in the uterus and the baby is born like any other baby.
•    Babies born through this technique are called test-tube babies. This term is actually misleading because babies cannot grow in test tubes.

PREGNANCY AND EMBRYONIC DEVELOPMENT
•    After implantation, finger-like projections appear on the trophoblast called chorionic villi which are surrounded by the uterine tissue and maternal blood.
•    The chorionic villi and uterine tissue become interdigitated with each other and jointly form a structural and functional unit between developing embryo (foetus) and maternal body called placenta.
•    The placenta facilitate the supply of oxygen and nutrients to the embryo and also removal of carbon dioxide and excretory/waste 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 several hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens, progestogens, etc.
•    In the later phase of pregnancy, a hormone called relaxin is also secreted by the ovary. Let us remember that hCG, hPL and relaxin are produced in women only during pregnancy.
•    In addition, during pregnancy the levels of other hormones like estrogens, progestogens, cortisol, prolactin, thyroxine, etc., are increased several folds in the maternal blood.
•    Increased production of these hormones is essential for supporting the fetal growth, metabolic changes in the mother and maintenance of pregnancy.
•    Immediately after implantation, the inner cell mass (embryo) differentiates into an outer layer calledectoderm and an inner layer called endoderm. A mesoderm soon appears between the ectoderm and the endoderm [triploblastic]. These three layers give rise to all tissues (organs) in adults.
•    It needs to be mentioned here that the inner cell mass contains certain cells called stem cells which have the potency to give rise to all the tissues and organs.
•    The human pregnancy lasts 9 months. In human beings, after one month of pregnancy, the embryo’s heart is formed. The first sign of growing foetus may be noticed by listening to the heart sound carefully through the stethoscope.
•    By the end of the second month of pregnancy, the foetus develops limbs and digits. By the end of 12 weeks (first trimester), most of the major organ systems are formed, for example, the limbs and external genital organs are well developed.
•    The first movements of the foetus and appearance of hair on the head are usually observed during the fifth month. By the end of about 24 weeks (end of second trimester), the body is covered with fine hair, eye-lids separate, and eyelashes are formed. By the end of nine months of pregnancy, the foetus is fully developed and is ready for delivery.

PARTURITION AND LACTATION
•    The average duration of human pregnancy is about 9 months which is called the gestation period. Vigorous contraction of the uterus at the end of pregnancy causes expulsion/delivery of the foetus. This process of delivery of the foetus (childbirth) is called parturition.
•    Parturition is induced by a complex neuroendocrine mechanism. The signals for parturition originate from the fully developed foetus and the placenta which induce mild uterine contractions called foetal ejection reflex. This triggers release of oxytocin from the maternal pituitary.
•    Oxytocin acts on the uterine muscle and causes stronger uterine contractions, which in turn stimulates 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 – parturition.
•    Soon after the infant is delivered, the placenta is also expelled out of the uterus. The mammary glands of the female undergo differentiation during pregnancy and starts producing milk towards the end of pregnancy by the process called lactation. This helps the mother in feeding the newborn.
•    The milk produced during the initial few days of lactation is called colostrum which contains several antibodies absolutely essential to develop resistance for the new-born babies.
•    Breast-feeding during the initial period of infant growth is recommended by doctors for bringing up a healthy baby.

SUMMARY
•    Humans are sexually reproducing and viviparous.
•    The male reproductive system is composed of a pair of testes, the male sex accessory ducts and the accessory glands and external genitalia.
•    Each testis has about 250 compartments called testicular lobules, and each lobule contains one to three highly coiled seminiferous tubules.
•    Each seminiferous tubule is lined inside by spermatogonia and Sertoli cells.
•    The spermatogonia undergo meiotic divisions leading to sperm formation, while Sertoli cells provide nutrition to the dividing germ cells.
•    The Leydig cells outside the seminiferous tubules, synthesise and secrete testicular hormones called androgens.
•    The male external genitalia is called penis.
•    The female reproductive system consists of a pair of ovaries, a pair of oviducts, a uterus, a vagina, external genitalia, and a pair of mammary glands.
•    The ovaries produce the female gamete (ovum) and some steroid hormones (ovarian hormones).
•    Ovarian follicles in different stages of development are embedded in the stroma.
•    The oviducts, uterus and vagina are female accessory ducts.
•    The uterus has three layers namely perimetrium, myometrium and endometrium.
•    The female external genitalia includes mons pubis, labia majora, labia minora, hymen and clitoris.
•    The mammary glands are one of the female secondary sexual characteristics.
•    Spermatogenesis results in the formation of sperms that are transported by the male sex accessory ducts.
•    A normal human sperm is composed of a head, neck, a middle piece and tail.
•    The process of formation of mature female gametes is called oogenesis.
•    The reproductive cycle of female primates is called menstrual cycle.
•    Menstrual cycle starts only after attaining sexual maturation (puberty).
•    During ovulation only one ovum is released per menstrual cycle.
•    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 coitus, sperms are transported to the junction of the isthmus and ampulla, where the sperm fertilizes the ovum leading to formation of a diploid zygote.
•    The presence of X or Y chromosome in the sperm determines the sex of the embryo.
•    The zygote undergoes repeated mitotic division to form a blastocyst, which is implanted in the uterus resulting in pregnancy.
•    After nine months of pregnancy, the fully developed foetus is ready for delivery.
•    The process of childbirth is called parturition which is induced by a complex neuroendocrine mechanism involving cortisol, estrogens and oxytocin.
•    Mammary glands differentiate during pregnancy and secrete milk after child-birth.
•    The new-born baby is fed milk by the mother (lactation) during the initial few months of growth.