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Human reproduction

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  • BioAdmin
    started a topic Human reproduction

    Human reproduction

    Reproduction is defined as a biological process in which an organism gives rise to young ones (offspring) similar to itself.
    The offspring grow, mature and in turn produce new offspring. Thus, there is a cycle of birth, growth and death.
    Reproduction enables the continuity of the species, generation after generation.

    Based on whether there is participation of one organism or two in the process of reproduction, it is of two types.
    When offspring is produced by a single parent with or without the involvement of gamete formation, the reproduction is asexual.
    When two parents (opposite sex) participate in the reproductive process and also involve fusion of male and female gametes, it is called sexual reproduction
    The primary sex organs are called gonads. Primary organs are testes in male and ovaries in females.
    The testis produces sperms and the male hormone testosterone
    ​The ovaries produces ova and the female hormone estrogen and progesterone

  • BioAdmin

    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.
    One ovum is released (ovulation) during the middle of each menstrual 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. Phases of the menstrual cycle

    Menstrual phase (1st to 5th day)- In the absence of fertilisation, the corpus luteum degenerates. This causes disintegration of the endometrium leading to discharge of blood carrying broken uterine tissue. This flow of blood is called menstruation.

    Follicular phase (Post menstrual or Proliferative phase) 6th to 13/14th day-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. Endometrial glands elongate and become corkscrew shaped. The arterioles become longer and coiled. Epithelial cells of fallopian tube get thickened and movement of cilia increases.
    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 14th day).

    Ovulatory phase (13/14th 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).

    Luteal phase (Post ovulatory, secretory or premenstrual phase) 15th to 28th day- 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. Uterine glands enlarge and releases nutritional fluids.
    Such an endometrium is necessary for implantation of the fertilised ovum and other events of pregnancy. During pregnanacy all events of
    the menstrual cycle stop and there is no menstruation.
    What happens during the menstrual cycle?
    1. The start of the menstrual cycle begins on the first day of bleeding. On about the 5th day, the brain begins producing hormones that cause a few follicles to grow and mature in the ovaries. These follicles then start producing the hormone estrogen.
    2. The lining of the uterus (endometrium) responds to the estrogen by becoming thicker and developing more blood vessels.
    3. Somewhere around the 14th or 15th day of the cycle, the most mature follicle bursts and releases an egg or ovum. This is called ovulation.
    4. The egg then enters the Fallopian tube.
    5. The ruptured follicle becomes the corpus luteum and begins to secrete the hormone progesterone.
    6. Progesterone increases the supply of blood and nutrients to the uterus and maintains the endometrial lining.
    7. If the egg unites with a sperm from a man, it becomes fertilized and travels down the Fallopian tube, finally settling into the lining of the uterus. If the egg does not become fertilized, it soon breaks down, and the follicle stops producing progesterone.
    8. The loss of progesterone causes the lining of the uterus to break apart and bleed. The menstrual period then begins, starting another cycle. Hormones and the menstrual cycle

    • During the menstrual phase, estrogen and progesterone levels are low, this stimulates the release of FSH and LH from the pituitary.
    • The increase in FSH stimulates the development of follicles.
    • The cells of the follicle (granulosa cells) secrete estrogen.
    • FSH and LH maintains more of estrogen and less of progesterone.
    • The estrogen levels increases and reaches a peak at about the 12th/ 13th day.
    • This sends a feedback to the pituitary and causes LH surge.The LH levels peak on 13/14th day.
    • This results in ovulation and development of corpus luteum. The corpus luteum secretes progesterone.
    • This maintains pregnancy if fertilisation takes place.
    • If fertilisation does not take place the low levels of estrogen and progesterone starts the secretion of FSH and LH from the pituitary.

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  • BioAdmin
    Picture Picture
    The female reproductive system consists of a pair of ovaries alongwith a pair of oviducts, uterus, cervix, vagina and the external genitalia located in pelvic region. These parts of the system alongwith a pair of the mammary glands are integrated structurally and functionally to support the processes of ovulation, fertilisation, pregnancy, birth and child care.
    Ovary: Ovaries are the primary female sex organs that produce the female gamete (ovum) and several steroid hormones (ovarian hormones). They are located one on each side of the lower abdomen Each ovary is about 2 to 4 cm in length and is connected to the pelvic wall and uterus by ligaments.
    Oviduct/fallopian tube: The oviducts, uterus and vagina constitute the female accessory ducts. Each fallopian tube is about 10-12 cm long and extends from the periphery of each ovary to the uterus, the part closer to the ovary is the funnel-shaped infundibulum. The infundibulum possess finger-like projections called fimbriae, which help in collection of the ovum after ovulation. The infundibulum leads to a wider part of the oviduct called ampulla. The last part of the oviduct, isthmus has a narrow lumen and it joins the uterus.
    Uterus: The uterus is single and it is also called womb. It is supported by ligaments attached to the pelvic wall. The uterus opens into vagina through a narrow cervix. The cavity of the cervix is called cervical canal which along with vagina forms the birth canal. The wall of the uterus has three layers of tissue. The external thin membranous perimetrium, middle thick layer of smooth muscle, myometrium and inner glandular layer called endometrium that lines the uterine cavity. ​

    ​The surface of the ovary is covered by a layer of columnar cells which constitutes the germinal epithelium. There is a layer of connective tissue on the outer surface called the tunica albuginea.
    Internally the ovary consists of a number of vesicular ovarian follicles embedded in the meshes of a stroma. The stroma of the ovary may contain interstitial cells resembling those of the testis, and is supplied with abundant blood vessels. The developing follicles are present near the periphery called as cortical stroma and the highly vascularised inner region called medullary stroma. The follicles are spherical aggregations of cells in the ovary that contains a single oocyte. These follicles grow from a small primordial follicle to a mature Graafian follicle. This process is called folliculogenesis.

    Picture Basic types of ovarian follicle include:
    1. Primordial follicles, the earliest stage of development;
    2. Growing follicles, including preantral or primary and small antralsecondary follicles.
    3. Mature, pre-ovulatory or Graafian follicles.

    Primordial follicle: These appear in the third month of foetal development. The oocyte is surrounded by a single layer ofsquamous follicular cells.

    Primary follicle- has a central oocyte surrounded by a single layer of cuboidal cells. These cells are called granulosa cells. The zona pellucida is also visible.

    Secondary follicle- The characteristic feature of this is development of cavities in-between the granulosa cells. Outer layer of theca is visible.

    Graffian follicle- This stage is characterized by the presence of a large follicular cavity or the antrum filled with liquor folliculi. The thecal layers become more prominent and the layer of granulosa cells become thinner.

    Atretic follicle: Some of the developing follicles degenerate during the process of folliculogenesis.

    Corpus haemorrhagicum: a ruptured graafian follicle containing a blood clot that is absorbed as the cells lining the follicle form the corpus luteum.

    Corpus luteum: The corpus haemorrhagicum, formed after rupturing of the follicle changes to form a yellow coloured mass of cells called corpus luteum. This structure secretes the hormone progesterone during the luteal phase.

    Corpus albicans: When the oocyte is not fertilised, the corpus luteum starts to degenerate and stops secreting progesterone. This then forms a mass of fibrous scar tissue and is ca
    Graafian follicle is a mature ovarian follicle. This characterised by the presence of a large cavity called as antrum. The antrum is filled with a fluid called liquor folliculi.
    The oocyte increases in size and is surrounded by a row of cells called the corona radiata. The oocyte has undergone the first meiotic division and is arrested in the metaphase stage. The oocyte shifts to a side and lies on a mass of cells called cumulus oophorous.
    The granulosa cells form a thin layer of cells towards the periphery.

    The connective tissue layers outside the follicle differentiate to form two distinct layers- the theca interna and the theca externa.
    Picture The process of egg formation or oogenesis begins before the birth of the female.
    Multiplication phase: The germinal epithelium produces many oogonia that develop to form the primary oocytes. These are cells that become larger and has large nucleus. These cells form a mass called egg nest.
    One cell enters the growth phase whereas other cells form the follicle cells surrounding the oocyte.
    Growth phase: The primary oocyte divides by meiosis to form two unequal cells-
    (1) cell with large amount of cytoplasm is the secondary oocyte and
    (2) cell with very less cytoplasm called the polar body.
    This includes vitellogenesis, formation of zone pellucida and increased activity of different organelles like mitochondria, golgi body etc.
    There will be around 2 million primary oocytes in the ovary of a new born female.
    Maturation phase: The secondary oocyte undergoes meiosis II and is arrested in the metaphase stage till ovulation. The division is complete only if fertilisation takes place. This gives rise to the ovum and another polar body.
    The polar bodies disintegrate.

    1. Process takes place in the seminiferous tubules.
    2. Process begins at puberty.
    3. Process continuous throughout the life time.
    4. Four functional, motile sperms are formed after the process.
    5. No polar bodies are formed during this process.
    6. The spermatozoa produced are minute, stramlined and yolkless.

    1. Process takes place in the ovaries.
    2. Process begins in the fetus.
    3. Process stops after menopause.
    4. One functional, non-motile ovum is produced after the process.
    5. Two to three polar bodies are produced during this process.
    6. The ovum is large, round with yolk.
    The germinal epithelium divides to form follicle cells that surround a primary oocyte to form the primary follicle. At birth the baby girl will have about a million primary follicles. The primary oocyte starts the meiotic division but is arrested at prophase I.
    At puberty, hormones from the pituitary stimulate the follicle to develop further. Each month several follicles start to develop but usually only one matures to form a graafian follicle. The remaining undergo atresia.
    The follicle cells of the primary follicle multiply and a number of fluid filled spaces appear between them. This is now a secondary follicle. The secondary follicle forms a mature follicle and migrates to the surface of the ovary. Eventually the follicle bursts and the secondary oocyte with its surrounding cells is released in the out of the ovary. This process is called ovulation.
    After ovulation the remaining follicle cells develop to form the corpus luteum.

    The secondary oocyte is the largest cell in the human body with a diameter of 14mm. The cytoplasm has small lipid droplets and the cytoplasm is called as ooplasm. It has a nucleus called the germinal vesicle. The ovum is enclosed by a thick, transparent envelope called the zona striata or zona pellucida. The space between the plasma membrane and the zona pellucida is called peri-vitelline space.
    To the outer surface of the zona pellucida there are two to three strata of cells derived from the follicle called corona radiata.

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  • BioAdmin

    Parts of the male reproductive system
    Picture The male reproductive system is located in the pelvis region. It includes a pair of testes along with accessory ducts, glands and the external genitalia.
    The primary reproductive organ- The testes are situated outside the abdominal cavity within a pouch called scrotum. The testes descend through the inguinal canal into the scrotum at the time of birth.
    FUNCTION: The scrotum helps in maintaining temperature of the testes 2–2.5 degree C lower than the normal internal body temperature. This is necessary for spermatogenesis.
    The failure of testes to descend into the scrotal sac is called chryptorchidism
    The internal organs of the male reproductive system, also called accessory organs, include the following:
    The epididymis is a long, coiled tube that rests on the backside of each testicle. It transports and stores sperm cells that are produced in the testes. It also brings the sperm to maturity, since the sperm that emerge from the testes are immature and incapable of fertilization.
    The vas deferens is a long, muscular tube that travels from the epididymis into the pelvic cavity, to just behind the bladder. The vas deferens transports mature sperm to the urethra, the tube that carries urine or sperm to outside of the body, in preparation for ejaculation.
    Ejaculatory ducts are formed by the fusion of the vas deferens and the seminal vesicles (see below). The ejaculatory ducts empty into the urethra.
    Urethra is the tube that carries urine from the bladder to outside of the body. In males, it has the additional function of ejaculating semen when the man reaches orgasm. When the penis is erect during sex, the flow of urine is blocked from the urethra, allowing only semen to be ejaculated at orgasm.
    • Seminal vesicles: These are sac-like pouches that attach to the vas deferens near the base of the bladder. The fluid of the seminal vesicles makes up most of the volume of a man's ejaculatory fluid, or ejaculate
    FUNCTION: They produce a sugar-rich fluid (fructose) that provides sperm with a source of energy to help them move.
    • Prostate gland: It is a walnut-sized structure that is located below the urinary bladder in front of the rectum. The urethra, which carries the ejaculate to be expelled during orgasm, runs through the center of the prostate gland.
    FUNCTION: The prostate gland contributes additional fluid to the ejaculate and also help to nourish the sperm.
    • Bulbourethral glands: Also called Cowper's glands, these are pea-sized structures located on the sides of the urethra just below the prostate gland. These glands produce a clear, slippery fluid that empties directly into the urethra.
    ​FUNCTION: This fluid serves to lubricate the urethra and to neutralize any acidity that may be present due to residual drops of urine in the urethra.
    Structure of testes
    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. The testis is covered by a dense covering.
    The tunica vaginalis is the outer serous covering of the testis.
    The Tunica Albuginea is the inner fibrous covering of the testis.
    It is a dense membrane, of a bluish-white color, composed of bundles of white fibrous tissue which interlace in every direction.
    It is covered by the tunica vaginalis, except at the points of attachment of the epididymis to the testis, and along its posterior border, where the spermatic vessels enter the gland.From the inners surface of the tunica albuginea, a series of fibrous septa extend towards the interior of the organ. These septa divide the testicle into several lobules. Each testis has about 250 compartments called testicular lobules. Within each lobule, there lay one to three coiled tubules known as seminiferous tubules.
    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.
    The seminiferous tubules open into a network of channels called the rete testis. Small efferent ductules connect the rete testis to the upper end of the epididymis.
    Seminiferous tubule
    Picture Picture
    Each seminiferous tubule consists of a basement layer formed of laminated connective tissue containing numerous elastic fibers with flattened cells between the layers and covered externally by a layer of flattened epithelioid cells.
    Within the basement membrane are epithelial cells arranged in several irregular layers. Among these cells may be seen the spermatozoa in different stages of development.
    (1) Lining the basement membrane and forming the outer zone is a layer of cubical cells, with small nuclei; some of these enlarge to become spermatogonia.
    (2) Within this next layer is a number of larger polyhedral cells, with clear nuclei, arranged in two or three layers; these are the intermediate cells or spermatocytes. Most of these cells are in a condition of karyokinetic division, and the cells which result from this division form those of the next layer, the spermatoblasts or spermatids.
    (3) The third layer of cells consists of the spermatoblasts or spermatids, and each of these, without further subdivision, becomes a spermatozoon. The spermatids are small polyhedral cells, the nucleus of each of which contains half the usual number of chromosomes.
    In addition to these three layers of cells others are seen, which are termed the supporting cells (cells of Sertoli). They are elongated and columnar, and project inward from the basement membrane toward the lumen of the tube. As development of the spermatozoa proceeds the latter group themselves around the inner extremities of the supporting cells. External genitilia- Penis

    The penis is the male external genitalia, made up of special tissue that helps in erection of the penis to facilitate insemination. The enlarged end of penis called the glans penis is covered by a loose fold of skin called foreskin.
    Male reproductive system Spermatogenesis
    Picture The development of the germ cells begins with the spermatogonia at the periphery of the seminal canal and advances towards the lumen. The cells form spermatocytes I (primary spermatocytes), spermatocytes II (secondary spermatocytes), spermatids and finally to mature sperm cells.
    Multiplication phase (spermatocytogenesis): The germinal epithelium divides mitotically to produce sperm mother cells called spermatogonium. In this division two daughter cells are formed - one remains as spermatogonium and the other proceeds to form the primary spermatocyte.
    The primary spermatocyte undergoes meiosis.
    Growth phase: The spermatocyte and the nucleus enlarges and gets ready for the maturation phase.
    Maturation phase: The primary spermatocytes undergo meiosis to form secondary spermatocytes with half the number of chromosomes. The secondary spermatocytes form spermatids.The spherical spermatid cells undergo differentiation to form the sperms.
    The differentiation of the spermatids to form sperms is called as spermiogenesis.
    The spermatozoa formed are fully developed but are not entirely motile. They derive nourishment from the sertoli cells and are released to the lumen. This process of release of sperms from the sertoli cells is called spermiation.
    ​These are pushed to the epididymis and attain full motility in the cauda epididymis.
    One spermatogonium produces four sperms.
    Meiosis reduces the chromosome number to half and thus maintains the chromosome number of the species.
    Meiosis also produces variations. ​HORMONAL CONTROL OF SPERMATOGENESIS
    Picture Spermatogenesis starts at the age of puberty due to significant increase in the secretion of gonadotropins.
    The increased levels of GnRH then acts at the anterior pituitary gland and stimulates secretion of two gonadotropins – follicle stimulating hormone and intestitial cell stimulating hormone.
    ​ICSH acts on the Leydig cells and stimulates synthesis and secretion of androgens. Androgens, in turn, stimulate the process of spermatogenesis. FSH acts on the Sertoli cells and stimulates secretion of some factors which help in the process of spermiogenesis.
    Sperm cell
    A mature spermatozoan is approximately 60um long, flagellated, motile cell. It consists of flat oval head and narrow tail.
    Head: It is flat and oval and consists of
    (1) anterior, acrosome that contains hydrolytic enzymes. These enzymes help in penetrating the egg membrane during fertilization and
    (2) posterior, large nucleus that contains a haploid set of chromosomes.
    Mid piece: It consists of a sheath of ring-shaped mitochondria grouped around the axoneme to provide the energy for the flagellar movement.
    Tail: The tail has 3 parts-
    1. A narrow neck with two centrioles,
    2. The middle piece which is composed of many mitochondria forming a spiral, that provides energy for to the sperm for motility.
    3. The principal piece of the tail that finally ends in end piece.
    ​The ejaculate
    The human male ejaculates about 200 to 300 million sperms during coitus of which, for normal fertility, at least 60 per cent sperms must have normal shape and size and at least 40 per cent of them must show vigorous motility.

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