NCUK Week 8: Reproductive System PDF

NCUK – WEEK 8 Part 1: Outline male and female reproductive systems. Part 2: Oogenesis and spermatogenesis. Part 3: Hormonal control of human menstrual cycle. Male reproductive system The Male Reproductive System Includes the testes and Accessory Structures The male rep...

NCUK – WEEK 8 Part 1: Outline male and female reproductive systems. Part 2: Oogenesis and spermatogenesis. Part 3: Hormonal control of human menstrual cycle. Male reproductive system The Male Reproductive System Includes the testes and Accessory Structures The male reproductive system includes the testes, which produce testosterone and sperm; glands that secrete substances that activate and nourish sperm; and tubes that store sperm and conduct them out of the body. Testosterone and sperm are produced nearly continuously, beginning at puberty and continuing until death.  The human male reproductive system  The testes hang beneath the abdominal cavity in the scrotum. Sperm pass from the testis to the epididymis, through the vas- deferens and urethra to the tip of the penis.  Along the way, fluids are added from the seminal vesicles, the prostate gland, and the bulbourethral glands.  Sperm Are produced in the testes The testes are located in the scrotum, a pouch that hangs outside the main body cavity.  This location keeps the testes about 1° to 6°F (0.5° to 3°C) cooler than the core of the body.  Cooler temperatures promote sperm development.  Each testis is nearly filled with coiled, hollow seminiferous tubules, in which sperm are produced.  Interstitial cells, which synthesize testosterone, are located in the spaces between the tubules.  Just inside the wall of each seminiferous tubule lie two major types of cells: (1) stem cells called spermatogonia or spermatogonial stem cells, which give rise to sperm. (2) much larger Sertoli cells, which nourish the developing sperm and regulate their growth. Spermatogenesis A spermatogonium divides by mitotic cell-division; one of its two daughter cells remains a spermatogonium, while the other enlarges and becomes a primary spermatocyte, which will undergo meiotic cell division followed by differentiation, producing haploid sperm. To clarity, only two pairs of chromosomes are shown; however, human cells have 23 pairs of chromosomes  Spermatogonia are diploid cells that undergo mitotic cell division, forming two types of daughter cells.  Daughter cells of one type remain spermatogonia, ensuring a steady supply throughout a man’s life. Daughter cells of the other type become committed to spermatogenesis, the processes that produce sperm.  A committed daughter cell passes through several rounds of mitotic cell division.  Each final offspring cell differentiates into a large cell called a primary spermatocyte, which then undergoes meiotic cell division.  At the end of meiosis I, each primary spermatocyte gives rise to two haploid secondary spermatocytes.  Each secondary spermatocyte undergoes meiosis II, producing two spermatids. Thus, each diploid primary spermatocyte generates a total of four haploid spermatids.  Spermatids differentiate into sperm without further cell division.  Spermatogonia, spermatocytes, and spermatids are enfolded in the Sertoli cells.  As spermatogenesis proceeds, the developing sperm migrate to the central cavity of the seminiferous tubule into which the mature sperm are released.  A human sperm is unlike any other cell of the body.  There is little cytoplasm, so the haploid nucleus nearly fills the sperm’s head.  Atop the nucleus lies a sac of enzymes called an acrosome  The enzymes dissolve protective layers that surround the egg, enabling the sperm to penetrate through them.  Behind the head is the mid-piece, packed with mitochondria.  The mitochondria provide the energy needed to move the tail, which is actually a long flagellum.  Whip-like movements of the tail propel the sperm through the female reproductive tract.  Hormones from the Anterior pituitary and testes Regulate Spermatogenesis * Spermatogenesis is stimulated by GnRH released from the hypothalamus. 1) GnRH causes the anterior pituitary to release LH and FSH. 2) LH stimulates the interstitial cells of the testes to produce testosterone. 3) The combination of testosterone and FSH stimulates the Sertoli cells, which promotes spermatogenesis.  Testicular function is regulated by negative feedback. *The Sertoli cells, when stimulated by FSH and testosterone, secrete a hormone called inhibin, which inhibits FSH release.  Testosterone itself inhibits the release of GnRH, LH, and FSH , which limits further testosterone production and sperm development.  If testosterone levels fall too low, then the negative feedback is reduced, allowing production of GnRH, LH, FSH, and testosterone to increase again.  This complex feedback process maintains relatively constant levels of testosterone and sperm production. Female reproductive system  The Female Reproductive System Includes the Ovaries and Accessory Structures - The female reproductive system consists of the ovaries and structures that accept sperm, conduct the sperm to the egg, and nourish the developing embryo. - the Ovary produces Eggs, Estrogen, and progesterone During the Menstrual Cycle.  The human female reproductive system  Eggs are produced in the ovaries and enter the uterine tube.  Sperm and egg usually meet in the uterine tube, where fertilization and very early development occur.  The early embryo implants in the lining of the uterus, where development continues.  The vagina receives sperm and serves as the birth canal.  Oogenesis: An oogonium enlarges to form a primary oocyte.  At meiosis I, almost all the cytoplasm is included in one daughter cell, the secondary oocyte. The other daughter cell is a small polar body that contains chromosomes but little cytoplasm.  At meiosis II, almost all the cytoplasm of the secondary oocyte is included in the egg, and a second small polar body discards the remaining “extra” chromosomes.  The first polar body may also undergo the second meiotic division.  The polar bodies eventually degenerate.  In humans, meiosis II does not occur unless a sperm penetrates the egg.  The Ovary produces Eggs, Estrogen, and progesterone During the Menstrual Cycle 1) In the mature ovary, each oocyte is surrounded by a layer of smaller cells. Together, the oocyte and these accessory cells make up a follicle. 2) Follicle development and ovulation are governed by interactions among hormones produced by the hypothalamus, anterior pituitary, and ovary during the menstrual cycle (from the Latin “mensis,” meaning “month”). 3) Roughly once a month, the hypothalamus secretes GnRH, which stimulates the anterior pituitary to release LH and FSH. 4) FSH stimulates about a dozen follicles to begin developing. 5) The follicle cells also release estrogen into the bloodstream. Usually, only one follicle completely matures during each menstrual cycle. 6) The maturing follicle secretes increasing amounts of estrogen, which stimulates a surge of LH that causes the primary oocyte to complete meiosis I, dividing into a single secondary oocyte and the first polar body. 7) The polar body is a small cell, with very little cytoplasm; it cannot be fertilized by sperm. The surge of LH also causes ovulation, as the follicle erupts through the surface of the ovary, releasing its secondary oocyte. 8) The secondary oocyte will not undergo meiosis II unless it is fertilized. 9) For convenience, we will refer to the ovulated secondary oocyte as the egg. 8) Some of the accessory follicle cells leave the ovary with the egg, but most remain in the ovary , where they enlarge, forming a temporary gland called the corpus luteum. 9) The corpus luteum secretes both estrogen and a second hormone called progesterone. 10) The combination of estrogen and progesterone inhibits further release of GnRH, LH,and FSH, thereby preventing the development of any more follicles. 11) If the egg is not fertilized, the corpus luteum will degenerate within a few days.  Hormonal control of the menstrual cycle The menstrual cycle consists of two related cycles (a) the ovarian cycle and (b) the uterine cycle.