UNIT 4: Homeostasis Chapter 10: The Endocrine System pg. 466 10.7: The Reproductive Hormones pg. 496-503 Gonads are the gland responsible for the production of sex hormones, as well as the egg and sperm cells, called testes in males and ovaries in females. Androgen are predominantly male sex hormones, including testosterone, that control sexual development and reproduction. Sexual reproduction requires both a male and female to produce sex gametes, which are used during fertilization to produce a zygote that has a full complement of genetic material similar to its species. The sex organs for a male are the testes, and the female it s the ovaries. These organs will secrete hormones, such as; androgens (predominantly in males) and estrogens, and progestins (predominantly in females) to regulate the development of their reproductive systems, sexual characteristics and mating behaviour. The Female Reproductive System Gonadotropin releasing Hormone (GnRH) is a hormone released by the hypothalamus that controls the release of LH and FSH from the anterior pituitary, which, in turn, control the synthesis and release of the male or female sex hormones in the gonads. Oogenesis is the production of eggs, or ova, from oocytes in the ovaries by two meiotic divisions. Menopause - is the end of a female s reproductive capability, after which menstruation ceases and female hormone levels drop. Menstrual cycle is the monthly cycle of events in a sexually mature female that prepares the uterus for the implantation of a fertilized egg.
The female reproductive system consists of a pair of ovaries that produce the ova or eggs, known as the sex gametes. Hormones are secrete from the ovaries; estrogen (steroid hormone) which stimulate and control the development and maintenance of the reproductive system. The Oviduct transport ova from each ovary and delivers them to the uterus. It is in this location that fertilization takes place when a sperm fuses with ova. The uterus is a cavity space with walls that contain smooth muscles known as the myometrium. The myometrium is lined by an endometrium, which forms a layer of connective tissue with embedded glands and a rich supply of blood vessels. The fertilized egg embeds itself in the endometrium where it develops from a zygote to embryo to fetus, then birth. At the lower end of the uterus is the cervix. The cervix is a muscular circular shaped sphincter. The cervix remains closed and supports the developing fetus until birth. The cervix must dilate 10 cm before birthing can take place. The outer cavity space is known as the vagina, sperm enters here and must travel pass the cervix, through the uterus and up the oviducts for fertilization to occur. Figure 1: The reproductive organs of a human female.
Oogenesis is the process that produces viable ova in the ovaries. This process is controlled by hormones produced by the pituitary gland. During Oogenesis oocytes are produced, which are immature eggs. These oocytes must undergo their first meiotic division. After the first division these oocytes become secondary oocytes and are non functional and are now known as a polar body. Ovaries contain million oocytes which can become non functional. Approximately 200 000 to 380 000 oocytes survive until the female reaches the age of maturity. These oocytes compete to become the one ova released to be later fertilized. About 1000 compete per menstrual cycle. This allows for approximately 380 ovulations or about 32 years between the ages of puberty to menopause. The cells that were not selected to be released and fertilized will form the corpus luteum, only if the egg is fertilized, sperm fuses to the nucleus of the egg, and a zygote is formed. The Menstrual Cycle Females go through the menstrual cycle regularly from puberty to menopause which has a 28 day cycle. The cycle is control by hormone secretions and interactions between the hypothalamus, pituitary, ovaries and the uterus. The hypothalamus releases GnRH which stimulates the pituitary gland to release FSH and LH into the blood stream and is transported to the target cells in the ovaries. This FSH causes the oocytes to begin meiosis and form a follicle. Only one follicle develops to maturity and is released at ovulation. FSH and LH stimulate the secretion of estrogen by the follicular cells; they first use negative feedback to stop the pituitary gland from secreting FSH. As time goes by estrogen secretion increases and peaks at day 12, which also has a positive feedback response on the hypothalamus and pituitary stimulating GnRH and eventually FSH and LH. On day 14 the event of ovulation occurs, before this event can occur, LH secretions increase causing the follicle to release an enzyme that digests its wall releasing an egg. The remaining follicles form the Corpus Luteum.
The luteal phase starts and prepares the uterus for a possible fertilized ovum. The corpus luteum secretes estrogens and large quantities of progesterone. Progesterone stimulates the development of the uterine lining and inhibits the contractions of the uterus. Progesterone also has a negative feedback o the hypothalamus and pituitary stopping the secretion of FSH and LH and which stops the follicles from developing. If fertilization does not occur the corpus luteum begins to breakdown and hormone secretions stop and menstruation will begin. Since progesterone secretion stop then GnRH and FSH and LH secretions are not longer inhibited. Flow Phase: Day 0 to approximately Day 4/5/6/7, depending on the individual. During this phase the old endometrium is shedding from the uterine lining. Follicular Phase: Day 5 to Day 13, during this phase the old endometrium has been shed and uterine contraction cease. A new endometrium begins to form, and oocytes begin to develop in both ovaries. Ovulation: Day 14 is the day of ovulation; one mature egg is released from one of the two ovaries. Luteal Phase: Day 15 to Day 28, the uterine lining continues to develop in preparation for a fertilized ovum. If the egg is not fertilized then the uterine lining will be to disintegrate. This cycle continues until an adult female reaches age of 40 s to early 50 s, then menopause sets in and hormones secretions change. Side affects of a decrease of estrogen and progesterone can be; hot flashes, headaches, and mood swings. Hormone replacement therapy (HRT) can be prescribed replace the estrogen and progesterone to lessen the symptoms but also has risks, such as; heart disease, breast cancer, and stroke. Estrous cycle occurs in mammals where the uterine lining is reabsorbed by the body if fertilization does not occur.
Figure 2: The ovarian and menstrual cycles of a human female.
Fertilization and Pregnancy Fertilization takes place in the upper third of the oviduct, if it is not fertilized within the first 12 to 24 hours the egg will begin to disintegrate. The sperm must travel from the vagina, through the cervix, through the uterus and up the oviduct for fertilization to occur. Once one sperm reaches the one egg, they will fuse together. The head of the sperm enter the cytosol of the egg, the sperm and egg pronuclei (haploid nuclei of a sperm or egg before fusion) then fuse becoming a zygote. Mitotic division begins and embryonic development. The first mitotic division takes lace in the oviduct and within seven days the embryo passes from the oviduct into the uterus, where it is embedded in the endometrium. After implantation, the embryo secretes human chorionic Gonadotropin hormone (hcg) that is responsible for maintaining the corpus luteum and preventing the immune system from rejecting the embryo. After implantation the embryo rapidly develops and the placenta forms to support the growth of the embryo. The corpus luteum secretes estrogen and progesterone, maintaining high levels, to maintain uterine lining and prevents menstruation. Mucus is secreted which plugs the opening of the cervix from the vagina, keeping bacteria, viruses, and sperms cells from entering. After 10 weeks the placenta takes over the secretion of progesterone as hcg drops. Corpus luteum regresses but still produces the hormone relaxin, which inhibits the contractions of the uterus until the time of birth.
The Male Reproductive System Spermatogenesis is the production and development of sperm cells in the testes. Male sex hormones are responsible for the development of secondary sex characteristics. The testes secrete androgens (steroid) that stimulate and control the development and maintenance of the male reproductive system. Testosterone, the primary male hormone, stimulates puberty and the secondary sex characteristics, such as; facial hair and body hair, muscle development, change in voice, and the development of sex drive. The development of testosterone is controlled by LH of the pituitary gland. The production of sperm is known as Spermatogenesis and is controlled by testosterone. Sperm is produced in the testes by cells called Spermatogonia cells. The testes are housed in a sack like structure called the scrotum. This is important for maintaining a slightly lower temperature then 37 o C, which is important for producing viable sperm. A testicle is made up of approximately 125 m of seminiferous tubules where spermatogenesis occurs. The process takes about 9 10 weeks to produce a mature sperm. Immature sperm is stored in the epididymis found at the back of the testis. An average male can produce 130 million sperm per day, although this count has dropped and evidence indicates it may be environmental. Immature sperm, known as spermatocytes are supported and nourished by Sertoli cells in the seminiferous tubules. Leydig cells located in the developing spermatocytes produce male sex hormones, particularly testosterone. Sperm will move from the epididymis, into the Vas deferens, by rhythmic muscular contractions. The sperm passes by a number of glands; prostate, Cowper s gland, and the seminal vesicle, which secrete fluids that make up the semen.
Figure 4: The reproductive system of a human male. Male reproductive hormones are maintained by a negative feedback system. If testosterone should decrease in the blood stream the hypothalamus response by increasing the release of GnRH. If testosterone becomes too high the production is stopped by inhibiting LH secretion also Sertoli cells secrete inhibin, which inhibits FSH secretion. Concentrations are returned to normal levels. Figure 5: The hormonal regulation of reproduction in the male, and the negative feedback systems that control the hormone levels.
Controlling Reproduction with Hormones Chapter 10: Summary pg. 506 Chapter 10: Self-Quiz pg. 507 Chapter 10: Review pg. 508-513