MCB 135K – Discussion



MCB 135K – Discussion

Summary of Reproductive Systems

The Reproductive System

1. The reproductive systems of both sexes share the same basic organization:

1. The gonads produce gametes (eggs or sperm) which unite during fertilization. They also produce steroid hormones essential for reproduction as well the growth and development of the entire body.

2. The genitalia are the external reproductive structures (penis in males, clitoris and vulva in females). They allow for passage of the sperm from the male into the female.

3. Ducts and accessory glands are the structures connecting the gonads to the genitalia. They are the site of final preparation of the gametes for fertilization, and in females also create the environment where fertilization occurs and the resulting embryo develops until birth.

2. The creation of gametes is called gametogenesis. The process follows the same general sequence in both males and females:

1. Gamete production begins with germ cells, which begin to develop and multiply in the embryo. They are diploid cells, containing 23 pairs of chromosomes for a total of 46 chromosomes. Germ cell proliferation occurs through mitosis, giving rise to more germ cells with the same number of chromosomes as the parent cells.

2. The path from germ cell to gamete occurs through meiosis, in which a germ cell replicates its DNA once but divides twice.

3. DNA replication occurs prior to the first replication, as occurs for mitosis. After replication is complete, each cell possesses four copies of each of the 23 chromosomes.

4. The first meiotic division gives rise to two daughter cells, each with two copies of each of the 23 chromosomes. These cells are called primary gametes. Each primary gamete has a total of 46 chromosomes, the same as most other cells in the body.

5. In the second meiotic division, each of these daughter cells divides again without replicating more DNA. As a result, each secondary gamete receives only one copy of each of the 23 chromosomes, or half the normal chromosomal contingent of other cells in the body.

6. The cells that arise from meiosis must then undergo extensive changes before they become actual gametes.

3. Besides gametogenesis, the other major function of the gonads is to produce steroid sex hormones, include androgens (such as testosterone), estrogens, and progesterone. Hormonal control of the reproductive system follows the same general pattern in men and women:

1. A group of neurons in the hypothalamus acts as the primary pacemaker of hormonal control of the reproductive system. These neurons produce bursts of gonadotropin relasing hormone (GnRH) every one to three hours. GnRH is a small peptide that travels via local blood circulation to affect cells in the nearby anterior pituitary. The frequency and magnitude of GnRH release is modulated by the brain in response to a wide variety of internal and external stimuli.

2. GnRH regulates the secretion of two peptide hormones from the anterior pituitary: follicle stimulating hormone (FSH) and luteinizing hormone (LH). Both hormones have trophic (stimulatory or growth-inducing) effects on the gonads, but on different types of cells. As such, they are collectively called gonadotropins. FSH acts on the gamete-producing cells to regulate gametogenesis. LH acts on the endocrine or hormone-producing cells, stimulating release of steroid sex hormones.

3. There are two levels of feedback control over production of steroid sex hormones. Gonadal hormones secreted into the body circulation eventually reach the brain and pituitary and suppress GnRH, FSH, and LH production. The gonadotropins also directly inhibit GnRH secretion via a shorter feedback loop.

Male Reproductive System

1. Anatomy of the male reproductive system:

1. The external genitalia of the male consist of the penis and scrotum.

2. The urethra is the common passageway for semen and urine.

3. The testes are the male gonads. Sperm travel from the testes to the urethra via the ductus deferens.

4. Accessory glands contribute secretions to the sperm as they travel along the pathway from the testes to the urethra. These glands include the prostate gland, the seminal vesicles, and the bulbourethral glands. Their secretions constitute most of the volume of the semen that the male eventually ejaculates.

2. Sperm production takes place within the testes:

1. The internal structure of the testes consists of coiled masses of seminiferous tubules. Germ cells (spermatogonia) embedded in the walls of the tubules give rise to developing sperm by the process of meiosis, becoming primary spermatocytes after the first meiotic division and secondary spermatocytes after the second division. Each secondary spermatocyte then undergoes a gradual transformation into mobile spermatids, losing most of its cytosol and developing an elongated flagellum.

2. Spermatid development is regulated and supported by Sertoli cells that are also embedded in the walls of the seminiferous tubules. FSH from the anterior pituitary stimulates the Sertoli cells to produce a variety of factors required for normal spermatogenesis. These include:

1. Androgen binding protein, which traps testosterone in the fluid surrounding the developing spermatids.

2. Inhibin, which selectively inhibits FSH release by the anterior pituitary.

3. The other major function of the testes is production of testosterone. This is handled by Leydig cells found in the interstitialspaces (outside the seminiferous tubules). Leydig cells are stimulated to produce testosterone by LH from the anterior pituitary. Testerone, in turn, travels back to the anterior pituitary to suppress release of LH.

Female Reproductive System

1. Anatomy of the female reproductive system:

1. The entire external genitalia of the female is called the vulva. This includes the labia majora, labia minora and clitoris.

2. The vagina connects the vulva to the uterus, serving both as a receptacle for sperm during sexual intercourse and as the passageway for the infant during birth. The urethra ends in a separate orifice. The cervix is the neck of the uterus that protrudes into the upper end of the vagina.

3. The walls of the uterus contain a thick layer of smooth muscle called the myometrium, which serve to expel the fetus during labor. The inner lining of the uterus, the endometrium, proliferates in preparation for implantation of the zygote and is then shed during each menstrual cycle.

4. Two Fallopian tubes, one on each side, connect the uterus to the ovaries. Fertilization typically occurs here.

5. Closely associated with the end of each Fallopian tube is a walnut-sized ovary, the female gonad.

2. Egg production takes place within the ovaries:

1. In the male, all phases of spermatid production from germ cell to gamete takes place continually over the entire lifespan. In the female, each phase takes place during specific periods. About 7 million germ cells, or oogonia, develop in the embryonic ovary as early as 5 months of gestation. Most of these cells then die, with about a half a million of them proceeding through the first stage of meiosis to become primary oocytes before birth.

2. After puberty, the primary oocytes then begin to develop a few at a time, with each contained in a follicle of surrounding supporting cells.

3. One or more ovarian follicles rupture at the midpoint of each menstrual cycle, releasing an egg and surrounding cells into the fallopian tube.

3. Hormone production in the ovaries occurs primarily within the developing follicles. The two major steroid hormones produced by the human ovaries are progresterone and estradiol, a form of estrogen. Production of these two hormones varies widely over the course of the menstrual cycle:

1. The first day of menstruation is usually labeled as the beginning of the cycle. At this point, rising levels of FSH stimulate the development of several follicles in the ovaries. These early follicles in turn begin to produce estrogen. Usually, only one of these follicles will develop past the early stage.

2. Estrogen levels continue to rise as the follicle develops. Progesterone eventually begin to rise as well. Up to a certain point, the estrogen being produced exerts negative feedback on both GnRH and gonadotropin secretion.

3. Around the 14th day of the cycle, the anterior pituitary abruptly changes its response to the persistently high estrogen levels. Rather than suppressing gonadotropin release, the estrogen now has a positive feedback effect. The reasons for this switch are still not clear.

4. The switch to positive feedback provokes a steep surge in LH, and to a lesser degree FSH. High LH levels in turn initiates the process of ovulation, whereby the follicle ruptures and egg is released.

5. After ovulation, the follicle collapses and the cells within it transform. This new structure is called the corpus luteum. The luteal cells produce more progesterone than estrogen, so blood levels of progesterone rise higher than estrogen levels after ovulation. The corpus luteum only remains viable for about 14 days, after which both estrogen and progesterone levels taper off. The declining levels of estrogen remove the negative feedback on GnRH and FSH production, allowing the cycle to begin over again.

Questions:

What hormones regulate ovulation and the menstrual cycle?

Where do they originate?

What are the effects of LH and FSH respectively?

How do FSH and LH interact with estrogen and progesterone to establish the menstrual cycle?

Menopause occurs primarily for what reason?

How does menopause affect the levels of estrogen, progesterone, LH, and FSH?

What are the major actions of estrogen?

What other gland can produce (a limited amount of) estrogen?

What are the most common symptoms of menopause?

Do these changes occur in all women?

Estrogen replacement therapy has been shown to prevent what symptoms associated with menopause?

Estrogen replacement may cause a slight increase in the risk of certain types of cancer, can the benefits of replacement therapy outweigh the risks?

What cell type secretes testosterone?

Where are these cells located?

What hormones stimulate testosterone secretion?

LH and FSH levels fluctuate in women, regulating the timing of the menstrual cycle. Do LH and FSH levels vary in men?

How does sperm production change with age?

Do testosterone levels change with age? How?

How does LH and FSH levels change in aging men?

What does it mean that testosterone in an "anabolic" steroid?

What is the relationship between testosterone and prostate cancer?

What is organotherapy?

What are the key observations from organotherapy that led to the development of endocrinology?

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