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Physiology 2130
Anita Woods

Reproductive System Section 14.1 – Objectives By the end of this section, you should be able to: • Describe the fetal development of the male and female reproductive systems. • Draw a diagram of the male reproductive system, showing all the structures. • List the functions of each of the male reproductive organs. • Define spermatogenesis and describe the sequence of events involved in the entire process. • Explain the control and regulation of the male reproductive system, including spermatogenesis and testosterone production. • List the functions of testosterone. • Draw a diagram of the female reproductive system, showing all the structures. • List the functions of each of the female reproductive organs. • Define oogenesis and describe the sequence of events involved in the entire process. • List the hormones of the ovaries and explain their function. • List the events in the menstrual cycle and describe how each hormone is involved in this cycle. Section 14.2 – Introduction • The sections that we have covered so far have all dealt with physiological systems whose primary function is to maintain homeostasis. • The combined functions of both male and female reproductive systems, on the other hand, are to pass on the genes of the individuals and to maintain the species. Section 14.3 – Fetal Development of the Reproductive System • Each cell in the body contains 23 pairs of chromosomes (shown below) – this includes a pair of sex chromosomes.  The sex chromosomes consist of a large X chromosome and a smaller Y chromosome.  All eggs contain an X chromosome while the sperm can carry either an X or Y chromosome. • The sex of the future baby is determined at the point of fertilization – if a sperm carrying the X chromosomes penetrates the egg carrying the X chromosome, then a female will develop (XX). • If, on the other hand, a sperm carrying the Y chromosomes fertilizes the X chromosome bearing egg, then a male will develop (XY). • However, development of each respective reproductive tract does not begin immediately after fertilization. Let's look at how this happens 1 Section 14.4 – Fetal Development of the Reproductive System • During the first 6 weeks of development, male and female embryos contain common (sometimes called indifferent) gonads.  These structures will eventually form the testes in the male or the ovaries in the female. • There are also two sets of primitive reproductive tracts: 1. The mesonephric or Wolffian Duct and 2. The paramesonephric or Mullerian Duct  With the correct cues, these structures will form either the male or female reproductive tracts. Let's have a closer look at these events. 2 Section 14.5 – Fetal Development of the Reproductive System • In the developing male embryo, at roughly 6 to 7 weeks the presence of the Y chromosome causes the indifferent gonads to develop into the testes. • However, in the developing female embryo, at about 9 weeks of development the XX chromosomes are activated and ovaries develop. • Once the testes or ovaries have developed, the reproductive tracts and external genitalia follow. • As mentioned on the previous page, the reproductive tracts develop from two primitive ducts: 1. The Wolffian duct 2. The Mullerian duct. Section 14.6 – Fetal Development of the Reproductive System • Male: o In the developing male fetus, at 7 weeks testicular cells begin to produce Mullerian inhibiting hormone (MIH) which causes the Mullerian duct to regress. o Then, at 9 weeks, testicular cells begin to produce testosterone.  This small surge in testosterone stimulates the Wolffian duct to develop into the epididymis, vas deferens, seminal vesicles, and the urethra.  The presence of testosterone also causes the development of the male external genitalia. • Female: o The development of the female reproductive tract and external genitalia requires no hormonal control. o In the developing female, since there is no Mullerian inhibiting hormone, the Mullerian duct develops into the fallopian tubes, uterus, cervix, and part of the vagina. o Also, since no testosterone is produced in the female fetus, the Wolffian duct regresses and female external genitalia develop. 3 Section 14.47 – The Reproductive Systems: Introduction (cont’d) • The male reproductive system:  Produce the sex steroid testosterone  Has effects during puberty  Produce sperm (called spermatogenesis), and to  Deliver it to the female vagina.As we will see, testosterone has many effects throughout the body, especially at puberty. • The female reproductive system:  More complex  Responsible for producing the sex steroid estrogen and the hormone progesterone.  Produces eggs, receives the sperm, and provides the optimal conditions for the development of the fetus. Section 14.8 – The Male Reproductive System: Structure 4 • The external genitalia: o Consists of the penis and scrotum.  The penis is made up of the urethra, glans penis, and the erectile tissue corpus spongiosum and corpus cavernosum.  The scrotum contains the testes where sperm and testosterone are produced. • After production, the sperm is stored in the epididymis. o During ejaculation, sperm travels through the ductus deferens (or vas deferens), mixes with fluid from the seminal vesicles, passes through the prostate, receives more fluid from the bulbourethral (Cowper's) gland, enters the urethra, and passes out through the penis. Section 14.9 - The Male Reproductive System: Structure (cont’d) • The Testes: • Consists of almost 1000 coiled seminiferous tubules, each of which is roughly 150 cm (60 inches) long.  After formation in the seminiferous tubule, the sperm move into the epididymis – another coiled tube that is roughly 6 meters (20 feet long).  The epididymis drains into the ductus deferens (or vas deferens).  Seminiferous tubules contain Sertoli cells and developing sperm.  Immediately outside the seminiferous tubule are the Leydig cells (also called interstitial cells). Section 14.10 – The Male Reproductive System: Function • We will examine the function of each structure by beginning with the smallest and working our way out toward the external environment. • Seminiferous tubules:  The site of spermatogenesis.  Spermatogenesis is regulated by sertoli cells (SC) o SCs maintain the developing sperm cells (called spermatogonia). o These cells also produce the hormone inhibin, secrete fluid that pushes the immature sperm to the epididymis, and form the blood-testis barrier (BTB).  The BTB isolates the developing sperm cells from the blood so that immune cells do not attack these genetically different cells.  If the BTB did not develop properly, immune cells would attack and destroy the developing sperm resulting in sterility. 5 • Leydig cells:  Located in the interstitial space between the seminiferous tubules  Produce testosterone. Section 14.11 – The Male Reproductive System: Function (cont’d) • The Epididymis:  The final maturation area and storage site for sperm. • The vas (or ductus) deferens:  Carries the sperm from the epididymis to the ejaculatory duct. • The Ejaculatory Duct:  Drains into the urethra. • Seminal Vesicles:  Contribute a large amount of fluid to the semen during ejaculation. The fluid, rich in fructose and enzymes, helps to maintain and nourish the sperm. • The Prostate Gland:  Also secretes enzymes and fluid that help to neutralize the acid environment of the urethra and vagina. • The Bulbourethral Gland (also called Cowper's Gland):  Also secretes a fluid that helps to neutralize the pH and lubricate the urethra and vagina to create an optimal environment for the sperm. • The Urethra:  Transports the sperm during ejaculation and drains the bladder during urination. 6 Section 14.12 – The Male Reproductive System: Spermatogenesis • As the male begins puberty and starts producing testosterone, sperm production (spermatogenesis) begins. • The spermatogonia, or germ cells:  Contain 46 chromosomes  Located at the outer edge of the seminiferous tubules  Divide by mitosis into two cells: 1. One will continue as a spermatogonia 2. The other will develop into a primary spermatocyte  Each still containing 46 chromosomes.  The primary spermatocyte divides by meiosis into two secondary spermatocyte (during first meiotic division) and then into four spermatid (during second meiotic division).  Throughout this process, the dividing cells, which are surrounded and nourished by the Sertoli cells, work their way to the center of the tubule.  The spermatids develop into sperm cells, where they are released by the Sertoli cells into the lumen of the seminiferous tubules.  This whole process takes 64 days to complete and ends with 4 sperm cells that contains 23 chromosomes each – half of the original number. • Video: Spermatogonia are the cells from which sperm cells arise. The spermatogonia divide by mitosis. One daughter cell remains a spermatogonia and the other becomes a primary spermacyte. The primary spermatocyte divides by meiosis to form secondary spermatocytes. Secondary spermatocytes divide again to form spermatids. The spermatids differentiate into sperm cells. 7 Section 14.13 – The Male Reproductive System: Spermatogenesis (cont’d) • Although not fully mature and able to swim, the sperm cell contains a head with an acrosome and nucleus, a midpiece with mitochondria, and a long tail or flagellum that will propel the sperm once it fully matures. • Complete maturation of the sperm takes another 12 days and occurs as the sperm are moved to the epididymis by the fluid that is secreted by the Sertoli cells. Section 14.14 – The Male Reproductive System: Control of Testicular Function • The functions of the testes are controlled by follicle stimulating hormone (FSH) and luteinizing hormone (LH, aka interstitial cell stimulating hormone or ICSH) released by the anterior pituitary gland in response to gonadotropin releasing hormone (GnRH) from the hypothalamus. • FSH acts on the Sertoli cells to promote spermatogenesis while also producing the hormone inhibin.  Inhibin feeds back to the anterior pituitary to decrease the release of LH and FSH.  LH stimulates the Leydig cells to produce the male sex hormone testosterone. o Testosterone, also essential for spermatogenesis, will feed back to both the hypothalamus and the anterior pituitary to decrease the production and secretion of LH and FSH. Section 14.15 – The Male Reproductive System: Testosterone • Testosterone:  Asteroid hormone produced by the Leydig cells in the testes.  Testosterone production involves a series of complex reactions (shown below) that begins with cholesterol and the formation of several intermediates ending in the production of testosterone.  Note the structural similarity between cholesterol and the testosterone. 8  Also of interest is the presence of progesterone, one of the female hormones, as one of the intermediates leading to testosterone production. . Section 14.16 – The Male Reproductive System: Testosterone Levels andAge • The levels of testosterone vary throughout life. • The diagram below shows that there is a rise in testosterone levels during fetal development.  This rise in testosterone is necessary for the development of the male reproductive tract and external genitalia in the fetus.  After birth there is another brief increase in testosterone – however, the function of this is unknown. • The levels of testosterone then remain low until puberty. 9 Section 14.17 – The Male Reproductive System: Testosterone • Puberty marks the transition from childhood (non-reproductive state) to adulthood (reproductive state).  At puberty, which in males occurs between the ages 9 to 14, the levels of gonadotropin releasing hormone (GnRH) begin to increase.  The cause of this increase in GnRH remains unclear, however, it leads to an increase in LH and FSH, which then cause testosterone levels to rise, as well. o This increase causes the final maturation of the male reproductive system to the point where mature sperm is produced.  Sexual maturity and peak testosterone levels are reached around the age of 16 to 18. • REFER to the graph picture above! Section 14.18 – The Male Reproductive System: Testosterone • Testosterone has a wide variety of functions in the body, including those listed below. 10 Section 14.19 – The Male Reproductive System: Testosterone • Beginning at roughly 40 years of age there is a slow decrease in the levels of testosterone. • At age 50 sperm production begins to decrease.  This period of decreased testosterone production is called andropause and may occur because the Leydig cells, which produce testosterone, no longer fully respond to LH.  Andropause is accompanied by decreased sexual desire and erectile ability, fatigue and depression, decreased lean body mass, increased obesity, and decreased bone density. • REFER to graph again! Section 14.20 – The Male Reproductive System: Review • The Testes:  Develop from the common or indifferent gonads in the presence of the XY chromosomes.  The testes then begin to produce Mullerian Inhibiting hormone (MIH) which cause the regression of the Mullerian duct.  Testosterone is then produced which causes the Wolffian duct to develop into the male reproductive tract.  The testes contain the Leydig cells, Sertoli cells, and the seminiferous tubules.  The seminiferous tubule converges and forms into the epididymis which drains into the vas deferens, which joins the urethra as it passes through the prostate. 11 • You should also know the function of every structure shown below. Section 14.21 - The Male Reproductive System—Review (cont’d) • Spermatogenesis:  The production of sperm  Relies on the two pituitary hormones  Luteinizing hormone (LH) and  Follicle stimulating hormone (FSH), as well as o FSH acts on the Sertoli cells to promote spermatogenesis, while LH stimulates the production of testosterone by the Leydig cells.  The male sex hormone testosterone.  Spermatogenesis takes place in the seminiferous tubules where the spermatogonia form into primary spermatocytes then into secondary spermatocytes and finally into spermatids.  The final maturation process will be completed by the time the sperm leave the epididymis. You should also be familiar with the functions of testosterone at this point (pic below). 12 Section 14.22 - The Female Reproductive System: Structure • The external genitalia:  Collectively called the vulva.  They include:  The clitoris: o Asmall mound of erectile tissue that is derived from the same embryonic tissue as the penis.  The labium majus (aka the labia majora): o Which surrounds the labia minus (aka the labia minora – also part of the vulva) o Both of these are folds of skin derived from the same embryonic tissue as the scrotum and the shaft of the penis, respectively.  The labium minora encloses the vaginal orifice – the opening to the vagina. Section 14.22 – The Female Reproductive System: Structure 13 • The female internal genitalia:  Includes:  The vagina: o The canal that receives the penis during intercourse and where sperm is deposited.  The sperm will travel through the cervix into the
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