Human_Physiology_Medical_School_Crash_Course_Unabridged_015_A_T

The chapter discusses the physiology of the reproductive system, focusing on the similarities and differences between male and female reproductive systems. The female reproductive system consists of the ovaries, which produce estrogen and progesterone, release egg cells, and support pregnancy. The menstrual cycle, which is divided into the follicular, ovulation, and luteal phases, plays a crucial role in the female reproductive system. The male reproductive system consists of the testes, which produce testosterone, stimulate sperm production, and develop secondary sexual characteristics. The scrotum helps regulate the temperature of the testes for optimal sperm production. Chapter 12, Reproductive System Physiology. This chapter involves the physiology of the reproductive system. While the reproductive system is markedly different among males and females, they do share some characteristics in common. Both have organs that secrete hormones and make gametes, which are the reproductive cells. Both have means for the gametes to get to where fertilization takes place. They differ, however, in key ways. While the male reproductive system is relatively static over time, the female reproductive system is cyclical. Men retain their fertility until they are elderly, while women are fertile over a relatively short period of time in their life cycle. This chapter will discuss each reproductive system separately. Female Reproductive System The female reproductive system consists of the ovaries, which make estrogen and progesterone, and produce female egg cells under the control of the pituitary system. Egg cells are released by the ovaries and travel to the fallopian tubes, where they are sometimes fertilized by sperm cells. The fertilized egg resides in the female uterus, where it implants and grows into an infant. If the egg cell isn't fertilized, it degenerates, and the lining of the uterus is shed each month in the menstrual cycle. This cycle occurs every 28 days or so, from the time a female is about 12 years of age, until about the age of 50, when menopause occurs, and the woman loses her ability to reproduce. The Menstrual Cycle The basis of the female reproductive system is the menstrual cycle. It starts with egg maturation and development in the female ovaries, and is called oogenesis. Follicles develop in both ovaries, but only one or two follicles develop to the point of being mature enough to release at the time of ovulation. Primordial follicles in the ovaries begin the process of oogenesis using transformation of the primordial follicles into primary oocytes. A female is born with all of the egg cells she will have for the duration of her life. These egg cells develop in a process known as oocytogenesis, which is complete around the time of a female's birth. The egg cells remain dormant until the onset of menarche, when the egg cells develop the ability to mature through meiosis. The development and maturation of follicles represents the start of the menstrual cycle. About 12 to 20 primary follicles start to develop because of elevated levels of follicle-stimulating hormone, or FSH. They all form secondary follicles. The primary follicles come from primordial follicles, which are the type of follicle made in the female fetus in utero. They are arrested in the prophase state of cellular division until puberty. At around the ninth day of the menstrual cycle, just one and rarely two secondary follicles are present for ovulation. The remaining follicles are reabsorbed back into the ovary. The dominant follicle produces large amounts of estrogen in the late follicular phase of the menstrual cycle. On the 14th day of the cycle, there is a surge of luteinizing hormone, or LH, from the pituitary gland because of positive feedback by the follicle's estrogen. This causes the remaining secondary follicle to turn into a tertiary follicle, which leaves the ovary about 24 to 36 hours later. The oocyte completes its first meiotic division, leaving a polar body and a secondary oocyte. The empty follicle forms the corpus luteum, which releases progesterone to support a potential pregnancy. Right after the meiosis I phase occurs, the haploid secondary egg begins meiosis II. The process continues until metaphase II and is stopped until the time of fertilization. When meiosis II occurs, a polar body and an ootid are created. Both polar bodies disintegrate when meiosis II is completed, leaving behind an ootid that develops into a mature egg cell. The polar bodies are only there to get rid of the extra haploid sets of chromosomes that are no longer necessary. The menstrual cycle happens hundreds of times in a woman's life cycle. There are three phases of the menstrual cycle, including the follicular phase, the ovulation phase, and the luteal phase. The first day of the menstrual cycle is defined as the first day of bleeding. The follicular phase is also called the proliferative phase. It is the phase that begins with follicles that start to mature into oocytes. This process continues until the time of ovulation, when only one mature egg is released. Estradiol is the main hormone operating at this stage. The pituitary gland is secreting follicle-stimulating hormone, or FSH, during this phase to support follicular development. FSH levels are highest in the first week of the follicular phase. FSH causes multiple follicles to start developing. These yet-to-be-mature follicles are known as entral follicles or graphion follicles. They compete with one another until just one or two are dominant. FSH causes the proliferation of granulosa cells in the developing graphion follicles and controls the expression of luteinizing hormone, or LH receptors, on these granulosa cells. A couple of days before the LH levels rise, about one or two follicles are defined as being dominant. The dominant follicle produces high levels of estrogen that transiently slow the LH and FSH production so that the lesser follicles die off and are reabsorbed. The high estrogen levels cause the lining of the endometrium to proliferate. Crypts inside the cervix are activated to produce mucus that is more advantageous for the sperm to travel through, and there is a decrease in acidity in the vagina, which is better for sperm cells. The basal body temperature of the woman decreases slightly because of high estrogen levels just before the LH surge. Ovulation is a short phase of the menstrual cycle. It happens when the mature ovarian follicle ruptures, discharging a mature oocyte. There is a surge of luteinizing hormone that triggers this release. After this, the LH levels drop markedly and the part of the follicle left behind begins to secrete progesterone. The luteal phase, or secretory phase, of the cycle is the last half of the menstrual cycle. It starts when the corpus luteum is made and ends when the woman is pregnant or the follicle fails to make any more progesterone, triggering menses. The major hormone in this phase of the cycle is progesterone. After ovulation, LH and FSH remain operative and cause the dominant follicle to turn into the corpus luteum, which secretes progesterone and some estrogen. Progesterone is crucial to the transformation of the endometrium into a mature tissue lining that is receptive to the implantation of the blastocyst should the egg become fertilized. It also causes a prolonged elevation of the woman's basal body temperature. The estrogen and progesterone made by the corpus luteum block the production of FSH and LH, causing atrophy of the corpus luteum. If there is no pregnancy to continue the support of the corpus luteum, it will eventually die and cease to make estrogen and progesterone. This increases FSH levels so that more follicles are recruited for the next cycle. At the same time, there isn't enough estrogen and progesterone around, so the lining of the uterus breaks down and bleeds, beginning the next cycle. If the embryo implants into the uterine wall, it makes human chorionic gonadotropin, or HCG, which is structurally like luteinizing hormone. It keeps the corpus luteum alive so that it can continue to make progesterone to support the pregnancy for 8 to 12 weeks. After this, the placenta takes over progesterone production. Male Reproductive System The basic unit of the male reproductive system is the testes, or testis. These are located in the scrotum, which is the sack of skin located between the upper thighs. The testes originate in the male fetus near the kidneys, but descend into the scrotum just prior to birth. The main hormone produced by the male testis is testosterone. Testosterone serves two major functions. It stimulates male sperm production and gives rise to the male's secondary sexual characteristics The testes are set side by side in a fleshy sack known as the scrotum. The function of having a scrotal sack is to keep the testes cooler than the normal body temperature. There are involuntary muscles in the scrotum that can draw the testes closer to the body when the testes are exposed to the cold. The muscles relax during warmer temperatures to keep the testes cooler by putting them away from the body. The testes need to be held at lower than normal body temperature to create the optimum environment for spermatogenesis. The two muscles regulating the temperature of the scrotal sack are the dartus muscle and the cremaster muscle. The dartus muscle is a layer of smooth muscle just beneath the skin of the scrotal sack. It is responsible for the wrinkling of the scrotal skin. It contracts in cold weather to help maintain the testicular temperature steady. The cremaster muscle is skeletal muscle linked to the testes and the spermatic cord. It is a continuation of the internal oblique muscle of the abdominal wall. Each testis contains over a hundred yards of extremely packed seminiferous tubules. About 90% of the weight of the testis is from the seminiferous tubules. These are the functional unit of the testis and is where the spermatogenesis takes place. The sperm are produced there but are moved rapidly from their site of origin to the retetestis for them to mature. There are several types of cells in the testes. There are interstitial cells, also called the cells of Lydig. They are the secretory cells of the testes and make testosterone and other male hormones. There are the Sertoli cells, which are sometimes referred to as nurse cells. They are activated by FSH because they have FSH receptors on their cell surface. Its function is to nurture sperm cells as they mature. The Sertoli cells also get rid of any unneeded sperm cells. The sperm cells leave the testes via the efferent ductules to enter the epididymis. The epididymis is formed by the juncture of the seminiferous tubules. It is a two-inch long structure that is coiled up behind the testes. It is inside of the epididymis that the sperm become mature and can swim. Sperm is stored there until ejaculation. There is smooth muscle in the epididymis that propel the sperm into the ductus deferens. There are three parts to the epididymis. They include the head, or caput, the body, which is highly convoluted, and the tail, which takes the sperm into the vas deferens. The function of the vas deferens is to send sperm to the outside of the body. It is also called the sperm duct. It starts in the scrotum and goes through the inguinal canal. It has smooth muscle to allow peristalsis during the time of ejaculation. The seminal vesicles are behind the bladder. Their function is to secrete fructose to provide an energy source for sperm and to create an alkaline environment which improves the mobility of sperm. They secrete 65 to 75 percent of the total volume of the ejaculate. The duct of each seminal vesicle connects with the ductus deferens to make the ejaculatory duct. There are two ejaculatory ducts with one on each side of the body. They empty into the urethra. The prostate gland is part of the male reproductive system. It surrounds the proximal urethra and has smooth muscle that contracts during ejaculation to help propel the sperm out of the urethra. It makes a clear alkaline fluid that is part of the semen. There are bulbourethral glands or Cowper's glands below the prostate that help in alkalinizing the seminal fluid neutralizing the acidity of the vagina. Cowper's glands secrete pre-ejaculate that lubricates the urethra for easy passage of sperm through the urethra. Semen is a fluid containing the sperm cells. About 2 to 5 percent of the volume of semen comes from the testes and contains 200 to 500 million sperm cells. The seminal vesicles make up 65 to 75 percent of seminal fluid. It contains amino acids, enzymes, fructose, prostaglandins, and phosphorylcholine. The prostate gland makes up to 25 to 30 percent of seminal fluid that contains acid phosphatase, prostate-specific antigen, fibrinolysin, proteolytic enzymes, and zinc. The bulbourethral glands contribute less than 1 percent of seminal fluid and secrete galactose and mucus. Hormone Regulation in Males The male hormones important in the reproductive tract include gonadotropin-releasing hormone or GnRH. This is secreted by the hypothalamus and contributes to the stimulation of pituitary secretion of follicle-stimulating hormone or FSH and luteinizing hormone or LH. LH is secreted after receiving the hypothalamus. It stimulates the lydic cells to make testosterone. Follicle-stimulating hormone is secreted by the anterior pituitary gland. Testosterone helps this hormone get through the bloodstream so that it can help make sirtoli cells, which reside in the seminiferous tubules of the testes. It aids in the maturation of sperm. Testosterone is the main male hormone and is crucial for sperm cell production. Sperm Cell Maturation The sperm cell divides a number of times as it develops and matures. It takes about 9 to 10 weeks to form and mature sperm. The first division of sperm cells happens using mitosis to divide one sperm cell into two identical sperm cells. The second division involves two cell divisions using meiosis. The result is secondary spermatocytes. The third division makes haploid cells that eventually mature to become gametes or male sperm cells. Each sperm cell has a set of chromosomes that carries either an X chromosome or a Y chromosome, which determines the gender of the fetus after fertilization. Key Takeaways The basic organ of the female reproductive system is the ovary. The ovary produces estrogen, progesterone, and female sperm. The menstrual cycle is a 28-day cyclical cycle that readies the body for fertilization. The male organ of reproduction is the testis. The testes contain several types of cells, including sperm cells and cells that secrete male hormone. Sperm cells go through several stages of maturation before becoming haploid gametes. Number 1. What part of the menstrual cycle happens first? A. Menstrual flow. B. Follicular phase. C. Ovulation. D. Luteal phase. Answer A. The first part of the menstrual cycle is the menstrual flow, which is referred to as Day 1 of the menstrual cycle. Number 2. Which hormone is secreted by the corpus luteum in the luteal phase of the menstrual cycle? A. Luteinizing hormone. B. Human gonadotropin-releasing hormone. C. Progesterone. D. Estrogen. Answer C. The luteal phase is marked by secretion of progesterone by the corpus luteum. Number 3. What happens to the uterine lining during the follicular phase of the menstrual cycle? A. The cells of the uterine lining become spongier. B. The uterine lining thickens. C. The uterine lining matures and grows a new layer for implantation. D. The uterine lining begins to shed. Answer B. During the follicular phase of the menstrual cycle, the uterine lining thickens. It doesn't mature until the luteal phase. Number 4. Which hormone triggers ovulation? A. Human chorionic gonadotropin. B. Luteinizing hormone. C. Follicle-stimulating hormone. D. Progesterone. Answer B. A surge of luteinizing hormone triggers the egg to be reached at the time of ovulation. Number 5. How does human chorionic gonadotropin, HCG, work to keep the corpus luteum functioning? A. It binds to HCG receptors on the corpus luteum, preventing cell death. B. It inhibits luteinizing hormone so the corpus luteum isn't reabsorbed. C. It inhibits progesterone so that the corpus luteum is triggered to make more progesterone. D. It is structurally similar to luteinizing hormone, so it prevents reabsorption of the corpus luteum. Answer D. Human gonadotropin-releasing hormone has a structural similarity to luteinizing hormone, so it binds to luteinizing hormone receptors on the corpus luteum. Number 6. What is the function of the dartus muscle in the male reproductive system? A. It propels sperm through the urethra. B. It propels sperm through the vas deferens. C. It regulates the temperature of the testes. D. It helps hold up the testes. Answer C. The dartus muscle contracts and relaxes involuntarily to control the temperature of the testes. Number 7. Which male reproductive organ contributes most to the volume of seminal fluid? A. Prostate gland. B. Testes. C. Bulbourethral glands. D. Seminal vesicles. Answer D. The seminal vesicles contribute to 65 to 75 percent of seminal fluid. Number 8. Which part of the male reproductive system produces fructose to nourish the sperm cells? A. Prostate gland. B. Seminal vesicles. C. Bulbourethral glands. D. Vas deferens. Answer B. The seminal vesicle produces fructose, which nourish the sperm cells during transit. Number 9. Which organ in the male reproductive cell is where the sperm cells mature? A. Testes. B. Epididymis. C. Prostate gland. D. Seminal vesicles. Answer B. The epididymis is the part of the male reproductive tract and is where sperm cells mature. Number 10. Which cells in the male reproductive cells produce testosterone? A. Sertoli cells. B. Anterior pituitary gland cells. C. Cells of Lydig. D. Bulbourethral cells. Answer C. The cells of Lydig or interstitial cells make testosterone.