Chapter 22 Reproductive Systems List the general functions of the male reproductive system
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Chapter 22Reproductive Systems
The primary male reproductive organ is the testis. The internal male accessory organs include the epididymis, vas deferens, seminal vesicle, prostate gland, and the bulbourethral glands. The external male accessory organs consist of the scrotum and the penis.
One to two months before birth, secreted testosterone signals the testes to descend. The gubernaculum aids in the descent by guiding the testes through the inguinal canal, and into the scrotum. The developing vas deferens suspends the testes in the scrotum.
Cryptorchidism is a condition in which the testes do not descend. Untreated, this condition will cause infertility.
The testis is an ovoid structure that is about five centimeters long and three centimeters in diameter and is enclosed by a tough white fibrous capsule. The connective tissue thickens and extends into the organ along its posterior border, forming thin septa that subdivides it into about 250 lobules. Each lobule contains a seminiferous tubule that courses posteriorly and unites to form a complex network of channels. These channels give rise to ducts that join the epididymis. The seminiferous tubules are lined with spermatogenic cells that produce sperm. Interstitial cells are located within the spaces between the seminiferous tubules and produce and secrete male sex hormones.
Sustentacular cells are responsible for nourishment, support, and regulation of the spermatogenic cells.
Meiosis is the process that halves the chromosome number for inclusion into gametes. Meiosis is a two-part process. The first meiotic division (meiosis I) separates the homologous chromosome pairs. These single chromosomes are replicated so that they consist of two complete DNA strands (chromatids). Each chromatid is attached together at a special region called a centromere. The second division of meiosis (meiosis II) separates those chromatids so that they become haploid (having one copy of each chromosome). At this stage the chromosomes are not replicated and each of the chromatids is considered a chromosome. The specific steps of meiosis I and meiosis II are:
The second meiotic division begins almost immediately. Prophase II—The chromosomes condense and reappear and move into positions midway between the poles of the developing spindle. Metaphase II—The chromosomes attach to spindle fibers and line up midway between them. Anaphase II—The centromeres separate and the chromatids move to opposite poles of the spindles. The chromatids are now called chromosomes. Telophase II—Each of the original two cells formed in meiosis I divide to form two cells. Therefore, each cell undergoing meiosis has the potential to produce four gametes.
In meiosis, any one of a person’s more than eight million combinations of 23 chromosomes can combine with any one of the more that eight million combinations of his or her mate, leading to more than seventy trillion combinations. Crossing over contributes even more variability.
During the embryonic stage, spermatogonia become active in response to hormones. Some undergo mitosis while others enlarge into primary spermatocytes. At puberty, testosterone causes the primary spermatocytes to undergo meiosis and yield secondary spermatocytes. Each secondary spermatocyte divides into two spermatids. These, in turn, mature into sperm cells.
A sperm cell is a tadpole-shaped structure with a flattened head, cylindrical body, and an elongated tail. The head is composed of a nucleus containing highly compacted chromatin. The acrosome contains enzymes that aid in penetrating the egg cell. The midpiece contains a central core and many mitochondria arranged in a spiral. The tail consists of several microtubules enclosed in a cell membrane extension.
It is a tightly coiled, threadlike tube that is about six meters long. It connects to ducts within a testis. It emerges from the top of the testis along its posterior surface and then courses upward to become the vas deferens. The epididymis receives the immobile, immature sperm. As they travel through the epididymis by peristaltic contractions, they mature.
The vas deferens passes upward along the medial side of a testis, through the inguinal canal, enters the abdominal cavity, and ends behind the urinary bladder. Just outside the prostate gland, it unites with the duct of a seminal vesicle. The fusion of these two ducts forms an ejaculatory duct.
See figure 22.1, page 837 for location. The seminal vesicles secrete a slightly alkaline fluid, rich in fructose and a variety of other nutrients.
See figure 22.1, page 831 for location. The prostate gland secretes a thin, milky fluid with an alkaline pH.
See figure 22.1, page 831 for location. The bulbourethral glands secrete a mucuslike fluid that lubricates the end of the penis.
Semen is the fluid conveyed by the male urethra to the outside as a result of sexual stimulation. It consists of sperm cells from the testes and secretions from the seminal vesicles, prostate gland, and bulbourethral glands.
Capacitation is the ability of sperm cells to fertilize an egg cell by changes that weaken the acrosomal membrane.
The scrotum is a pouch of skin and subcutaneous tissue hanging from the lower abdominal region. The subcutaneous tissue contains a layer of smooth muscle that makes up the dartos muscle. A medial septum divides the scrotum into two chambers, each of which enclose a testis. Each chamber has a serous membrane, which aids the testis and epididymis to move smoothly within the scrotum.
The penis is a cylindrical organ that conveys urine and semen through the urethra. It is also specialized to enlarge and stiffen so that it can be inserted into the female vagina during sexual intercourse. The body is composed of three columns of erectile tissue, which include a pair of dorsally located corpora cavernosa and a single corpus spongiosum below. Skin, a thick layer of subcutaneous tissue, and a layer of elastic tissue enclose the penis. A tough capsule of white fibrous connective tissue surrounds each column. The corpus spongiosum is enlarged at its distal end to form a sensitive, cone-shaped glans penis. The glans covers the ends of the corpora cavernosa and bears the external urethral meatus. A prepuce or foreskin may or may not be present. This depends entirely upon whether or not the male has been circumcised.
During sexual stimulation the smooth muscles become relaxed. At the same time, parasympathetic nerve impulses pass from the sacral portion of the spinal cord to the arteries leading into the penis, causing them to dilate. These same impulses also stimulate the veins leading away from the penis to constrict. Because of this, arterial blood under relatively high pressure enters the vascular space, and the flow of venous blood away is reduced. Thus, blood accumulates in the erectile tissues, and the penis swells, elongates, and becomes erect.
Emission is the movement of sperm cells from the testes and secretions from the prostate gland and seminal vesicles into the urethra, where they are mixed to form semen. Ejaculation is the forcing of the semen through the urethra to the outside.
As the urethra fills with semen, sensory impulses are stimulated and pass into the sacral portion of the spinal cord. In response, motor impulses are transmitted from the cord to certain skeletal muscles at the base of the erectile columns of the penis causing them to contract rhythmically. This increases the pressure within the erectile tissue and aids in ejaculation.
In response to the release of GnRH (gonadotropin releasing hormone), the anterior pituitary releases luteinizing hormone (LH), which is called interstitial cell-stimulating hormone (ICSH) in the male, and follicle-stimulating hormone (FSH). The ICSH promotes the development of the interstitial cells that secrete male hormones. FSH causes the supporting cells of the germinal epithelium to respond to the effects of the male sex hormone testosterone. Then, in the presence of FSH and testosterone, these supporting cells stimulate the spermatogenic cells to undergo spermatogenesis, producing sperm cells.
The collection of male sex hormones are called androgens. Testosterone is a specific androgen.
The phase and development in which androgen production increases rapidly and the male becomes reproductively functional is called puberty.
Testosterone stimulates enlargement of the testes (the primary male sex characteristic) and accessory organs of the reproductive system. It is also responsible for male secondary sex characteristics including:
Testosterone is regulated by a negative feedback system that is responsive to blood concentration levels. As the concentration levels increase, the hypothalamus inhibits GnRH release. If the blood concentration levels drop, the hypothalamus releases more GnRH.
The primary reproductive organs are the ovaries. The other parts of the reproductive system are called the accessory organs.
The ovaries are held in place by several ligaments. The broad ligament is attached to the ovary, uterine tube, and uterus. The suspensory ligament is attached to the upper end, and the ovarian ligament arises from the broad ligament and attaches to the lower end of the ovary.
During fetal development, the ovaries descend from behind the parietal peritoneum to just below the pelvic brim.
The ovary consists of a medulla composed of loose connective tissue and numerous blood vessels, lymphatic vessels, and nerve fibers. The cortex is composed of more compact tissue and has a somewhat granular appearance due to the presence of ovarian follicles. A layer of cuboidal epithelial cells covers the free surface of the ovary. Just beneath this epithelium is a layer of dense connective tissue.
A primordial follicle is a small group of cells in the outer ovarian cortex with a single, large cell (primary oocyte) surrounded by a layer of flattened epithelial cells (follicular cells).
At puberty, some of the primary oocytes undergo meiosis. This division yields one secondary oocyte and one polar body (first polar body).
In response to increasing levels of FSH, the primordial follicles enlarge, and the surrounding cells undergo mitosis, forming a stratified epithelium of granulosa cells. Glycoprotein forms the zona pellucida, separating the oocyte from the granulosa cells. The follicle is now called a primary follicle. As the ovarian cells outside the follicle divide, they organize into two layers called the inner vascular layer and the outer fibrous layer. When six to twelve layers have formed, the irregular spaces join to become a single cavity (antrum) and the oocyte is pressed against one side. At the stage, the follicle is known as a secondary follicle.
At puberty, the anterior pituitary gland secretes a greatly increased amount of FSH and the ovaries enlarge in response. During this time, primary follicles are being formed from some of the primordial follicles. As it matures, the oocyte of a primary follicle grows larger, and the follicular cells surrounding it divide actively by mitosis. The cells become organized into layers, and soon a cavity appears in the cellular mass that becomes filled with a clear follicular fluid. This fluid bathes the oocyte. This fluid-filled cavity continues to enlarge, and the oocyte is pressed to one side. Over time, the follicle reaches a diameter of ten millimeters or more and bulges outward on the surface of the ovary like a blister. The oocyte within a mature follicle is a large, spherical cell, surrounded by the zona pellucida and is enclosed by the corona radiata. As many as twenty follicles may start this process but only one reaches full development.
Ovulation is the process by which a secondary oocyte is released from its follicle.
See figure 22.19, page 848. The uterine tubes function to carry the egg cell from the ovary to the uterus.
The uterus is a hollow muscular organ shaped somewhat like an inverted pear. It is located medially within the anterior portion of the pelvic cavity, above the vagina, and is usually bent forward over the urinary bladder. The upper two-thirds, called the body, has a dome-shaped top and is joined by the uterine tubes that enter its wall at its broadest part. The lower one-third is called the cervix, which extends downward into the upper portion of the vagina. The uterine wall is thick and consists of three layers. The endometrium forms the inner mucosa layer, lining the uterine cavity with columnar epithelium and contains numerous tubular glands. The myometrium is a thick muscular layer made up of bundles of smooth muscle fibers. The perimetrium is an outer serosal layer, which covers the body of the uterus and part of the cervix.
The vagina is a fibromuscular tube extending from the uterus to the outside. The vaginal wall consists of three layers. The inner layer (mucosal layer) consists of stratified squamous epithelium and forms the vaginal rugae; the middle layer (muscular layer) consists of longitudinal and circular patterns of smooth muscle; the outer layer (fibrous layer) consists of dense fibrous connective tissue and elastic fibers.
The labia majora are the outer rounded folds of skin, adipose tissue, and smooth muscle. The labia minora are the flattened inner folds of connective tissue and stratified squamous epithelium.
See figure 22.18, page 847. The clitoris is a small projection about two centimeters long and one-half centimeter in diameter at the anterior end of the vulva between the labia minora. The clitoris corresponds to the male penis and has a similar structure. It is composed of two columns of erectile tissue, called corpora cavernosa. At its anterior end, a small mass of erectile tissue that is richly supplied with sensory nerve fibers forms a glans.
The vestibule is a space found between the labia minora and contains the urethral and vaginal openings.
During sexual stimulation, parasympathetic nerve impulses cause dilation of the arteries associated with the erectile tissues found in the clitoris and vaginal entrance. This causes the tissues to swell and at the same time, the vagina expands and elongates. The vestibular glands begin to secrete mucous into the vestibule when stimulation is sufficient.
Orgasm is the culmination of stimulation of the sensory nerve fibers in the clitoris that causes the pleasurable sense of physiological and psychological release. In the male, it is also followed with ejaculation.
GnRH allows the anterior pituitary gland to release threshold levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). This leads to ovulation in the female and the resulting menstrual cycle.
The menstrual cycle is characterized by regular, recurring changes in the uterine lining, which culminate in menstrual bleeding.
The menstrual cycle is initiated by increasing levels of GnRH, FSH, and LH. These cause secretion of estrogen and progesterone.
The anterior pituitary gland secretes FSH to stimulate follicle maturation and LH to stimulate ovulation. The uterine wall thickens and follicular and thecal cells change into corpus luteum cells and secrete estrogen and progesterone. If the egg cell is not fertilized, the corpus luteum cells cease their secretions. This causes the blood vessels in the uterine lining to constrict and the lining sloughs off, producing the menstrual flow.
Menopause is the period of life when the menstrual cycle ceases completely.
Movement of the sperm cells is aided by the lashing of its tail and muscle contractions of the uterus and uterine tubes (caused by the prostaglandins in semen). In the first part of the menstrual cycle, the uterus and cervix contain a thin, watery fluid that aids the sperms’ movement and survival.
The mammary glands are located in the breasts, which overlie the pectoralis major muscles and extend from the second to the sixth ribs and from the sternum to the axillae. Each gland is composed of fifteen to twenty lobes. Each lobe contains glands called alveolar glands and a duct that leads to the nipple and opens to the outside. Dense connective tissue and adipose tissue separate the lobes from each other. Suspensory ligaments extend inward from the dermis of the breast to the fascia, helping to support the weight of the breast.
Contraception is a way to avoid fertilization of an egg cell following sexual intercourse or to prevent implantation of a blastocyst.
See table 22.5, page 863 for the complete list.
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