Reproductive System – Anatomy, Structure, Functions

Overview of the Male and Female Reproductive Systems

The human reproductive system functions to produce human offspring, with the male providing sperm and the female providing the ovum.

The reproductive system or genital system is a set of organs within an organism that work together to produce offspring. Many non-living substances, such as fluids, hormones, and pheromones, are important accessories to the reproductive system. Unlike most organ systems, the sexes of differentiated species often have significant differences. These differences allow for a combination of genetic material between two individuals and thus the possibility of greater genetic fitness of the offspring.

The Reproductive Process

Human reproduction takes place as internal fertilization by sexual intercourse. During this process, the erect penis of the male is inserted into the female’s vagina until the male ejaculates semen, which contains sperm, into the vagina. The sperm travels through the vagina and cervix into the uterus for potential fertilization of an ovum. Upon successful fertilization and implantation, the gestation of the fetus occurs within the female’s uterus for approximately nine months (pregnancy). Gestation ends with labor resulting in birth. In labor, the uterine muscles contract, the cervix dilates, and the baby passes out through the vagina. Human babies and children are nearly helpless and require high levels of parental care for many years. One important type of parental care is the use of the mammary glands in the female breasts to nurse the baby.

The Male Reproductive System

The human male reproductive system is a series of organs located outside of the body and around the pelvic region. The primary direct function of the male reproductive system is to provide the male gamete or spermatozoa for fertilization of the ovum. The major reproductive organs of the male can be grouped into three categories. The first category is sperm production and storage. Production takes place in the testes, housed in the temperature-regulating scrotum. Immature sperm then travel to the epididymis for development and storage. The second category, the ejaculatory fluid-producing glands, includes the seminal vesicles, prostate, and vas deferens. The final category, used for copulation and deposition of the spermatozoa (sperm) within the female, includes the penis, urethra, vas deferens, and Cowper’s gland.

Organ Systems Involved

The hypothalamic-pituitary-gonadal axis plays a major role in promoting sexual maturity, sperm production and the development of secondary sex characteristics. It maintains spermatogenesis and sexual function throughout the male’s lifetime. The hypothalamus secretes GnRH into the hypothalamo-hypophyseal portal system to stimulate the anterior pituitary. GnRH is a peptide hormone released by hypothalamic neurons in a pulsatile fashion. It acts on the gonadotrophs of the anterior pituitary via the binding and activation of a G protein receptor, which stimulates the anterior pituitary through inositol 1,4,5-triphosphate (IP3) activation (which increases intracellular calcium) to release FSH and LH. GnRH is inhibited by testosterone, estrogen, estradiol, and prolactin.[rx]

In response, the anterior pituitary secretes LH and FSH into the blood. These gonadotropic hormones act on membrane receptors in the Leydig and Sertoli cells of the testes respectively. Both hormones come from the same glycoprotein family and consist of identical alpha subunits, but their different beta-subunit differentiates their functions. Both exert their physiologic effects by binding and activating a G protein receptor, which activates adenylyl cyclase and increases cellular cAMP levels, to stimulate Sertoli and Leydig cells. LH stimulates Leydig cells in the interstitium of the testes to produce testosterone from cholesterol. LH promotes desmolase, which is the initial rate-limiting enzyme that converts cholesterol into pregnenolone. This goes on to produce two key weak androgen intermediates: dehydroepiandrosterone (DHEA) and androstenedione. The enzyme 17-beta-hydroxysteroid dehydrogenase completes the conversion of androstenedione to testosterone. Testosterone acts on the hypothalamus and anterior pituitary via negative feedback to decrease the secretion of LH and FSH. Testosterone can also exert some effect on Sertoli cells, found in the periphery of the seminiferous tubules of testes. FSH and testosterone can stimulate Sertoli cells to release androgen-binding protein (ABP), which provides testosterone to germ cells during spermatogenesis. FSH stimulates Sertoli cells to promote sperm production and release inhibin B and MIS. Inhibin serves as the negative feedback control that Sertoli cells exert on the hypothalamic-pituitary system to decrease FSH release.[rx]

Before puberty, the levels of androgens and gonadotropins typically remain low and constant. Once puberty occurs, the hypothalamus releases GnRH in a pulsatile fashion every one to two hours to maintain amounts of FSH, LH, and plasma testosterone, all of which regulate each other to maintain hormonal balance. In the third decade of life, testosterone levels are found to decline.[rx][rx][rx]

Although a majority of testosterone production in men comes from the Leydig cells in the testes, the adrenal cortex contributes some androgen production. Similar to the hypothalamic-pituitary-gonadal axis, the adrenal glands are also controlled by the hypothalamus and anterior pituitary to form the hypothalamic-pituitary-adrenal axis. The hypothalamus releases corticotrophin-releasing hormone (CRH), which stimulates the release of adrenocorticotropic hormone (ACTH) from the anterior pituitary. ACTH stimulates the enzyme desmolase to convert cholesterol into pregnenolone in the adrenals, similar to testosterone synthesis in the testes. Specifically, the zona reticularis of the adrenal medulla is responsible for generating the weak androgens DHEA and androstenedione, which go on to be converted to testosterone or estradiol peripherally.[rx]

Function

The function of the male reproductive system is to produce androgens such as testosterone that maintain male reproductive function and to promote spermatogenesis and transport into the female reproductive system for fertilization. The testes act as both endocrine and exocrine organs in that they are responsible for androgen production and sperm production and transport.

Mechanism

Spermatogenesis starts at puberty with the germ cells found in the basement membrane of the seminiferous tubules of the testes. Sertoli cells stimulated by FSH help regulate spermatogenesis. One cycle of spermatogenesis begins approximately every 13 days; however, spermatogenesis is not consistently synchronous throughout all seminiferous tubules. The first stage of spermatogenesis begins with mitosis of diploid spermatogonia into primary spermatocytes. These spermatocytes undergo meiosis I to produce haploid secondary spermatocytes, which undergo meiosis II to form haploid spermatids. The most primitive spermatocytes are found peripherally in the seminiferous tubules and mature by migrating towards the lumen. Spermatids transform into spermatozoa by reducing cytoplasm. These spermatozoa are still immotile and are released into the tubules to travel to the epididymis for maturation. The epididymis is a coiled structure consisting of a head, body, and tail. The tail eventually joins with the vas deferens, providing an outlet for mature sperms to ejaculate. In the epididymis, the sperm takes about twelve days to mature and develop motility. They are then stored in the tail of the epididymis until ejaculation occurs. A mature sperm consists of a head, midpiece, and tail. The head contains the nucleus with very little cytoplasm. An acrosome or cap covers the head and is filled with lysosomes, which aids with fertilization. The midpiece contains abundant mitochondria to provide energy for the flagellum or tail of the sperm.

During sexual arousal (physical or psychological), vasodilation brings blood to the penis. The penis contains corpora cavernosa and a corpus spongiosum where blood flows along to enlarge and erect the penis. As sexual stimulation continues, blood continues to flow to the genitals, and the testes enlarge in preparation for ejaculation.

When ejaculation occurs, smooth muscle contractions of the epididymis push sperm into the ductus deferens (vas deferens), which sit in the spermatic cord. The ductus deferens deliver the sperm to the ejaculatory duct by joining with the seminal vesicle duct near the prostate. The seminal vesicles produce fructose, which provides the energy for sperm motility. It is released within a fluid that mixes with the sperm to form semen. Once in the ejaculatory duct, the semen passes through the prostate, which secretes an alkaline fluid that helps thicken the semen so sperm can better stay within the female reproductive system. The semen then passes the bulbourethral glands or Cowper’s glands, which release a thick fluid that lubricates the urethral opening and clears the urethra of any urine residue. The semen then can enter the female vaginal canal, allowing the sperm to travel to and fertilize a potential egg within the female reproductive system.[rx][rx][rx]

The Human Male Reproductive System: Cross-sectional diagram of the male reproductive organs.

Only our species has a distinctive mushroom-capped glans, which is connected to the shaft of the penis by a thin tissue of frenulum (the delicate tab of skin just beneath the urethra). One of the most significant features of the human penis is the coronal ridge underneath the gland around the circumference of the shaft. Magnetic imaging studies of heterosexual couples having sex reveal that during coitus, the typical penis expands to fill the vaginal tract, and with full penetration can even reach the woman’s cervix and lift her uterus. This combined with the fact that human ejaculate is expelled with great force and considerable distance (up to two feet if not contained), suggests that men are designed to release sperm into the uppermost portion of the vagina. This may be an evolutionary adaptation to expel the semen left by other males while at the same time increasing the possibility of fertilization with the current male’s semen.

The Female Reproductive System

The human female reproductive system is a series of organs primarily located inside the body and around the pelvic region. It contains three main parts: the vagina, which leads from the vulva, the vaginal opening, to the uterus; the uterus, which holds the developing fetus; and the ovaries, which produce the female’s ova. The breasts are also a reproductive organ during parenting but are usually not classified as part of the female reproductive system. The vagina meets the outside at the vulva, which also includes the labia, clitoris, and urethra. During intercourse, this area is lubricated by mucus secreted by the Bartholin’s glands. The vagina is attached to the uterus through the cervix, while the uterus is attached to the ovaries via the Fallopian tubes. At certain intervals, approximately every 28 days, the ovaries release an ovum that passes through the Fallopian tube into the uterus.
If the ova are fertilized by sperm, it attaches to the endometrium and the fetus develops. In months when fertilization does not occur, the lining of the uterus called the endometrium, and unfertilized ova are shed each cycle through a process known as menstruation.

Organ Systems Involved

Female Reproductive Organs

Ovaries

• The ovaries are female gonads, the site of gametogenesis, and the secretion of sex hormones. The outer cortex of each ovary is the site of follicular development, while the inner medulla of each contains blood vessels and connective tissue.[rx]

Fallopian Tubes

• The vulva describes the external female genitalia: labia majora, labia minora, clitoris, vulvar vestibule, urethrethral meatus, vaginal orifice.[rx] The labia majora are lateral to the labia minora, fusing anteriorly to make up the mons pubis (a layer overlying the pubic symphysis). The vulvar vestibule is the area medial to the labia minora and is the location of the urethra and vaginal openings.[rx] Bartholin’s glands open lateral to the vaginal opening.[rx]
• The vagina is a flexible, fibromuscular tubular structure extending from the vulvar vestibule to the uterine cervix. The distal vagina is the introitus. The anterior vagina abuts the posterior bladder wall while the posterior vagina abuts the anterior rectum.[rx][rx][rx][rx][rx]
• The uterus consists of the corpus (body) and cervix. The superior aspect of the uterine corpus is the fundus, while the inferior portion adjacent to the cervix is called the isthmus/lower uterine segment. The uterine walls contain three distinct layers: the endometrium, myometrium, and the serosa. The endometrium lines the uterine cavity; its thickness and structure vary with hormonal stimulation. The myometrium consists of smooth muscle fibers and is the middle and thickest layer of the uterine wall. The serosa is the outermost lining of the uterus.[rx]
• The uterine cervix is a tubular structure contiguous with the uterine cavity and the vagina, acting as a conduit between the two. The inferior cervix opens into the upper vagina at the cervical os. The lining of the cervix that protrudes into the vagina is called the ectocervix and consists of columnar epithelium. The lining of the inside of the cervical canal is the endocervix, composed of stratified squamous epithelium. The region where the ecto- and endocervix meet, characterized by the transformation from columnar to squamous epithelium, is the transformation zone. The transformation zone is the most frequent location for cervical dysplasia and malignant transformation.
• Fallopian tubes provide a passageway for oocytes to travel from the ovaries into the uterine cavity. The part of each tube closest to the ovary contains fimbria: finger-like projections that help move the expelled oocyte further into the tube—the fimbria transition into the ampulla, the part of the tube with the widest lumen. The ampulla becomes the isthmus as the lumen narrows and projects towards the uterus. The tube then passes into the uterus, where it becomes the interstitial portion.[rx] This opening is where the traveling oocyte exits the tube and enters the uterine cavity.

Uterus

• The uterus consists of the corpus (body) and cervix. The superior aspect of the uterine corpus is the fundus while the inferior portion adjacent to the cervix is called the isthmus/lower uterine segment. The uterine walls contain three distinct layers: the endometrium, myometrium, and the serosa. The endometrium lines the uterine cavity; its thickness and structure vary with hormonal stimulation. The myometrium consists of smooth muscle fibers and is the middle and thickest layer of the uterine wall. The serosa is the outermost lining of the uterus.[rx]
• The uterine cervix is a tubular structure contiguous with the uterine cavity and the vagina, acting as a conduit between the two. The inferior cervix opens into the upper vagina at the cervical os. The lining of the cervix that protrudes into the vagina is called the ectocervix and consists of columnar epithelium. The lining of the inside of the cervical canal is the endocervix, composed of stratified squamous epithelium. The region where the ecto- and endocervix meet, characterized by the transformation from columnar to squamous epithelium, is the transformation zone. The transformation zone is the most frequent location for cervical dysplasia and malignant transformation.

Vagina

• The vagina is a flexible, fibromuscular tubular structure extending from the vulvar vestibule to the uterine cervix. The distal vagina is the introitus. The anterior vagina abuts the posterior bladder wall while the posterior vagina abuts the anterior rectum.[rx][rx][rx][rx][rx]

Vulva

• The vulva describes the external female genitalia: labia majora, labia minora, clitoris, vulvar vestibule, urethrethral meatus, vaginal orifice.[rx] The labia majora are lateral to the labia minora, fusing anteriorly to make up the mons pubis (a layer overlying the pubic symphysis). The vulvar vestibule is the area medial to the labia minora and is the location of the urethra and vaginal openings.[rx] Bartholin’s glands open lateral to the vaginal opening.[rx]

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