Human Reproductive Anatomy and Gametogenesis

Male Reproductive Anatomy

The male reproductive system is a series of organs and glands that produce sperm, fluids, hormones, and enable the ejaculation of sperm.

Learning Objectives

Diagram the structures of human male reproductive anatomy

Key Takeaways

Key Points

  • The male gonads, or testes, produce sperm within the seminiferous tubules; the sperm are housed here until they are nearly mature, at which point they enter the epidydimis for full maturation.
  • The testes are housed in the scrotum, an external sac that keeps the sperm at a temperature lower than that of the body.
  • At ejaculation, sperm leave the epidydimis and enter the vas deferens, a duct which carries the sperm out of the body through the urethra, along with the fluids of various glands of the male reproductive system.
  • The seminal vesicles produce a thick fluid that is alkaline in order to protect sperm from the acidic nature of the female vagina; it also contains sugars to nourish the sperm.
  • The prostate gland produces the force necessary to push the sperm out of the epididymis at ejaculation, while the bulbourethral gland emits a fluid just prior to ejaculation that neutralizes acid from any urine left over in the urethra.
  • During sexual arousal, the spongy tissue inside the penis (the corpus spongiosum) fills with blood, causing the penis to become erect and hard; after ejaculation, the blood flows back out of the penis, leaving it flaccid.

Key Terms

  • epididymis: a narrow, tightly-coiled tube connecting the efferent ducts from the rear of each testicle to its vas deferens, where sperm are stored during maturation
  • prostate gland: a gland in male mammals surrounding the urethra just below the urinary bladder that controls the release of urine from the bladder and produces a secretion that is the fluid part of semen
  • seminiferous tubule: any of many threadlike structures, located in the testes, that are the specialized areas of sperm production

Human Reproductive Anatomy

The reproductive tissues of male and female humans develop similarly in utero until a low level of the hormone testosterone is released from male gonads. Testosterone causes the undeveloped tissues to differentiate into male sexual organs. Primitive gonads become testes; other tissues produce a penis and scrotum in males.

Male Reproductive Anatomy

In the male reproductive system, the scrotum houses the testicles or testes, providing passage for blood vessels, nerves, and muscles related to testicular function. The testes, a pair of male reproductive organs, produce sperm and male sex hormones, including the steroid testosterone. Coiled in each testis are seminiferous tubules that produce sperm.

Sperm

Sperm are immobile at body temperature; therefore, the scrotum and penis are external to the body so that a proper temperature is maintained for motility. In land mammals, the pair of testes must be suspended outside the body at about 2° C lower than body temperature to produce viable sperm.

Sperm develop in the seminiferous tubules that are coiled inside the testes. The walls of the seminiferous tubules are composed of the developing sperm cells, with the least-developed sperm at the periphery of the tubule and the fully-developed sperm in the lumen. The sperm cells are mixed with “nursemaid” cells called Sertoli cells which protect the germ cells and promote their development. Other cells mixed in the wall of the tubules are the interstitial cells of Leydig; these cells produce high levels of testosterone once the male reaches adolescence.

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Male reproductive system: The reproductive structures of the human male are shown. Sperm are produced in the seminiferous tubules, mature in the epididymis, and then are forced out into the vas deferens and out of the body through the urethra during ejaculation.

Sperm consist of a flagellum (as a tail), a neck that contains the cell’s energy-producing mitochondria, and a head that contains the genetic material. When the sperm have developed flagella, or lash-like appendages that protrude from the cell body, and are nearly mature, they leave the testicles and enter the epididymis. This structure lies along the top and posterior portion of the testes; it is the site of sperm maturation. The sperm leave the epididymis and enter the vas deferens, which is the duct in the testicle that carries sperm from the epididymis to the ejaculatory duct.

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Structure of a human sperm: Human sperm, visualized using scanning electron microscopy, have a flagellum, neck, and head.

Semen is a mixture of sperm and spermatic duct secretions (about 10 percent of the total), along with fluids from accessory glands, that contribute most of the semen’s volume. An ejaculate will contain from two to five milliliters of fluid with from 50–120 million sperm per milliliter. The bulk of the semen comes from the accessory glands associated with the male reproductive system, including the seminal vesicles, the prostate gland, and the bulbourethral gland.

Seminal vesicles, penis, prostate, and bulbourethral gland

The seminal vesicles are a pair of glands that lie along the posterior border of the urinary bladder. The glands make a solution that is thick, yellowish, and alkaline. As sperm are only motile in an alkaline environment, a basic pH is important to reverse the acidity of the vaginal environment. The solution also contains mucus, fructose (a sperm mitochondrial nutrient), a coagulating enzyme, ascorbic acid, and local-acting hormones called prostaglandins.

The penis is an organ that drains urine from the renal bladder and functions as a copulatory organ during intercourse. The penis contains three tubes of erectile tissue running through the length of the organ. These consist of a pair of tubes on the dorsal side, called the corpus cavernosum, and a single tube of tissue on the ventral side, called the corpus spongiosum. This tissue, when engorged with blood, becomes erect and hard, in preparation for intercourse. The organ is inserted into the vagina, culminating with an ejaculation, which is the forcible ejection of semen from the urethra. An orgasm is a two-stage process: first, glands and accessory organs connected to the testes contract; second, semen (containing sperm) is expelled through the urethra during ejaculation. After intercourse, the blood drains from the erectile tissue and the penis becomes flaccid.

The walnut-shaped prostate gland surrounds the urethra, the connection to the urinary bladder. It has a series of short ducts that directly connect to the urethra. The gland is a mixture of smooth muscle and glandular tissue. The muscle provides much of the force needed for ejaculation to occur.

The bulbourethral gland, or Cowper’s gland, is an exocrine gland which secretes a clear fluid known as pre-ejaculate that is generated upon sexual arousal. This gland releases its secretion prior to the release of the bulk of the semen. It neutralizes any acid residue in the urethra left over from urine. This usually accounts for a couple of drops of fluid in the total ejaculate and may contain a few sperm. Withdrawal of the penis from the vagina before ejaculation to prevent pregnancy may not work if sperm are present in the bulbourethral gland secretions.

Female Reproductive Anatomy

The female reproductive structures produce eggs, support a growing embryo, and provide a birth canal to the fetus.

Learning Objectives

Diagram the structures of human female reproductive anatomy

Key Takeaways

Key Points

  • The external anatomy of the female reproductive system is referred to as the vulva; it includes the labia minora, which protects the vagina and urethra, and the labia majora, which surrounds it.
  • Internal female reproductive structures include ovaries, oviducts, the uterus, and the vagina.
  • The eggs develop in structures called follicles, which are located on the surface of the ovaries; at maturity, one of the follicles will rupture and release the egg, which is captured by the fimbrae of the oviduct.
  • If fertilization occurs, it generally does so in the oviduct; the fertilized egg then travels down the oviduct and enters the uterus, where it will implant in the lining of the uterus, known as the endometrium.
  • If fertilization does not occur, the endometrium of the uterus will slough off at the end of the menstrual period and is shed through the vagina, which is also the opening through which the penis enters during intercourse and through which the baby will exit during birth.

Key Terms

  • clitoris: a small sensitive elongated erectile organ at the anterior part of the vulva in female mammals, homologous with the penis
  • ovary: a female reproductive organ, often paired, that produces ova and in mammals secretes the hormones estrogen and progesterone
  • vagina: the passage leading from the opening of the vulva to the cervix of the uterus for copulation and childbirth in female mammals
  • uterus: an organ of the female reproductive system in which the young are conceived and develop until birth; the womb
  • vulva: the external female sexual organs, collectively

Female reproductive anatomy

Female reproductive anatomy includes both external and internal structures. Among the external structures are the vulva, which consists of the mons pubis, clitoris, labia majora, labia minora, and the vestibular glands. The vulva is an area associated with the vestibule that includes the structures found in the inguinal (groin) area of women. The mons pubis is a round, fatty area that overlies the pubic symphysis. The clitoris is a structure with erectile tissue that contains a large number of sensory nerves and serves as a source of stimulation during intercourse. The labia majora are a pair of elongated folds of tissue that run posterior from the mons pubis and enclose the other components of the vulva. The labia majora derive from the same tissue that produces the scrotum in a male. The labia minora are thin folds of tissue centrally located within the labia majora. These labia protect the openings to the vagina and urethra. The mons pubis and the anterior portion of the labia majora become covered with hair during adolescence; the labia minora is hairless. The greater vestibular glands are found at the sides of the vaginal opening and provide lubrication during intercourse.

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Female reproductive anatomy: The reproductive structures of the human female are shown. Several structures are on the outside of the body, such as the labia, vulva, and clitoris, while others, such as the ovaries and uterus, are internal.

Internal female reproductive structures include ovaries, oviducts, the uterus, and the vagina. An ovary consists of a medulla and cortex: the medulla contains nerves and blood vessels to supply the cortex with nutrients and remove waste. The outer layers of cells of the cortex are the functional parts of the ovaries. The cortex is made up of follicular cells that surround eggs. During the menstrual cycle, a batch of follicular cells develops, preparing the eggs for release. At ovulation, one follicle ruptures and one egg is released.

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Oocyte development: Oocytes develop in (a) follicles, located in the ovary. At the beginning of the menstrual cycle, the follicle matures. At ovulation, the follicle ruptures, releasing the egg. The follicle becomes a corpus luteum, which eventually degenerates. The (b) follicle in this light micrograph has an oocyte at its center.

The oviducts, or fallopian tubes, extend from the uterus in the lower abdominal cavity to the ovaries, but they are not in contact with the ovaries. The lateral ends of the oviducts flare out into a trumpet-like structure and have a fringe of finger-like projections called fimbriae. When an egg is released at ovulation, the fimbrae help the non-motile egg enter into the tube, the passage to the uterus. The walls of the oviducts are ciliated (covered in cilia ) and are primarily smooth muscle. The cilia beat toward the middle, while the smooth muscle contracts in the same direction, moving the egg toward the uterus. Fertilization usually takes place within the oviducts. The embryo is moved toward the uterus for further development. It usually takes the egg or embryo a week to travel through the oviduct.

The uterus, a structure about the size of a woman’s fist, is lined with an endometrium that is rich in blood vessels and mucus glands. The uterus supports the developing embryo and fetus during gestation. The thickest portion of the wall of the uterus is made of smooth muscle. Contractions of the smooth muscle in the uterus aid in passing the baby through the vagina during labor. A portion of the lining of the uterus sloughs off during each menstrual period if an egg has not been fertilized; it builds up again in preparation for an implantation. Part of the uterus, called the cervix, protrudes into the top of the vagina, which functions as the birth canal.

The vagina is a muscular tube that serves several purposes. It allows menstrual flow to leave the body, is the receptacle for the penis during intercourse, and serves as the vessel for the delivery of offspring. It is lined by stratified squamous epithelial cells to protect the underlying tissue.

Gametogenesis (Spermatogenesis and Oogenesis)

Spermatogenesis and oogenesis are both forms of gametogenesis, in which a diploid gamete cell produces haploid sperm and egg cells, respectively.

Learning Objectives

Distinguish between spermatogenesis and oogenesis

Key Takeaways

Key Points

  • Gametogenesis, the production of sperm (spermatogenesis) and eggs (oogenesis), takes place through the process of meiosis.
  • In oogenesis, diploid oogonium go through mitosis until one develops into a primary oocyte, which will begin the first meiotic division, but then arrest; it will finish this division as it develops in the follicle, giving rise to a haploid secondary oocyte and a smaller polar body.
  • The secondary oocyte begins the second meiotic division and then arrests again; it will not finish this division unless it is fertilized by a sperm; if this occurs, a mature ovum and another polar body is produced.
  • In spermatogenesis, diploid spermatogonia go through mitosis until they begin to develop into gametes; eventually, one develops into a primary spermatocyte that will go through the first meiotic division to form two haploid secondary spermatocytes.
  • The secondary spermatocytes will go through a second meiotic division to each produce two spermatids; these cells will eventually develop flagella and become mature sperm.

Key Terms

  • spermatocyte: a male gametocyte, from which a spermatozoon develops
  • oocyte: a cell that develops into an egg or ovum; a female gametocyte
  • polar body: one of the small cells that are by-products of the meiosis that forms an egg
  • mitosis: the division of a cell nucleus in which the genome is copied and separated into two identical halves. It is normally followed by cell division
  • meiosis: cell division of a diploid cell into four haploid cells, which develop to produce gametes

Gametogenesis (Spermatogenesis and Oogenesis)

Gametogenesis, the production of sperm and eggs, takes place through the process of meiosis. During meiosis, two cell divisions separate the paired chromosomes in the nucleus and then separate the chromatids that were made during an earlier stage of the cell’s life cycle, resulting in gametes that each contain half the number of chromosomes as the parent. The production of sperm is called spermatogenesis and the production of eggs is called oogenesis.

Oogenesis

Oogenesis occurs in the outermost layers of the ovaries. As with sperm production, oogenesis starts with a germ cell, called an oogonium (plural: oogonia), but this cell undergoes mitosis to increase in number, eventually resulting in up to one to two million cells in the embryo.

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Oogenesis: The process of oogenesis occurs in the ovary’s outermost layer. A primary oocyte begins the first meiotic division, but then arrests until later in life when it will finish this division in a developing follicle. This results in a secondary oocyte, which will complete meiosis if it is fertilized.

The cell starting meiosis is called a primary oocyte. This cell will begin the first meiotic division, but be arrested in its progress in the first prophase stage. At the time of birth, all future eggs are in the prophase stage. At adolescence, anterior pituitary hormones cause the development of a number of follicles in an ovary. This results in the primary oocyte finishing the first meiotic division. The cell divides unequally, with most of the cellular material and organelles going to one cell, called a secondary oocyte, and only one set of chromosomes and a small amount of cytoplasm going to the other cell. This second cell is called a polar body and usually dies. A secondary meiotic arrest occurs, this time at the metaphase II stage. At ovulation, this secondary oocyte will be released and travel toward the uterus through the oviduct. If the secondary oocyte is fertilized, the cell continues through the meiosis II, completing meiosis, producing a second polar body and a fertilized egg containing all 46 chromosomes of a human being, half of them coming from the sperm.

Spermatogenesis

Spermatogenesis occurs in the wall of the seminiferous tubules, with stem cells at the periphery of the tube and the spermatozoa at the lumen of the tube. Immediately under the capsule of the tubule are diploid, undifferentiated cells. These stem cells, called spermatogonia (singular: spermatagonium), go through mitosis with one offspring going on to differentiate into a sperm cell, while the other gives rise to the next generation of sperm.

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Spermatogenesis: During spermatogenesis, four sperm result from each primary spermatocyte, which divides into two haploid secondary spermatocytes; these cells will go through a second meiotic division to produce four spermatids.

Meiosis begins with a cell called a primary spermatocyte. At the end of the first meiotic division, a haploid cell is produced called a secondary spermatocyte. This haploid cell must go through another meiotic cell division. The cell produced at the end of meiosis is called a spermatid. When it reaches the lumen of the tubule and grows a flagellum (or “tail”), it is called a sperm cell. Four sperm result from each primary spermatocyte that goes through meiosis.

Stem cells are deposited during gestation and are present at birth through the beginning of adolescence, but in an inactive state. During adolescence, gonadotropic hormones from the anterior pituitary cause the activation of these cells and the production of viable sperm. This continues into old age.