Animal Reproductive Structures and Functions

Learning Objectives

  • Identify and describe functions of key anatomical reproductive structures present in various types of animals, including the spermatheca, the cloaca, the ovary and related structures, and the testes and related structures
  • Compare and contrast the process, products, and locations of male and female gametogenesis in mammals
  • Describe roles of hormones in gametogenesis, ovulation, and implantation
  • Explain how various medical interventions affect reproductive cycles and fertilization

The information below was adapted from OpenStax Biology43.2

Diversity of Animal Reproductive Anatomy

The reproductive structures of many animals are very similar, even across different lineages. In animals ranging from insects to humans, males produce sperm in testes and sperm are stored in the epididymis until ejaculation. Females produce eggs that mature in the ovary. When they are released from the ovary, they travel to the uterine tubes for fertilization (in animals that reproduce via internal fertilization) or are released in the aqueous environment (in animals that reproduce via external fertilization). While this general pathway is the same, there are some differences in different types of animals:

  • In some invertebrate species, including many insects and some mollusks and worms, the female has a specialized sac, called a spermatheca, which stores sperm for later use, sometimes up to a year. Fertilization can be timed with environmental or food conditions that are optimal for offspring survival.
  • In non-mammal vertebrates, such as most birds and reptiles, there is a common body opening, called a cloaca which functions in the digestive, excretory and reproductive systems. Mating between birds usually involves positioning the cloaca openings opposite each other for transfer of sperm from male to female (ducks are rare among birds in that males have a penis).
  • Mammals have separate openings for the systems in the female, and placental mammals have a uterus for support of developing offspring. The uterus has two chambers in species that produce large numbers of offspring at a time, while species that produce one offspring, such as primates, have a single chamber.

The video below provides a great overview of the information described above and below:

 

The information below was adapted from OpenStax Biology 43.3

Mammalian Reproductive Anatomy

We’ll now focus on mammalian reproductive anatomy, using humans as a model.

Male Reproductive Anatomy

In the male reproductive system, the scrotum houses the testicles or testes (singular: testis), which produce sperm and some reproductive hormones. Sperm are immobile at body temperature; therefore, the scrotum and penis are external to the body, as illustrated below, 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. Infertility can occur in land mammals when the testes do not descend through the abdominal cavity during fetal development.

The reproductive structures of the human male. Illustration shows a cross section of the penis and testes. The urethra is an opening that runs through the middle of the penis to the bladder. The testes, located immediately behind the penis, are covered by the scrotum. Seminiferous tubules are located in the testes. The epididymis partly surrounds the sac containing the seminiferous tubules. The Vas deferens is a tube connecting the seminiferous tubules to the ejaculatory duct, which begins in the prostate gland. The prostate gland is located behind and below the bladder. The seminal vesicle, located above the prostate, also connects to the seminal vesicle. The bulbourethral gland connects to the ejaculatory duct where the ejaculatory duct enters the penis. Image credit: OpenStax Biology.

Sperm are produced in the seminiferous tubules inside the testes.  The sperm cell production is mediated by two different types of cells: “nursemaid” cells called Sertoli cells which protect the germ cells and promote their development, and cells of Leydig which produce high levels of testosterone once the male reaches adolescence and regulate sperm development.

When the sperm have developed flagella and are nearly mature, they leave the testicles and enter the epididymis, where sperm mature. During ejaculation, the sperm leave the epididymis and enter the vas deferens, which carries the sperm, behind the bladder, and forms the ejaculatory duct with the duct from the seminal vesicles.

Semen is a mixture of sperm and spermatic duct secretions and fluids from accessory glands that contribute most of the semen’s volume. The bulk of the semen comes from the accessory glands associated with the male reproductive system. These are the seminal vesicles, the prostate gland, and the bulbourethral gland, all of which are illustrated above.

  • The seminal vesicles are a pair of glands that make thick, yellowish, and alkaline solution. 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 source of energy for the sperm cells), a coagulating enzyme, ascorbic acid (vitamin C), and local-acting hormones called prostaglandins (may help stimulate smooth muscle contractions in the uterus). The seminal vesicle glands account for 60 percent of the bulk of semen.
  • The 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 glandular tissue makes a thin, milky fluid that contains citrate (stimulates sperm motility), enzymes, and prostate specific antigen (PSA). PSA is a proteolytic enzyme that helps to liquefy the ejaculate several minutes after release from the male. Prostate gland secretions account for about 30 percent of the bulk of semen.
  • The bulbourethral gland releases its secretion prior to the release of the bulk of the semen. The mucous secretions of this gland help lubricate and neutralize 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.

This table briefly summarizes the major organs, locations, and functions of mammalian male reproductive anatomy:

Organ Location Function
Scrotum External Carry and support testes
Penis External Deliver urine, copulating organ
Testes External Produce sperm and male hormones
Seminal vesicles Internal Contribute to semen production
Prostate gland Internal Contribute to semen production
Bulbourethral glands Internal Clean urethra at ejaculation

 

This video provides a concise overview of the anatomy and function of the male reproductive system:

Female Reproductive Anatomy

A number of reproductive structures are exterior to the female’s body. These include the breasts and the vulva. Internal female reproductive structures include ovariesoviducts, the uterus, and the vagina, shown below.

The reproductive structures of the human female. The vagina is wide at the bottom, and narrows into the cervix. Above the cervix is the uterus, which is shaped like a triangle pointing down. Fallopian tubes extend from the top sides of the uterus. The Fallopian tubes curve back in toward the uterus, and end in fingerlike appendages called fimbriae. The ovaries are located between the fimbriae and the uterus. The urethra is located in front of the vagina, and the rectum is located behind. The clitoris is a structure located in front of the urethra. The labia minora and labia majora are folds of tissue on either side of the vagina. (credit a: modification of work by Gray’s Anatomy; credit b: modification of work by CDC)

Ovaries are the site of egg development. Egg development occurs in structures called follicles, which are lined with specialized cells called follicular cells that surround the egg and promote egg development. During the menstrual period, a batch of follicular cells develops and prepares the eggs for release. At ovulation, one follicle ruptures and one egg is released, as illustrated below. The ruptured follicle, which remains in the ovary, is then called the corpus luteum, which secretes hormones that prevent menstruation until the egg has had time to be fertilized. If fertilization and implantation in the uterine wall occurs, then the corpus luteum continues to prevent menstruation; if fertilization does not occur, then the corpus luteum degenerates and menstruation occurs.

Oocyte maturation within a follicle, followed by ovulation (follicle rupture). The follicle becomes a corpus luteum after ovulation and degenerates if the egg is not fertilized. Image credit: MartaFF – http://www.lainfertilidad.com/hormona-foliculo-estimulante-fsh, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=39272613

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 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 egg enter into the tube and passage to the uterusFertilization (the union of sperm and egg) usually takes place within the oviducts and the developing embryo is moved toward the uterus for development. It usually takes the egg or embryo a week to travel through the oviduct.

This video provides a concise overview of the anatomy and function of the female reproductive system:

Gametogenesis (Spermatogenesis and Oogenesis)

Gametogenesis, the production of sperm and eggs, takes place through the process of meiosis (see the Biology 1510 website page on Cell Division for help with this often confusing concept).  Meiosis produces haploid cells with half the number of chromosomes normally found in diploid cells. The production of sperm is called spermatogenesis and the production of eggs is called oogenesis. While both processes produce gametes, there are some important differences:

Spermatogenesis, illustrated below, occurs in the seminiferous tubules in the testes. Sperm stem cells (called spermatogonia) are present at birth but are inactive until puberty, when hormones from the anterior pituitary cause the activation of these cells and the continuous production of sperm. Sperm production continues into old age. To produce sperm, a cell called a spermatocyte (a precursor to sperm) undergoes meiosis to produce four haploid spermatids (immature sperm). Once the spermatid develops a flagellum, (a tail that allows it to swim), it is called a sperm cell. Four sperm cells result from each spermatocyte that goes through meiosis.

During spermatogenesis, four sperm result from each primary spermatocyte. Spermatogenesis begins when the 2n (diploid) spermatogonium undergoes mitosis, producing more spermatagonia. The spermatogonia undergo meiosis I, producing haploid (1n) secondary spermatocytes, and meiosis II, producing spermatids. Differentiation of the spermatids results in mature sperm. Image credit: OpenStax Biology.

This video (beginning at 2:13) provides an overview of spermatogenesis and will help you visualize the process of sperm production and development in the locations where they occurs:

Oogenesis, illustrated in below, occurs in the the ovaries.  Egg stem cells, called oogonia, divide by mitosis during embryonic development to produce up to 2 million oocytes (a precursor to the egg). The oocytes start the process of meiosis and then pause during meiotic prophase I; this means that a female mammal is born with every single egg she will be able produce during her lifetime already present (in an immature form) in her ovaries. This situation is very different from males, whose spermatogonia (the sperm equivalent to oogonia) do not begin producing spermatocytes (the sperm equivalent to oocytes) until puberty.

Unlike spermatogenesis which produces four sperm from each spermatocyte, only a single egg is produced from each oocyte that goes through meiosis. Once a female enters puberty, anterior pituitary hormones cause some of the follicles to begin developing, causing the oocyte inside the follicle to finish the first meiotic division. The oocyte divides unequally, so that almost all of the cytoplasm goes into only one daughter cell rather than evenly distributed into both. The smaller cell is called a polar body, and normally dies. After completing meiosis I, the oocyte pauses again, this time during metaphase II. Though several follicles are activated during each cycle, only one will release an oocyte. The released oocyte will begin traveling through the oviduct, still arrested in meiosis II.  If the oocyte is fertilized by a sperm, it will finish meiosis II and undergo unequal cytokinesis (cell division) to produce a fertilized egg (an embryo) and another polar body.  This process is illustrated below:

Oogenesis begins when the 2n oogonium undergoes mitosis, producing a primary oocyte. The primary oocytes arrest in prophase I before birth. After puberty, meiosis of one oocyte per menstrual cycle continues, resulting in a 1n secondary oocyte that arrests in metaphase II and a polar body. Upon ovulation and sperm entry, meiosis is completed and fertilization occurs, resulting in a polar body and a fertilized egg. Image credit: OpenStax Biology.

Thus while both spermatogenesis and oogenesis make gametes, there are some big differences between the processes:

  • Egg production begins during embryonic development (before birth), then is arrested during meiosis until puberty; sperm production does not begin until puberty
  • Egg production is not actually completed until after fertilization (!), while sperm production is complete prior to ejaculation
  • Egg production results in only a single egg from each egg stem cell; sperm production results in four sperm from each sperm stem cell.
  • Once an individual enters puberty, sperm production is continuous in a “conveyor belt” process; egg production occurs one-at-a-time at each menstrual cycle.

Hormonal Control of Gametogenesis

The information below was adapted from OpenStax Biology 43.4

The human male and female reproductive cycles are controlled by the interaction of hormones from the hypothalamus and anterior pituitary with hormones from reproductive tissues and organs. When the reproductive hormone is required, the hypothalamus sends a gonadotropin-releasing hormone (GnRH) to the anterior pituitary. This causes the release of follicle stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary into the blood. Although FSH and LH are named after their functions in female reproduction, they are produced and play important roles in controlling reproduction in both sexes.

Reproductive hormones in males

Spermatogenesis is controlled by FSH, LH, and testosterone:

  • FSH enters the testes and stimulates spermatogenesis
  • LH enters the testes and stimulates production of testosterone
  • Testosterone further stimulates spermatogenesis. It is also the hormone responsible for the secondary sexual characteristics that develop in the male during adolescence, including a deepening of the voice, the growth of facial, axillary, and pubic hair, and the beginnings of the sex drive.

A negative feedback system occurs in the male with rising levels of testosterone acting on the hypothalamus and anterior pituitary to inhibit the release of GnRH, FSH, and LH. The Sertoli cells produce the hormone inhibin, which is released into the blood when the sperm count is too high. This inhibits the release of GnRH and FSH, which will cause spermatogenesis to slow down. If the sperm count reaches 20 million/ml, the Sertoli cells cease the release of inhibin, and the sperm count increases.

This video describes human male reproductive anatomy and hormonal control of gametogenesis:

Reproductive hormones in females

The control of reproduction in females is more complex. Oogenesis is controlled by FSH, LH, estrogen, and progesterone.

  • FSH stimulates development of egg cells, called ova, which develop in structures called follicles. Follicle cells produce the hormone inhibin, which inhibits FSH production. Follicles also produce estradiol and progesterone.
  • LH also plays a role in the development of ova, induction of ovulation, and stimulation of estradiol (a form of estrogen) and progesterone production by the ovaries.
  • Estrogens (such as estradiol) and progesterone are released from the developing follicles. Estrogen is the reproductive hormone in females that assists in endometrial regrowth, ovulation, and calcium absorption; it is also responsible for the secondary sexual characteristics of females such as breast development, flaring of the hips, and a shorter period necessary for bone maturation. Progesterone assists in endometrial re-growth and inhibition of FSH and LH release.

These hormones together regulate the ovarian and menstrual cycles. The ovarian cycle governs the preparation of endocrine tissues and release of eggs, while the menstrual cycle governs the preparation and maintenance of the uterine lining. These cycles occur concurrently and are coordinated over a 22–32 day cycle, with an average length of 28 days:

  • The first half of the ovarian cycle is the follicular phase. Slowly rising levels of FSH and LH cause the growth of follicles on the surface of the ovary. This process prepares the egg for ovulation. As the follicles grow, they begin releasing estrogens. Estrogen levels increase over the course of the follicular phase as the follicles continue to develop.  In the menstrual cycle, menstrual flow occurs at the beginning of the follicular phase when estrogen levels are low (when the follicles are only just beginning to develop); rising levels of estrogen then cause the endometrium to proliferate (grow), replacing the blood vessels and glands that deteriorated during the end of the last cycle.
  • Ovulation occurs just prior to the middle of the cycle (approximately day 14), when the high level of estrogen produced by the developing follicles causes FSH and especially LH to rise rapidly, then fall. The spike in LH causes ovulation: the most mature follicle ruptures and releases its egg. The follicles that did not rupture degenerate and their eggs are lost. The level of estrogen decreases when the extra follicles degenerate.
  • Following ovulation, the ovarian cycle enters its luteal phase, and the menstrual cycle enters its secretory phase, both of which run from about day 15 to 28. The luteal and secretory phases refer to changes in the ruptured follicle. The cells in the follicle undergo physical changes and produce a structure called a corpus luteum. The corpus luteum produces estrogen and progesterone. The progesterone facilitates the regrowth of the uterine lining and inhibits the release of further FSH and LH. The uterus is being prepared to accept a fertilized egg, should it occur during this cycle. The inhibition of FSH and LH prevents any further eggs and follicles from developing, while the progesterone is elevated. The level of estrogen produced by the corpus luteum increases to a steady level for the next few days. Estrogen enhances the effects of progesterone.
  • It takes about seven days for an egg to travel through the fallopian tube from the ovary to the uterus, and it must be fertilized while in the fallopian tube. If no fertilized egg is implanted into the uterus, the corpus luteum degenerates and the levels of estrogen and progesterone decrease. The endometrium begins to degenerate as the progesterone levels drop, initiating the next menstrual cycle. The decrease in progesterone also allows the hypothalamus to send GnRH to the anterior pituitary, releasing FSH and LH and starting the cycles again.The figure below visually compares the ovarian and uterine cycles as well as the hormone levels controlling these cycles.
The figure below visually compares the ovarian and uterine cycles as well as the hormone levels controlling these cycles.

Rising and falling hormone levels result in progression of the ovarian and menstrual cycles. The uterine cycle is divided into menstrual flow, the proliferative phase and the secretory phase. The ovarian cycle is separated into follicular and luteal phases. At day zero the uterine cycle enters the menstrual phase and the ovarian cycle enters the follicular phase. Menstruation begins, and the follicle inside the uterus begins to grow. The level of the pituitary hormone FSH rises slightly, while LH levels remain low. The levels of ovarian hormones estradiol and progesterone remain low. After menses the uterine cycle enters the proliferative phase and the follicle continues to grow. The level of the ovarian hormone estradiol begins to rapidly rise. Toward the end of the proliferative phase, levels of the pituitary hormones FSH and LH rise as well. Around day fourteen, just after the levels of estrogen, progesterone and estradiol reach their peak, ovulation occurs. The follicle ruptures, releasing the oocyte. The ovarian cycle enters the luteal phase. The follicle grows into a corpus luteum and then degenerates. The uterus enters the secretory phase. Progesterone levels increase and estradiol levels, which had dropped after ovulation, increase as well. Toward the end of the secretory phase estrogen and progesterone levels decrease, reaching their baseline levels around day 28. At this point menstruation begins. Image credit: modification of work from OpenStax Biology and OpenStax Anatomy and Physiology; modification of work by Mikael Häggström)

This video describe human female reproductive anatomy and hormonal control of gametogenesis:

Contraception and Birth Control

The information below was adapted from OpenStax Biology 43.5

The prevention of a pregnancy comes under the terms contraception or birth control. Strictly speaking, contraception refers to preventing the sperm and egg from joining. Both terms are, however, frequently used interchangeably.

The diagram below illustrates common methods of contraception and the typical rates that occur. A failure rate is the number of pregnancies resulting from the method’s use over a twelve-month period.

By Center for Disease Control and Prevention – http://www.cdc.gov/reproductivehealth/UnintendedPregnancy/PDF/effectiveness_of_contraceptive_methods.pdf, Public Domain, https://commons.wikimedia.org/w/index.php?curid=27189006

Barrier methods, such as condoms, cervical caps, and diaphragms, block sperm from entering the uterus, preventing fertilization. Spermicides are chemicals that are placed in the vagina that kill sperm. Sponges, which are saturated with spermicides, are placed in the vagina at the cervical opening. Combinations of spermicidal chemicals and barrier methods achieve lower failure rates than do the methods when used separately.

Natural family planning is based on the monitoring of the menstrual cycle and having intercourse only during times when the egg is not available. A woman’s body temperature may rise a degree Celsius at ovulation and the cervical mucus may increase in volume and become more pliable. These changes give a general indication of when intercourse is more or less likely to result in fertilization. Withdrawal involves the removal of the penis from the vagina during intercourse, before ejaculation occurs. This method with has a high failure rate due to the possible presence of sperm in the bulbourethral gland’s secretion, which may enter the vagina prior to removing the penis.

Hormonal methods use synthetic progesterone (sometimes in combination with estrogen), to inhibit the hypothalamus from releasing FSH or LH, and thus prevent an egg from being available for fertilization. The method of administering the hormone affects failure rate. The most reliable method, with a failure rate of less than 1 percent, is the implantation of the hormone under the skin. The same rate can be achieved through using an intrauterine device (IUD). IUDs are inserted into the uterus and establish an inflammatory condition that prevents fertilized eggs from implanting into the uterine wall. Some IUDs also release progesterone. Emergency contraception, also known as “Plan B” is also a hormone-based method of contraception. One common misconception about emergency contraception is that it prevents implantation after fertilization; however, like other contraceptive methods, it does not induce abortion (it has no impact after fertilization).

This video provides a quick overview of hormone-based birth control, with emphasis on emergency contraception:

During a vasectomy, a section of the vas deferens is removed, preventing sperm from being passed out of the body during ejaculation and preventing fertilization. The equivalent process in women is called a tubal ligation; it is analogous to a vasectomy in males in that the oviducts are severed and sealed. Tubal ligation and vasectomy are considered permanent prevention, while other methods are reversible and provide short-term contraception.