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 a process that begins with two gametes–eggs and sperm–and ends with a zygote, which is a fertilized egg. In animals ranging from insects to humans, males produce sperm in testes, and sperm are stored in the epididymis until ejaculation. Sperm are small, mobile, low-cost cells that occur in high numbers. Females produce an ovum or egg that matures in the ovary. Eggs are large cells that require a substantial investment of time and energy to form, are non-mobile, and are rare relative to sperm numbers. When the eggs 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).

The first half of Hank Green’s video below has a nice summary of these and other ideas we’ve discussed previously, and the second half introduces human reproductive anatomy before we take a deep dive into the structures and functions via dynamic hormonal regulation:

For our purposes, all sexual reproducers have females with ovaries that produce large eggs, which subsequently travel down a uterine tube, and males with testes that produce small, plenteous sperm, stored in an epididymus. Of course, beyond this general anatomy, there are some interesting differences in different types of animals:

  • In some invertebrate species, including many insects and some mollusks and worms, the female has a spermatheca: a specialized sac 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.
  • Non-mammal vertebrates, such as most birds and reptiles, have a cloaca: a single body openingw hich 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 a rare exception, where the 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 information below was adapted from OpenStax Biology 43.3

Mammalian (Human) Reproductive Anatomy, Gametogenesis, and Hormonal Regulation

The remainder of today’s content focus on mammalian reproduction, featuring humans as a model organism. We’ll first look at females, followed by males, emphasizing the structures, the process of gametogenesis, and hormonal control of reproduction.

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. Hormones are dynamic (changing), so this process can be trickier to understand than basic anatomy or gametogenesis.

Hormonal changes are the center of the fascinating biology of reproduction. The human male and female reproductive cycles are both controlled by the interaction of hormones from the hypothalamus and anterior pituitary with hormones from reproductive tissues and organs: the hypothalamus sends a gonadotropin-releasing hormone (GnRH) to the anterior pituitary, and follicle stimulating hormone (FSH) and luteinizing hormone (LH) are release from the anterior pituitary into the blood as a result. 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.

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.

Diagram of major female reproductive organs Image modified by Khan Academy from OpenStax, CC BY 4.0

Humans females become capable of reproduction at sexual maturity, which follows puberty. During puberty, the hypothalamus in the brain signals the pituitary gland to produce two hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). In females, FSH and LH stimulate the ovaries to produce the female sex hormones, estrogen and progesterone. This results in the development of secondary sex characteristics (such as breasts) and causes the ovaries to begin producing mature eggs.

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

Organ Function
Ovaries Produces and develops eggs
Fallopian tubes (oviducts) Transports egg to uterus, acts as site of fertilization
Uterus Supports a developing embryo
Cervix Allows passage between the uterus and the vagina
Vagina Receives penis during intercourse, acts as birth canal, passes menstrual flow
Breasts Produce and deliver milk

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 cycle, 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 to the ovaries, but they are not in direct physical 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.

 

Female Gametogenesis: Oogenesis

Oogenesis, the process of producing an egg cell, occurs in the the ovaries.  Egg stem cells, called oogonia, divide by mitosis to produce up to 2 million oocytes (a precursor to the egg). The process of oogenesis begins while the female is still an embryo undergoing development: the oocytes start the process of meiosis and then pause during meiotic prophase I. Because this process occurs during embryonic development, 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.

The oocyes remain in meiotic prophase I until the onset of puberty, when a series of events can lead to egg maturation:

  1. The anterior pituitary hormones, FSH and LH, cause some of the follicles to begin developing and oocyte inside the follicle to finish the first meiotic division.
  2. After completing meiosis I, the oocyte pauses again, this time during metaphase II.
  3. 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.
  4. 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. (If it is not fertilized, the oocyte degrades without completing meiosis II.)

The process of oogenesis 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.

 

One final point: when an oocyte undergoes meiosis, it produces only a single egg (again, this is different from spermatogenesis, which produces four sperm from each spermatocyte). 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.

Hormonal control of oogenesis

Oogenesis is controlled by FSH, LH, estrogen, and progesterone.

  • FSH stimulates development of egg cells that develop in structures called follicles, which are located within the ovaries.
  • LH also promotes development and maturation of eggs and induction of ovulation.
  • Estrogen is the reproductive hormone in females that assists in ovulation and regrowing the lining of the uterus; it is also responsible for the secondary sexual characteristics of females such as breast development.
  • 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 follicle which is most mature 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 cells in the follicle undergo physical changes and produce a structure called a corpus luteum, which produces estrogen and progesterone. The progesterone facilitates the regrowth of the uterine lining and inhibits the release of further FSH and LH. The uterus becomes prepared to accept a fertilized egg, should fertilization occur. The inhibition of FSH and LH by progesterone prevents any further eggs and follicles from developing. 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.
    • If a fertilized egg implants in the endometrial lining of the uterine wall, the embryo produces a hormone called human chorionic gonadotropin (hCG) that maintains the corpus luteum. The ovary continues to produce progesterone at high levels, and the menstrual cycle is arrested for the duration of the pregnancy. Because hCG is unique to pregnancy, it is the hormone detected by pregnancy tests.

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. Image credit: modification of work from OpenStax Biology and OpenStax Anatomy and Physiology; modification of work by Mikael Häggström)

This video provides a great overview of the human female reproductive system, emphasizing many of the points described above:

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.

Diagram of male reproductive organs
Image from OpenStax, CC BY 4.0

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

 

Male Gametogenesis: Spermatogenesis

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.

Hormonal Control of Spermatogenesis

The information below was adapted from OpenStax Biology 43.4

Just like oogenesis, spermatogenesis is controlled by FSH, LH. Testosterone also plays a role in spermatogenesis:

  • FSH stimulates spermatogenesis in the testes
  • LH stimulates testosterone production
  • 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.

While this doesn’t occur in a monthly cycle as in females, the hormones do interact in a feedback cycle which initiates during puberty: In response to signals from the hypothalamus that begin at the onset of puberty in males, the pituitary gland produces FSH. FSH enters the testes to begin facilitating spermatogenesis, which is the production of sperm cells (gametes) by meiosis. LH, made by the pituitary, also enters the testes to stimulate the production and release of testosterone into the blood. Testosterone stimulates spermatogenesis as well as the development of male secondary sex characteristics that include 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 when sperm counts get too high (over about 20 million/ml): rising testosterone levels cause Sertoli cells to release the hormone inhibin, which acts on the hypothalamus and pituitary gland to inhibit the release of FSH and LH.  The inhibition causes spermatogenesis to slow down until proper levels are again achieved. Once the sperm levels are reduced, the Sertoli cells stop releasing inhibin, and the sperm count increases.

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

How and when are gametes made?

As you’ve just seen in the two videos the production of sperm and eggs takes place through the process of meiosis, but there are some big differences between the processes to make eggs versus sperm:

  • When gametes start to form: Egg production begins during embryonic development (before birth), then is arrested during meiosis until puberty; sperm production does not begin until puberty
  • When gametes finish being made: Egg production is not actually completed until after fertilization (!), while sperm production is complete prior to ejaculation
  • How many gametes are made from a gamete stem cell: Egg production results in only a single egg from each egg stem cell; sperm production results in four sperm from each sperm stem cell.
  • Rate of production: 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.

Contraception and Birth Control

The information below was adapted from OpenStax Biology 43.5

Methods of contraception to prevent pregnancy have varying probabilities of success. In the diagram below, the failure rate is the given as the percent of women who become pregnant during the first year of use of that method.

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 present. 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).

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.

Work through the methods in this diagram to determine the most effective strategies for preventing pregnancy. Methods in combination, such as spermicidal chemicals and barrier, prevent pregnancy more effectively than do the methods when used separately.

What causes failure? Some sperm are present in pre-ejaculate secretions, so without a barrier, a some sperm may still enter the vagina. Hormonal methods vary in success by hormone delivery method. A method not pictured is emergency contraception, also known as “Plan B.” This hormone-based method of contraception works against the egg and the sperm simultaneously, and before the fertilization occurs.

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

 

The video below provides a great overview of the information described above. We recommend it as supplemental viewing if you’d like to review the topics covered in this reading: