REPRODUCTIVE SYSTEM OVERVIEW

The reproductive system is regarded as one of the most important systems for our survival as a species, as it secures the succession of the human race. Furthermore, without this system operating in both of our parents, we would never have had the opportunity to experience life.

The health of the reproductive system does not only reflect the genetic viability of future offspring, but also of our overall health in general. Reproductive organs are responsible for producing the vast majority of sex hormones in the body which play a huge role in maintaining aspects of growth, regeneration, energy production and cognition. These hormones also impact our emotional state of being which plays a role in maintaining optimal mental health, irrespective of whether one is sexually active or not.

Understanding the nature of the reproductive system is therefore an important part of piecing together how the body functions as a whole. The following overview of the reproductive system gives insight into its inner workings for both of the biologic sexes.

Hypothalamic-Pituitary-Gonadal Axis

The hypothalamic-pituitary-gonadal axis (HPG) refers to a neurological circuit that forms between two components of the brain (the hypothalamus and the pituitary gland) and the reproductive organs. This axis is connected via the nervous system, allowing the brain and reproductive system to communicate.

One of the main functions of the HPG is hormone production. Hormones are required on mass in both male and female reproductive organs in order to ensure proper functionality, as well as optimal growth and regeneration. The sex organs are delicate by nature; comparable to the likeness of flowers, the sex organs of plants. The male reproductive system requires a perpetual supply of testosterone, while the female reproductive system requires a continuous supply of estrogen. Without this continuous hormonal input, many areas of both reproductive tracts would perish.  

Both male and female reproductive organs produce their own supply of testosterone and estrogen respectively. The ovaries produce estrogen that supply the entire reproductive tract in females and the testis produce all the testosterone that is required by the male reproductive system. The release of sex hormones is controlled by hormonal signaling in the brain, which is often timed according to the circadian rhythm of the body. At the right moment, the hypothalamus releases gonadotropin-releasing hormone, which signals to the pituitary to release hormones referred to as gonadotropins. These are Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH). Both LH and FSH service the reproductive tract, promoting the release of sex hormones as well as instructing the cells to maintain specific functions. Some of these include sperm production, menstruation, preparing the lining of the uterus for gestation and more.[1]

A small percentage of sex steroid hormones are also produced in the adrenal glands through neurological stimulation on a separate axis (the hypothalamic-pituitary-adrenal axis), as well as in the skin.

Female Reproductive System

The female reproductive system is vital for creating steroidal hormones that contribute toward the growth and development of a woman and her egg cells[2]. It also acts as a secure vessel for housing the vulnerable fetus during its development into a new born child.

Primary Sex Organs

The main components of the female reproductive tract are the ovaries and the uterus.

• The Ovaries house egg cells and are also the main site for sex hormone production, critical for maintaining reproductive functionality as well as many important aspects of feminine biology at large.
• The Uterus is responsible for generating a lining, known as the endometrium, which ultimately serves as a nurturing medium for fetal development. Once fertilization occurs, the uterus transforms into the womb, complete with a placenta that nourishes and sustains the developing infant in utero.

These count as the primary sex organs and are the most vital for reproduction.

Secondary Sex Organs

Secondary sex organs are considered to be less important than primary sex organs. However, their function is still a requirement for optimal reproductive functionality.

The secondary sex organs consist of the following:

• The Vulva is a term used to describe the external portion of the female reproductive system, including the entrance to the vaginal canal, the labia, the clitoris, and the opening of the urethra. The labia refer to the inner and outer folds of skin that line the sides of the vaginal opening. [3]
• The Vaginal Canal and Cervix together form the entrance into the female reproductive tract. The vaginal canal receives the penis during reproduction to allow for fertilization to take place. It expands inwardly in size when the female is aroused in order to accommodate the shape and size of the erect penis during intercourse. The cervix is a smaller canal that rests above the vaginal canal. Where the two meet, there is a very small opening (the cervical opening) that allows for some of the male ejaculate to pass through into the uterus; while simultaneously preventing anything hazardous from entering the delicate inner portions of the reproductive tract. Technically, the cervix forms a part of the uterus, however it serves the same basic function of the vaginal tract in terms of facilitating fertilization.
• The Clitoris is an accessory sex organ that rests above the urethral opening, forming the top of the entrance into the vaginal canal. 
• Bartholin’s Glands are two pea-sized glands that rest on either side a little way in from the vaginal opening. They are responsible for lubricating the tract and form the female equivalent of the bulbourethral gland in males.
• Skene’s Gland or the Paraurethral Gland is the literal female equivalent of the prostate gland in males[4]. Few realize that women have their own prostates and that it performs similar actions to that of the male prostate. The paraurethral gland can be found a little way along the female urinary tract from the urethral opening, positioned between the urethra and the vaginal tract.
• The Fallopian Tubes are thick extensions from either side of the uterus that connect to the ovaries. They appear to hold onto the ovaries with finger-like projections. These “fingers” aid the movement of egg cells from the ovaries during ovulation, speeding up the process as they move through the fallopian tubes into the uterus.
• Breasts are secondary sex organs that become fully functional post gestation, during breast feeding. Prior to that point, they serve no use other than perhaps to help attract a mate by contributing to the overall feminine physique.

Functions

The female reproductive system serves 5 main essential functions that are necessary for the optimal continuation of the human species.

1. Hormone Production

Without adequate hormonal supply, the reproductive tract would not be able to grow, develop or function. The following hormones are vital to ensure that all organs in the female reproductive system are functioning optimally:

• Estrogen is the dominant hormone required to balance growth in the female reproductive tract and ensure that all of its functions are carried out. It is produced by follicular cells inside the ovaries, which contain pre-mature egg cells inside of them. Estrogen ensures that the endometrial cells in the uterus and the secretory cells in the vaginal canal grow and function optimally. A spike in estrogen initiates ovulation as well as the excessive proliferation of the uterine wall in order to prepare it for gestation[5].
• Progesterone is required to balance the effects of estrogen and also contributes toward reproductive function. While estrogen stimulates growth of the endometrium, progesterone is required to maintain the endometrial vascular system and to ensure that the endometrium stops growing after ovulation, remaining intact should successful fertilization occur. Progesterone also maintains the endometrial lining of the womb during pregnancy and is key to sustaining a healthy immune function in the reproductive tract, especially when a developing fetus is present.[6]
• Testosterone is also used in small amounts to service certain reproductive organs, such as the paraurethral gland, as well as to balance the effects of estrogen throughout the body as a whole, however the bulk of it is transformed into estrogen in the ovaries. In females, estrogen levels ought to be much higher than testosterone. These two hormones form an important ratio that, by contrast, is inverted in men.

2. Development & Perpetuation of Female Characteristics

During fetal development and again later on during puberty, female hormones become active in shaping the physical characteristics of the feminine form.

In the fetus, estrogens ensure the development of the reproductive tract which continues to grow after birth and during adolescence. At puberty, the reproductive tract becomes fully active for the first time due to increased hypothalamic-pituitary stimulation, thereby inducing the uterine cycle (described below) and increasing the levels of hormones produced by the ovaries.

The increased levels of female sex hormones during puberty enhance female weight distribution, bone structure, height, hip proportions, female bodily hair and its distribution as well as promote breast growth and maturation.

3. Egg Cell Preparation & the Uterine Cycle

Another prime function of the female reproductive system is to protect the ovarian reservoir of pre-mature egg cells and offer chances for fertilization to occur. The uterine cycle is a perpetual process that occurs in the female reproductive tract in order to serve these functions. Another vital function that this cycle serves is ensuring a continuous supply of female sex hormones to service the entire tract.

The cycle tends to span 28 days on average, however some women may have a shorter or longer cycle, depending on their biological clock. It allows for pre-mature egg cells to mature into viable egg cells, which may then be fertilized during reproduction. If no fertilization occurs, the eggs cells are then excreted from the body along with the lining of the uterus during menstruation. Another name for the uterine cycle would be the menstrual cycle.[7]

Menstruation & the Follicular Phase

The follicular phase consists of the menses, followed by the proliferative phase in which estrogen gradually increases and stimulates the growth of the uterine lining. During in the follicular phase, follicular cells simultaneously undergo a transformation that is vital for ensuring reproductive homeostasis.

Follicular cells contain egg cells inside of them and are important for their maturation, as well as for supplying the right amounts of estrogen and progesterone throughout the uterine cycle. The numbers of follicular cells, with the egg cells they carry, are set when the female reproductive tract is created in utero; of which there are normally more than enough to last a woman for her lifetime. These cells are premature and are also referred to as primordial follicular cells. A stable baseline level of estrogen is generated by the other primordial follicular cells in the ovaries, while a stable level of progesterone and testosterone is maintained by theca cells.

During menstruation, a set of primordial follicular cells begin to mature in preparation for the next ovulation. This coincides with relatively low hormone levels, mildly elevated progesterone compared to estrogen and the degeneration of the previous month’s mature follicular cell. Out of the few that are selected, only one is chosen to evolve into a mature follicular cell.

Ovulation

As the chosen follicular cell grows to maturity, the levels of estrogen it produces scale up with its size. When the cell reaches full maturity, estrogen dramatically declines which causes a surge of both LH and FSH from the pituitary. High levels of LH cause the mature follicular cell to burst open and release the mature egg cell, which is then ferried off into the uterus where it finds a comfortable spot in the lining and waits for a viable sperm cell to fertilize it. This process is better known as ovulation.

Ovulation occurs in the middle of the menstrual cycle[8]. Before ovulation, the accompanying spike in estrogen usually stimulates secretory action from the mucosal cells lining the vaginal tract creating a mid-cycle discharge.

The Luteal Phase

The remainder of the follicular cell (still residing in the ovary) starts to heal and changes in response to LH, becoming a structure known as the corpus luteum. This change occurs within a matter of hours from the time of ovulation, when the egg cell is violently released from its follicular encasement.

The corpus luteum starts to produce high levels of progesterone alongside small amounts of testosterone. Most of the testosterone is converted into estrogen in order to maintain a balanced supply of gonadotropins from the pituitary, as well as to ensure the survival of the reproductive organs. When progesterone peaks in the luteal phase, it causes the levels of LH to decrease and the levels of FSH to increase slightly. This increase in FSH stimulates for a set of new primordial follicular cells to start maturing for the next up-coming menstruation period.

Eventually, the corpus luteum is broken down to nothing by the body’s immune system, a process that can take a few weeks to months to complete. In those who have a robust immune system, the process may be quicker. Occasionally, some residue scar tissue is left behind, the frequency and amount of which varies from female to female. When both estrogen and progesterone drop back down to their usually low levels, the cycle repeats.

4. Gestation

Gestation occurs when an egg cell is fertilized. Hypothetically, fertilization can only occur any time after ovulation and before menstruation; however experience reveals that women are capable of falling pregnant at nearly all times of their cycle. Each woman’s cycle is unique to their biology and therefore it’s difficult to predict based off a theoretical model.

Nevertheless, after a successful round of intercourse when all the conditions miraculously line up, sperm travel through the cervix and into the uterus where they find and fertilize a viable egg cell. The female reproductive tract is highly hostile toward sperm cells as well, being the wrong pH for sperm viability. The immune system also tends to treat sperm cells like foreign hazardous genetic material, working swiftly to neutralize them, break them down and remove them. Sperm cells counteract these defenses by being large in number as well as through the protection offered by compounds found in the male ejaculate.

Once an egg cell has been fertilized by a sperm cell, it fuses and quickly begins to divide. The lining of the uterus gets thicker, the cervix forms a plug and eventually the wombs final structure comes into being. The uterine cycle ends in order to make way for gestation. The womb grows in line with the development of the fetus, causing nearly all other organs in the abdomen to shift up to accommodate the size.

5. Menopause

Menopause refers to the time in every woman’s life when the menstrual cycle comes to an end and fertilization is no longer possible. It falls in line with the natural aging process and is important to ensure that offspring are not born with congenital defects. Women that give birth later on in life increase their child’s chance of being born with such defects as the aging process tends to deplete her resources and deprive the fetus. Furthermore, an accruement of DNA damage is highly associated with aging and therefore the genetic viability of potential offspring declines as a woman ages.

Reproductive hormones experience a surge in a woman’s 30’s. This surge is preceded by a slow but steady decline. During the late 40’s up to late 50’s, a woman’s uterine cycle begins to get erratic as hormone levels get lower, often with longer, irregular spaces in between menstrual periods. This is known as pre-menopause. Eventually the menstrual cycle comes to a standstill altogether and hormone levels remain at a very low baseline. Hormones proceed to decline until they are no longer produced by the reproductive organs, only being produced by the adrenal glands and the skin. If a woman lives a long and prosperous life, hormones will end up only being produced in the skin. The phase after menopause is known as post-menopause.

Read more about menopause in our article on Hormone Replacement Therapy.

Male Reproductive System

Like the female reproductive system, the male reproductive tract is there to ensure that there is an adequate supply of male sex hormones which promote optimal growth, regeneration and reproductive function in men. It also ensures that sperm cells are produced and kept safe until they can attempt to fertilize a female egg cell.

Primary Sex Organs

The primary component of the male reproductive system consists solely of the testicles.

• The Testicles are internal glands which are housed externally by the scrotum. They are responsible for producing and maintaining the health and vitality of sperm cells. Sperm are created by testicular Sertoli cells. Leydig cells in the testis are also responsible for producing up to 95% of the testosterone in the male body, which is vital for ensuring the survival of the reproductive tract as well as masculine growth, development and physical characteristics.

Secondary Sex Organs

The secondary sex organs are considered less important than the testicles, yet they are all still a requirement for successful reproductive function.

Male secondary sex organs consist of tubes and glands that follow on from the testicles, each of which contributes unique portions to the male ejaculate as it makes its way out of the tract. They comprise of the following structures:

• The Epididymis is a very long, yet highly compact tube that rests behind and extends above each testicle. It is neatly contained inside its own structure and is responsible for storing as well as maturing sperm cells. Freshly created sperm make their way from the testis into the epididymis where they are concentrated and develop motility or the ability to propel themselves forward. Sperm motility is repressed until intercourse, where they are released from the epididymis into the vas deferens.[9]
• The Vas Deferens is merely a thin connecting tube that transports sperm from the epididymis, all the way along the reproductive tract. The seminal vesicle, prostate and bulbourethral glands all connect to the vas deferens along the way, contributing their secretions to the ejaculate. Smooth muscles lining the vas deferens contract during intercourse to ensure that sperm are propelled along the tract without any difficulty.[10]
• The Seminal Vesicle is a gland that contributes the bulk portion of seminal fluid to the ejaculate, comprising 70% or more of it in total. The secretions from this gland consist of various nutrients that ensure the survival of sperm outside the tract. Some components of this fluid also interact with the female reproductive tract on the other side of ejaculation, helping to soften the mucosal lining of the cervix which is known to be deleterious to sperm cell survival. The seminal fluid also promotes less contractility in the female tract to ensure that sperm are not pushed back out the moment they enter.[11]
• The Prostate contributes up to one sixth of the male ejaculate and is the next secretory gland along the vas deferens in the male reproductive tract. It rests below the bladder and also contracts to prevent urine from entering the tract during intercourse. Further nutrients are supplied to the sperm from the prostate, specifically large quantities of zinc which keeps the ejaculate sterile and offers antioxidant support for prolonged survival of sperm outside the tract. Prostatic secretions are also vital for liquefying the ejaculate, ensuring that it does not clump together soon after leaving the tract and allowing for the ejaculate to spread further. This is important, as seminal fluid that doesn’t contain prostatic secretions is only able to make it roughly halfway along the female reproductive tract.[12]
• The Bulbourethral Gland or Cowper’s Gland is the male equivalent of the female bartholin’s glands. This gland is there to lubricate the remaining portion of the male reproductive tract and does not contribute to the ejaculate. Usually the gland secretes a clear substance that forms the pre-ejaculate, however some men may also expel actual ejaculate with this fluid during intercourse. [13]
• The Phallus refers to the erect penile shaft and is the last portion of the reproductive tract that the sperm need to traverse before exiting. It plays a vital role in reproduction, ensuring that the male ejaculate is expelled as far along the female reproductive tract as possible. The penile shaft is lined with smooth muscle, known as the corpus cavernosum. This muscle remains contracted to keep the penis in its usual flaccid or non-erect state. When stimulated, nerve impulses feedback to the brain via the HPA, which in turn signals for the blood vessels in the shaft to dilate, fill with blood and relax these muscles, resulting in a healthy erection.[14]

Functions

The male reproductive system serves 5 main functions that are important for ensuring the continued survival of the human race. They are as follows:

1. Hormone Production

Unlike the female reproductive system, the male reproductive system majors on producing testosterone.

• Testosterone is created inside testicular Leydig cells through the transformation of cholesterol into progesterone, which is then further converted into testosterone. This production is caused by the release of LH from the pituitary. This male sex hormone governs most of the reproductive functions in men, such as sperm production, sex organ secretions and male libido. It is also responsible for the development of male characteristics, such as increased height, facial hair and the deepening of the voice during puberty.[15] As a hormone, testosterone has a reputation for enhancing overall growth in the body as a whole, more so than other sex hormones.
• Progesterone is present in small amounts in the male reproductive system and have an inhibitory effect on testosterone production. It is required more for regulating other vital cellular processes in the body.[16]
• Estrogen is created in the male reproductive tract in very small amounts via the conversion of testosterone by an enzyme known as aromatase. When kept in very low amounts, estrogen regulates growth by inducing apoptosis (programmed cell death), which keeps the exuberant growth-promoting effects of testosterone under control. Estrogen is also required for the maturation of the epididymis during puberty, as well as for maintaining its functions with regard to storing and maturing sperm[17].

2. Development & Perpetuation of Male Characteristics

During fetal development and again during puberty, testosterone is responsible for promoting the development of the male reproductive tract. In puberty, the tract matures and male characteristics emerge: the testicles descend, certain glands grow to their full size (like the prostate), the voice deepens, the bones and muscles grow into a masculine phenotype, the last of pre-pubescent fat is lost and facial hair begins to grow.

To accommodate these phases of rapid growth, normal testosterone is converted into a more potent form known as dihydrotestosterone (DHT). DHT has an action that is 10 times more potent than testosterone, able to stimulate growth at a rapid speed. The increase in testosterone levels is brought about through a sudden surge of the gonadotropins LH and FSH from the pituitary.

3. Sperm Cell Production

While the Leydig cells are responsible for testosterone production, the Sertoli cells in the testis are involved in the production and maturation of sperm cells. The testicles comprise of very fine, tightly wound tubes known as the seminiferous tubules. During fetal development, a set number of primordial germ cells populate these tubules. When a man becomes fertile later on in life, these cells become active and begin to transform into mature sperm cells, a process known as spermatogenesis. A man has more than enough of these primordial cells to keep producing sperm for longer than he can remain alive; however, overall reproductive function declines before the final decades of life.

Testosterone and FSH stimulate the action of sertoli cells, which serve to initiate spermatogenesis and nourish these primordial cells with the nutrients they need. Primordial germ cells become spermatogonium cells, which are actually stem cells that can continuously divide.

Spermatogonium cells form new types of stem cells when they divide, with A1, A2, A3 and A4 types being present in the testis. The A4 spermatogonium cells, are the cells that go on to becoming type B spermatogonium cells. B spermatogonium transform into spermatocytes which begin to generate the basic sperm cell structure internally.

Primary spermatocytes are further divided to form secondary spermatocytes which produce spermatids. Spermatids are connected to the membranes of secondary spermatocytes and eventually detach before maturing into actual sperm cells. Through this process of continual division, the newly formed cells are moved from the outer layer of the seminiferous tubule further and further inward until eventually spermatids break away into the center of the tube to form sperm cells. Sperm cells are then ejected toward the epididymis for full maturation, where they become mobile and able to “swim.”[18]

While the number of primordial germ cells that began this process are set, they can keep dividing in this manner at the many parts of this process to sustain a continual supply of sperm. It takes up to 74 days for sperm to mature, with pauses in-between the process. Not all parts of the testis produce sperm at once, with only a few pockets being active at any given time.[19]

4. Fertilization

Naturally, the entire design of the male reproductive tract complements that of the female reproductive tract. The phallus allows for deeper penetration into the vaginal canal, from which sperm cells are expelled from the tip at the point of ejaculation. The secretions from the various parts of the reproductive tract ensure that the ejaculate goes as far as it can and ensures the prolonged survival of sperm. [20]

Sperm that make it through the cervical opening enter the uterus and from there they are on their own. Their motility helps them to “swim” inside the uterus until a few are lucky enough to find a fertile female egg cell. The environment inside the female reproductive tract serves to dissolve the sperm, especially sperm that are no longer protected by the nourishing fluids of the ejaculate. Of sperm cells that find their way to the egg, only one is chosen while the rest tend to perish.

When the sperm meets the egg, a literal “spark of life” is generated through the chemical interaction of calcium from the egg and zinc from the sperm[21]. This spark initiates the fusion of the two as well as the beginning of gestation.

5. Andropause

Andropause is the male equivalent of menopause, yet unlike menopause, it is not a sudden event that puts a halt to reproductive function for the rest of a man’s life. Instead, andropause refers to the gradual decline of sex hormones as well as reproductive function. Men can maintain sexual function for as long as they live, yet traditionally other faucets of the aging process catch up with the reproductive system.[22]

Common age-related issues that result in sexual impotence include cardiovascular problems, low testosterone levels, less libido and energy, impaired nutrient absorption and lower urinary tract symptoms.

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Sources:

• [1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4498991/
• [2] https://pubmed.ncbi.nlm.nih.gov/30725817/
• [3] https://www.ncbi.nlm.nih.gov/books/NBK537132/
• [4] https://pubmed.ncbi.nlm.nih.gov/6546868/
• [5] https://www.ncbi.nlm.nih.gov/books/NBK538260/
• [6] https://www.ncbi.nlm.nih.gov/books/NBK558960/
• [7] https://www.britannica.com/science/menstruation/Hormonal-control-of-menstrual-cycle
• [8] https://www.ncbi.nlm.nih.gov/books/NBK441996/
• [9] https://www.researchgate.net/publication/300839314_Physiology_of_the_Epididymis
• [10] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3749770/
• [11] https://www.ncbi.nlm.nih.gov/books/NBK499854/
• [12] https://www.ncbi.nlm.nih.gov/books/NBK279291/
• [13] https://onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2605.2005.00499.x
• [14] https://onlinelibrary.wiley.com/doi/full/10.1002/j.1939-4640.2003.tb02742.x
• [15] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6044347/
• [16] https://pubmed.ncbi.nlm.nih.gov/15669543/
• [17] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC179885/
• [18] https://www.ncbi.nlm.nih.gov/books/NBK10095/
• [19] https://www.britannica.com/science/spermatogenesis
• [20] https://www.ncbi.nlm.nih.gov/books/NBK538429/
• [21] https://www.nature.com/articles/srep24737
• [22] https://pubmed.ncbi.nlm.nih.gov/10687877/

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