Making Babies, Part 1
The true goal for any organism is to survive long enough to procreate and ensure the continuation of the species. In order for human beings to reproduce, a spermatozoon must be introduced into the female reproductive tract and fertilize an egg. This creates new life.
Testes
Similar in function to the female ovaries, the testes are where sperm form. It is also where the genital ducts begin.
Anatomy
Suspended from the perineum of the male pelvis, the bilateral testes are covered in skin called the scrotum. The scrotum functions as more than just a case for the testes; the scrotum and its thin underlying muscles regulate the temperature of the testes by either retracting and pulling the testes closer to the pelvis (to warm the testes) or relaxing and allowing the testes to descend farther away from the body (to lower the temperature of the testes).
Beneath the skin of the scrotum is a dense connective tissue capsule that surrounds each testis called the tunica albuginea. This connective tissue also accumulates at the posterior portion of the testes to form the mediastinum testis. From this point connective tissue septa emanate throughout the testis to divide the tissue into lobes. Each divided lobe contains 1–4 seminiferous tubules, which are the sites of sperm germ cells as well as where the sperm develops.
Cells
In addition to the germ cells, two other cell types are present in the testes that play essential roles in spermatozoa generation as well as the maintenance of secondary male sexual characteristics, which includes greater body hair, heavier bone structure and muscle mass, lower body fat percentage when compared to females, and the development of male genitalia.
Sertoli cells (nurse cells) are located throughout the seminiferous tubules and function to sustain and protect the developing sperm. As sperm germ cells undergo meiotic cell divisions and genetic recombination, the resulting sperm are genetically and immunologically different from the male in which they are produced. If the Sertoli cells did not create a blood-testis barrier, the newly formed spermatozoa would elicit an immune response and be destroyed. Additionally, these cells produce a fructose-rich secretion that nourish the sperm in their protected environment within the lumen of the seminiferous tubule.
Outside of the seminiferous tubules, in the interstitium of the testis, are the interstitial cells of Leydig. These cells are endocrine in nature and produce the androgen testosterone.
Intratesticular Tract
The pathway sperm use to make their way from the testes to the female reproductive tract is collectively called the male genital ducts or reproductive tract. After the sperm are formed in the seminiferous tubules, they move toward the mediastinum testis and straight, narrow terminal portions of the seminiferous tubules, which are called the tubuli recti (literally translated “straight tubes”).
These short, straight passages allow the spermatozoa to move into an anastomotic (interconnecting) maze of passages in the mediastinum testis (rete testis). These allow the sperm to move into the next portion of the genital duct system, the efferent ductules. There are 10–20 of these ductules that transfer spermatozoa from the testes into the first portion of the extratesticular tract, the epididymis.
Spermatogenesis
Spermatogenesis is the umbrella term that encompasses all of the cellular and molecular processes that change male germ cells (spermatogonia) into free, mobile sperm.
Spermatocytogenesis
During this phase, the spermatogonia divide in order to reproduce themselves as well as to produce the next stage of cells in the developmental process, those being the primary spermatocytes. While the spermatogonia are situated at the basal region of the seminiferous tubules between adjacent Sertoli cells, the primary spermatocytes migrate toward the lumen and through the tight junctional complexes that separate the lumen and the tubule from the rest of the male testes.
So far, the cell divisions have been accomplished using mitosis or cloning of the cells. In later stages, meiosis must occur to produce cells with half of the genetic material (haploid or 1N cells). The main cells visible in the seminiferous tubule are the spermatogonia (with its dark nucleus in the basal area) and the primary spermatocyte (with its larger, vesiculated nucleus, due to condensing chromatin). The next cellular intermediate is formed by the first meiotic chromosomes dividing into 2 haploid secondary spermatocytes. These 2 cells rapidly divide during the second meiotic division into 4 spermatids.
Spermatidogenesis
This process of division into spermatids (called spermatidogenesis) is so rapid that secondary spermatocytes are not typically visible in a histological preparation. Although meiosis has occurred to transform a single primary spermatocyte into 4 early spermatids, only the nuclei have divided. The cytoplasm of these 4 daughter cells remains attached to one another via cytoplasmic bridges.
Spermiogenesis
This final stage in sperm formation converts the rounded early spermatids into late spermatids, which are then separated from the other daughter cells and released into the lumen of the seminiferous tubule as spermatozoa. Four distinct stages occur to shape and form all of the cellular and molecular components of the spermatozoa prior to their undocking from the Sertoli cell.
The first phase of spermiogenesis is called the Golgi phase, during which enzymes are released and provide a means by which the head of the sperm may fuse with and be inserted into the egg. While this is happening at the head of the sperm, at the opposite side, centrioles are forming into a microtubule-organizing unit to produce the base of the flagellum (the axoneme), which also course through the core of the flagellum itself.
The cap phase is next. This stage is marked by the movement of all the granules to just above the nucleus of the spermatid, forming the acrosome (acrosomal cap). Mitochondria are also moved into the area of the flagellum base to provide energy that drives the movement of the flagellum and propel the spermatozoa.
In the third stage, tail formation, the microtubules extend to push the plasma membrane outward and form the elongated structure that provides mobility to the spermatozoa. The spermatids at this point are oriented so that their now tapered heads are directed to the lumen of the seminiferous tubule.
During the final maturation phase, the excess cytoplasm is shed and the spermatids are freed from their sibling cells into independent cells. Any other remaining cytoplasm is removed while the spermatozoa are in the lumen of the seminiferous tubule by a process called spermiation (sperm release). However, at this point, spermatozoa are immobile and remain incapable of fertilizing an egg.
Extratesticular Reproductive Tract
Once the spermatozoa have left the efferent ductules, they enter the extratesticular ducts. The first portion of these ducts is found within the scrotum. The later portion is part of the spermatic cord that rises to the pelvis and enters the lower abdomen to join with the urethra and exit the body.
Epididymis
The first portion of the extratesticular duct is the epididymis, which stores sperm and reabsorbs fluid. This highly coiled tube possesses a head, which receives spermatozoa from the efferent ductules. From the superior portion of the testis, the body of the epididymis extends downward before forming the tail at the inferior portion of the testis. This is primarily where the spermatozoa are stored for possibly up to 2–3 months. The epithelial cells of the epididymis are also well suited to assist in the reabsorption of materials in the reproductive tract. Extending into the lumen are long cellular processes called stereocilia. Stereocilia do not assist in movement of materials; in fact, these processes are actually extremely long microvilli that function by increasing the cellular surface area for maximal reabsorption of material.
Vas Deferens
Beginning at the tail of the epididymis, the vas deferens extends upward from the scrotum in the spermatic cord and enters the body from an opening in the lower pelvis. This is the thickest of the ducts in the male reproductive tract because of its extremely thick layers of smooth muscle. These become active during ejaculation and generate peristaltic contractions that propel spermatozoa along and out of the male reproductive tract.
Ejaculatory Duct
As the vas deferens approaches the urethra (the common duct for urine and sperm), it is joined by a duct from the accessory sex gland known as the seminal vesicle. When this union occurs, this terminal portion of the vas deferens is renamed the ejaculatory duct.
Urethra
The final duct of the male reproductive tract is the urethra, which expels urine. Urine is harmful to sperm; therefore, several accessory sex glands are located throughout this portion of the male reproductive tract to not only nourish but also protect the sperm.
Glands of the Male Reproductive Tract
Since sperm are essentially a nucleus with half a complement of chromosomes, mitochondria (to provide energy for flagellar movement), and a bag (acrosome) of enzymes to be used to penetrate the oocyte (female egg), they have no means of generating or processing materials to use as fuel for the mitochondria. They instead must obtain these materials from the secretions of the accessory sex glands. Additionally, for protection against the acidic urine that may remain in the urethra, other secretions will neutralize and buffer the urethral environment and protect and feed the spermatozoa that are nearing the end of their journey through the male reproductive tract.
Seminal Vesicles
These accessory glands provide the majority of fluid in the total volume of semen (approximately 70 percent is produced here). This secretory material contains a fructose-rich fluid that will be used by the sperm to power their propulsion through the remainder of the male reproductive tract and freely swim through the female reproductive tract.
Prostate Gland
Situated around the first portion of the urethra, the prostate is about the size of a walnut. This gland secretes materials into the prostatic urethra that are primarily alkaline in nature and reduce the acidity of both the male and female reproductive tract to lengthen the viability of the sperm. In addition, the prostate contributes the bulk of the remaining volume of semen.
Bulbourethral Glands
Located at the base of the penis and around the membranous/spongy urethral boundary, the bulbourethral glands (Cowper’s glands) produce a lubricating fluid that is excreted at the initiation of an orgasm and precedes the semen out of the penis during an ejaculation.
External Genitalia
During embryonic and early fetal development, the gonads (ovaries and testes) and the external genitalia in males and females are indistinguishable. The same developmental tissue is used to make either male or female external genitalia. For females, the embryonic tissue remains open and does not fuse along the midline. However, in male development, under the control of male hormones, this embryonic tissue closes up into the scrotum and the penis.
Scrotum
As mentioned previously, this is the compartment of skin that encases the testicles and regulates the temperature of the male gonads. Along the midline of the scrotum, between the two testicles, is a raised line running the length of the scrotum called the scrotal raphe, which is the seam where the embryonic tissue fused together to form a sack (into which the testicles descend). Also during early development, the gonads begin to form in a similar manner in the same abdominal location. When the process of shaping the embryonic tissue into a testis is nearing completion, the testes descend through an inguinal opening and drop into the scrotum.
Penis
The male sex organ, the penis, consists of four tubes, wrapped within a tube or connective tissue surrounded by skin. Three of the inner tubes are spongy columns of tissue that can be rapidly filled with blood during arousal to generate an erection. The veins that drain these spongy columns of erectile tissue close during an erection and open following an ejaculation and orgasm. The two larger columns, which are arranged side by side on the dorsal aspect of the penis, are the corpus cavernosa. The smaller column is located just ventral to the junction of the two larger columns and will also contain the penile urethra.
The end of the penis is enlarged into the glans penis (head of the penis). At birth, this is covered by an extension of skin (foreskin) from the shaft of the penis. The penis can expand outward from this extension of skin during an erection.
What is circumcision?
Circumcision is the surgical removal of the flap of skin (foreskin) at the head of the penis, frequently done shortly after birth, for social, religious, or aesthetic reasons. Little evidence has been demonstrated to show a health benefit either way.
Male Sex Hormones
Male hormones that must shift the indifferent gonad and genitalia toward a male pathway and that must maintain that physiology are critical for normal male development and function. Early in embryonic development, testis-determining factor (TDF) hormone is produced that will initiate a cascade of molecular and cellular switches, resulting in male development.
One important function of TDF is to initiate the expression of testosterone by the interstitial cells of Leydig, which continue male sexual development. This hormone is used to maintain these activities in the adult. Low testosterone levels (Low T) result in energy loss, increased body fat deposition, and potential erectile dysfunction and/or infertility.
However, with the indifferent embryonic tissue, it isn’t sufficient to turn on the male genes. The tissue that would have been used to form the female reproductive tract must be inactivated. This is also a cellular event triggered by TDF. The cells that facilitate this female “off” signal are the Sertoli cells. These cells make a hormone that shuts off the process for female reproductive development. Considering how this “off” signal is essential, logic dictates that the default pathway for gender development in humans is femaleness (unless the Y chromosome is present to shut down this pathway and turn on the male developmental mechanisms).