Exercise 10: Reproductive System (10.2, 10.3, 10.4, 10.5)
Scrotum
The testes (singular: testis), the gamete-producing organs of the male, are situated outside the body in a sac known as the scrotum.
The scrotum is composed of skin, connective tissue, and a subcutaneous layer of smooth muscle called the dartos muscle.
The dartos muscle contracts when it is cold, thus the scrotum becomes taut and wrinkled.
They are located externally because sperm production requires a temperature of about 34°C (about 94°F), which is lower than body temperature.
Along the midline, the scrotum has a connective tissue septum that divides it into 2 chambers, one for each testis.
Spermatic Cord
The layer of connective tissue deep to the scrotum extends up into the pelvic cavity to form a structure known as the spermatic cord.
The spermatic cord passes into the pelvic cavity through the small 4-cm long passageway called the inguinal canal, which has 2 openings on each end:
External inguinal ring: is the inferior opening from the scrotum into the inguinal canal.
Internal inguinal ring: is the superior opening from the inguinal canal into the pelvic cavity.
Notice that the spermatic cord surrounds the:
Cremaster muscle: a smooth muscle tissue that's meshed with the spermatic cord. This muscle tissue contracts to control the testes’ distance from the body and, therefore, their temperature. Do not confuse this with the dartos muscle which is located subcutaneously in the scrotum.
Testicular artery: supplies oxygenated blood to the testes. Arises from the abdominal aorta.
Pampiniform venous plexus: A group of tendril-shaped veins that surrounds the testicular artery in the spermatic cord. Aside from draining deoxygenated blood from the testes:
It also acts as a countercurrent heat exchanger, which means the warm blood flowing from the testicular artery loses some of its heat from the cooler blood traveling the opposite direction via this plexus.
By the time arterial blood reaches the testes, it is 1.5 to 2.5oC cooler than the core body temp.
Without this plexus, warm arterial blood would heat the testes and inhibit sperm production.
Once these veins pass the inguinal canal back into the pelvic cavity they converge to form the testicular vein.
The right testicular vein drains into the inferior vena cava
The left testicular vein drains into the left renal vein
A duct that transports sperm is known as the ductus deferens/vas deferens.
Testicular nerves are mixed sensory and motor nerves that innervates the testes.
Lymphatic vessels for draining lymph.
Deep to the cremaster muscle we find 2 more sheaths:
The superficial tunica vaginalis, a serous membrane, which produces serous fluid.
The deep tunica albuginea, a connective tissue that divides the interior of each testis into 250-300 wedge-shaped lobules.
Each lobule contains about 1-3 tightly coiled seminiferous tubule where spermatogenesis or the formation of sperm cells, takes place.
Spermatic Ducts
The seminiferous tubules converge in a network near the superior part of the testis to form a structure called the rete testis, a place where sperm partially matures in.
The rete testis exits the testis to carry sperm cells to the first segment of the duct system of the male reproductive tract, called the efferent ductules, which have clusters of ciliated cells that help drive the sperm cells along.
About 12 small efferent ductules then carry sperm cells to the epididymis (plural: epididymides), which is a site of sperm maturation and storage.
There are 3 portions of the epididymis:
The initial club-like head, the long middle body, and the final slender tail.
Immature sperm produced by the seminiferous tubules migrate through each of these regions of the epididymis to finish their maturation, after which they move to a long muscular tube called the ductus deferens/vas deferens.
Stored sperm cells remain fertile for 40-60 days, but if they become too old without being ejaculated, they disintegrate and the epididymis reabsorbs them, along with the 90% of fluid secreted by the testis.
Notice that the ductus deferens ascends through the pelvic cavity and curls posteriorly and superiorly around the urinary bladder.
On the posterior surface of the urinary bladder, the ductus deferens widens to form its terminal portion, called the ampulla of ductus deferens.
Vasectomy, the surgical method of male contraception, consists of cutting out a short portion of the ductus deferens to interrupt the passage of sperm cells.
The duct leaving the ampulla merges with a duct leaving a gland called the seminal vesicle.
Together, the merged ducts form a short passage known as the ejaculatory duct.
This duct passes through a gland on the inferior surface of the urinary bladder—the prostate gland (be sure not to call it the “prostrate”).
Here its contents drain into the first segment of the urethra, the prostatic urethra.
Recall from the urinary unit that the prostatic urethra becomes the membranous urethra as it exits the prostate and is located by the pelvic diaphragm, it then becomes the spongy urethra as it enters the corpus spongiosum of the penis. The urethra terminates at the tip of the penis at the external urethral orifice.
Accessory Exocrine Glands
The male reproductive tract includes 3 exocrine glands:
Seminal vesicles: paired glands posterior to the urinary bladder. It empties into the ejaculatory duct. Associated with each ductus deferens.
Prostate gland: surrounds the prostatic urethra and ejaculatory duct. It empties directly into the prostatic urethra.
The seminal vesicles and the prostate gland together produce about 90 percent of the volume of semen, a thin milky secretion that contains chemicals to nourish and activate sperm.
Bulbourethral glands: a set of 2 small brownish spherical glands located at the root of the penis, that empties into the membranous urethra. The bulbourethral glands produce:
An alkaline secretion released prior to the release of sperm during ejaculation, which protects the sperm by neutralizing the acidity of residual urine in the urethra.
A clear slippery fluid that lubricates the head of the penis in preparation for intercourse.
Penis
The penis is the male copulatory organ.
Half of the penis is located internally and the other half is externally visible:
Internal: root of penis
Externally visible: shaft or body of penis and glans penis
It is composed of 3 erectile bodies consist of vascular spaces that fill with blood during an erection:
The single and ventral corpus spongiosum
It surrounds the spongy urethra and enlarges distally to form the glans penis.
The paired and dorsal corpora cavernosa (singular: corpus cavernosum)
The skin is loosely attached to the penile shaft, allowing for expansion during erection. It continues over the glans as the prepuce or foreskin, which is often removed by circumcision.
Spermatogenesis is the process of developing spermatozoa that occurs within the seminiferous tubules in the male testes.
It begins for the first time in puberty and continues throughout adulthood.
The process occurs within the walls of the seminiferous tubules in the testes and takes about 2 to 2.5 months to complete.
It begins near the outer portion of the seminiferous tubules, proceeds toward the center, and finally ends when the sperm cells are released into the lumen of the seminiferous tubules.
It takes about 70 days for a spermatogonium to become mature spermatozoa.
A young man produces ab out 300,000 sperm per minute, or 400 million per day.
One of the hormones that regulates spermatogenesis is testosterone.
It is secreted by interstitial cells (cells of Leydig), located between the seminiferous tubules.
Sustentacular cells (Sertoli cells) compose the walls of the seminiferous tubules. Their functions include:
Physically support and nourish the developing sperm cells (DSCs)
Control the movement of the DSCs
Remove wastes
Produce the fluid that fills the lumen of the seminiferous tubules
Each sustentacular cell is joined tightly to another by tight junctions like rivets that anchor cell membranes together.
This tight seal helps maintain the blood-testis barrier, which has 2 specific functions:
It protects developing sperm cells from potentially harmful substances by preventing proteins and other large molecules in the blood from coming in direct contact with developing sperm cells.
It prevents the immune system from being exposed to sperm cell antigens not found on any other body cells. If this were to occur, the immune system would view sperm cells mistakenly as foreign and respond by making antisperm antibodies in an effort to destroy the sperm cells.
Stages of Spermatogenesis
This flowchart shows the stages in spermatogenesis:
Spermatogonium (SG)
→ primary spermatocyte (PS)
→ secondary spermatocyte (SS)
→ early spermatids (EST)
→ late spermatids (LST)
→ spermatozoa or sperm cells (S)
The spermatogonium (plural: spermatogonia)is a stem cell found in the outer wall of the seminiferous tubules within the testes.
It undergoes cell division to produce 2 new copies of itself.
One of these new cells remains in the outer wall as a new spermatogonium
The other cell differentiates into a slightly larger cell called a primary spermatocyte.
This cell contains the total number or diploid (2n) number of chromosomes found in all human body cells: 46.
The primary spermatocyte enters a special process called meiosis, in which a cell undergoes 2 cell divisions to produce 4 final cells, each containing half the number of chromosomes as the original cell.
This half-set of chromosomes is called the haploid (n) number.
The only purpose of meiosis is to produce gametes (sperm, ova).
Because the total chromosome number is reduced by half, meiosis is called “reduction division.”
The primary spermatocyte divides to produce 2 secondary spermatocytes.
Each of these cells contains a set of 23 replicated chromosomes.
Each of the 2 secondary spermatocytes then divides to produce 2 new spermatids.
In total, 4 spermatids are produced, each containing the haploid (n) number, or 23 chromosomes.
Last, the spermatids develop into spermatozoa (singular: spermatozoon) via spermiogenesis.
Spermiogenesis
Spermiogenesis is the last part of the spermatogenesis process, in which a spermatid is transformed into a spermatozoa. Highlights of this metamorphosis are:
Acrosome formation: A sac covers the head of the sperm cell and contains the digestive enzymes used to penetrate the outer portion of the ova in the fertilization process. Acrosome is a lysosome in the form of a thin cap covering the apical half of the nucleus.
Note: A lysosome is a membrane covered organelle that contains enzymes.
Mitochondrial reproduction: The mitochondria replicate themselves and cluster to form a tightly coiled spiral around the neck of the sperm cell. They will be used to supply large amounts of adenosine triphospate (ATP) to propel the sperm cell by moving its flagellum.
Flagellum formation: The centrioles form the microtubules/axoneme that make up the flagellum (plural: flagella).
Sperm Cell
The spermatozoon or sperm cell has 3 main parts:
Head: has a nucleus that contains the haploid (n) number of chromosomes (23) and acrosome.
Midpiece: contains many mitochondria that produce large amounts of ATP to fuel the whipping of the tail.
Tail or flagellum: contains microtubules/axoneme and is used to propel the sperm cell.
Gametogenesis is the process of producing sperm cells by the testes (spermatogenesis) and oocytes by the ovaries (oogenesis).
Spermatogenesis, begins with stem cells located at the outer edge of the seminiferous tubules along the basement membrane called spermatogonia.
Before puberty, these cells undergo repeated rounds of mitosis to increase their numbers.
As puberty begins, some spermatogonia divide into 2 different cells:
one cell that remains in the basement membrane as a spermatogonium
another cell that becomes a diploid primary spermatocyte.
The primary spermatocyte then undergoes meiosis I and divides into 2 haploid secondary spermatocytes that migrate closer to the lumen of the tubule.
The two secondary spermatocytes undergo meiosis II and give rise to 2 haploid spermatids.
A spermatid divides no further, but undergoes the transformation of spermiogenesis, in which it differentiates into a single haploid spermatozoon.
Note that most of the stages of spermatogenesis are identifiable on a microscope slide of the seminiferous tubules.
On the inner edge of the tubule near the lumen are small, round cells with little cytoplasm. These are the spermatids.
As we move to deeper layers of the tubule wall, we can see primary spermatocytes, and on the outer edge we can see the cuboidal spermatogonia.
In between the tubules are small clusters of cells called interstitial (or Leydig) cells.
These cells make testosterone, a hormone required for spermatogenesis to take place.
The spermatozoa then move to the epididymis to mature into functional gametes and to complete spermiogenesis.
Notice that the mucosal lining of the epididymis is pseudostratified columnar epithelium with long projections called stereocilia.
These projections are nonmotile microvilli that help the sperm cells to:
complete spermiogenesis
absorb excess fluid
pass nutrients to the developing cells
Notice also the smooth muscle cells lining the outside of the epididymis tubule, which function to propel the sperm cells along as they mature.
By the time the cells reach the end of the tail of epididymis, they are mature sperm cells (or spermatozoa) that contain 3 parts:
The head, in which the DNA resides
The midpiece, which contains mitochondria
The flagellum/tail, which contains the axoneme/microtubules
As mentioned before, spermatogenesis: the development of sperm cells, occurs within the seminiferous tubules in the male testes. This process begins at puberty and continues throughout adult life. The group of hormones that regulates this process is the same basic set that regulates the ovarian cycle in the female.
It all begins when the hypothalamus in the brain releases into the blood a peptide called gonadotropin-releasing hormone (GnRH).
In adult males, GnRH is released in regular bursts every 60–90 minutes.
It travels through the blood to target cells within the anterior lobe (adenohypophysis) of the pituitary gland, which has receptors for GnRH.
The binding of GnRH induces a response in these cells to make 2 gonadotropin hormones:
Follicle-stimulating hormone (FSH)
Luteinizing hormone (LH)
Like any hormone, these chemical messengers are released into the bloodstream and travel to their respective targets, which contain receptors for these hormones.
In the male reproductive system:
FSH targets the sustentacular cells within the walls of the seminiferous tubules in the testes. Remember, sustentacular cells support and nourish the spermatogenic cells.
The binding of FSH also induces these cells to secrete 2 proteins:
Androgen-binding protein (ABP)
Inhibin
At the same time, LH binds to receptors on the interstitial cells, located between the seminiferous tubules.
After binding, LH induces these cells to produce the hormone testosterone, which has many different targets.
It is responsible for producing the secondary sex characteristics in the male (deepening of voice, enlargement of the genitals, growth of body hair, among other things).
In this case, the testosterone binds to the protein ABP to form a complex.
Once formed, this complex induces spermatogenic cells to develop into sperm cells.
The newly developed sperm cells are released into the lumen of the seminiferous tubule, where they travel through the male duct system until they are released from the body through the external urethral orifice of the penis.
Negative feedback is used to shut down the pituitary gland.
As the levels of testosterone rise, they inhibit the hypothalamus and the pituitary gland.
The result is that production of GnRH ceases, w/c leads to no production of either FSH or LH.
This inhibition is enhanced by production of the hormone inhibin by the sustentacular cells.
It has the same effect on the hypothalamus and pituitary gland as did testosterone.
Once the testosterone and inhibin levels fall, inhibition is lost, and the cycle begins again. In the adult male, this cycle results in a steady level of testosterone production that fluctuates within a normal range.