Male Reproductive System: Primary Organs, Ducts, Glands, and Thermoregulation

Course Administration and Evaluations

• Spot Evaluations: Students are encouraged to fill out spot evaluations for the class. These are available online and the process begins next week.

• Logistics: An email regarding evaluations should be sent to students this morning.

• Anonymity: Evaluations are anonymous. The instructor does not see any results until after grades are posted. Even after posting, the instructor only sees raw data. Evaluation comments are only identifying if the student chooses to put their name in them.

Classification of Male Reproductive Organs

• Primary Sex Organs (Gonads): These are the testes, which are responsible for producing sperm cells and hormones.

• Secondary Sex Organs: These include the glands associated with the reproductive system and the ducts/tubes used for transport.

The Testes: Anatomy and Function

• Terminology: The term "testes" (singular: testis) is the formal anatomical name. The word "testicles" technically means "little testes."

• General Function:   - Production of sperm cells.   - Endocrine function: Production of hormones such as testosterone.

• Tunica Vaginalis: A serous membrane that covers or surrounds the testes. This lining creates a cavity that was originally continuous with the body cavity during development.

• Internal Structure:   - Modules: The testis is separated into segments or modules, similar to a segmented door entry.   - Seminiferous Tubules: Each module contains between one to several seminiferous tubules. These are long, highly twisted, and numerous. This is the specific site where sperm cells and interstitial cells are produced.

Spermatogenesis and Sperm Movement

• Spermatogonia: These are stem cells found at the outer edge of the seminiferous tubule.   - Cell Types: They produce an "A cell" and a "B cell."   - A Cell: Functions as a stem cell to stay at the edge and maintain the population.   - B Cell: Undergoes meiosis to eventually become four sperm cells.

• Timeline: This process is active from puberty until death.

• Movement of Immature Sperm: Immature sperm cells are pushed through the tubules by fluid pressure rather than their own motility. This fluid is produced by cells associated with the tubules.

The Duct System

• Efferent Ductules: Transport sperm from the seminiferous tubules toward the epididymis.

• Epididymis:   - A highly coiled, narrow tube that curves along the posterior side of the testis.   - Size: Although the testis is roughly the size of a grape, the epididymis is approximately $6.0\text{ meters}$ long when stretched out.   - Pathway: Sperm enter the tail region and move forward via fluid pressure.   - Maturation and Storage: This is the site for sperm maturation. Sperm are stored here for approximately $1.0$ to $3.0$ weeks. If not used, they are broken down and their components are reabsorbed.

• Ductus Deferens (Vas Deferens):   - The term "vas" means vessel, though this is a duct, not a blood vessel.   - Vasectomy: A surgical procedure where the ductus deferens is cut and both ends are closed off. Sperm cells are still made but are broken down since they cannot leave the body.   - Pathway: The duct moves from the scrotum, over the top of the pubic bone, and around behind the bladder.   - Anatomical Relationship: It loops over the ureter.   - Historical Development: The long, looped pathway (described by the instructor as going from Turlock to Ceres by way of Tracy) exists because the testes originally form in the abdomen and descend to the scrotum during development, pulling the duct with them.

• Ejaculatory Duct: Formed where the ductus deferens meets the duct of the seminal vesicle. This duct carries both sperm and glandular secretions.

• Urethra:   - Prostatic Urethra: The portion passing through the prostate gland.   - Membranous Urethra: The middle portion.   - Penile (Spongy) Urethra: The portion passing through the penis.

• Sperm Motility: Sperm cells do not become mobile (using their tails) until they reach the female reproductive tract. Until then, they are moved by fluid and muscular pressures.

Accessory Glands and Seminal Fluid

• Seminal Vesicles (Seminal Glands):   - Paired glands (one on each side) located behind the bladder.   - They produce approximately $2/3$ of the total volume of semen.   - Components:     - Fructose: A fruit sugar used by sperm mitochondria to produce $ATP$ for power.     - Prostaglandins: Chemicals that can cause muscle contractions in the female reproductive tract.     - Fibrinogen: A clotting factor that causes semen to clot after ejaculation.

• Prostate Gland:   - A single gland surrounding the prostatic urethra.   - Produces approximately $1/3$ of the semen volume.   - Components:     - Alkaline secretions: Help neutralize the slightly acidic environment of the female reproductive tract.     - Fibrinolysin and Prostate Specific Antigen (PSA): These help liquefy the clotted semen, allowing sperm cells to break free from the viscous fluid.

• Bulbourethral Glands:   - Small, paired glands located near the bulb of the penis in the floor of the pelvis.   - These glands are active during the excitation phase (erection).   - They secrete an alkaline mucus that neutralizes the acidic urethra and provides lubrication for the tube and for copulation.

The Scrotum and Thermoregulation

• Structure: A pouch of skin that is an extension of the abdominal wall, divided in half so each testis is in its own compartment.

• Musculature:   - Dartos Muscle: A thin layer of smooth muscle just under the skin. It wrinkles the skin to decrease surface area or relaxes to increase it, aiding in heat regulation.   - Cremaster Muscle: Deeper muscle continuous with the abdominal muscles. It raises the testes closer to the body (to warm them) or lowers them (to cool them) in response to temperature.

• Thermoregulation Requirements:   - Sperm production is temperature-sensitive and originated in ectotherms (cold-blooded animals).   - Internal body temperature is $37.0^{\circ}\text{C}$ ($98.6^{\circ}\text{F}$), which is too high for viable sperm and would lead to sterility.   - The ideal temperature for sperm production is approximately $35.0^{\circ}\text{C}$.

• Pampiniform Plexus: A network of veins surrounding the testicular artery.   - Countercurrent Heat Exchange: High-temperature ($37.0^{\circ}\text{C}$) blood in the artery loses heat to the cooler ($35.0^{\circ}\text{C}$) blood returning to the body through the veins. By the time arterial blood reaches the testis, it is cooled to $35.0^{\circ}\text{C}$.

Anatomy of the Penis

• External Parts:   - Shaft (Body): The visible length.   - Glans Penis: The expanded tip (apex).   - Prepuce (Foreskin): A layer of skin covering the glans. Circumcision is the removal of this skin.

• Internal Erectile Columns:   - Corpus Spongiosum: A single column that surrounds the urethra. It contains less vascular space than the other columns to prevent the urethra from being pinched shut during erection, which would block the passage of semen.   - Corpora Cavernosa: Two larger, paired columns of erectile tissue above the corpus spongiosum with large vascular spaces that fill with blood during excitation to cause erection.

• Anchoring Structures:   - Bulb of the Penis: The base of the corpus spongiosum.   - Crus (plural: Crura): The base of the corpora cavernosa, which anchors the penis to the connective tissue of the pubic bone. This anchoring is necessary for the functional use of the erect structure.

• The Baculum (Os Penis):   - A bone found within the penis of many mammal groups.   - While humans do not have a baculum, our closest relatives (chimpanzees and orangutans) have very small ones.   - Mnemonic for mammals with a baculum: "P.R.I.C.C." or "P.R.I.C.B."     - P: Primates (most, though not humans or those with monogamous/short copulation times).     - R: Rodents (rats, etc.).     - I: Insectivores (shrews, moles, voles).     - C: Carnivores (dogs, cats, bears, etc.).     - C (or B): Chiroptera (Bats).

• Note on Dogs: Dogs have a baculum and a specific clotting/plugging mechanism which, along with the bone, can cause them to become "stuck together" during mating.

Questions & Discussion

• Discussion on Circumcision: The instructor notes there is no strictly medical reason for circumcision. It became common in the United States in the $1940\text{s}$ as births moved to hospitals and due to a specific doctor's book which argued that circumcising both males and females would prevent masturbation later in life.

• Comparison with other mammals: In rodents, the clotting mechanism in semen forms a physical plug to prevent a second male's sperm from reaching the eggs, ensuring paternity. Humans lack this plug, but still require the clotting/liquefying cycle for sperm to break free from the seminal fluid effectively.