Basic knowledge of male reproductive anatomy and physiology for men's health

Anatomy and Physiology of Male Genitals

The male genitalia are divided into internal and external genitalia. Internal genitalia include the testes, vas deferens, and accessory glands. External genitalia include the scrotum and penis.

1. Penis

(1) Anatomy: The penis is divided into three parts: the glans, the body, and the root. The glans is relatively large, with the sagittal urethral orifice at its anterior end. The narrowing at the junction of the glans and the body is called the coronal sulcus. The penis is mainly composed of two corpora cavernosa and one corpus spongiosum, covered by fascia and skin. Each corpus cavernosum is covered by a thick fibrous membrane called the tunica albuginea. The interior of the corpora cavernosa is a spongy structure composed of connective tissue, elastic fibers, and smooth muscle, containing many cavities that directly connect to blood vessels; penile erection occurs when these cavities are filled with blood. The skin of the penis is thin, soft, and highly elastic. It folds forward and then inward and backward along the glans penis, forming a double-layered, ring-shaped fold called the foreskin. The frenulum connects the foreskin to the urethral opening on the ventral midline of the glans penis. In adults, if the foreskin covers the urethral opening, it is called phimosis; if the foreskin is too long and cannot be retracted, it is called paraphimosis.

The average length of a normal adult penis is 6.55 cm, with a minimum of 3.7 cm and a maximum of 10.6 cm. Penile length is not related to height. When the penis is erect, both its length and circumference can approximately double. In a flaccid state, smaller penises tend to increase in size more significantly upon erection, while larger penises tend to increase in size less significantly.

The tunica albuginea of ​​the penis, from superficial to deep, consists of: the superficial perineal fascia (superficial penile fascia), containing the superficial dorsal penile artery and vein; the deep penile fascia, thick and elastic, covering all the corpora cavernosa, gradually thinning towards the glans penis, through which the deep dorsal penile artery and vein, and the dorsal penile nerve pass; and the tunica albuginea, which surrounds each corpus cavernosum and forms a septum between the two corpora cavernosa. The deep penile arteries pass through each of the two corpora cavernosa. The penis is primarily supplied with blood by the dorsal penile artery and the deep penile artery.

The sensory nerves of the penis are mainly the dorsal penile nerve, which innervates the skin, foreskin, and frenulum of the penis. Therefore, during circumcision, a nerve block anesthesia is administered at the base of the penis. The motor nerves of the penis originate from the hypogastric plexus and the 2nd and 3rd sacral nerves, accompanying the arteries into the corpora cavernosa. They primarily regulate erectile function and are therefore the erectile nerves of the penis. Damage to these nerves can cause erectile dysfunction.

The penile lymphatic system is divided into superficial and deep groups. Superficial lymphatic vessels collect lymph from the foreskin, penile skin, and subcutaneous tissue, forming a main dorsal trunk that runs along the superficial dorsal vein of the penis to the inguinal lymph nodes on both sides. Trauma or cold stimulation can cause inflammation of the superficial dorsal lymphatic vessels of the penis. Deep lymphatic vessels collect lymph from the glans penis and corpora cavernosa, running along the deep dorsal vein of the penis to the deep inguinal lymph nodes on both sides, and then through the femoral canal to the external iliac lymph nodes.

(2) Physiology: The main physiological function of the penis is sexual intercourse. The completion of sexual intercourse requires a full erection of the penis. Penile erection is the result of blood engorgement of the corpora cavernosa and is also a major indicator of male sexual arousal. It is known that the corpora cavernosa are vascular sinuses composed of many smooth muscles. Normally, these vascular sinuses are basically closed. When a sexual nerve impulse is received, the smooth muscles contract, hindering the outflow of blood into the vascular sinuses, resulting in high blood engorgement and erection. The erection of the blood vessels is controlled by nerve impulses. The speed of erection initiation, the degree of erection, and the duration of erection are related to the body's functional state, androgen levels, as well as mental and psychological factors, thus exhibiting certain differences between individuals.

2. Scrotum

(1) Anatomy: The scrotum is located below and behind the penis, and is a skin sac. The scrotum is actually a continuation of the abdominal wall, and its tissue layers are consistent with those of the abdominal wall. From the outside in, the layers are: skin, dartos fascia, superficial perineal fascia, external spermatic cord fascia, cremaster muscle, internal spermatic cord fascia, and tunica vaginalis. The inner scrotum lining forms the scrotal septum, dividing the scrotum into two chambers, each containing one testis.

The scrotal skin is thinner, softer, and more delicate than normal skin, with darker local pigmentation and good elasticity. Many sweat glands, sebaceous glands, and pubic hairs are distributed within the scrotal skin. The septum is prominently protruding, forming the midline of the scrotum.

The lower layer of the skin is the dartos fascia, composed of smooth muscle fibers, connective tissue, and elastic fibers, richly containing blood vessels. These muscle fibers can contract and relax in response to changes in external temperature to regulate the temperature within the scrotum. Below the external spermatic cord fascia is the tunica vaginalis, which consists of a thin layer of striated muscle tissue and loose elastic tissue. Its main function is to suspend the testis and protect it from trauma and cold. In fact, the tunica vaginalis is a continuation of the processus vaginalis of the peritoneum. Normally, the processus vaginalis serves as the passage for testicular descent and should close after the testis descends into the scrotum during fetal development. However, if its closure is abnormal and communicates with the abdominal cavity, or due to certain diseases, hydrocele can occur. The scrotum has a rich blood supply, mainly from the external pudendal artery, the posterior scrotal artery, and the external spermatic artery. The veins of the scrotum run parallel to the arteries, draining into the internal pudendal vein and the dorsal penile vein. Lymph from the scrotum drains into the inguinal lymph nodes.

(2) Physiology: The scrotum is the outer protective layer of the testes, and its main function is temperature regulation. When it's cold or the skin is stimulated by external factors, the scrotal wall contracts and rises to protect the internal temperature of the testes. When it's hot, the scrotum relaxes, and the sweat glands secrete large amounts of sweat to dissipate heat, keeping the temperature of the testes constant at 1.5-2 degrees Celsius lower than the abdominal cavity. This ensures the normal spermatogenic function of the testes.

3. Testes

(1) Anatomy: Shape and size of the testes: The testes are located in the scrotum, one on each side. In adults, each testis weighs 10-15 grams, is 4-5 cm long, and 3-4 cm thick. The normal testicular volume in adult males in my country is 15-25 ml.

The testis is a slightly flattened oval body with a smooth surface, divided into inner and outer surfaces, anterior and posterior margins, and upper and lower ends. The anterior margin is free, while the posterior margin has blood vessels, nerves, and lymphatic vessels entering and exiting, and is in contact with the epididymis and the lower segment of the vas deferens (testicular part). The testes grow rapidly with sexual maturity, but shrink and become smaller in older men as their sexual function declines.

Structure of the testes: The surface of the testes is covered by a thick fibrous membrane called the tunica albuginea. Along the posterior margin of the testis, the tunica albuginea thickens and protrudes into the testis to form the mediastinum. From the mediastinum, numerous connective tissue septa emerge, dividing the testicular parenchyma into many lobules. Each lobule contains coiled seminiferous tubules, the epithelium of which produces sperm. The connective tissue between the tubules contains cells that secrete sex hormones, called interstitial cells. The seminiferous tubules combine to form straight tubules, which enter the mediastinum and interweave to form the reticulum testis. From the reticulum testis, 12-15 efferent ductules emerge from the upper part of the posterior margin of the testis and enter the epididymis.

(2) Physiology: Sperm production: The epithelium of the seminiferous tubules in the testis is the base for sperm production. The diameter of seminiferous tubules in adults is 150-250 micrometers, and each tubule is 30-70 centimeters long. If the lengths of all seminiferous tubules in the testes were added together, the total length would reach 250 meters. The epithelium of the seminiferous tubules is composed of spermatogonia and Sertoli cells. Beginning of puberty, the testes are stimulated by gonadotropins from the pituitary gland, causing spermatogonia to proliferate and develop into sperm.

Spermatogenesis can be divided into three stages. Stage 1 is the spermatogonial proliferation stage. After about five mitotic divisions, a portion of a spermatogonium develops into a primary spermatocyte; the other portion remains as a stem cell and continues to undergo mitosis. Stage 2 is the spermatocyte meiosis stage. During this stage, spermatocytes evolve into spermatids, undergoing two cell divisions. The first division forms two secondary spermatocytes, and the second maturation division forms two spermatids. The number of chromosomes in these spermatids is reduced by half, resulting in hemiploidy. The third stage is the metamorphosis of spermatocytes into spermatids. Spermatocytes no longer divide; they simply evolve into sperm. At this stage, the sperm is a specialized cell, divided into a head and a tail. The head participates in fertilization, while the tail provides whip-like motility. The development from a spermatogonium to a mature sperm constitutes one reproductive cycle, taking 64-74 days. One spermatogonium, through this reproductive cell cycle, produces approximately one hundred spermatids. An adult produces about 10 million spermatids per day per gram of testicular cells.

Secretion of male hormones: The interstitial cells between the seminiferous tubules in the testes secrete male hormones, the most important of which is testosterone. During embryonic development, testosterone promotes the differentiation of the external genitalia primordia into males; during puberty, it promotes the maturation of sex organs and the appearance of secondary sexual characteristics; in adulthood, it maintains the normal function of reproductive organs and sexual characteristics.