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The penis is the male organ of copulation (sexual intercourse). When erect, the stiffness of the organ allows it to penetrate into the vagina and deposit semen into the female reproductive tract.
The shaft of the penis surrounds the penile urethra ( [link] ). The shaft is composed of three column-like chambers of erectile tissue that span the length of the shaft. Each of the two larger lateral chambers is called a corpus cavernosum . Together, these make up the bulk of the penis. The corpus spongiosum surrounds the spongy, or penile, urethra. The end of the penis, called the glans penis , has a high concentration of nerve endings, resulting in very sensitive skin that influences the likelihood of ejaculation (see [link] ). The skin from the shaft extends down over the glans and forms a collar called the prepuce (or foreskin). The foreskin also contains a dense concentration of nerve endings, and both lubricate and protect the sensitive skin of the glans penis. A surgical procedure called circumcision , often performed for religious or social reasons, removes the prepuce, typically within days of birth.
Testosterone, an androgen, is a steroid hormone produced by Leydig cells or interstitial cells . In male embryos, testosterone is secreted by Leydig cells by the seventh week of development. This early release of testosterone results in the anatomical differentiation of the male sexual organs.
The continued presence of testosterone is necessary to keep the male reproductive system working properly. Maintaining these normal concentrations of testosterone promotes spermatogenesis, whereas low levels of testosterone can lead to infertility. In addition testosterone is also released into the blood circulation and plays an important role in muscle development, bone growth, the development of secondary sex characteristics, and maintaining libido (sex drive) in both males and females. In females, the ovaries secrete small amounts of testosterone.
The regulation of testosterone concentrations throughout the body is critical for male reproductive function. The regulation of production of testosterone begins outside of the testes. The hypothalamus and the pituitary gland in the brain control testosterone production. The hypothalamus releases a hormone called gonadotropin-releasing hormone (GnRH) . Binding of GnRH to its receptors on the anterior pituitary gland stimulates release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) . These two hormones are critical for reproductive function in both men and women. In men, FSH promotes spermatogenesis. LH binds to receptors on Leydig cells in the testes and influences the production of testosterone.
Gametes are the reproductive cells that combine to form offspring. Organs called gonads produce the gametes, along with the hormones that regulate human reproduction. The male gametes are called sperm. Spermatogenesis, the production of sperm, occurs within the seminiferous tubules that make up most of the testis. The scrotum is the muscular sac that holds the testes outside of the body cavity.
Spermatogenesis begins with mitotic division of spermatogonia (stem cells) to produce primary spermatocytes that undergo the two divisions of meiosis to become secondary spermatocytes, then the haploid spermatids. During spermiogenesis, spermatids are transformed into spermatozoa (formed sperm). Upon release from the seminiferous tubules, sperm are moved to the epididymis where they continue to mature. During ejaculation, sperm exit the epididymis through the ductus deferens, a duct in the spermatic cord that leaves the scrotum. The ampulla of the ductus deferens meets the seminal vesicle, a gland that contributes fructose and proteins, at the ejaculatory duct. The fluid continues through the prostatic urethra, where secretions from the prostate are added to form semen. These secretions help the sperm to travel through the urethra and into the female reproductive tract. Secretions from the bulbourethral glands protect sperm and cleanse and lubricate the penile (spongy) urethra.
The penis is the male organ of copulation. Columns of erectile tissue called the corpora cavernosa and corpus spongiosum fill with blood when sexual arousal activates vasodilatation in the blood vessels of the penis. Testosterone regulates and maintains the sex organs and sex drive, and induces the physical changes of puberty. Interplay between the testes and the endocrine system precisely control the production of testosterone with a negative feedback loop.
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