the penis system

Descent of the testes

Testes develop behind the parietal peritoneum near the kidneys and descend into the scrotum 1-2 months before birth under the influence of testosterone.

Gubernaculum

Fibrous cord that guides the testes through the inguinal canal during descent.

Spermatic cord

Contains the ductus deferens, blood vessels, lymphatic vessels, nerves, and connective tissue that support each testis.

Functions of the male internal accessory organs

Nurture, transport, and support sperm and produce the fluids that make up semen.

Male internal accessory organs

Epididymis, ductus (vas) deferens, ejaculatory ducts, seminal vesicles, prostate gland, bulbourethral glands, and urethra.

Epididymis

Long, tightly coiled tube on the superior surface of each testis where sperm mature and are stored before entering the ductus deferens.

Histology of the epididymis

Lined with pseudostratified columnar epithelium containing nonmotile stereocilia.

Functions of the epididymis

Promotes sperm maturation, stores sperm, and transports sperm to the ductus deferens.

Ductus (vas) deferens

Muscular tube approximately 45 cm long that transports sperm from the epididymis to the ejaculatory duct.

Histology of the ductus deferens

Lined with pseudostratified columnar epithelium and surrounded by thick smooth muscle for sperm transport.

Seminal vesicles

Paired glands attached to the ductus deferens near the urinary bladder that contribute most of the semen volume.

Secretions of the seminal vesicles

Alkaline fluid containing fructose and prostaglandins.

Functions of seminal vesicle secretions

Fructose provides energy for sperm, prostaglandins promote movement through the female reproductive tract, and alkaline fluid helps neutralize acidity in the male and female reproductive tracts.

Prostate gland

Single gland surrounding the proximal urethra just inferior to the urinary bladder.

Structure of the prostate gland

Composed of tubular glands embedded in connective tissue and smooth muscle.

Prostate gland secretion

Thin, milky, alkaline fluid that enhances sperm motility and contributes to semen volume.

Bulbourethral (Cowper's) glands

Small paired glands inferior to the prostate that secrete alkaline mucus before ejaculation.

Functions of bulbourethral gland secretions

Lubricate the urethra and neutralize acidic urine remaining in the urethra before ejaculation.

Semen

Fluid ejaculated through the urethra containing sperm and secretions from the seminal vesicles, prostate gland, and bulbourethral glands.

Components of semen

Sperm cells plus secretions from the seminal vesicles, prostate gland, and bulbourethral glands.

Male external reproductive organs

Scrotum and penis.

Scrotum

Skin-covered sac divided into two chambers, each containing one testis and one epididymis.

Dartos muscle

Smooth muscle within the scrotal wall that contracts or relaxes in response to temperature changes.

Function of the dartos muscle

Maintains the testes approximately 5°F (3°C) below body temperature for optimal sperm production and survival.

Scrotal septum

Internal partition that divides the scrotum into right and left chambers.

Penis

External organ that transports urine and semen through the urethra and becomes erect during sexual intercourse.

Erectile tissues of the penis

Two corpora cavernosa and one corpus spongiosum.

Corpora cavernosa

Paired erectile bodies primarily responsible for penile rigidity during erection.

Corpus spongiosum

Erectile tissue surrounding the urethra that prevents urethral compression during erection and expands distally to form the glans penis.

Glans penis

Distal enlargement of the corpus spongiosum that contains numerous sensory receptors.

Prepuce (foreskin)

Fold of skin covering the glans penis that may be removed during circumcision.

Erection

Parasympathetic stimulation causes nitric oxide release, dilation of penile arteries, compression of veins, increased blood filling of erectile tissue, and penile enlargement.

Nitric oxide (NO)

Neurotransmitter released during parasympathetic stimulation that relaxes smooth muscle and dilates penile arteries.

Orgasm

Pleasurable physiological and psychological climax accompanied by emission and ejaculation.

Emission

Movement of semen into the urethra.

Ejaculation

Forceful expulsion of semen from the urethra.

Nervous system control of ejaculation

Sympathetic nerve impulses trigger emission and ejaculation.

Spermatogenesis

Process of producing sperm cells within the seminiferous tubules of the testes.

Location of spermatogenesis

Occurs in the seminiferous tubules of the testes.

Seminiferous tubules

Coiled tubules within the testes where sperm are produced.

Sustentacular (Sertoli) cells

Large supporting cells that span the seminiferous tubules and nourish, protect, and regulate developing sperm cells.

Functions of Sertoli cells

Support and nourish developing sperm, form the blood-testis barrier, stimulate spermatogenesis in response to FSH and testosterone, secrete inhibin, and secrete androgen-binding protein (ABP).

Blood-testis barrier

Tight junctions between Sertoli cells that protect developing sperm from the immune system and help maintain the proper environment for spermatogenesis.

Androgen-binding protein (ABP)

Protein secreted by Sertoli cells that binds testosterone and maintains a high testosterone concentration within the seminiferous tubules to promote spermatogenesis.

Sequence of spermatogenesis

Spermatogonia → Primary spermatocytes → Secondary spermatocytes → Spermatids → Spermatozoa.

Spermatogonia

Diploid stem cells that divide by mitosis to replenish themselves and produce primary spermatocytes.

Primary spermatocytes

Diploid cells that undergo Meiosis I to produce secondary spermatocytes.

Secondary spermatocytes

Haploid cells produced by Meiosis I that undergo Meiosis II to form spermatids.

Spermatids

Haploid immature sperm cells that develop into spermatozoa during spermiogenesis.

Spermatozoa

Mature sperm cells capable of fertilizing an egg.

Spermiogenesis

Process by which spermatids differentiate into mature spermatozoa without further cell division.

Chromosome number in mature sperm

23 chromosomes (haploid).

Purpose of meiosis in spermatogenesis

Reduces the chromosome number from 46 to 23 so fertilization restores the normal diploid number.

Migration of developing sperm

Developing sperm move from the outer edge of the seminiferous tubule toward the lumen as they mature.

Hypothalamic hormone regulating reproduction

Gonadotropin-releasing hormone (GnRH).

GnRH (Gonadotropin-releasing hormone)

Hormone released by the hypothalamus that stimulates the anterior pituitary to secrete LH and FSH.

Anterior pituitary gonadotropins

Luteinizing hormone (LH/ICSH) and follicle-stimulating hormone (FSH).

Luteinizing hormone (LH/ICSH)

Stimulates interstitial (Leydig) cells of the testes to produce testosterone.

Interstitial (Leydig) cells

Cells located between the seminiferous tubules that produce testosterone in response to LH.

Follicle-stimulating hormone (FSH)

Stimulates Sertoli cells to support spermatogenesis and respond to testosterone.

Requirements for spermatogenesis

Normal spermatogenesis requires both FSH and testosterone acting on Sertoli cells.

Inhibin

Hormone secreted by Sertoli cells that inhibits FSH secretion by the anterior pituitary.

Negative feedback of testosterone

Testosterone inhibits secretion of GnRH from the hypothalamus and LH from the anterior pituitary.

Negative feedback of inhibin

Inhibin decreases FSH secretion from the anterior pituitary to regulate sperm production.

Androgens

Group of male sex hormones responsible for male reproductive development and secondary sex characteristics.

Testosterone

Primary androgen produced mainly by interstitial (Leydig) cells of the testes.

Site of testosterone production

Interstitial (Leydig) cells of the testes; small amounts are produced by the adrenal cortex.

Testosterone secretion during life

Begins during fetal development, decreases after birth, remains low during childhood, rises sharply at puberty, and continues throughout adult life.

Functions of testosterone

Stimulates development of male reproductive organs, supports spermatogenesis, produces male secondary sex characteristics, increases muscle and bone growth, and maintains libido.

Male secondary sex characteristics

Deepened voice, facial and body hair growth, increased muscle mass, increased bone growth, broader shoulders, and thicker skin.

Hormonal control of spermatogenesis

GnRH stimulates LH and FSH release; LH stimulates Leydig cells to produce testosterone; FSH and testosterone stimulate Sertoli cells to support spermatogenesis; testosterone and inhibin provide negative feedback.

Pathway of sperm

Testis (seminiferous tubules) → Epididymis → Ductus deferens → Ejaculatory duct → Urethra → Outside the body.

Accessory gland secretions added to sperm

Seminal vesicle fluid is added first, followed by prostatic fluid, then bulbourethral gland secretions before ejaculation.