Lecture 21
Male Reproductive System
· Testes
o Male gonads – combined endocrine and exocrine glands that produce sex hormones and sperm
o Oval-shaped and slightly flattened
o Divided into 200 to 300 wedge-shaped lobules
o Each lobule contains one to three seminiferous tubules
§ Seminiferous tubules are slender ducts up to 1 meter long in which sperm are produced
§ Between the seminiferous tubules are clusters of interstitial cells, the source of testosterone
o The epithelium of the seminiferous tubules consists of:
§ several layers of germ cells in the process of becoming sperm
§ a smaller number of sustentacular cells which protect the germ cells and promote their development
· germ cells depend on sustentacular cells for nutrients, waste removal, growth factors, and other needs
· the rate of sperm production depends on a hormone called inhibin, secreted by sustentacular cells
· Scrotum
o The pouch in which the testes are contained is the scrotum
o It is separated into left and right compartments by an internal median septum
§ It prevents infection of one of the testes from spreading to the other
o The spermatic cord is a cord of connective tissue that passes upward behind the testis, across the anterior side of the pubis, and through an opening called the inguinal ring in the muscles of the groin
§ It contains the ductus deferens (vas deferens), blood and lymphatic vessels, and the testicular nerves
o The testes must be held at 35 degrees Celsius to produce viable sperm
§ The core body temperature is 37, so the scrotum holds the testes away from the core of the body
§ There are three mechanisms for regulating the temperature of the testes
· The cremaster muscle contracts and draws the testes closer if external temperatures are cold
· The dartos muscle contracts when cold, making the scrotum taught and wrinkled, and reducing surface area
· The pampiniform plexus is an extensive network of veins.
o They help to prevent the testes from warming as blood is carried into the testes by the testicular arteries
· Spermatogenesis and Sperm
o Spermatogenesis is the process of sperm production
§ Three events:
· Remodeling a relatively large germ cell into a small, mobile cell with a flagellum
· Reduction of the chromosome number by ½ so that when an egg cell and sperm cell combine they do not get a doubling of the chromosome number with each generation
· A shuffling of the genes so that each chromosome of the sperm carries new gene combinations that did not exist in the chromosomes received from one’s parents
· The reduction in chromosome number and shuffling of genes occur as a result of meiosis
o Meiosis produces four haploid daughter cells that subsequently differentiate into sperm
o Spermatozoon
§ Two parts:
· Head
o Contains 3 structures:
§ Nucleus – fills most of the head and contains a haploid set of condensed, genetically inactive chromosomes
§ Acrosome – lysome in the form of a thin cap covering the apical half of the nucleus
· It contains enzymes that are later used to penetrate the egg if the sperm is successful
§ Basal body of the flagellum – nestled in an indentation at the posterior end of the nucleus
· Tail
o Divided into 3 regions:
§ Midpiece – a cylinder about 5 to 9 micrometers long and half as wide as the head
· Contains numerous mitochondria that produce ATP needed for the beating of the tail
§ Principal piece – 40 to 45 micrometers long and constitutes most of the tail
§ Endpiece - 4 to 5 micrometers long
· Spermatic Ducts
o After leaving the testes, the sperm travel through a series of spermatic ducts
§ Efferent ductules – small tubes that carry sperm to the epididymis
§ Epididymis – site of sperm maturation and storage
· Adheres to the posterior side of the testis
· Reabsorbs about 90% of the fluid secreted by the testis
· Sperm are physiologically immature when they leave the testis, but mature as they travel through the epididymis
· Sperm are stored here and are reabsorbed if they become to old without being ejaculated
§ Ductus (vas) deferens – This tube carries sperm from the epididymis upward into the pelvic cavity, where it turns medially and approaches the urinary bladder
· Where it passes between the bladder and ureter, it turns downward behind the bladder and widens into a terminal ampulla
§ Ejaculatory duct – This tube runs from the ampulla through the prostate gland and empties into the urethra
· Accessory Glands
o Seminal vesicles
§ These are a pair of glands dorsal to the urinary bladder
§ One is associated with each ductus deferens
§ The duct empties into the ejaculatory duct
§ The secretion constitutes about 60% of the semen
o Prostate
§ This is a structure immediately below the urinary bladder
§ It surrounds the urethra and ejaculatory duct
§ The milky secretion contributes about 30% of the semen
o Bulbourethral gland
§ These are brownish, spherical glands about 1 cm in diameter
§ They are found at the inner end of the penis
§ During sexual arousal, they produce a clear slippery fluid that lubricates the head of the penis in preparation for coitus
§ It neutralizes the acidity of residual urine in the urethra
· Semen
o This is a complex mixture of sperm and glandular secretions
o A typical ejaculation discharges 2 to 5 m of semen
§ 10% consists of sperm and fluid from the spermatic ducts
§ 30% is from the prostate
§ 60% is from the seminal vesicles
§ sperm count is normally 50 to 120 million sperm/mL
o Ejaculation:
§ Peristalsis of the ductus deferens drives sperm from the epididymis to the ampulla
§ Contraction of the ampulla then discharges the sperm into the prostatic urethra, where they mix the secretions with the accessory glands
§ Semen in the urethra triggers a reflex contraction of the bulbospongiosus muscle which ensheaths the root of the penis
· Rhythmic compression of the urethra by this muscle expels the semen from the penis
· Most of the sperm are contained in the first 1 mL of ejaculate, followed by prostatic fluid and seminal fluid
· Penis
o The function of the penis is to deposit semen in the vagina
o About half is the internal root
o The other half is externally visible, including shaft and glans
§ The glans is the expanded head at the distal end of the penis
§ The prepuce, or foreskin, is skin that is loosely attached to the skin continues over the glans
§ The glans and prepuce have sebaceous glands that produce a secretion called smegma
o The penis consists of three cylindrical bodies called erectile tissues
§ These fill with blood during sexual arousal
§ They cause enlargement of the penis and an erection
§ Corpus spongiosum – single erectile body that encloses the urethra in the penis
· It passes along the ventral side of the penis
· It expands at the distal end to fill the entire glans
§ Corpus cavernosum – Pair of erectile bodies on each side of the dorsal surface of the penis
§ All three cylinders are spongy in appearance and contain blood sinuses called lacunae
o The penis receives blood from a pair of internal pudental arteries
§ The dorsal artery supplies blood to the skin, fascia, and corpus spongiosum
§ The deep artery travels through the core of the corpus cavernosum
§ When the deep arteries dilate, the lacunae fills with blood and the erectile tissues swell.
§ When the penis is flaccid, most of its blood comes from the dorsal arteries
§ The deep dorsal vein is the blood vessel that drains blood from the penis
o Nerves
§ The penis is richly innervated by sensory and motor nerve fibers
§ Parasympathetic fibers extend from S2 to S4 of the spinal cord and are involved in an autonomic reflex arc that causes erection in response to direct stimulation of the penis
§ Sympathetic fibers from T12 to L2 dilate penile arteries and can induce erection in response to input from special senses and sexual thoughts