Chapter 27: Male Reproductive System Learning Outcomes
Contrast meiosis with mitosis.
1. Mitosis: a body cell doubles its DNA and then divides to produce two genetically identical daughter cells -Basis for division of single-cell fertilized egg, growth of an embryo, all postnatal growth, and tissue repair -Consists of four stages: prophase, metaphase, anaphase, telophase •DNA duplicated in interphase but sister chromatids remain attached at centromere •Prophase-Chromatin condenses to chromosomes •Metaphase-Chromosomes line up on metaphase plate •Anaphase-Sister chromatid separate at centromere •Telophase-Followed by cytokinesis Mitosis: - somatic cell division - produces 2 daughter cells genetically identical to the parent cell - diploid daughter cells - no crossing over 2. Meiosis produces four gametes (haploid cells), each with only half the DNA of the diploid body cells -Combining male and female gametes with half the genetic material produces an embryo with the same number of chromosomes as each of the parents •Prophase I •Metaphase I -Homologues chromosomes line up -Tetrads -Crossing-Over •Anaphase I •Telophase I -Homologs separate •Prophase II •Metaphase II •Anaphase II -Sister chromatids separate at centromere •Telophase II Meiosis: - sex cell division - produces 4 daughter cells that are genetically different from the parent cell - haploid daughter cells - includes crossing over
Describe the blood and nerve supply to the penis.
Blood Supply: Each internal iliac artery gives way to an internal prudendal artery. Internal Prudendal (Penile) Artery- enters the root of the penis and divides in two. One branch: Dorsal Artery- travels dorsally along the penis, not far beneath the skin, supplying blood to the skin, fascia and corpus spongiosum. Other Branch: Deep Artery- travels through the core of the corpus cavernosum. Gives off smaller branches: Helicine Arteries- penetrate the trabeculae and empty into the lacunae. *When the penis is flaccid most of the blood supply comes from the dorsal arteries. *When the deep artery dilates, the lacunae fill with blood and the penis becomes erect. *A median deep dorsal vein drains blood from the penis. Nerve Supply: Penis is richly innervated by sensory and motor nerve fibers. The glans and foreskin have an abundance of tactile, pressure and temperature receptors. The receptors lead by way of a pair of prominent dorsal nerves to the internal pudendal nerves, then via the sacral plexus to segments S2-S4 of the spinal cord
Explain how these govern erection and ejaculation.
Excitement Phase •Stimuli, such as touch, enter the sacral spinal cord via the internal pudendal nerve -Or •Stimuli, such as vision (or dreams), enter sacral spinal cord from brain •Parasympathetic fibers in the pelvic nerve stimulate release of nitric oxide (NO) •NO causes relaxation of deep arteries in corpora cavernosa and relaxation of lacunae •Dilation of deep arteries fills lacunae causing an erection •Bulbourethral glands secrete fluid Ejaculation 1.Emission -Lumbar sympathetic fibers cause •Peristalsis of ductus deferens •Secretion from prostate and seminal vesicles 2.Expulsion -Semen in urethra activates somatic and sympathetic reflexes -Somatic efferent's •Contract bulbocavernosa muscle to expel semen -Sympathetic efferent's •Contract internal urethral sphincter so urine cannot enter urethra and semen cannot enter bladder •Add additional prostatic and seminal secretions •Sympathetic signals constrict internal pudendal artery and reduce blood flow to penis
Describe the stages of meiosis.
Meiosis 1: - the homologous pairs of chromosomes are separated when the cell divides - the result is two cells, each containing 23 chromosomes (not 23 pairs) that consist of duplicated sister chromatids held together at a centromere •Prophase I - pairing up (synapsis) of homologous replicated chromosomes to form a tetrad with two sets of two sister chromatids - crossing over occurs to ensure continued genetic diversity - nuclear envelope breaks down •Metaphase I - lining up of homologous pairs of each tetrad on either side of the cell's midline, forming a double line of chromosomes - this lining process is random with respect to whether the original maternal/paternal chromosome of a pair is on the left or right side (random alignment termed independent assortment) -crossing-over - spindle fibers attach to centromeres •Anaphase I - homologous pairs of chromosomes are pulled to opposite ends of the cell, a process called reduction division - each daughter cell receives only half the starting number of chromosomes (only 23 chromosomes of the original 23 pairs) - each chromosome still with 2 sister chromatids •Telophase I - nuclear division finishes and nuclear envelopes re-form - a cleavage furrow forms in the cell, and the cytoplasm divides (cytokinesis) to produce two new cells - each daughter cell contains 23 replicated chromosomes only with 2 sister chromatids each -Homologs separate Meiosis 2 -the sister chromatids separate and become single chromosomes in haploid cells •Prophase II - breaking of nuclear envelope - chromosomes collecting together - no crossing over •Metaphase II - spindle fibers extending from centrioles to centromeres - replicated chromosomes (composed of sister chromatids) are aligned to form a single line along the cell midline •Anaphase II - sister chromatids pulled apart - single-stranded chromosomes separated to opposite poles of the cell •Telophase II - reformation of nuclear membrane - formation of cleavage furrow - division of cytoplasm - produces for haploid daughter cells containing only 23 total chromosomes (22 autosomes and 1 sex chromosome, either X or Y) - mature into secondary oocytes in females - mature into sperm in males
Describe the anatomy of the scrotum, testes, and penis.
Scrotum •Pouch of skin, muscle, and fibrous connective tissue containing the testes •Internal median septum divides scrotum into right and left compartments •Perineal raphe: medial seam on scrotum surface that marks location of median septum -Extends anteriorly along ventral side of penis and posteriorly to anus •Spermatic cord—bundle of fibrous connective tissue containing the ductus deferens, blood and lymphatic vessels, and testicular nerve -Continues through inguinal canal into pelvic cavity -External inguinal ring: inferior entrance to inguinal canal -Internal inguinal ring: superior exit to pelvic cavity •Human testes reside in the scrotum because sperm production occurs at 35C not 37C. •Scrotum has three mechanisms to regulate temperature of testes: 1. Cremaster: strips of the internal abdominal oblique muscle -In cold temperatures, contracts and draws testes upward toward body -In warm temperatures, relaxes suspending testes further from body 2. Dartos fascia: subcutaneous layer of smooth muscle -Contracts when cold, wrinkling the scrotum, holding testes against warm body -Reduces surface area of the scrotum and heat loss 3. Pampiniform plexus: an extensive network of veins from the testes that surrounds the testicular artery and spermatic cord -Removes heat from the descending arterial blood Testes •Testes (testicles)—combined endocrine and exocrine glands that produce sex hormones and sperm •Covered anteriorly and laterally by tunica vaginalis •Tunica albuginea: white fibrous capsule on testes •Connective tissue septa divides testes into 250 to 300 wedge-shaped lobules •Seminiferous tubules—ducts where sperm are produced -One to three in each lobule •Rete testis—a network on the posterior side of the testis that collects sperm from seminiferous tubules -Sperm flow with fluid secreted by nurse cells -Sperm do not swim while in the male reproductive tract Seminiferous Tubules •Each tubule lined with a thick germinal epithelium of germ cells (becoming sperm) and nurse cells -Nurse cells in between germ cells •Protect the germ cells, and promote their development •Germ cells depend on them for nutrients, waste removal, growth factors, and other needs •Interstitial endocrine cells between tubules produce testosterone. Penis •Penis serves to deposit semen in the vagina •Half of the penis is an internal root •Half is an externally visible shaft and glans (head) •Skin over shaft loosely attaches to allow expansion •Extends over glans as prepuce (foreskin) that is removed by circumcision •Three cylindrical bodies of spongy erectile tissue have blood sinuses called lacunae that fill with blood during sexual arousal and account for erection •Single corpus spongiosum along ventral side of penis -Encloses spongy (penile) urethra -Distal end enlarges and forms the glans penis -Proximal end is a dilated bulb ensheathed by bulbospongiosus muscle •Two corpora cavernosa -Diverge like arms of a Y -Each arm, called a crus, attaches penis to pubic arch -Covered with ischiocavernosus muscle
Define primary sex organs, secondary sex organs, and secondary sex characteristics.
Sex Organs -Primary sex organs (gonads) •Produce gametes (testes or ovaries) -Secondary sex organs: organs other than gonads that are necessary for reproduction •Male—system of ducts, glands; penis delivers sperm cells •Female—uterine tubes, uterus, and vagina receive sperm and harbor developing fetus Secondary Sex Characteristics -Develop at puberty -Both sexes •Pubic and axillary hair and their associated scent glands, and the pitch of the voice -Male •Facial hair, coarse and visible hair on the torso and limbs, relatively muscular physique. -Female •Distribution of body fat, breast enlargement, and relatively hairless appearance of the skin.
Describe the pathway taken by a sperm cell from its formation to its ejaculation, naming all the passages it travels.
Spermatic Ducts •Efferent ductules-About 12 small ciliated ducts collecting sperm from rete testes and transporting it to epididymis •Duct of the epididymis (head, body, and tail) -Site of sperm maturation and storage (fertile for 40 to 60 days) -Contains a single coiled duct, 6 m long, adhering to posterior of testis -Sperm mature as they travel through the duct -If not ejaculated, they disintegrate and epididymis reabsorbs them. •Ductus (vas) deferens -Muscular tube, 45 cm long, passing up from scrotum through inguinal canal to posterior surface of bladder -Duct widens behind the bladder and widens into the terminal ampulla -Duct ends by uniting with duct of the seminal vesicle -Thick wall of smooth muscle well innervated by sympathetic nerve fibers •Ejaculatory duct -2 cm duct formed from ductus deferens and seminal vesicle; passes through prostate to empty into urethra
Describe the sequence of cell types in spermatogenesis, and relate these to the stages of meiosis.
Spermatogenesis •Spermatogonia lie along periphery of seminiferous tubules and divide by mitosis to produce: -Type A spermatogonium •Remains in tubule wall as stem cell -Type B spermatogonium •Migrates away from wall and is on its way to producing sperm -Type B spermatogonium enlarges and becomes a primary spermatocyte •Primary spermatocyte undergoes meiosis I which gives secondary spermatocytes •Each secondary spermatocyte undergoes meiosis II dividing into two spermatids (four total) •Spermiogenesis—four spermatids undergo transformations in which they differentiate into a spermatozoa (sperm.) -Gain acrosome -Gain flagellum -Lose excess cytoplam
Describe the composition of semen and functions of its components.
•60% seminal vesicle fluid, 30% prostatic fluid, and 10% sperm and spermatic duct secretions •Prostate produces an alkaline, thin, milky white fluid containing: -Citrate -a nutrient source for sperm -A clotting enzyme -which causes semen to stick to the wall vagina -Ions that buffer the acidity of the vagina -Prostate-specific antigen -a protease that dissolves the clot so the sperm can continue up the female tract •Seminal vesicles contribute viscous yellowish fluid containing: -Fructose and other carbohydrates and citrate -nutrient source -Prostaglandins -thin mucus plug and stimulate reverse peristalsis -Proteins involved in clotting
Define and describe male climacteric and the effect of aging on male reproductive function.
•Adulthood -Testosterone sustains the male reproductive tract, sperm production, and libido -Inhibin from nurse cells suppresses FSH output from the pituitary, reducing sperm production without reducing LH and testosterone secretion •Testosterone secretion declines with age -Peak secretion at 7 mg/day at age 20; declines to one-fifth of that by age 80 •Decline in number and activity of interstitial endocrine cells •Andropause(male climacteric) may occur -A period of declining reproductive function that may be first seen in early 50s -Although sperm counts decline, men can still father children throughout old age
Identify which of the male and female external genitalia are homologous to each other.
•Genital tubercle becomes the head (glans) of the penis or glans clitoris •Pair of urogenital folds encloses male urethra helping to form the penis or forms the labia minora •Pair of labio-scrotal folds becomes either scrotum or labia majora •Male and female organs that develop from the same embryonic structure are homologous -Penis is homologous to the clitoris -Scrotum is homologous to the labia majora •By week 12, either male or female genitalia are distinctly formed
Describe the descent of the gonads and explain why it is important.
•Gonads initially develop high in the abdominal cavity and then migrate into the pelvic cavity (ovaries) or scrotum (testes) -Gubernaculum—embryonic connective tissue cord extending from gonad to pelvic cavity floor •In the male, it passes between the internal and external abdominal oblique muscles into the scrotal swelling -Vaginal process: fold of peritoneum that extends into the scrotum -Inguinal canal: pathway of low resistance through the groin created by gubernaculum and vaginal process •Most common site of hernia in males •Descent of the testes begins as early as 6 weeks -In seventh month, testes pass through the inguinal canal into the scrotum guided by the gubernaculum -Testes accompanied by elongating testicular arteries and veins, lymphatic vessels, nerves, spermatic ducts, and extensions of internal abdominal oblique muscle.
Describe or draw and label a sperm cell.
•Head -Nucleus contains haploid set of chromosomes -Acrosome—enzyme cap over the apical half of the nucleus that contains enzymes that penetrate the egg •Tail -Divided into three regions 1.Mid-piece contains mitochondria, produces ATP for flagellar movement 2.Principal piece is axoneme surrounded by sheath of supporting fibers •Constitutes most of tail 3.End-pieces very narrow tip of flagella
Describe the role of nurse cells in spermatogenesis.
•Immune system is 'educated' in childhood, but spermatogenesis begins at puberty •When primary spermatocyte undergoes meiosis, it becomes genetically different than self •Nurse cells protect it from the body's immune system: blood-testis barrier (BTB) •As the primary spermatocyte moves toward the lumen of the seminiferous tubule surrounded by nurse cells, tight junctions form behind it
Explain how the Y chromosome determines the response of the fetal gonad to prenatal hormones.
•Initially, a fetus is sexually undifferentiated •Gonads begin to develop at 5 or 6 weeks as gonadal ridges •Two sets of ducts adjacent to each gonadal ridge •SRY gene (sex-determining region of Y chromosome) found only in males initiates development of testes •Testes begin to secrete testosterone at 8 to 9 weeks, stimulates development of male ducts and degeneration of female ducts •Estrogen levels are always high in pregnancy •Female development of a fetus occurs whenever there is an absence of androgen hormones -Not because estrogen is present
Explain the role of the sex chromosomes in determining sex.
•Our cells contain 23 pairs of chromosomes -22 pairs of autosomes -1 pair of sex chromosomes (XY males; XX females) •Males produce half Y-carrying sperm and half X-carrying sperm •All eggs carry the X chromosome •Sex of child determined by type of sperm that fertilizes mother's egg -X-carrying sperm fertilizes the egg: female -Y-carrying sperm fertilizes the egg: male
Describe the resulting changes in the male body.
•Pubic and axillary hair and their associated scent glands, and the decreased pitch of the voice •Facial hair, coarse and visible hair on the torso and limbs, relatively muscular physique.
Identify the most fundamental biological distinction between male and female.
•Sexual reproduction is biparental, meaning offspring receives genes from two parents •Male and female gametes (sex cells) combine their genes to form a zygote (fertilized egg) -One gamete has motility: sperm (spermatozoon) •Parent producing sperm considered male •Parent with a Y chromosome is male -Other gamete contains nutrients for developing embryo: egg (ovum) •Parent producing eggs considered female •Anyone lacking a Y chromosome is female •Male reproductive system serves to produce sperm and introduce them into the female body -Males have a copulatory organ (penis) for introducing their gametes into the female reproductive tract •Female reproductive system produces eggs, receives sperm, provides for gametes' union, harbors fetus, and nourishes offspring -Females have a copulatory organ (vagina) for receiving the sperm
Describe the hormonal control of puberty.
•Testes secrete testosterone in first trimester of fetal development •Then testes becomes dormant until puberty •As hypothalamus matures in late childhood, gonadotropin-releasing hormone (GnRH) begins to be secreted •GnRH stimulates anterior pituitary cells to secrete: -Luteinizing hormone (LH) sometimes called interstitial cell-stimulating hormone (ICSH) •Stimulates interstitial endocrine cells to produce testosterone •Nurse cells also secrete inhibin, which selectively inhibits FSH secretion and thus reduces sperm production without reducing testosterone secretion •Testosterone has negative feedback effects that reduce GnRH secretion and pituitary sensitivity to GnRH •Testosterone also stimulates the libido and the development of secondary sex organs and characteristics.
State the names, locations, and functions of the male accessory reproductive glands.
•Three sets of accessory glands in male reproductive system: seminal vesicles, prostate, bulbourethral glands 1. Seminal vesicles •Pair of glands posterior to bladder •Empties into ejaculatory duct •Forms 60% of semen 2. Prostate •Surrounds urethra and ejaculatory duct just inferior to the bladder •Empty through about 20 pores in the prostatic urethra •Thin milky secretion forms 30% of semen 3. Bulbourethral glands •During sexual arousal, they produce a clear slippery fluid that lubricates the head of the penis in preparation for intercourse •Protects the sperm by neutralizing the acidity of residual urine in the urethra