d.midpiece.

e.axoneme.

.Which of the following statements about testosterone is the most accurate?

a.It is produced by the exocrine testis.

b.It exists mainly in the unbound or "free" form in the circulation.

c.It is regulated by follicle-stimulating hormone.

d.Its production is increased by excess prolactin.

e.One of its metabolites is dihydrotestosterone (DHT).

.Processes that must occur for a sperm to normally fertilize an egg include all of the following EXCEPT:

a.development of motility.

b.acrosome reaction.

c.capacitation.

d.zona pellucida binding.

e.sexual intercourse.

.After ejaculation the contents of the vas deferens are:

a.returned to the seminal vesicles.

b.maintained in the ampulla.

c.released into the ejaculatory ducts.

d.propelled back into the epididymis.

e.released into the bladder.

Answers

1.b. Wolffian ducts. Müllerian ducts regress in the male. The indifferent gonad migrates to the urogenital ridge to become the testis. The gubernaculum testis is associated with the testis.

2.c. Spermatogenic cells. In humans, interstitial tissue takes up 20% to 30% of the total testicular volume, whereas germ line cells constitute the remainder (70%-80%).

3.c. Luteinizing hormone (LH) and FSH. Estradiol and testosterone are not anterior pituitary hormones, although they influence pituitary function. TSH and ACTH are derived from the anterior pituitary but do not act directly on the testicle.

4.c. The seminal vesicles. At least 65% to 70% of ejaculate volume is derived from the seminal vesicles, with the remainder from the vas deferens (sperm) and prostatic secretions. Periurethral Cowper glands also

contribute a small amount of fluid to the ejaculate.

5.d. The internal spermatic veins. The pampiniform plexus of veins forms from the internal spermatic or gonadal veins. Deferential veins follow the vas deferens and empty into the hypogastric veins. These veins are spared during varicocele ligation surgery.

6.c. Low, because exogenous testosterone inhibits pituitary production of FSH and LH. Negative feedback inhibition maintains homeostatic balance in the pituitary-gonadal axis. Excess testosterone from any source acts on the anterior pituitary to reduce the production of LH and FSH. Without appropriate FSH production, azoospermia results in most men taking anabolic steroids, but the effect varies based on the dose, frequency, and duration of the drugs.

7.a. Sertoli-Sertoli cells. Sertoli-Sertoli tight junctions are the strongest cellcell interactions in the body and give the blood-testis barrier its name. They form a high-resistance barrier that prevents the deep penetration of most drugs, molecules, and electron-opaque tracers into the seminiferous epithelium from the testicular interstitium. Sertoli cell tight junctions also segregate premeiotic germ cells (spermatogonia) from meiotic and postmeiotic germ cells.

8.d. Stressful blood draw. All of the choices can cause elevated serum prolactin levels. However, pathologically high prolactin levels are associated with low LH, FSH, and testosterone levels and reduced semen quality. Because anterior pituitary function is preserved, the elevated prolactin is likely spurious and due to a stressful blood draw. The measurement should be repeated with a blood draw at least 2 hours after awakening from sleep.

9.e. Spermatogonia, spermatocyte, spermatid. Proceeding from the least to

the most differentiated, they are named dark type A spermatogonia (Ad); pale type A spermatogonia (Ap); type B spermatogonia (B); preleptotene primary spermatocytes (R); leptotene primary spermatocytes (L); zygotene primary spermatocytes (z); pachytene primary spermatocytes (p); secondary spermatocytes (II); and Sa, Sb1, Sb2, Sc, Sd1, and Sd2 spermatids.

.b. Testosterone. Inhibin and activin regulate FSH secretion. Prolactin, in excess, can downregulate LH but is not the primary regulator of LH

secretion.

.a. 12 mL. Although ethnic and racial differences do exist, a normal testis volume is generally above 16 mL and averages 20 mL.

.e. Caput, corpus, and cauda. Anatomically, the epididymis is divided into three regions: the caput, the corpus, and the cauda epididymis. On the basis of

histologic criteria, each of these regions can be subdivided into distinct zones separated by transition segments.

.c. It is a process that occurs only in gametes. Crossing over of sister chromatids, exchange of genetic material, creation of unique daughter

cells, and halving of chromosomal number are the hallmarks of meiosis.

Meiosis occurs only in gametes whereas mitosis occurs in somatic cells.

.a. First trimester gestation, 2 months of age, puberty. Testosterone peaks occur during early gestation and at approximately 2 months of age; a third spike occurs at puberty. Testosterone levels fall about 1%/year after age 30 and may fall more dramatically in the seventh and eighth decades of

life.

.c. Caput epididymis. Studies in men with congenital absence of the vas deferens or epididymal obstruction from vasectomy generally report better sperm motility in caput versus cauda epididymal sperm.

.c. Cellular remodeling and nuclear compaction of spermatid DNA. In spermiogenesis, massive remodeling of postmeiotic germ cells (spermatids) occurs that involves nuclear compaction, tail and acrosome development, and cytoplasmic stripping of the mature germ cell. This is a postmeiotic process, and cell division, or cytokinesis, does not occur.

.e. Increased capacity for glycolysis. Sperm undergo many metabolic changes during epididymal transit. Animal studies describe an increased capacity for glycolysis, changes in intracellular pH and calcium content, modification of adenylate cyclase activity, and alterations in cellular phospholipid and phospholipid-like fatty acid content. Sperm motility increases with epididymal transit.

.e. Type A spermatogonia. Type A spermatogonia are the only true stem cell in the testis because they can either self-renew or differentiate to become sperm. All of the other cell types listed do not share this dual potential.

.d. Testis. Sperm spend 45 to 60 days developing in the testis and 2 to 12 days in the epididymis. They are not routinely found in the seminal vesicle, and spend only seconds in the ejaculatory ducts and urethra during

ejaculation.

.d. Midpiece. The middle segment of sperm is highly organized, consisting of helically arranged mitochondria that surround a set of outer dense fibers and

the 9 + 2 microtubular structure of the axoneme. The sperm head contains an acrosome and the nucleus, and the tail contains the axoneme.

.e. One of its metabolites is dihydrotestosterone (DHT). Testosterone is produced by the endocrine testes, exists mainly in bound forms in plasma, and is regulated by LH, not FSH. Testosterone is converted to DHT in target tissue by 5-α reductase. It is also converted to estrogen by aromatase enzymes.

Excess prolactin decreases testosterone production.

.e. Sexual intercourse. For physiologic egg fertilization, all of the processes listed must occur. Intrauterine insemination (IUI) can replace sexual intercourse and improve the odds of natural conception. The process of in vitro fertilization (IVF)–intracytoplasmic sperm injection (ICSI) avoids many of these physiologic processes by simply injecting a sperm from any

reproductive-tract source into a mature oocyte or egg.

.d. Propelled back into the epididymis. Studies have shown that after sexual stimulation or ejaculation, the contents of the vas deferens are propelled back toward the cauda epididymis, because of differences in the amplitude, frequency, and duration of vas deferens contractility along its length.

Chapter review

1.The steroid hormones testosterone, dihydrotestosterone, and estradiol are translocated to nuclear DNA recognition sites and regulate the transcription of target genes.

2.The hypothalamus is anatomically linked to the pituitary gland by both a portal vascular system and neuronal pathway. The hypothalamic hormone GnRH (sometimes called LHRH) stimulates the secretion of luteinizing hormone and follicle-stimulating hormone.

3.GnRH output exhibits three types of rhythmicity: (1) seasonal, peaking in the spring; (2) circadian, peaking in the early morning; and (3) pulsatile, periodic peaks per 24 hours. The importance of pulsatile GnRH secretory pattern is demonstrated when a GnRH agonist is given; this eliminates the pulsatile activity and therefore suppresses GnRH output.

4.Both androgens and estrogens regulate LH secretion through negative feedback.

5.Increased levels of prolactin abolish gonadotropin pulsatility by interfering with the episodic GnRH release, thus causing a reduction in LH and FSH.

6.Testosterone is metabolized to dihydrotestosterone (5-α reductase) and estradiol (aromatase).

7.Sertoli cells produce inhibin, which inhibits FSH, and activin, which stimulates FSH. The presence of these hormones accounts for the relatively secondary independence of FSH from GnRH secretion. Sertoli cells produce androgen-binding protein, which maintains high levels of androgen in the seminiferous tubules and epididymis.

8.Negative feedback for testosterone occurs at the hypothalamus; for estrogens, it occurs at the pituitary.

9.The SRY (sex-determining region Y gene) on the short arm of the Y chromosome is a critical gene for sex determination.

10.Dihydrotestosterone masculinizes the external genitalia; müllerianinhibiting substance prevents müllerian duct development, that is, the uterus, fallopian tubes, and proximal third of the vagina.

11.A slow but progressive decline in testosterone production occurs with age > 30 years.

12.Androgen production during the early male neonate's life is thought to hormonally imprint the hypothalamus, liver, and prostate.

13.The blood-testis barrier is more appropriately termed blood-seminiferous tubule barrier; it allows sperm development to occur in an immunologically privileged way.

14.An insult to the testes, such as biopsy, torsion, or trauma, will not induce anti-sperm antibodies if the event occurs before puberty.

15.The X chromosome appears to be important for spermatogenesis.

16.The azoospermic factor region (AZF) is located on the long arm of the Y chromosome. Deletions in this region are found in some patients with abnormal spermatogenesis. Microdeletions in the AZF region are subdivided into a, b, and c. Sperm may be found in AZFc microdeletions but not in AZFa and AZFb.

17.With paternal age, there are increases in structural chromosomal abnormalities in sperm and autosomal-dominant mutations leading to specific (sentinel) phenotypes in offspring.

18.Lymph from the caput and corpus epididymis travels the same route as that for the testes. Lymph from the cauda epididymis joins the vas deferens and terminates in the external iliac nodes. There is a bloodepididymis barrier that extends from caput to cauda epididymis.

19.The epididymis serves to transport sperm, store it, increase fertility, and

promote motility maturation. Epididymal function is temperature and androgen (DHT) dependent. Sperm fertility maturation is achieved at the level of the late corpus or early cauda epididymis.

20.Semenogelin, a component of semen that results in semen coagulation, is produced in the seminal vesicle. Seminal vesicle secretions have an alkaline pH and contain antioxidant enzymes, fructose, vitamin C, flavins, and prostaglandins.

21.At least 65% to 70% of ejaculate volume is derived from the seminal vesicles,

22.Testosterone peaks occur during early gestation and at approximately 2 months of age, and a third spike occurs at puberty.

23.Sperm spend 45 to 60 days developing in the testis and 2 to 12 days in the epididymis.