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Chapter 9 • Urinary System

ChapterSummary

•The kidney has 3 major regions: the hilum, cortex, and medulla.

-The hilum is the point of entrance and exit forthe renal vessels and ureter. The upper expanded portion of the ureter is called the renal pelvis, and divides into 2 or 3 major calyces and several minor calyces.

-The cortex has several renal columns that penetrate the entire depth of the kidney.

-The medulla forms a series of pyramids that direct the urinary stream into a minor calyx.

•The uriniferous tubule is composed of the nephron and collecting tubule.

-The nephron contains the glomerulus (a tuft of capillaries interposed between an afferent and efferentarteriole). Plasma filtration occurs here. Bowman's capsule has an inner visceral and outer parietal layer. The space between is the urinary space. The visceral layer is composed of podocytes resting on a basal lamina, which is fused with the capillary endothelium. The parietal layer is composed of simple squamous epithelium that is continuous with the proximal tubule epithelial lining. The proximal convoluted tubule is the longest and most convoluted segment ofthe nephron. Most of the glomerular filtrate is reabsorbed here. The loop ofHenle extends into the medulla and has a thick

and thin segment. It helps to create an osmotic gradient important for concentration ofthe tubular filtrate. The distal convoluted tubule reabsorbs sodium and chloride from the tubular filtrate.

-The collecting tubules have a range of cells from cuboidal to columnar. Water removal and urine concentration occur here with the help ofthe antidiuretic hormone. The blood supply is via renal artery and vein.

•The vasa rectae supply the medulla. They play an important role in maintaining the osmotic gradient. The juxtaglomerular apparatus OGA) is composed ofjuxtaglomerular cells, which are myoepithelial cells in the afferent arteriole. They secrete renin. The JGA also contains Polkissen cells (function unknown), located between afferent and efferent arterioles, and the macula densa. Macula densa cells are located in the wall ofthe distal tubule, located near the afferent arteriole. They sense sodium concentration in tubular fluid.

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Chapter 10 • Male Reproductive System

Spermatogenesis

The spermatogenic cells (germinal epithelium) are stacked in 4 to 8 layers that occupy the space between the basement membrane and the lumen of the semi­ niferous tubule. The stem cells (spermatogonia) are adjacent to the basement membrane. As the cells develop, they move from the basal to the luminal side ofthe tubule.

At puberty the stem cells resume mitosis, producing more stem cells as well as differentiated spermatogonia (type A and B) that are committed to meio­ sis. Type B spermatogonia differentiate into primary spermatocytes that enter meiosis. Primaryspermatocytes (4n, diploid) pass through a long prophase ( 10 days to 2 weeks) and after the first meiotic division form 2 secondary spermato­ cytes (2n, haploid). The secondary spermatocytes rapidly undergo the second meiotic division in a matter of minutes (and are rarely seen in histologic sec­ tions) to produce the spermatids (ln, haploid).

The progeny ofa single maturing spermatogonium remain connected to one an­ other by cytoplasmic bridges throughout their differentiation into mature sperm.

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Figure 1-10-3. Seminiferous tubule surrounded by a basement membrane (A) and myoepithelial cells

Spermatogonia (B) lie on the basement membrane, primary spermatocytes (C), and spermatozoa (D) are inside the blood testis barrier.

Sertoli cells (arrow) have elongate, pale-staining nuclei.

Spermiogenesis

Spermiogenesis transforms haploid spermatids into spermatozoa. This process of differentiation involves formation of the acrosome, condensation, and elon­ gation of the nucleus; development of the flagellum; and loss of much of the cytoplasm.

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Chapter 10 • Male Reproductive System

Sertoli Cells and the Blood-Testis Barrier

Sertoli cells are tall columnar epithelial cells. These multifunctional cells are the predominant cells in the seminiferous tubule prior to puberty and in elderly men but comprise only 10% of the cells during times of maximal spermatogenesis.

•Irregular in shape; the base adheres to the basal lamina and the apical end extends to the lumen. The nucleus tends to be oval with the long axis oriented perpendicular to the basement membrane.

•The cytoplasmic extensions make contact with neighboring Sertoli cells via tight junctions, forming the blood-testis barrier by separating the seminiferous tubule into a basal and an adlumenal compartment.

•Have abundant smooth endoplasmic reticulum, some rough endoplas­ mic reticulum, a well-developed Golgi complex, lysosomes, microtu­ bules, and microfilaments. Microtubules and microfilaments likely func­ tion to aid in the release of the spermatozoa.

•Secrete androgen-binding protein that binds testosterone and dihy­ drotestosterone. High concentrations of these hormones are essential for normal germ-cell maturation. The production of androgen-binding protein is stimulated by follicle-stimulatinghormone (FSH receptors are on Sertoli cells).

The blood-testis barrier is a network of Sertoli cells which divides the seminif­ erous tubule into a basal compartment (containing the spermatogonia and the earliest primary spermatocytes) and an adlumenal compartment (containing the remaining spermatocytes and spermatids). The basal compartment has free ac­ cess to material found in blood, while the more advanced stages of spermato­ genesis are protected from blood-borne products by the barrier formed by the tight junctions between the Sertoli cells. The primary spermatocytes traverse this barrier by a mechanism not yet understood.

Interstitial Tissues ofthe Testis

The interstitial tissue lying between the seminiferous tubules is a loose network of connective tissue composed of fibroblasts, collagen, blood and lymphatic ves­ sels, and Leydig cells (also called interstitial cells). The Leydig cells synthesize testosterone.

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Section I • Histology and Cell Biology

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Figure 1-10-8. Ductus deferens with thick layers of smooth muscle

ACCESSORY GLANDS

SeminalVesicles

The seminal vesicles are a pair of glands situated on the posterior and inferior surfaces of the bladder. These highly convoluted glands have a folded mucosa lined with pseudostratified columnar epithelium. The columnar epithelium is rich in secretory granules that displace the nuclei to the cell base.

The seminal vesicles produce a secretion that constitutes approximately 70% of human ejaculate and is rich in spermatozoa-activating substances such as fruc­ tose, citrate, prostaglandins, and several proteins. Fructose, which is a major nu­ trient for sperm, provides the energy for motility. The duct ofeach seminal vesicle joins a ductus deferens to form an ejaculatory duct. The ejaculatory duct traverses the prostate to empty into the prostatic urethra.

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Section I • Histology and Cell Biology

Table 1-10-1. Diseases ofthe Prostate

men

Two types: bacterial and nonbacterial

Both forms may be asymptomatic or may present with lower back pain and urinary sym ptoms

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Chapter 10 • Male Reproductive System

PENIS

The penis is comprised of 3 cylindrical bodies of erectile tissue: 2 corpora cav­ ernosa and a single corpus spongiosum through which the urethra runs. The cylinders are each surrounded by a dense fibrocollagenous sheath, the tunica albuginea.

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Figure 1-10-11. Penis Cross-Section

Penile urethra is in the corpus spongiosum (CS) that is ventral to a pair ofcorpora cavernosa (CC).

Erectile bodies are surrounded by fibrous tissue (arrow and arrowhead}.

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Chapter 10 • Male Reproductive System

ChapterSummary

•The testes contain seminiferous tubules and connective tissue stroma. Seminiferous tubules are the site of spermatogenesis. The epithelium contains Sertoli cells and spermatogenic cells.

-Sertoli cells synthesize androgen-binding protein and provide the blood­ testis barrier.

-Spermatogenic cells are germ cells located between Sertoli cells. They include spermatogonia, primary and secondary spermatocytes, spermatids, and spermatozoa.

Spermatozoa number about 60,000 per mm3 of seminal fluid. Each one has a head, which contains chromatin. Atthe apex ofthe nucleus is the acrosome. The tail contains microtubules.

•Interstitial cells of Leydig are located between the seminiferous tubules in the interstitial connective tissue. They synthesize testosterone and are activated by luteinizing hormone from the anterior pituitary.

•The genital ducts are composed oftubuli recti, rete testis, efferent ductules, ductus epididymis, ductus deferens, and ejaculatory ducts.

-Spermatozoa undergo maturation and increased motility within the ductus (vas) epididymis.

-Spermatozoa are stored in the efferent ductules, epididymis, and proximal ductus deferens.

•The urethra extends from the urinary bladderto the tip of the penis. The prostatic urethra is composed of transitional epithelium and the distal urethra of stratified epithelium.

•Seminal vesicles secrete alkaline, viscous fluid rich in fructose. They do not store spermatozoa.

•Secretions from the prostate gland are rich in citric acid, lipids, zinc, and acid phosphatase.

•Bulbourethral gland secretes mucous fluid into the urethra for lubrication priorto ejaculation.

•The penis is composed of 3 cylindrical bodies of erectile tissue: corpora cavernosa, corpus spongiosum, and trabeculae of erectile tissue. The corpora cavernosa is surrounded by the tunica albuginea.