General Human Histology. The Urinary System

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2 General Human Histology Lecture 8 Assist. Prof. Ahmed Anwar Albir The Urinary System Collecting Tubules & Ducts Urine passes from the distal convoluted tubules to collecting tubules that join each other to form larger, straight collecting ducts, which widen gradually as they approach the tips of the medullary pyramids (Figures 2 and 3). Collecting tubules composed of a simple cuboidal epithelium, convey and modify the ultrafiltrate from the nephron to the minor calyces of the kidney. The distal convoluted tubules of several nephrons join to form a short connecting tubule that leads into the collecting tubule (Figure 3). Collecting tubules are about 20mm long and have three recognized regions (Figure 3): cortical, medullary, and papillary. Cortical Collecting Tubules They located in the medullary rays and are composed of two types of cuboidal cells (Figures 3 and 14): 1. Principal Cells: They have oval, centrally located nuclei, a few small mitochondria, and short, sparse microvilli. The basal membranes of these cells display numerous infoldings. 2. Intercalated Cells: They display numerous apical vesicles 50 to 200nm in diameter, and an abundance of mitochondria. The nuclei of these cells are 1

3 round and centrally located. The functions of principal cells are not known, but intercalated cells actively transport and secrete hydrogen ions against high cocentration gradients, thus modulating the acid-base balance of the body. Figure 14. Photomicrograph of the kidney cortex in a monkey illustrating renal corpuscle (R), medullary ray (M), and crosssectional profiles of the uriniferous tubules. Medullary Collecting Tubules They are of larger caliber because they are formed by the union of several cortical collecting tubules. Those in the outer zone of the medulla are similar to the cortical collecting tubules in that they display both principal and intercalated cells (Figure 15), whereas tubules of the inner zone of the medulla have principle cells only (Figure 16). In the medulla, collecting ducts are a major component of the urineconcentrating mechanism. 2

4 Figure 15. The medulla of the kidney displays the simple cuboidal epithelium of the collecting ducts. Figure 16. The uriniferous tubule and its vascular supply and drainage. 3

5 Papillary Collecting Tubules (Ducts of Bellini) They are each formed by the confluence of several medullary collecting tubules. These are large ducts, 200 to 300µm in diameter, and they open at the area cribrosa of the renal papilla to deliver the urine that they convey into the minor calyx of the kidney. These ducts are lined by tall columnar principal cells only (Figure 17). Figure 17. Papilla, longitudinal section through an area adjacent to a calyx. Renal Interstitium The space between uriniferous tubules and blood and lymph vessels is called the renal interstitium. It occupies a very small volume in the cortex but increases in the medulla. The renal interstitium contains a small amount of connective tissue with fibroblasts, some collagen fibers and, mainly in the medulla, a highly hydrated ground substance rich in proteoglycan. In the medulla the secreting cells interstitial cells are found. They contain cytoplasmic lipid droplets and are implicated in the synthesis of prostaglandins and prostacyclin. 4

6 Effects of Adrenal Steroids Steroid hormones of the adrenal cortex, mainly aldosterone, increase distal tubular absorption of sodium from the filtrate and thus decrease sodium loss in the urine. Aldosterone also facilitates the elimination of potassium and hydrogen ions. This hormone is crucial in maintaining electrolyte balance in the body. General Functions of the Kidney The kidneys play a role in excretion as well as in regulation of body fluid composition and volume. Specifically, they regulate solute components (e.g., sodium, potassium, chloride, glucose, amino acids) and acid-base balance. Thus, during the summer, when a great deal of fluid is lost through perspiration, the urinary output is reduced in volume and increased in osmolarity. During the winter months, when fluid loss through perspiration is minimal, the urinary output is increased in volume and the urine is dilute. In addition, the kidneys excrete detoxified end products, regulate the osmolality of urine, and secrete substances such as erythropoietin, medullipin I, renin, and prostaglandins. Finally, the kidneys regulate blood pressure and aid in the conversion of vitamin D to dihydroxycholecalciferol, which appears to control calcium transport. Excretory Passages The excretory passages of the urinary system consist of the minor and major calyces, the pelvis of the kidney, the ureter, the single urinary bladder, and the single urethra. 5

7 Calyces The renal papilla of each renal pyramid fits into a minor calyx, a funnel- shaped chamber that accepts urine leaving the ducts of Bellini at the area cribrosa (Figure 1). The portion of the apex of the pyramid that projects into the minor calyx is covered by transitional epithelium, which as a barrier, separating the urine from the underlying interstitial connective tissue. Deep to the lamina propria is a thin muscular coat composed entirely of smooth muscle. This muscular layer propels the urine into a major calyx, one of three or four larger funnel-shaped chambers, each of which collects urine from two to four minor calyces. The major calyces are similar in structure to the minor calyces as well as to the expanded proximal region of the ureters, the renal pelvis. The walls of the excretory passages thicken from the minor calyces to the urinary bladder. Ureter The ureters deliver urine from the kidneys to the urinary bladder (Figure 18). Each ureter is about 3 to 4mm in diameter, is approximately 25 to 30cm long. The ureters are hollow tubes consisting of: A mucosa, which lines the luman and presents several folds that project into the lumen when the ureter is empty but are absent when the ureter is distended. The transitional epithelial lining, three to five cell layers in thickness, overlies a layer of dense, irregular fibroelastic connective tissue, which constitutes the lamina propria. As always, the epithelium is separated from the underlying lamina propria by a basal lamina. 6

8 A muscular coat (muscularis) of the ureter is composed of two predominantly inseparable layers of smooth muscle cells. The outer layer is arranged circularly and the inner layer is longitudinally disposed. This arrangement is true for the proximal two thirds of the ureter, but in the lower third, near the urinary bladder, a third muscle layer, whose fibers are oriented longitudinally, is added onto the existing surface of the existing muscle coat. Hence, the muscular fiber orientation in the lower one third of the ureter is outer longitudinal, middle circular, and inner longitudinal. A fibrous, connective tissue covering (adventitia or fibrosa) of the ureter is composed of fibroelastic connective and adipose tissue. It contains numerous blood vessels, and small nerves. Figure 18. Ureter (transverse section). 7

9 Urinary Bladder The urinary bladder stores urine until it is ready to be voided. The layers of the wall of the bladder are similar to those of the ureter but the thick muscular coat of the bladder is distinctive (Figure 19). The wall consists of: the transitional epithelium and lamina propria are as in the ureter but the epithelium has more cell layers, the lamina propria is wider and the loose connective tissue in its deeper zone contains many elastic fibrers. A muscularis is a thick coat. It is more or less in three layers of smooth muscle fibers, the middle layer is the widest. Actually, the muscle fibers are arranged in anastomosing bundles between which is loose connective tissue. In a section, the groups of muscle fibers are seen in various planes and three layers are difficult to distinguish. A serous layer (serosa) on the superior surface is composed of superficial connective tissue and peritoneal mesothelium. Figure 19. Urinary Bladder (superior part). 8

10 The lower part of the bladder has an adventitia (fibrosa) which merges with connective tissue of adjacent structures. Urethra The urethra is a tube that carries the urine from the bladder to the exterior. In men, sperm also pass through it during ejaculation. In women, the urethra is exclusively a urinary organ. Male Urethra The male urethra is 15 to 20cm long, and its three regions are named according to the structures through which it passes. 1. The prostatic urethra, 3 to 4cm long, lies entirely in the prostate gland. It is lined by a transitional epithelium and receives the openings of many tiny ducts of the prostate, the prostatic utricle (a rudimentary homologue of the uterus), and the paired ejaculatory ducts. 2. The membranous urethra is only 1 to 2cm long. This segment is so named because it passes through the perineal membrane (urogenital diaphragm). It is lined by stratified columnar epithelium interspersed with patches of pseudostratified columnar epithelium. Surrounding this part of the urethra is a sphincter of striated muscle, the external sphincter of the urethra. The voluntary external striated sphincter adds further closing pressure to that exerted by the involuntary urethral sphincter. The latter is formed by the continuation of the internal longitudinal muscle of the bladder. 3. The spong urethra (penile urethra), the longest portion of the urethra (15cm long), passes through the length of the penis, terminating at the tip of the glans penis as the external urethral orifice. This segment is so 9

11 named because it is located in the corpus spongiosum of the penis. It is lined by stratified columnar epithelium interspersed with patches of pseudostratified columnar and stratified squamous, nonkeratinized epithelia. The enlarged terminal portion of the urethra in the glans penis (the navicular fossa) is lined by stratified squamous, nonkeratinized epithelium. The lamina propria of all three regions is composed of a loose fibroelastic connective tissue with a rich vascular supply. It houses numerous glands of Littre, whose mucous secretion lubricates the epithelial lining of the urethra. Female Urethra The female urethra is about 4 to 5cm in length and 5 to 6mm in diameter. It extends from the urinary bladder to the external urethral orifice just above and anterior to the opening of the vagina. It is lined by a transitional epithelium near the bladder and by a stratified squamous nonkeratinized epithelium along the remainder of its length. Interspersed in the epithelium are patches of pseudostratified columnar epithelium. The mucosa is arranged in elongated folds because of the organization of the fibroelastic lamina propria. Along the entire length of the urethra are numerous clear, mucous-secreting glands of Littre. A thin, vascular, erectile coat surrounds the mucosa, resembling the corpus spongiosum of the male. The muscular layer of the urethra is continuous with that of the bladder but is composed of two layers only, an inner longitudinal and an outer circular smooth muscle layer. The mid part of the female urethra is surrounded by an external striated voluntary sphincter. 10