Histology 2016-2017

Department of Anatomy &Histology: Dr.Rajaa Ali

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URINARY SYSTEM III

Kidney: Cortex.

Presence of:

  • Malpighian or renal corpuscles;
  • Proximal convoluted tubules with brush border (darkly stained);
  • Distal convoluted tubules without brush border (lightly stained).

Distal Convoluted Tubule

  • The distal convoluted tubule (DCT) is shorter and less convoluted than the PCT and is found in the cortex.
  • It is lined by simple cuboidal epithelium (no brush border; Fig.).
  • It actively reabsorbs sodium ions from the tubular fluid. This reabsorptive activity is directly coupled with the secretion of hydrogen or potassium ions into the tubular fluid. (One hydrogen ion or one potassium ion is secreted for every sodium ion reabsorbed.) This process of reabsorption and secretion is controlled by the hormone aldosterone secreted by the adrenal cortex.
  • Thus, DCT is involved in maintenance of acid-base balance in body fluids.

Collecting Tubule and Collecting Duct (Fig.)

  • The collecting tubule begins in the medullary ray as the continuation of DCT. As it enters the medulla it is joined by several other collecting tubules to form a larger duct, the duct of Bellini or the papillary duct which opens on the apex of the pyramid (renal papilla).
  • The collecting tubules are lined by simple cuboidal epithelium with distinct cell boundaries and clear pale cytoplasm.
  • The papillary ducts are larger, wider and lined by tall columnar cells with pale-staining cytoplasm.
  • The collecting tubules and ducts are not normally permeable to water. However, in the presence of antidiuretic hormone (ADH) secreted by the neurohypophysis, they become permeable to water. As a result, water is drawn out of the collecting ducts because of the hypertonic tissue fluid in the medullary interstitium.

  • Thus, the function of the collecting tubules and ducts is to concentrate the urine and conduct it to the calyces.

Fig. : uriniferous ( renal ) tubule.

Kidney: Medulla.

Presence of:

  • Collecting tubules (cuboidal epithelium) and collecting ducts
  • (columnar epithelium);
  • Thin segments of loop of Henle(squamous epithelium);
  • Thick segments of loop of Henle (cuboidal epithelium);
  • Vasa recta (endothelium).

JUXTAGLOMERULAR APPARATUS (JGA)

  • The JGA is involved in regulation of blood pressure and is made of three components (Fig.):

1. Juxtaglomerular (JG) cells

  • The JG cells are modified smooth muscle cells present in the tunica media of afferent arteriole at the point of contact with the DCT. The internal elastic lamina of the afferent arteriole disappears at the site of JG cells.
  • EM structure of these cells shows the characteristic features of protein secreting cells. The secretory granules contain the enzyme, renin.
  • JG cells are sensitive to the pressure of blood in the afferent arteriole (mechanoreceptor).

2. Macula densa

  • Macula densa is the specialized region in the wall of the DCT which comes into intimate contact with the JG cells of afferent arteriole. In this region the lining epithelial cells are modified to become macula densa.
  • The cells of macula densa are taller and their nuclei are denser and close to one another. The basement membrane is very thin.
  • The cells of the macula densa are sensitive to concentration of sodium ions in the fluid present in DCT (osmoreceptor/ chemoreceptor).

3. Lacis (network)/Polkissen cells

  • They are the extraglomerularmesangial cells found at the vascular pole of the renal corpuscle in close relationship with the macula densa.
  • The function of these cells are not fully understood. They may be involved in production of hormone, erythropoietin, which promotes erythropoiesis in bone marrow.

Mode of Action of JGA

A decrease in concentration of sodium ions in the fluid of DCT (due to low blood pressure resulting in low fi ltration rate) indirectly stimulates the JG cells to release renin.


Similarly, a decrease in blood pressure directly stimulates the JG cells to release renin, which activates the angiotensin–aldosterone mechanism promoting an increase in blood pressure.

Fig. : juxtaglomerular apparatus.

URETER

GENERAL FEATURES

Ureters are muscular tubes that conduct urine from renal pelvis to the urinary bladder by peristaltic contraction ofsmooth muscle present in the wall.

STRUCTURE

The wall of the ureter is made of three coats (Box 13.3). From inner to outer, they are:

1. Mucosa

  • It consists of transitional epithelium and the supporting underlying connective tissue, the lamina propria rich in elastic fi bres.
  • The mucosa is thrown into longitudinal folds which give a star-shaped appearance to the lumen in cross section.

2. Muscle coat

  • It consists of smooth muscle fi bres arranged into two layers in the upper two-third of ureter, an inner longitudinal and an outer circular layers.
  • An additional outer longitudinal layer is added in the lower third of ureter.

3. Adventitia

It is the outermost coat surrounding the muscular coat and is made of loose connective tissue containing bloodvessels, lymphatics and nerves.

URINARY BLADDER

GENERAL FEATURES

  • Urinary bladder is a muscular bag, where urine is stored temporarily and is discharged out periodically via urethra during micturition.
  • Empty bladder lies within the pelvis behind the pubic symphysis and resembles a four sided pyramid. Its normal capacity is about 200 to 300 ml.

Ureter.

Presence of:

  • Transitional epithelium;
  • Starshaped lumen;
  • Muscle coat made up of inner longitudinal and outer circular layers of smooth muscle.

STRUCTURE

Urinary bladder has following three coats:

1. Mucosa

  • It consists of transitional epithelial lining and the underlying connective tissue, the lamina propria.
  • The transitional epithelium is also called urothelium because it is found only in the urinary system. It lines the conducting passages from the minor calyx to the upper part of urethra. It is specially adapted to stretch, so as to permit great distension of the bladder without damage to its surface integrity.
  • When the bladder is empty the mucosa exhibits folds and its epithelium is thick consisting of fi ve or six cell layers.
  • The surface cells of the epithelium are rounded and bulge into the lumen. These cells often contain two nuclei or even are polyploid. The plasma membrane of these superficial cells are specially thickened to form an osmotic barrier between the toxic urine and tissue fluids.
  • When the bladder is distended with urine, the mucosal folds disappear and the epithelium becomes thin consisting of three or four cell layers. The superficial cells become flattened.

2. Muscle coat

It is made of three loosely arranged ill-defined layers of smooth muscle fi bres, namely, inner longitudinal, middlecircular and outer longitudinal. The muscle fi bres around the internal urethral orifice form an internal sphincter.

3. Adventitia

It is made of fi broelastic connective tissue carrying blood vessels, nerves and lymphatics. The superior surface ofthe bladder is covered by mesothelium of peritoneum, forming serosa instead of adventitia.

Urinary Bladder.

Presence of :

  • Ttransitional epithelium;
  • Thick muscle coat made up of discrete bundles of smooth muscle
  • separated by connective tissue;
  • Glands are absent.

URETHRA

The urethra is a tube that carries urine from the bladder to the exterior.

In the male, semen also passes through it during ejaculation. So, it is considered as part of male reproductive system .

In the female, urethra is exclusively a part of urinary system; it is described here.

Female Urethra

The female urethra is a dilatable tube (4 cm long) whose mucous membrane is thrown into longitudinal folds.

It is lined by stratified squamous epithelium except near the bladder where it is lined by transitional epithelium.

Many small urethral glands (mucous) homologous to male prostate open into the urethra.

External to mucosa there is a thin layer of spongy erectile tissue surrounded by smooth muscle fi bres arranged into innerlongitudinal and outer circular layers.