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Permeability of the Thin Segment
The descending portion of the thin segment is permeable, permitting free passage or equilibrationof salt and water between the lumen of the nephron and the peritubular connective tissue.
Distal Thick Segment
The thick, straight, ascending segment of the distal tubule is the third part of the loop of Henle and includes both medullary and cortical portions, with the latter in the medullary rays. The straight segment of the distal tubule, like the ascending thin segment, transports ions from the tubular lumen to the interstitium. The large, cuboidal cells of the distal tubule have extensive basal-lateral plications and numerous large mitochondria associated with these basal folds. They have considerably fewer and less well-developed microvilli than proximal tubule cells.
The lateral margins of the cells are indistinct. The nucleus is located in the apical portion of the cell and sometimes, especially in the straight segment, causes the cell to bulge into the lumen.
The Distal ConvolutedTubule Exchanges Na+ for K+ Under Aldosterone Regulation
The distal convolutedtubule is short segment that responsible for:
-Reabsorption of Na+ and secretion of K+ in order to conserve Na+
-Continued reabsorption of bicarbonate ion
-Aldosterone secreted by the adrenal gland increases the reabsorption of Na+ and secretion of K+. This has the effect of increasing blood volume and blood pressure in response to increased blood Na+ concentration.
Antidiuretic hormone (ADH), can act on the terminal portion of the distal convoluted tubule to increase the permeability of the tubule to water, thereby producing a more concentrated urine. The action of this hormone is more significant in the collecting tubules and collecting ducts.
Collecting Tubules and Collecting Ducts
Both the collecting tubules and ducts are composed of a simple epithelium. The arched and cortical collecting tubules have flattened cells, somewhat squamous to cuboid in shape. The medullary collecting ducts have cuboidal cells, with a transition to columnar cells as the ducts increase in size.
Cytologically, two distinct types of cells are present in the collecting system:
Light cells, also called collecting duct or CD cells, are the principal cells of the system. They are cells with basal infoldings, true infoldings rather than processes that implicate with those of adjacent cells. They possess a single cilium and have relatively few short microvilli.
Dark cells, also called intercalated (1С) cells, occur in considerably smaller numbers. They have many mitochondria, and their cytoplasm appears more dense. Numerous vesicles are present in the apical cytoplasm.
INTERSTITIAL CELLS
The connective tissue of the kidney parenchyma, called interstitial tissue, surrounds the components of the nephrons, the ducts, and the blood and lymphatic vessels. In the cortex, two types of interstitial cells are recognized: cells that resemble fibroblasts, found between the basement membrane of the tubules and the adjacent peritubular capillaries, and occasional macrophages.
In the medulla, the principal interstitial cells resemble myofibroblasts. Prominent lipid droplets in the cytoplasm appear to increase and decrease in relationship to the diuretic state. Some experimental evidence suggests that these cells may secrete a hormone-like material that reduces blood pressure. In addition, it has been reported that prostaglandins and prostacyclin are synthesized in the interstitium.
HISTOPHYSIOLOGY OF THE KIDNEY
The Countercurrent Multiplier System Creates Hypertonic Urine
The ability to excrete a hypertonic urine depends or combination of three morphologic specializations:
-Loop of Henle
-Vasa recta,which form a loop paralleling the loop Henle
-Arrangement of the collecting ducts in the medulla
A Standing Gradient of Ion Concentration Produces Hypertonic Urine
The loop of Henle is the structure that allows the creation and maintenance in the medullary interstitium of a gradient of ion concentration that increases from the corticomedullary junction to the renal papilla. The concentration of NaCl in the interstitium gradually increases down the length of the loop of Henle and, consequently, through the thickness of the medulla from the corticomedullary junction to the papilla.
Collecting Ducts and Vasa Recta Are Countercurrent Exchangers
Vasa Recta
The efferent arterioles of the renal corpuscles of most of the cortex branch to form the capillary network that surrounds the tubular portions of the nephron in the cortex, the peritubular capillary network. The efferent arterioles of the juxtamedullary renal corpuscles form several unbranched arterioles that descend into the medullary pyramid. These arteriolae rectae make a hairpin turn deep in the medullary pyramid and ascend as the venulae rectae. Together, the descending arterioles and the ascending venules are called the vasa recta.
Countercurrent exchange occurs between the collecting system and vasa recta
Figure 92. Diagram showing movement of substances into and out of the nephron and collecting system
Because the ascending limb of the loop of Henle has a high level of transport activity and because it is impermeable to water, the modified filtrate that ultimately reaches the distal convoluted tubule is hypotonic. When ADH is present, the distal convoluted tubules, the collecting tubules and the collecting ducts are highly permeable to water. Within the cortex, the modified filtrate equilibrates to isotonicity in the distal convoluted tubule, partly by loss of water to interstitium and partly by addition of ions other than Na+ and Cl- to the filtrate. In the medulla, increasing amounts of water leave the filtrate as the collecting ducts pass through the increasingly hypertonic interstitium on their course to the papillae.
The vasa recta also form loops in the medulla parallel the loop of Henle. The vasa recta also form countercurrent exchange system in the following manner. As the arterial vessels descend through the medulla, the blood loses water to the interstitium and gains salt from the interstitium, so that at the tip of the loop, deep in the medulla, the blood essentially in equilibrium with the hypertonic interstitial fluid.
As the venous vessels ascend toward the corticomedullary junction, the process is reserved; the hypertonic blood loses salt to the interstitium and gains water from the interstitium. This passive countercurrent exchange of water and salt between the blood and the interstitium occurs without expenditure of energy by the endothelial cells.
Excretory passages
The urine that excreted at the tip of the papilla flows sequentially
-to the minor calyx of that papilla
-to a major calyx
-to the renal pelvis
-through the ureter to the urinary bladder
-through the urethra, where it is voided
All of these excretory passages except the urethra have the same general structure, namely, a mucosa, a muscularis, and an adventitia (or in some regions serosa)
Transitional epithelium
It lines all of the excretory passages. Transitional epithelium is essentially impermeable to salts and water. The apical surface of the cell is very irregular, with deep clefts penetrating into the apical region. Modified areas of plasma membrane called plaques are seen on the luminal surface of the cells.
Connective tissue and muscle
In the ureters and urethra there are usually two layers of smooth muscle beneath the lamina propria:
The inner layer is arranged in a loose spiral described as a longitudinal layer.
The outer layer is arranged in a tight spiral described as a circular layer
The smooth muscle of the urinary passages is mixed with connective tissue, so that it forms parallel bundles rather than pure muscular sheets. Peristaltic contractions of the smooth muscle move the urine from the minor calyces through the ureter to the bladder.
The smooth muscle of the bladder wall is less regularly arranged than that of the tubular portions of the excretory passages, and thus, there is more random mixing of muscle and collagen bundles. Contraction of the smooth muscle of the bladder muscle compresses the whole viscus and forces the urine into the urethra.
URETERS
The ureters conduct urine from the renal pelvis to the bladder and follow an oblique path through the wall of the bladder. Contraction of the smooth muscle of the bladder wall also compresses the openings of the ureters into the bladder. This is important in protecting the kidney from the spread of infection from the bladder and urethra.
The terminal portion of the ureters is a thick outer layerof longitudinal muscle.
URINARY BLADDER
The bladder is a distensible reservoir for urine; its size and shape change as it fills. The smooth muscle of the bladder wall forms an internal sphincter, a ring-like arrangement of muscle around the opening of the urethra. The triangular region defined by these three openings the trigone, is relatively smooth.
URETHRA
In the male, the urethra is about 20 cm long and serves as the terminal duct of both the urinary and genital systems. It has three distinct segments:
- Prostatic urethraextends for 3-4 cm from the neck of the bladder through the prostate gland
-Membranous urethra extends for about 1 cm from the apex of the prostate gland through the body wall
-Penile urethra extends for about 15cm through the length of the penis to open the body surface at the glans penis.
Male reproductive system
The male reproductive system consists of the testes, epididymides and genital ducts, accessory reproductive glands, and penis. The accessory glands include the seminal vesicles, prostate, and bulbourethral glands. The primary function of the testis is the production of sperm, the male gamete.
Figure 93. Schematic diagram demonstrating the components of the male reproductive system
TESTIS
Figure 94. Diagram and photomicrograph of a sagittal section of the human testis
The Testes Produce Sperm and Androgens
Androgens, especially testosterone, are steroid hormones secreted by the interstitial cells in the testes. They influence embryonic development, sexual maturation, and reproductive function.
The Testes Have an Unusually Thick Connective Tissue Capsule, the Tunica Albuginea
A thick fibrous connective tissue capsule, the albuginea, covers each testis. The inner part of this capsule is the tunica vasculosa, a loose connective tissue layer that contains blood vessels. Each testis is divided into approximately 250 lobules by incomplete connective tissue septa that project from the capsule.
Along the posterior surface of the testis, the albuginea thickens and projects inward as the mediastinum testis.
Each Lobule Contains Several Highly Convoluted Seminiferous Tubules
Each lobule contains 1—4 seminiferous tubules, which produce the sperm, and a connective tissue stroma in whichthe interstitial or Leydig cells are contained. Each tubule within the lobule forms a convoluted loop. The ends of the loop in the vicinity of the mediastinum are called the tubuli recti or straight tubule and are continuous with the rete testis anastomosing channel system within the mediastinum.
The Seminiferous Tubules Consist of a Seminiferous Epithelium Surrounded by a Tunica Propria
The seminiferous epithelium is a complex stratified epithelium composed of two basic populations of cells:
-Spermatogenic cells, which regularly replicate and differentiate into mature sperm
-Sertoli cells, also known as supporting or sustentacular cells.
Sertoli cells
Figure 95. Schematic drawing of the Sertoli cell and its relationship to the adjacent spermatogenic cells
The Sertoli cells are columnar cells with complex apical and lateral processes that surround the adjacent spermatogenic cells and fill the spaces between them. The Sertoli cells give structural organization to the tubules as they extend through the full thickness of the seminiferous epithelium.
The Sertoli –Sertoli junctional complex divides the seminiferous epithelium into basal and luminal compartments.
Spermatogonia and early primary spermatocytes are restricted to the basal compartment, i.e., between the Sertoli-Sertoli junctions and the basal lamina. More mature spermatocytes and spermatids are restricted to the luminal side of the Sertoli-Sertoli junctions. The function of Sertoli cells is in the exchange of metabolic substrates and wastes between the developing spermatogenic cells and the circulatory system. In addition, Sertoli cells phagocytized and break down any spermatogenic cells that fail to differentiate completely.
The Sertoli-Sertoli junctional complex is the site of the blood-testis barrier.
Plasma proteins and circulating antibodies are excluded from the lumen of the seminiferous tubules. The exocrine secretory products of the Sertoli cells, particularly androgen-binding protein (ABP), can become highly concentrated in the lumen. Most importantly, the barrier isolates the genetically and, therefore, antigenically different haploid germ cells (secondary spermatocytes, spermatids, and sperm) from the immune system of the adult male. Antigens produced by or specific to the sperm are prevented from reaching the systemic circulation.
The tunica propria also called the lamina propria, is a multilayered connective tissue that lacks typical fibroblasts. In man, it consists of three to five layers of myoid cellsand collagen fibrils external to the basal lamina of the seminiferous epithelium. Rhythmic contractions of the myoid cells create peristaltic waves that help move spermatozoa and testicular fluid through the seminiferous tubules to the excurrent duct system.
Leydig Cells
Leydig cells are large, polygonal, acidophilic cells that typically contain lipid droplets. The enzymes necessary for the synthesis of testosterone from cholesterol are associated with the sER. Leydig cells secrete testosterone.
INTRATESTICULAR DUCTS
Straight Tubules, the Tubuli Recti, Join the seminiferous Tubules with the Rete Testis of the mediastinum
At the end of each seminiferous tubule there is an abrupt transition to the tubuli recti or straight tubules. This short terminal section of the seminiferous tubule is lined only by Sertoli cells. Near their termination, the tubuli rectinarrow, and their lining changes to a simple cuboidal epithelium.
The straight tubules empty into the rete testis, a complex series of interconnecting channels within the highly vascular connective tissue of the mediastinum. A simplecuboidal or low columnar epithelium lines the channels of the rete testis. These cells have a single apical cilium and few apical microvilli.
EXCURRENT DUCT SYSTEM
Efferent Ductules
The Efferent Ductules Connect the Rete Testis to the Epididymis
In man, approximately 20 efferent ductules (ductuli efferentes) connect the channels of the rete testis at the superior end of the mediastinum to the proximal portion of the ductus epididymis. They are highly coiled and form 6-10 conical masses.
The efferent ductules are lined with a pseudostratified columnar epithelium. Interspersed among the columnar cells are a few basal cells and intraepithelial lymphocytes. The tall columnar cells are generally ciliated.
The smooth muscle forms a layer in which the cells are arrayed as a circular sheath in the wall of the ductule. Transport of the sperm in the efferent ductules is effected largely by both ciliary action and contraction of this fibromuscular layer.
Ductus Epididymis
Figure 96. Photomicrograph of the human ductus epididymis
The Ductus Epididymis Is a Highly Coiled Tube in Which Sperm Undergo Further Maturation
The epididymislies along the posterior surface of testis. It consists of the ductus epididymis and its associated vascularized connective tissue, smooth muscle, and a fibrous connective tissue tunic. The ductus epididymis is a highly coiled tube. It is divided a head (caput), a body (corpus), and a tail (cauda). Sperm mature during their passage through the epididymis. This maturation, like their earlier differentiation, is androgen dependent. During this process, changes occur in the sperm plasma membrane, including the addition to the glycocalyx of glycoproteins secreted by the epididymal epithelial cells. The ductus epididymis is lined with a pseudostratified columnar epithelium. It contains principal cells (tall) and basal cells (short). Numerous, long microvilli called stereocilia extend from the luminal surface of the principal cells. Basal cells are the stem cells of the duct epithelium.
Epididymal Cells Function in Both Absorption and Secretion
The Smooth Muscle Coat
In the head of the epididymis and most of the body, the smooth muscle coat consists of a thin layer of circular smooth muscle. In the tail, inner and outer longitudinal layers are added. These three layers are then continuous with the three smooth muscle layers of the ductus deferens, the next component of the excurrent duct system.
Ductus deferens
The ductus deferens (vas deferens), the terminal portion of the excurrent system, ends in the prostatic urethra
The ductus deferens is a direct continuation of the epididymis. It ascends along the posterior border of the testis, close to the testicular vessels nerves. The vas deferens then descends to the pelvis, to the level of the bladder, where it connects to the prostatic urethra. The distal end of the ductus deferens enlarges to form the ampulla. It is joined there by the duct of the seminal vesicle continues through the prostate gland to the urethra as ejaculatory duct. The ampulla has taller, branched mucosal folds. There is no muscularis layer in the wall of the ejaculatory tract.
The ductus deferens is lined with a pseudostratified columnar epithelium. The tall columnar cells also have long microvilli that extend into the lumen. The rounded basal cells rest onthe basal lamina. The lumen of the duct does not appear smooth. It appears to be thrown into deep longitudinal folds throughout most of its length, probably due to contraction.
Accessory sex glands
Seminal vesicles
The paired seminal vesicles secrete the fluid that is rich in fructose. The seminal vesicles are paired, elongate tubular glands that have a muscular and fibrous coat. The pseudostratified columnar epithelium contains tall, nonciliated columnar cells and short round cells. Prostaglandins are synthesized in large amounts in the seminal vesicles. Contraction of the smooth muscle coat of the seminal vesicles during ejaculation discharges their secretion into the ejaculatory ducts and helps to flush sperm out of the urethra.
Prostate Gland
Figure 98. Schematic drawings of the zones of the human prostate gland
The Prostate, the Largest Accessory Sex Gland, Secretes Acid Phosphatase, Fibrinolysin, and Citric Acid
The prostate consists of a collection of 30—50 tubuloalveolar glands that surround the proximal urethra. The glands are arranged in three concentric layers: a mucosal layer, a submucosal layer, and the most peripheral layer containing the main prostatic glands. The mucosal glands secrete directly into the urethra; the other two layers have ducts that open into the prostatic sinuses located on either side of the crest of the posterior wall of the urethra.
The epithelium is generally columnar but may have patches that are cuboidal, squamous, or pseudostratified. It secretes protein enzymes. The secretions are pumped into the urethra at the time of ejaculation by contraction of the fibromuscular tissue of the prostate. The epithelium of the prostate is also depended on testosterone for normal morphology and function.
Bulbourethral Glands
The Bulbourethral Glands Secrete Preseminal Fluid
The paired bulbourethral glands are pea-sized structures located in the urogenital diaphragm. The glands are compound tubuloalveolar glands that structurally resemble mucus secretory glands. It is lined by simple columnar epithelium.
Erotic stimulation causes release of the secretion, which constitutes the major portion of the preseminal fluid and probably serves as a lubricant of the penile urethra.
Semen
Semen contains fluids and sperm from the testis and secretory products from the epididymis, vas deferens, pros seminal vesicles, and bulbourethral glands.
The volume of the average ejaculate of semen is 3 mL. This normally contains up to 100 million sperm mL, of which it is estimated that 20% are morphologic abnormal and nearly 25% are immotile.
Penis
Figure 99. Photomicrograph of the histologic section of the penis
The penis is the termination of both the urinary system and the reproductive excurrent duct system in the male. The urethra, which originates at the bladder and extends through the penis, carries both semen and urine to the exterior.
Erection of the penis involves the filling of the vascular sinuses of the corpora cavernosa and spongiosum
The penis consists principally of two dorsal masses of erectile tissue, the corpora cavernosa, and a ventral mass erectile tissue, the corpus spongiosum, in which the urethra is embedded. A dense fibroelastic layer, the tunica albuginea, binds the three together and forms a capsule around each one. The corpora cavernosa are lined with vascular endothelium.
The skin of the penis is thin and loosely attached to the underlying loose connective tissue except at the glans, where it is very thin and tightly attached. There is a thin layer of smooth muscle that is continuous with the dartos layer of the scrotum.
The Female Reproductive System
The Female Reproductive System Consists of Internal Sex Organs and External Genital Structures
The internal organs are the ovaries, oviducts, uterus, and vagina.
The external genitalia include the mons pubis, labia majora and minora, clitoris, vestibule and opening of the vagina, and external urethral orifice and mammary glands.
Figure 100. Schematic drawing of the female internal sex organs
OVARY
Production of Gametes and Production of Steroid Hormones Are the Two Major Functions of the Ovary
Two major groups of steroid hormones are secreted by the ovaries. They are the estrogens and the progestogens.
Estrogens promote growth and maturation of internal and external sex organs and are responsible for the typical female characteristics.
Progestogens prepare the internal sex organs, mainly the uterus, for pregnancy by promoting secretory changes in the endometrium. Both hormones play an important role in the menstrual cycle by preparing the uterus for implantation of a fertilized ovum.
Ovarian Structure
The ovaries are paired, almond-shaped, pinkish-white structures. The surface of the ovary is covered by tunica albuginea. Each ovary is attached to the mesovarium. The ovary is composed of a cortex and a medulla.
-Medulla or medullary region located in the central portion of the ovary contains loose connective tissue, a mass of relatively large contorted blood vessels, lymphatic vessels, and nerves.
Figure 101. Schematic drawing of a section through the ovary
-Cortex or cortical region found in the peripheral portion of the ovary surrounding the medulla contains the ovarian follicles embedded in a richly cellular connective tissue. Scattered smooth muscle fibers are present in the stroma around the follicles.
Follicle Development
Three basic types of ovarian follicles can be identified, based on developmental state:
1. Primordial follicles
2. Growing follicles (primary and secondary (or antral) follicles)
3. Mature or Graafian follicles
The Primordial Follicle Is the Earliest Stage of Follicular Development
Primordial follicles first appear in the ovaries during the third month of fetal development. Early growth of the primordial follicles is independent of gonadotropin stimulation. The primordial follicles are found in the stroma of thecortex just beneath the tunica albuginea. A single layer of squamous follicular cells surrounds the oocyte.
The Primary Follicle Is the First Stage of the Growing Follicle
Initially, the oocyte enlarges and the surrounding flattened follicular cells proliferate and become cuboidal.As the oocyte grows, a homogeneous, acidophilic layer called the zona pellucida appears between the oocyte and the adjacent follicular cells. Follicular cells undergo stratification to form the granulosa layer of the primary follicle.
Connective Tissue Cells Form the Theca Layers of the Primary Follicle
-The theca interna is the inner, highly vascularized layer of cuboidal secretory cells. Cells of the theca interna possess a large number of luteinizing hormone (LH) receptors.
-The theca externa is the outer layer of connective tissue cells.
Maturation of the Oocyte Occurs in the Primary Follicle
The oocytes exhibit specialized secretory vesicles known as cortical granules. They are located just beneath the plasma membrane.
The Secondary Follicle Is Characterized by a Fluid-Containing Antrum
Liquor folliculi includes estrogen. In the secondary follicle begins to form a single cavity called the antrum. The follicle is now identified as a secondary or antral follicle. The granulosa cells form the cumulus oophorus that projects into the antrum. The cells of the cumulus oophorus that immediately surround the oocyte and remain with it at ovulation are referred to as the corona radiata.
The Graafian Follicle Contains the Mature Secondary Oocyte
The mature follicle, also known as a Graafian follicle, has a diameter of 10 mm or more. The stratum granulosum appears to become thinner as the antrum increases in size. The oocyte and cumulus cells are gradually loosened from the rest of the granulosa cells in preparation for ovulation. The cumulus cells immediately surrounding the oocyte now form a single layer of cells of the corona radiata.
The granulosa and thecal cells then undergo luteinization and produce progesterone.
Ovulation
Figure 102. Ovulation
Ovulation is the process by which an oocyte is released from the Graafian follicle.
A combination of hormonal changes and enzymatic effects is responsible for the actual release of the secondary oocyte at the middle of the menstrual cycle. The factors include
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