Urinary System

I. OVERVIEW (Figure 8-1 ). Intermediate mesoderm forms a longitudinal elevation along the dorsal body wall called the nephrogenic cord, which forms the pronephros, mesonephros, and metanephros (adult kidney). The homeobox genes, Lim-1 and Pax-2, appear to be important in this early stage of kidney development.

A. The pronephros is not functional and completely regresses.

B. The mesonephros is functional for a short period and completely regresses, except for the mesonephric (wolffian) duct.

C. The metanephros (adult kidney) develops from two different sources:

1. The ureteric bud is an outgrowth of the mesonephric duct. This outgrowth is regulated by WT-1 (an anti-oncogene), GDNF (glial cell line-derived neurotrophic factor), and c-Ret (a tyrosine kinase receptor).

2. The metanephric mesoderm is a condensation of mesoderm near the ureteric bud.

II. DEVELOPMENT OF THE KIDNEY (Table 8-1 and Figure 8-2)

A. Development of the metanephros

1. The ureteric bud penetrates the metanephric mesoderm and undergoes repeated divisions to form the collecting duct, minor calyx, major calyx, renal pelvis, and ureter.

2. The metanephric mesoderm forms metanephric vesicles (or blastema) that differentiate into primitive renal tubules. This differentiation is regulated by FGF-2 (fibroblast growth factor), BMP-7 (bone morphogenetic protein), and Wnt-11.

3. The primitive renal tubules eventually form the renal glomerulus, renal (Bowman's) capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, and connecting tubule.

B. Relative ascent of the kidneys

1. The fetal metanephros is located in the sacral region, whereas the adult kidney is located at vertebral levels T12-L3. The change in location results from a disproportionate growth of the embryo caudal to the metanephros.

2. During the relative ascent, the kidneys rotate 90 degrees medially, causing the hilum to orientate medially.

C. Blood supply of the kidneys

1. The blood supply changes as the metanephros undergoes its relative ascent. The

Intermediate mesoderm

Formation Intermediate Mesoderm

Nephrogenic

Figure 8-1. Formation of the nephrogenic cord as the embryo goes through craniocaudal and lateral folding.

Nueral Tube Formation

Neural tube

Gut tube

Intermediate mesoderm

Nephrogenic

Neural tube

Gut tube

Figure 8-1. Formation of the nephrogenic cord as the embryo goes through craniocaudal and lateral folding.

metanephros will receive its blood supply from arteries at progressively higher levels until the definitive renal arteries develop at L2.

2. Arteries formed during the ascent may persist and are called supernumerary arteries. Supernumerary arteries are end arteries; therefore, damage :o them will result in necrosis of kidney parenchyma.

III. DEVELOPMENT OF THE UROGENITAL SINUS (Figure 8-2A) A. Urinary bladder

1. The urinary bladder develops from the upper end of the urogenital sinus, which is continuous with the allantois.

2. The allantois normally degenerates and forms a fibrous cord in the adult called the urachus or median umbilical ligament.

Table 8-1

Development of the Kidney

Embryo

Adult Derivative

Ureteric bud

Metanephric mesoderm

Collecting duct Minor calyx Major calyx Renal pelvis Ureter

Renal glomerulus Renal (Bowman's) capsule Proximal convoluted tubule Loop of Henle Distal convoluted tubule Connecting tubule

Minor Calyx

(T) Connecting tubule

Minor calyx

Major calyx

Renal pelvis

Collecting duct

Minor calyx

Major calyx

Renal pelvis

Metanephric vesicles

Lobe

Ureter

Metanephric vesicles

Distal convoluted tubule

(T) Connecting tubule

Minor Calyx

Bowman's (4) capsule

Glomerulus (5)

Proximal convoluted (3) tubule

Bowman's (4) capsule

Glomerulus (5)

Loop of Henle (§) Collecting duct

Lobe

Ureter

Figure 8-2. Formation of the adult kidney. (A) Lateral view of the embryo showing the ureteric bud (UB; shaded) budding off the mesonephric duct (MD) and the metanephric mesoderm (MM). The arrow indicates the ascent of the kidney. AL = allantois; HG = hindgut; UG = urogenital sinus. (B) Lateral view of a fetal kidney. Shaded area indicates structures formed from the ureteric bud. Note the lobulated appearance of a fetal kidney. The lobulation disappears during infancy as the kidney grows through elongation of the proximal convoluted tubules and loops of Henlc. (C) Enlarged view of the rectangle shown in B, illustrating a collecting duct (Shaded) derived from the ureteric bud and those structures derived from the metancphric vesicle. Structures numbered ©through ©make up a nephron. (B and C from Dudek RW, Fix JF: BRS Embryology, 2nd ed. Baltimore, Williams & Wilkins, 1998, p 165.)

3. The trigone of the bladder is formed by the incorporation of the lower end of the mesonephric ducts into the posterior wall of the urogenital sinus.

B. Inferior two thirds of the vagina (see Chapter 9, Figures 9-1 and 9-2)

1. The paramesonephric ducts project into the dorsal wall of the urogenital sinus and induce rhe formation of the sinovaginal bulbs.

2. The sinovaginal bulbs fuse to form the vaginal plate, which canalizes and develops into the inferior two thirds of the vagina.

C. Female urethra and vestibule of the vagina

1. These are formed from the lower end of the urogenital sinus.

2. Outgrowths from these structures form the urethral, paraurethral, and greater vestibular glands.

D. Male urethra

1. The prostatic urethra, membranous urethra, and proximal part of the penile urethra form from the lower end of the urogenital sinus.

2. The distal part of the penile urethra forms from an ectodermal ingrowth called the glandular plate, which becomes canalized to form the navicular fossa.

IV. DEVELOPMENT OF THE SUPRARENAL GLAND

A. Cortex

1. The cortex forms from two episodes of mesoderm proliferation; the first episode forms the fetal cortex, and the second episode forms the adult cortex.

2. The fetal cortex is present at birth bur regresses by the sccond postnatal month.

3. The zona glomerulosa and zona fasciculata of the adult cortex are present at birth, but the zona reticularis is not formed until 3 years of age.

B. Medulla

1. The medulla forms when neural crest cells aggregate at the medial aspect of the fetal cortex and eventually become surrounded by the fetal cortex.

2. The neural crest cells differentiate into chromaffin cells. Chromaffin cells can be found in extra suprarenal sites at birth, but these sites normally will regress completely by puberty. In a normal adult, chromaffin cells are found only in the suprarenal medulla.

V. CLINICAL CORRELATIONS

A. Renal agenesis occurs when the ureteric bud fails to develop.

1. Unilateral renal agenesis a. Unilateral renal agenesis is relatively common; therefore, a physician should never assume that a patient has two kidneys.

b. It is more common in males.

C. It is asymptomatic and compatible with life because the remaining kidney hypertrophies.

2. Bilateral renal agenesis a. Bilateral renal agenesis is relatively uncommon.

b. It causes oligohydramnios during pregnancy, which allows the uterine wall to compress the fetus, resulting in Potter's syndrome (deformed limbs, wrinkly skin, and abnormal facial appearance) (see Chapter 5 III CI).

C. It is incompatible with life unless a suitable donor is available for a kidney transplant.

B. Horseshoe kidney (Figure 8-3A) occurs when the inferior poles of both kidneys fuse. During the ascent, the horseshoe kidney gets trapped behind the inferior mesenteric artery.

C. Urachal fistula occurs when the allantois persists, thus forming a direct connection between the urinary bladder lumen and outside of the body ar the umbilicus. This condition is associated with urine drainage from the umbilicus.

Deformed Vagina

D- Ectopic ureteric orifices

1. In males, the ectopic ureter usually opens into the neck of the bladder or prostatic urethra.

2. In females, the ectopic ureter usually opens into the neck of the bladder or vestibule of the vagina.

3- Incontinence is the common complaint because urine continually dribbles from the urethra in males and from the urethra or the vagina in females.

1. Wilms' tumor is the most common primary renal tumor of childhood.

2. The tumor presents as a large, solitary, well-circumscribed mass that on cut section is soft, homogeneous, and tan to gray.

3. Wilms' tumor is interesting histologically, because this tumor tends to recapitulate different stages of embryologic formation of the kidney. Three classic histologic areas are described: (1) a stromal area, (2) a blastemal area of tightly packed embryonic cells, and (3) a tubular area.

F. Polycystic disease of the kidneys (Figure 8^3 C, D)

1. Polycystic disease of the kidneys occurs when the loops of Henle dilate, forming large cysts that severely compromise kidney function.

2. It is a relatively common hereditary disease.

3. It is associated clinically with cysts of the liver, pancreas, and lungs.

4. Treatment consists in dialysis and kidney transplantation.

G. Congenital adrenal hyperplasia (CAH; see Chapter 9 IV CI and Figure 9-4 A)

1. CAH is caused most commonly by mutations in genes for enzymes involved in adrenocortical steroid biosynthesis (e.g., 2 ^hydroxylase deficiency, 11P-hydroxylase deficiency).

2. In 21-hydroxylase deficiency (90% of all cases), there is virtually no synthesis of the aldosterone or Cortisol, so that intermediates are funneled into androgen biosynthesis, thus elevating androgen levels.

3- The elevated levels of androgens lead to virilization of a female fetus ranging from

Figure 8-3. (A) Photograph of a horseshoe kidney. U = ureter; BW = bladder wall. (B) Photomicrograph of Wilms' tumor. This tumor is characterized histologically by recognizable attempts to recapitulate embryonic development of the kidney. In this regard, the following three components are seen: (1) metanephric blastema elements (bias) consisting of clumps of small, tightly packed embryonic cells; (2) stromal elements (str); and (3) epithelial elements, generally in the form of abortive attempts at forming tubules (t) or glomemli. (C) Photograph of an infant with polycystic kidney. (D) Photomicrograph of polycystic kidney showing large, fluid-filled cysts (CY) throughout the substance of the kidney. Between the cysts, some functioning nephrons can be observed. ( E) Photomicrograph of a pheochromocytoma. The neoplastic cells have abundant cytoplasm with small centrally located nuclei. The cells are generally separated into clusters separated by a slender stroma and numerous capillaries. (F) Photomicrograph of a neuroblastoma. The neoplastic cells are small, primitive-looking cells with dark nuclei and scant cytoplasm. The cells are generally arranged as solid sheets and some cclls arrange around a central fibrillar area forming Homer-Wright pseudorosettes (*). (A, from Stevenson RE: Human Malformations and Related Anomalies. New York, Oxford University Press, 1993. C, From Papp Z: Atlas of Fetal Diagnosis. New York, Elsevier, 1992, p 178. B, D, E, and F, From East Carolina University, Department of Pathology slide collection.)

mild cliroral enlargement to complete labioscrotal fusion with a phalloid organ (i.e., female pscudointcrscxuality)

4. Because Cortisol cannot be synthesized, negative feedback to the adenohypophysis does not occur; thus, ACTH continues to stimulate the adrenal cortex and results in adrenal hyperplasia.

5. Because aldosterone cannot be synthesized, the patient presents with hyponatremia ("salt-wasting") with adjoining dehydration and hyperkalemia.

6. Treatment includes immediate infusion of intravenous saline and long-term steroid hormone replacement, both Cortisol and mineralocorticoids ^«-fludrocortisone).

H. Pheochromocytoma (Figure 8-3E)

1. Pheochromocytoma is a relatively rare neoplasm that contains both epinephrine and norepinephrine.

2. It occurs mainly in adults 40-60 years old.

3- It is generally found in the region of the adrenal gland, but may be found in extra suprarenal sites.

4. Associated signs and symptoms include persistent or paroxysmal hypertension, anxiety, tremor, profuse sweating, pallor, chest pain, and abdominal pain.

5. Common laboratory findings are increased urine vanillylmandelic acid (VMA) and metanephrine levels, inability to suppress catecholamines with clonidine, and hyperglycemia.

6. Treatment is by surgery or phenoxybenzamine (an «-adrenergic antagonist).

I. Neuroblastoma (Figure 8-3F)

1. Neuroblastoma is a common extracranial neoplasm containing primitive neuroblasts of neural crest origin.

2. It occurs mainly in children.

3- It is found in extra-adrenal sites usually along the sympathetic chain ganglia (60%) or within the adrenal medulla (40%).

4. It contains small cells arranged in Homer-Wright pseudorosettes.

5. It involves increased urine VMA and metanephrine levels.

6. It metastasizes widely.

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