Embryology, anatomy, and anomalies of the adrenal gland

Embryology, anatomy, and anomalies of the adrenal gland

Embryology, Anatomy, and Anomalies of the Adrenal Gland By Harold A. Mitty EMBRYOLOGY H E ADRENAL GLAND is composed of two separate functional units,...

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Embryology, Anatomy, and Anomalies of the Adrenal Gland By Harold A. Mitty EMBRYOLOGY

H E ADRENAL GLAND is composed of two separate functional units, the cortex and the medulla. Although these units have completely separate origins, they unite within a single capsule during fetal life. The separate nature of cortex and medulla is well illustrated in fish and other lower forms of animal life in which these structures exist as distinct glands.

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The Cortex

The cortical primordium of the adrenal gland arises from developing peritoneal epithelium or coelomic mesoderm. These mesenchymal cells are located on the posterior trunk at the angle of the genital ridge and the root of the mesentery. ',2 This group of cells grows rapidly and penetrates the retroperitoneal mesenchyme near the cranial end of the mesonephros to form the primitive cortex by the fifth week of embryologic development. This primitive cortex (provisional cortex, fetal zone, transitional zone, X-zone) accounts for the bulk of the cortex during fetal life. At 7 weeks (12 mm embryo), a second wave of cells migrate from the cortical primordium to envelope the primitive cortex and form a permanent cortex. At the end of the 8th week, the cortical mass separates from the peritoneal mesothelium and becomes encapsulated by connective tissue. The gland is now much larger than the kidney. The primitive cortex atrophies immediately after birth. This change appears to be related to the birth process because it occurs at the same rate in premature and term infants. This process is seen as vascular engorgement, so marked as to resemble hemorrhage. As a result of this atrophy, the adrenal glands lose a third of their weight by the 2nd week after birth. The primitive cortex is completely resorbed by the end of the 1st year. The outer or permanent cortex begins to differentiate after birth. By the 3rd year, differentiation into zona glomerulosa, fasciculata, and reticularis is complete.

The Medulla

Ectodermal cells from the neural crest migrate forward to become the primitive sympathetic ganglia."? Some of these cells do not differentiate into neurons but instead become endocrine cells. They are called chromaffin cells or pheochromoblasts because they stain brown when exposed to chromic acid salts. This reaction is due to the presence of epinephrine and norepinephrine in the cells. These cells are widely dispersed in the embryo. At about the 10th week of development, a group of chromaffin cells from the primitive sympathetic ganglia migrate and invade the medial aspect of the fetal adrenal cortex. By the 18th week, these cells have achieved their position central to the cortex. During fetal development, most medullary functions originate from the larger paraganglionic masses. Significant adrenal medullary function is not evident until about the 11th week of development. After birth, much of the paraganglionic tissue atrophies and the adrenal medulla predominates. Some of the paraganglionic chromaffin tissue may remain as discrete structures near the origin of the celiac and of the superior mesenteric artery. Accessory medullary tissue may also be found with sympathetic nerves as well as near the origin of the inferior mesenteric artery, where it is called the organ of Zuckerkandl. It is of interest that during gestation and at birth the catecholamine content of the medulla and paraganglionic tissue is almost all norepinephrine. By 2 years of age this changes, so that 80% of the medullary output is epinephrine." ANATOMY

The adrenal gland may be described as having superior, middle, and basal portions, or tail, From the Department of Radiology. The Mount Sinai School of Medicine of the CUy University ofNew York. Harold A. Mitty: Professor ofRadiology. Address reprint requests to Harold A, Mitty, MD, I Gustave L. Levy PI, New York, NY 10029. © 1988 by Grune & Stratton, Inc. 0037-l98X/88/2304-0007$5.00/0

Seminars in Roentgenology, Vol XXIII, No 4 (October). 1988: pp 271-279

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body, and head. The head is the most inferior portion and contains most of the medulla, with a corticomedullary ratio of 4:1. The body has a small amount of medulla, resulting in a corticomedullary ratio of 15:1. The tail is essentially all cortex. Microscopic remnants may also be found in the course of the sympathetic chain. Anatomic Relationships

The adrenal glands are located at the level of the 11th or 12th rib lateral to the vertebrae in the extreme superoposterior portion of the retroperitoneal perirenal space.' The left gland extends as low as L-l, often joining the anterior portion of the renal vascular pedicle where the adrenal vein drains into the renal vein. The glands are attached to the inner surface of the anteromedial and superior aspects of the perirenal fascia. Like the kidney, the glands are surrounded by fatty areolar tissue, which is useful in separating them

HAROLD A. MITTY

from adjacent structures such as the kidneys, aorta, and liver (Fig 1). On the right, this areolar tissue thins and practically disappears between the gland and the posterior aspect of the inferior vena cava. It is important to recognize that the kidney is not fixed to the perirenal fascia. For this reason, deep inspiration or the upright position often causes discernible separation of the adrenal and kidney. This separation maneuver, particularly during sonography, can be employed to help demonstrate whether a mass is of renal or adrenal origin. The inferior portion of the right gland is located anteromedial to the upper pole of the kidney; the superior portion of the gland extends cephalad to the kidney. The anteromedial aspect of the right adrenal is directly posterior to the inferior vena cava and is nonperitonealized. The anteromedial portion is also posterior to the liver, its superior aspect approaching the bare area. The inferior portion is beneath the peritoneal

Fig 1. Coronal MRI examination of the normal retroperitoneum. (A) The triangUlar right adrenal is seen as a low signal structure in the perirenal fat adjacent to the liver (open arrow). The left adrenal is slightly more superior in location than usual (black arrow). (8) Note the close relationship between the left adrenal (arrow) and the slightly anterior splenic vein (arrowheads).

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adreno l--~~-­

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Pancreas 1m.if.F.l.~rn---;-- L. adre naI

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~~---">"--c olon Fig 2. Sagittal diagrams of adrenal reletionships. (A) Right side. The right adrenal is both suprarenal and anterior to the kidney. Note the relationship to the posterior aspect of the liver. A large right adrenal mass may thin the liver by extrinsic pressure. (6) Left side. This section is near the left renal hilum. The left gland is anreromedlal and may have little or no suprarenal extension. Note the relationship to the posterior aspect of the pancreas.

B

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HAROLD A. MITIY

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reflection, sometimes lying behind the duodenal loop (Fig 2) . On the left, most of the gland is anteromedial to the kidney , frequently extending to the renal hilum. The lower position of the left gland relative to the kidney makes the term suprarenal a misnomer. The left adrenal gland is situated lateral or posterolateral to the aorta. The upper 2/3 of the gland is covered by the peritoneum of the lesser sac posterior to the stomach. The lower third of the left gland lies posterior to the pancreatic body and splenic vessels (Fig 1). This portion of the gland is thus nonperitonealized .

Cross-Sectional Anatomy Since CT has become the ma instay of adrenal diagnostic imaging, familiarity with cross-sectional anatomy of the retroperitoneum is essential (Fig 3). MRI in this plane exhibits the same relationships. The ability to demonstrate structures in the coronal and sagittal planes with MRI adds a new dimension to the study of this area. The images obtained by CT and MRI in the cross-sectional plane demonstrate the adrenals as thin folded structures that have an anteromedial ridge and two posterior or posterolateral limbs . These limbs (wings) are close together in the

superior aspects of the gland, but as the adrenal extends inferiorly they become more separated so that the gland appears to straddle the anteromedial surface of the upper pole of the kidney. The angles between the limbs may exceed 1200 at the base of the gland. It is important to emphasize that the features demonstrated on cross-sectional images of the adrenal gland vary, depending on the level of the section (Figs 4 and 5). At the most cephalad level, the gland usually has a linear appearance with a direct AP or a slanted anterior to posterolateral orientation. Sections at the midportion of the gland usually produce an inverted V-shaped structure. The inferior sections show the inverted Y with wider angles. The medial wing is usually shorter than the lateral wing. In some cases, the anteromedial ridge is not developed or is small, resulting in a cross-sectional image of an inverted V-shaped gland. Normal Area and Weight

The average area of the right adrenal gland as measured at postmortem venography is 7.5 ern" (range 3.8 to 12.2 em") and that of the left gland is 9 ern! (range 5.7 to 17 ern"). The glands are thin (3 to 6 mm in width) and thicker in the head

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...... Le ...... Right adrenal gland Crus of diaphragm Left adrenal gland Fig 3. Cross-sectional diagram through the adrenal glands . Tha right gland lies postarior to the vena cava. Both glands are relatively anteromedial to the kidneys.

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The normal weight of the adrenal gland is difficult to establish. Because the size of the glands varies in response to st ress and debilitating illness, postmortem studies ma y be misleading . A more accurate assessment of normal adrenal weight is obtained from the glands of people dying suddenly in accidents. In such instances, the adrenal weight ranges from 4 to 5 g.6 Vasculature Arterial Supply There is a rich arterial supply of about 50 threadlike vessels called the arteriae comitantes (Fig 6) . They enter the superior, medial , and inferior edges of the gland and may be divided into groups , depending on their source. I The three main supplying vessels are (l) the inferior adrenal artery, arising from the renal artery, (2) the middle adrenal artery, a branch of the aorta, and (3) the superior adrenal artery, a branch of the inferior phrenic artery.

Fig 4. CT scans through normal right adrenal glands. (A) The gland appears as a linear structure between the liver and crus of the diaphram (arrow) just posterior to the vena cava . (B) In another patient. both wings of the right gland are demonstrated as an inverted Y structure (arrow).

or inferior portio n owing to the presence of the medullary tissue. There is also considerable var iab ility in length , with a norm al range of 4 to 6 em . Similarly, the width also varies, with a range of 2 to 3 em. This normal variation explains in part why a clinically hyperfunctioning gland may not appear larg er than normal.

Venous Drainage Each gland is drained by a single cent ral vein which emerges from the hilum (Figs 6 and 7) . On t he right, the vein enters the posterior aspect of the vena cava; on the left, it drains into the superior aspect of the renal vein. Emissary veins connect the central main adrenal veins with a pericapsular adrenal plexus . These connections are quite well developed, particularly on the left. Venous communications through these emissary veins may lead to opacification of adjacent gonadal veins, the splenic vein, and the renal veins. Emissary veins provide collateral drainage in case of adrenal or renal vein occlusion . The reverse is also true; the ad renal veins may become a collateral pathway for drainage of these adjacent veins. A good example is often seen in portal hypertension when the connection from the portal system through the splenic vein enlarges to become a spontaneous splenoadrenalrenal shunt. These enlarged collaterals may be mistaken for an adrenal mass on an unenhanced CTscan. 7 Histology The adrenal gland is covered by a capsule of fibroelastic tissue. The cortex may be divided into three zones.

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HAROLD A. MITIY

A

Fig 5. CT scans through normal left adrenal gland. (A) Scan through the cephalad portion produces a linear appearance (arrow). (B) At a lower level. the gland has an inverted Y shape (arrow).

Rl mid.adrenala. - --;L-----;..,_.

~:.-..--~-- L.

mid adrenal a.

.l,....---~*"-L. adrenal 1 -~Ji~~'1i

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Fig 6. Vascular anatomy. The arteriel supply may vary. with one or more brenches predominating. The right vein exits the anterior margin of the gland to enter the posterior aspect of the vena cava. which has been cut away in this diagram.

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Fig 7. Venous communications. Left adrenal venogram shows opacification of the splenic and portal veins. This normal communication between the portal and systemic circulations may enlarge in patients with portal hypertension and become a spontaneous splenoadrenal-renal shunt.

1. The zona glomerulosa is the outermost layer and receives its name from the spherical grouping of cells surrounded by fibrous tissue. This zone is responsible for the production of aldosterone. The zona glornerulosa may become thickened during a lowsodium diet. 2. The zona fasciculata is the middle layer of the cortex and is characterized by the presence of parallel sheets of fibers that separate vertical columns of cells. This zone is under the influence of ACTH from the pituitary gland and becomes thickened during serious stress. 3. The zona reticularis is the innermost layer and is adjacent to the medulla. It consists of sheets of fibrous tissue with interspersed groups of cells. The darker staining cells of this zone produce cortisol from cholesterol precursors. ANOMALIES

Accessory Adrenal Tissue

Fig 8. Left adrenal venogram in a patient with renal agenesis. The adrenal vein has joined with a gonadal vein to form a common vein which enters the vena cava.

The absence of an adrenal gland is rare, but the presence of accessory adrenal tissue is quite common. Accessory cortical or medullary tissue may be found. Only if both are found together can this be called a true accessory adrenal gland.

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HAROLD A. MITIY

Fig 9.

Longitudinal ultrasonogram of right

adrenal gland in a neonate. The adrenal is large

compared to the normal kidney. (Courtesy of Dr H-C. Yeh.)

Most of these are in the region of the celiac plexus. Graham" described accessory adrenal glands in 16% of postmortem examinations. It is more common to find accessory adrenal cortical tissue close to the parent gland. These foci arise from fragments of tissue split off from the cortical anlage. Occasionally this tissue is embedded in the renal cortex. When the adrenal lies within the renal substance, the term adrenalrenal heterotopia may be used. Accessory cortex may also be found in the retroperitoneum along the genitourinary tract. Accessory adrenal tissue has been found in the genitalia. Extra-adrenal cortical tissue is still capable of producing hormones that may be clinically significant under appropriate circumstances. Isotopic studies using 131Iodocholesterolor its derivatives may be useful in localizing residual or ectopic cortical tissue. Renal Agenesis

It is important to remember that the normal adrenal gland develops independently of the kidney. For this reason, a normal adrenal gland may be present in patients with renal agenesis. It is

said that such glands tend to be more oval or round owing to the absence of extrinsic pressure from the kidney. We have had the opportunity to study one such case by venography and this gland had a basically normal shape (Fig 8). This left-sided gland had a common drainage with the gonadal vein into the vena cava. In cases of nonascended or pelvic kidney, the adrenal develops in its normal location. The normal adrenal gland is quite large in the newborn (Fig 9), and may be mistaken for normal kidney during sonography in neonates with renal agenesis or renal ectopia." Congenital Hypoplasia

Congenital adrenal hypoplasia is a rare condition. The criteria for this diagnosis include laboratory findings of adrenal insufficiency, exclusion of a defect in steroid metabolism, and no response to ACTH. The cause of this condition is unknown and should not be confused with Addison's disease. The condition has been ascribed to maternal steroid use, prolonged pregnancy, and preeclampsia.

REFERENCES 1. Williams DL, Warwick R: The suprarenal glands. In: Gray's Anatomy, (ed 36). Philadelphia: Saunders, 1980 2. Forsham PH, Melmon KL: The adrenals, In: Williams RH (ed): Textbook of Endocrinology, (ed 4). Philadelphia: Saunders, 1968

3. Manger WM, Gifford RW Jr. Pheochromocytoma. New York: Springer, 1977 4. West GB, Shepherd DM, Hunter RB: Adrenaline and noradrenaline concentration in adrenal glands at different ages and in some diseases. Lancet 1951;261:966-967

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5. Mitty HA, Yeh HC. Adrenal anatomy. Radiology of the Adrenals With Sonography and CT. Philadelphia: Saunders, 1982 6. Holmes RO, Moon HD, Rinehart JE: A morphologic study of the adrenal gland with correlations of body size and heart size. Am J Patho!1951;27:724-730 7. Mitty HA, Cohen BA, Sprayregens S, et al: Adrenal

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pseudotumors on CT due to dilated portosystemic veins. AJR 1983;141:727-730 8. Graham LS: Celiac accessory adrenal glands. Cancer 1953;6:149-152 9. Silverman PM, Carroll BA, Moskowitz PS: Adrenal sonography in renal agenesis and dysgenesis. AJR 1980;134:600-604