The Adrenal Glands
Anatomy
The paired adrenal glands, or suprarenal glands, are found in proximity to the kidneys. The left adrenal gland is found anterior to the cranial pole of the left kidney and the right adrenal gland is found medial to the right kidney.
The glands average 2-3 cm in length and 1 cm in width, with the left larger than the right. The gland is composed of a capsule, a cortex, and a medulla. The zona fasciculata of the cortex is the thickest portion of the gland and is consistently thicker in females than in males (Prasad and Sinha 1981). Accessory adrenal cortical nodules occur in goats and may be located intracapsularly or extracapsu- larly. These represent cortical cells separated from the germinal cortex during embryonic development (Prasad and Sinha 1980). Extracapsular nodules are usually close to the main glands.Physiology
The endocrine and physiologic functions of the adrenal gland in the goat are essentially the same as those in other domestic ruminants, and mammals in general. Seasonal variations of corticoid levels in Angora goats have been measured. The baseline plasma level is quite variable, with the range of means between 2.2 and 10.6 ng/mL. A distinct increase is noted, however, in the autumn months, associated with the breeding season in females. Mean autumn plasma corticoid levels in does reach 18 ng/mL, with individual measurements as high as 26 ng/mL, while the mean in males remains less than 10 ng/mL (Wentzel et al. 1979).
Pathology
Neoplasia occurs in the caprine adrenal gland. Adrenal cortical adenomas have been identified at necropsy predominantly in castrated male goats older tha.n 4.5 years of age. The tumor is virtually non-existent in females and intact males. Castration is thought to predispose the goat to the tumor (Richter 1958; Altman et al. 1969). The tumor has been recognized more frequently in Angora goats than in dairy breeds, but this probably reflects that Angora wethers are more likely to be maintained for production to advanced age than castrated dairy goats.
No clinical syndromes have been associated with these tumors, except for one reported case in which a 6-year-old castrated Toggenburg wether was presented for udder enlargement and lactation, and subsequently diagnosed with adrenal cortical adenoma post mortem. Ante mortem, the goat had increased serum concentrations of estradiol 17β, incomplete suppression of cortisol secretion by dexamethasone, and an exaggerated response to adrenocorticotropic hormone (ACTH) administration (Lofstedt et al. 1994).Less common are tumors of the medulla. There is one published report of a pheochromocytoma identified in the adrenal medulla of a 10-year-old Nubian buck at necropsy. Again, no clinical signs were associated with the tumor ante mortem (Lairmore et al. 1987). The author (DMS) has seen similar gross and histologic lesions at necropsy in the adrenal medullae of aged goats, but the existence of functional pheochromocytomas that secrete excessive epinephrine and produce hypertension in living goats remains to be documented.
There is a report of presumed hereditary caprine pheochromocytoma from Finland in which the tumor was identified in a doe, its dam, and its granddam at ages 10, 13, and 15, respectively. All three goats were said to have shown repeated, sudden attacks of nervousness and anxiety (De Gritz 1997). There is also a report of two does, aged 20 and 10 years, that were presented for lactation, though neither had been bred. The abnormal lactation in both does was attributed to acidophilic pituitary adenomas, but both goats also coincidentally had pheochromocytomas (Miller et al. 1997).
Histologically, evidence of adrenal cortical hyperplasia associated with stress in chronic disease has been reported in goats with chronic paratuberculosis (Rajan et al. 1980) and prolonged trypanosomosis (Mutayoba et al. 1988).
Clinical Disease
Classic types of hyperadrenocorticism (Cushing's syndrome) or hypoadrenocorticism (Addison's disease) have not been recognized in goats.
Hyperadrenocorticism, however, is believed to play a role in the complex pathophysiology of a distinct form of non-infectious abortion peculiar to the Angora goat in South Africa (Wentzel et al. 1975). Affected does that abort the characteristic edematous fetuses associated with this condition show adrenal cortical hyperplasia histologically and maintain high adrenal corticosteroid levels throughout gestation, whereas normal does show a drop in corticosteroid levels in the third trimester. This condition, inherited abortion of Angoras, is discussed further in Chapter 13.It is well recognized that as a breed, Angora goats do not handle stress as well as other goat breeds or sheep. In South Africa, death of Angora goats stressed during periods of unexpected cold weather is a major constraint on production. These stress-related deaths are associated with hypoglycemia. Van Rensburg (1971) reported that selection of Angora goats for fine mohair production was linked to selection for reduced adrenal function. This has prompted a good deal of research to determine what role the adrenal gland might play in the hypoglycemia seen in stressed and dying Angora goats.
Experimentally, Angora goats given insulin to lower their blood glucose did not, when compared to Boer goats and Merino sheep, respond with the appropriately increased cortisol production necessary for gluconeogenesis and restoration of blood sugar under stress conditions (Engelbrecht et al. 2000). Furthermore, it was demonstrated in that study using adrenal cell cultures that the ACTH receptor on the adrenocortical cell membrane of Angora goats cannot adequately stimulate the cAMP signaling mechanism required for enhanced glucocorticoid production under stress. In addition, it was reported that the activity of the cytochrome P450c17 enzyme in Angora goats differed from that in Boer goats and Merino sheep, and that difference may contribute to the lowered production of glucocorticoids in Angora goats (Engelbrecht and Swart 2000).