Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

Chapter 19
Adrenocortical Function
Björn P. Meij
Department of Clinical Sciences of Companion Animals
Faculty of Veterinary Medicine
Utrecht University
Utrecht, The Netherlands
Jan A. Mol
Department of Clinical Sciences of Companion Animals
Faculty of Veterinary Medicine
Utrecht University
Utrecht, The Netherlands
I. ANATOMICAL CONSIDERATIONS
II. PHYSIOLOGY OF ADRENOCORTICAL HORMONES
A. Steroid Nomenclature
B. Biosynthesis
C. Regulation of Secretion
D. Transport
E. Metabolic Breakdown and Excretion
F. Steroid Receptors and Actions
G. Physiological Variation, Stress, and
the Immune System
III. ADRENOCORTICAL DISEASES
A. Hypoadrenocorticism
B. Hyperadrenocorticism
IV. ASSESSMENT OF ADRENOCORTICAL FUNCTION
A. Routine Laboratory Diagnostics
B. Tests of Basal Adrenocortical Function
C. Dynamic Tests
D. Tests for Differential Diagnosis
E. Collection and Handling of Samples
F. Protocols
REFERENCES
I . ANATOMICAL CONSIDERATIONS
In mammals, the adrenal glands are bilateral structures
located craniomedial to the kidneys. The adrenal is, in fact,
made up of two glands of different embryological origin.
The center of the gland, the medulla, comprises coalesced
chromaffin cells of neuroectodermal origin that secrete epinephrine
or norepinephrine. The surrounding adrenal cortex
arises from mesoderm and can be divided histologically into
three zones: (1) zona glomerulosa (or arcuata), (2) zona fasciculata,
and (3) zona reticularis. The cells of the zona glomerulosa
are arranged in ovoid groups immediately inside
the connective tissue capsule. The region may be particularly
obvious (e.g., sheep) or difficult to discern (e.g., small
rodents). The zona fasciculata accounts for most of the adrenal
cortex ( 60%) and appears histologically in radial lines.
The inner zona reticularis comprises networks of cell cords
surrounding the large blood sinuses. A notable feature of
the ferret adrenal gland is that islands of cortical cells can be
found among cells of the medulla. These cells resemble those
of the inner part of the zona fasciculata ( Holmes, 1961 ).
In birds, the adrenal glands are also separate, encapsulated
organs. The glands lie cranial to the kidney, often
wholly or partly covered by the gonads. The adrenocortical
and chromaffin tissues are intermingled to varying
degrees, depending on the species. The basic unit of avian
adrenocortical tissue is a cord of a double row of parenchymal
cells, with their long axes in the transverse plane of
the strand. The cords radiate from the center of the gland,
branch, and anastomose frequently. Groups of chromaffin
cells lie between the cords and are also ensheathed within
the connective tissue reticulum ( Chester-Jones, 1987 ).
Blood flow of the adrenals is centripetal. In species with
separated medulla and cortex, this results in high medullary
corticoid concentrations that induce an enzyme in the
chromaffin cells, converting norepinephrine to epinephrine.
Therefore, species with anatomically separated medulla and
cortex predominantly secrete epinephrine and species with
intermingled chromaffin and adrenocortical tissue secrete
predominantly norepinephrine ( Marks, 1983 ).
II . PHYSIOLOGY OF ADRENOCORTICAL
HORMONES
The secretion of the mammalian adrenal cortex comprises
three main categories of hormones, which can be related
to some extent to the above-described anatomical zonation.
The zona glomerulosa produces mineralocorticoids
(aldosterone and deoxycorticosterone), which maintain salt
balance. The cells of the zona fasciculata secrete glucocorticoids
(cortisol and corticosterone), which are primarily
involved in carbohydrate metabolism. The third category
of adrenocortical hormones, the androgens (e.g., androstenedione),
is produced in the zona reticularis. This zone
to a minor degree also secretes glucocorticoids and other
hormones such as progesterone and estrogens.
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