If severe enough, periods of acute stress in animals may be associated with the release of catecholamine hormones (noradrenaline and adrenaline) into the circulation; a response termed the acute humoral adrenergic stress response. The release of catecholamines from the sites of storage, the chromaffin cells, is under neuroendocrine control, the complexity of which appears to increase through phylogeny. In the agnathans, the earliest branching vertebrates, the chromaffin cells which are localized predominantly within the heart, lack neuronal innervation and thus catecholamine secretion in these animals is initiated solely by humoral mechanisms. In the more advanced teleost fish, the chromaffin cells are largely confined to the walls of the posterior cardinal vein at the level of the head kidney where they are intermingled with the steroidogenic interrenal cells. Catecholamine secretion from teleost chromaffin cells is regulated by a host of cholinergic and non-cholinergic pathways that ensure sufficient redundancy and flexibility in the secretion process to permit synchronized responses to a myriad of stressors. The complexity of catecholamine secretion control mechanisms continues through the amphibians, reptiles and birds although neural (cholinergic) regulation may become increasingly important in birds. Discrete adrenal glands are present in the non-mammalian tetrapods but unlike in mammals, there is no clear division of a steroidogenic cortex and a chromaffin cell enriched medulla. However, in all groups, there is an obvious intermingling of chromaffin and steroiodogenic cells. The association of the two cell types may be particularly important in the amphibians and birds because like in mammals, the enzyme catalysing the methylation of noradrenaline to adrenaline, PNMT, is under the control of the steroid cortisol.

The autonomic nervous system and chromaffin tissue: Neuroendocrine regulation of catecholamine secretion in non-mammalian vertebrates

1.1 The stress concept The word stress is used extensively to name a situation of tension that can be applied to living organisms such as animals or plants but also to ecosystems or in geological phenomena. Nevertheless, the concept of biological stress is closely connected to the historic development of the meaning of this word by Hans Selye after his short paper in Nature (Selye, 1936), following a first approach by Walter Cannon who restricted the physiological changes of stress and injuries to the effects of catecholamines and the adrenal medulla. Other key contributions of Selye to the stress associated concepts were the word stressor, meaning the agent causing stress effects and the non-specificity of the neuroendocrine response, even after positive or negative stressors (Szabo, 1998). Being such a general and widely used concept, the term stress has received many definitions, some of them trying to characterize the phenomenon, others focusing the elicited response and others even including the types of stressors, i.e. symbolic or real (physical, chemical, pathogenic). Nevertheless, in all of them some key elements are included: A source or stressor, the nonspecific reaction and the neuroendocrine response. As a relevant physiological mechanism, the stress response by itself is not inherently bad. For example, glucocorticoids are released in animals in response to situations that are not normally regarded as stressful, including courtship, copulation and hunting. In addition, hormones which increase during stress periods, are also part of the reproductive process and induce hormonal cascades causing parturition in some species (Moestl and Palme, 2002). As an example, brine shrimp Artemia exposed under gnotobiotic conditions to a nonlethal heat shock increases the expression of Heat Shock Protein-70 (HSP-70), thus inducing a non-specific molecular stress response. When Artemia was challenged with pathogenic bacteria, Vibrio campbellii and Vibrio proteolyticus, a cross-protection against pathogens was observed if an appropriate combination of heat application and recovery treatment was applied (Sung et al., 2007).

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The Endocrine Response to Stress - A Comparative View

Morphological and histological studies of the adrenal gland of the adult White Leghorn chicken (Gallus domesticus) were performed. Macro- and microscopic observations of azan- or formal-dichromate stained (to observe the chromaffin reaction of adrenal medulla) sections were conducted. The right and left adrenal glands differed in shape, position, weight, length, width and thickness. The adrenal glands were encapsulated with a thin connective tissue containing blood vessels. The adrenal parenchyma was composed of two main tissue types, the cortex and medulla; these were mixed throughout the organ. The adrenal gland could be divided into a subcapsular layer, peripheral zone, and central zone. The whole subcapsular layer was occupied by medulla, and the proportion of medulla in the central zone (49.7%) was larger than that in the peripheral zone (24.8%). Approximately 60% of the adrenal gland was cortex, 39% was medulla, and the remaining 1% was sinusoids. The adrenal cortico-medullary ratio in the adult chicken was approximately 1.6:1. Cortical cells were arranged in columns with a small, round to slightly oval, eccentric nucleus approximately 4 μm in diameter. Medullary cells were polygonal in shape with a large, spherical, centrally placed nucleus approximately 5 μm in diameter.

Morphological and Histological Studies on the Adrenal Gland of the Chicken (Gallus domesticus)

The morphology of the adrenal gland has been studied for a number of animal species all over the world, yet the detailed data about ostrich chick has not been reported. In the present study, the morphological features of the adrenal gland in African ostrich chicks were investigated by means of gross anatomy, light and electron microscope. Differences between the left and right adrenal glands were found in shape, size and location. The interrenal tissue and chromaffin cell interdigitated irregularly. The interrenal tissue was divided into a peripheral zone (PZ) and a central inner zone (CZ), and the PZ was further distinguished into an outer area (subcapsular zone, SCZ) and an inner area (IZ). The cellular arrangement in these zones showed evident zonation that resembled the mammalian. This phenomenon had been previously described only for the pelicanus. The cytoplasm of interrenal cells in SCZ was stained lightly than in IZ and CZ, and contained several vacuoles. Additionally, unlike CZ cells, SCZ cells appeared to contain more mitochondria and less lipid droplets. Two types of chromaffin cells: epinephrine cells and norepinephrine cells could be detected. The type 1 granules possessed a central core and a variable distance between membrane and core; the type 2 granules had an eccentric core, which leant to one side of granule and sticked to the membrane, giving a lager lacouna appearance in another side of the granule.

The morphological study on the adrenal gland of African ostrich chicks.

Histocytological and histochemical features of the adrenal gland of Adult Egyptian native breeds of chicken (Gallus Gallus domesticus)

The purpose of this review is to explore the world literature on the avian adrenal medulla from the last 20 years. Unlike the mammalian adrenal medulla, the adrenal gland in birds has chromaffin cells mixed with cortical cells. Studies have investigated the ultrastructure (both transmission and scanning electron microscopy), biochemistry, and physiology (partic- ularly interactions with other endocrine glands) of the avian adrenal medulla. Although progress has been made, it is apparent that research on the avian adrenal medulla still lags behind work on the mammalian organ.

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Monisha Mukherjee

Papers

Avian adrenal medulla: cytomorphology and function