reno cortical tissue exists as a series of bodies embedded in the posterior cardinal
veins. The distribution of the int errenal and chromaffin tissues in the major vertebrate groups is shown in Fig. 2.6 and 2.7.
Anatomically, there appears to be an evolutionary trend towards a closer association of the chromaffin and adrenocortical tissue as one ascends the phyletic scale,
but the biological significance of this is unknown. For detailed descriptions of the
anatomy of the adrenal glands in vertebrates the monographs by H ARTMAN and
BROWNELL (1949) and CHESTER JONES (1957) should be consulted.
a) Chromaffin Tissue
This tissue is named because of its brown staining reaction with bichromate or
chromic acid. Tissue identified in this manner is not only associated with the adrenal endocrine secretions, but also exists in other parts of the body, particularly
in the ganglia of the sympathetic nervous system. Indeed the adrenal medulla is
anatomically homologous to the sympathetic nerve ganglia, but discharges its products directly into the circulation, instead of towards a post-ganglionic nerve fibre.
Chromaffin tissue secretes two principal hormones: adrenaline (or epinephrine) and
noradrenaline (or norepinephrine). These, along with dopamine (which may act as
their metabolic intermediate), are termed catecholamines. Adrenaline acts mainly
as an endocrine hormone, while noradrenaline may have the role of a hormone,
a neural transmitter, or an intermediate in the formation of adrenaline. The
proportions of these two hormones in the endocrine chromaffin tissue vary considerably in different vertebrate groups, and even within the same species. Among
the mammals, the percentage of the combined total of the hormones existing as
noradrenaline, ranges from 86% in whales to 2% in rabbits, while in frogs and
toads it is about 50%, and in the chicken and dogfish, Squalus acanthias, 70%.
Histological evidence suggests that each hormone is stored in a different type of
cell. The structure of adr enaline is shown in Table 2.3. It is formed in the chromaffin tissue from accumulated tyrosine, which is oxidi zed to dopa which, in turn,
is decarboxylated to form dopamine. Dopamine is taken into granules in the cells
where it is stored or converted by P-hydroxylation to no radrenaline, which, in
turn, may undergo N-methylation to form adrenaline. The latter two products can
be released from the granules in response to nerve stimuli conveyed through the
splanchnic nerves that supply the gland. Reflex release can be initiated from other
parts of the nervous system such as the sciatic nerve and the vagus, the centre for
control being situated in the brain. The neural release of the catecholamine hormones is initiated by a variety of stressful, uncomfortable and unpleasant conditions including emotion, fear, pain, excesses of temperature, anoxia, a drop in the
blood pressure, a low blood sugar level and physiological experiments. The adrenal
chromaffin tissue thus affords, in addition to the pituitary, another site where
nerves and endocrines may int eract. Denervation does not result in degeneration
of the chromaffin tissue. In addition to responding to neural stimuli, it is thought
to discharge small amounts of its products continually into the circulation.
Animals survive adequately after ablation of the endocrine chromaffin tissue,
even though the secretions in certain circumstances facilitate physiological adap70
veins. The distribution of the int errenal and chromaffin tissues in the major vertebrate groups is shown in Fig. 2.6 and 2.7.
Anatomically, there appears to be an evolutionary trend towards a closer association of the chromaffin and adrenocortical tissue as one ascends the phyletic scale,
but the biological significance of this is unknown. For detailed descriptions of the
anatomy of the adrenal glands in vertebrates the monographs by H ARTMAN and
BROWNELL (1949) and CHESTER JONES (1957) should be consulted.
a) Chromaffin Tissue
This tissue is named because of its brown staining reaction with bichromate or
chromic acid. Tissue identified in this manner is not only associated with the adrenal endocrine secretions, but also exists in other parts of the body, particularly
in the ganglia of the sympathetic nervous system. Indeed the adrenal medulla is
anatomically homologous to the sympathetic nerve ganglia, but discharges its products directly into the circulation, instead of towards a post-ganglionic nerve fibre.
Chromaffin tissue secretes two principal hormones: adrenaline (or epinephrine) and
noradrenaline (or norepinephrine). These, along with dopamine (which may act as
their metabolic intermediate), are termed catecholamines. Adrenaline acts mainly
as an endocrine hormone, while noradrenaline may have the role of a hormone,
a neural transmitter, or an intermediate in the formation of adrenaline. The
proportions of these two hormones in the endocrine chromaffin tissue vary considerably in different vertebrate groups, and even within the same species. Among
the mammals, the percentage of the combined total of the hormones existing as
noradrenaline, ranges from 86% in whales to 2% in rabbits, while in frogs and
toads it is about 50%, and in the chicken and dogfish, Squalus acanthias, 70%.
Histological evidence suggests that each hormone is stored in a different type of
cell. The structure of adr enaline is shown in Table 2.3. It is formed in the chromaffin tissue from accumulated tyrosine, which is oxidi zed to dopa which, in turn,
is decarboxylated to form dopamine. Dopamine is taken into granules in the cells
where it is stored or converted by P-hydroxylation to no radrenaline, which, in
turn, may undergo N-methylation to form adrenaline. The latter two products can
be released from the granules in response to nerve stimuli conveyed through the
splanchnic nerves that supply the gland. Reflex release can be initiated from other
parts of the nervous system such as the sciatic nerve and the vagus, the centre for
control being situated in the brain. The neural release of the catecholamine hormones is initiated by a variety of stressful, uncomfortable and unpleasant conditions including emotion, fear, pain, excesses of temperature, anoxia, a drop in the
blood pressure, a low blood sugar level and physiological experiments. The adrenal
chromaffin tissue thus affords, in addition to the pituitary, another site where
nerves and endocrines may int eract. Denervation does not result in degeneration
of the chromaffin tissue. In addition to responding to neural stimuli, it is thought
to discharge small amounts of its products continually into the circulation.
Animals survive adequately after ablation of the endocrine chromaffin tissue,
even though the secretions in certain circumstances facilitate physiological adap70