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Endocrine system

(systema endocrinum)

The activity of cells and organs is subject to constant upon the number and affinity of corresponding receptors
control by higher-order regulatory mechanisms. The ner- on the target cell. Hormones are transported by body
vous, endocrine and immune systems exert a stimulatory, fluids (blood, lymph, interstitial fluid). The transfer of
inhibitory or synergistic effect on target cells that carry information by the endocrine system can therefore take
specific surface receptors. These systems share the ability up to minutes or hours.
to secrete chemical messenger molecules that bind to the The endocrine system comprises three organisational
cell receptors, thereby initiating a chain of responses. forms:
Chemical signals are transmitted in various ways.
· endocrine glands,
• Nerve cells communicate with receptor cells at · groups or clusters of endocrine cells and
synapses, where they release specific molecules (neu- · individual endocrine cells.
rotransmitters) that have a short duration of action.
• Endocrine cells secrete a range of chemical media- Endocrine glands include the hypophysis, epiphysis, thy-
tors comprising peptides, proteins and glycoproteins roid gland, parathyroid gland, adrenal gland and pancreatic
(hormones); these are either released into the blood- islets. These organs are characterised by the absence of
stream (endocrine signalling) to act on cells at a a secretory duct. They have a rich vascular supply and a
distant location, or they pass through the interstitial fenestrated capillary endothelium. Endocrine glands are
fluid to influence nearby cells (paracrine signalling). controlled primarily by feedback mechanisms.
• Tissue cells secrete signalling molecules that act as Clusters of endocrine cells are organ components that
local chemical mediators (e.g. histamine, leukot- act predominantly to regulate the function of that organ
rienes, prostaglandins, interleukins). (e.g. follicular epithelial cells and corpus luteum of the
ovary, Leydig cells of the testis). Endocrine cell clusters
The nervous and endocrine systems are closely intercon- are also regulated by feedback loops.
nected. Hypothalamic neurons release stimulatory and Individual endocrine cells occur in surface and glan-
inhibitory peptides that influence the function of cells in dular epithelia (e.g. gastrointestinal and respiratory tracts).
the adenohypophysis. Furthermore, neurosecretory prod- These are referred to as the diffuse neuroendocrine system
ucts pass via the hypothalamo-hypophyseal tract (tractus (DNES) and include APUD (amine precursor uptake and
hypothalamohypophysialis) into the neurohypophysis decarboxylation) cells. In the gastrointestinal tract, intra-
where they are stored and released as required (neurose- epithelial DNES cells exert local regulatory effects via the
cretion). The hypothalamus also acts on the autonomic paracrine mode of signalling (enteroendocrine system).
nervous system, thus influencing metabolic processes in
peripheral tissues and organs. Hypothalamo-hypophyseal system
The nervous and endocrine systems employ different The hypothalamus and the two components of the
mechanisms to convey their chemical signals to target cells. hypophysis (adenohypophysis, neurohypophysis) have
Neural transmission of information to effector cells occurs a central regulatory role within the endocrine system
rapidly at special junctions (synapses). Neurotransmitters (Figures 9.1 and 9.2). Releasing and inhibiting hormones
released from presynaptic axonal nerve terminals have a (RH, IH) synthesised by neurosecretory cells in the hypo-
stimulatory or inhibitory effect on the postsynaptic target thalamus are transported to the adenohypophysis, where
cell membrane. In contrast, the actions of the endocrine they influence the synthesis and release of hormones that
system are relatively slow and diffuse. Endocrine cells are regulate the function of other endocrine organs (tropic
typically, though not always, specialised to produce a sin- hormones). Effector hormones produced by neurons of
gle type of hormone. The effect of the hormone depends the hypothalamus are conveyed by axonal transport to

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