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Endocrine system (systema endocrinum) 167

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9.6 Epiphysis (horse). The epiphysis
is subdivided by delicate connective
tissue septa that surround pinealocytes,
interstitial cells (astrocytes) and
sympathetic nerve fibres. Azan stain
(x480).

pineal cells, capillaries and paravascular channels. The The significance of the epiphysis in non-seasonal breed-
chromatin of the oval nuclei of the interstitial cells is more ers is incompletely understood. The onset of puberty is
dense than that of pinealocytes. Like the astrocytes of the associated with a reduction in pineal activity.
brain, the interstitial cells exhibit extensively branching
cytoplasmic processes containing bundles of intermediate Species variation
filaments. In lower vertebrates, rather than being a compact struc-
With increasing age, calcium deposits appear within the ture, the epiphysis of fish and amphibians is a saccular
glial regions of the epiphysis and in spaces within the pia evagination referred to as the intracranial epiphysis. An
mater. These are referred to as corpora arenacea (brain additional component, the parapineal organ or parietal
sand). organ, manifests as a projection of the epiphyseal sac or
The epiphysis is richly innervated with sympathetic as a separate, more rostrally located, projection from the
nerve fibres. The perikarya of these nerve fibres are roof of the third ventricle. The parietal organ comes to
located in the cranial cervical ganglion. Within the ter- lie, at the end of a stalk, under the epidermis. In some
minals of sympathetic nerve axons are vesicles containing animals (e.g. frogs) it can be visualised at the occiput
noradrenaline and neuropeptide Y. with the naked eye. The ultrastructure of the cells of the
The mammalian pineal gland is influenced by daylight, parietal organ is similar to that of the photoreceptors of
about which it receives information from the optic system: the retina, with a lamellated sensory process and a basal
retina → suprachiasmatic nucleus → cerebrospinal tracts afferent synaptic connection (it has not been definitively
to intermediolateral nucleus in the first thoracic segments established whether this ‘parietal eye’ is photoreceptive).
of the spinal cord → preganglionic fibres → cranial cervi- It is assumed that these primitive photoreceptors are
cal ganglion → postganglionic sympathetic fibres to the homologues of the photoreceptors of mammals.
epiphysis.
The synthetic activity of pinealocytes is regulated by
noradrenaline released from the terminals of the sym- Thyroid gland (glandula thyroidea)
pathetic axons. Within the pinealocytes, melatonin is The thyroid gland produces the hormones tetraiodothyro-
produced by methylation of serotonin. Epiphyseal activ- nine (thyroxin, T ), tri-iodothyronine (T ) and calcitonin.
4 3
ity is controlled by the day–night cycle: the concentration Synthesis and secretion of thyroxin and tri-iodothyronine are
of melatonin in the blood increases during the dark phase, controlled by thyroid-stimulating hormone (TSH) produced
while light inhibits melatonin synthesis. The circadian mel- in the adenohypophysis. Calcitonin is regulated by a nega-
atonin rhythm regulates seasonal reproduction, providing tive feedback loop, driven by plasma calcium concentrations.
information about changes in day length. In laboratory Thyroxin and tri-iodothyronine bind to receptors in the
animals (golden hamster), the effects of photoperiod on cell nucleus and stimulate transcription. In this way, they
gonadal function can be simulated by artificial manipu- regulate intracellular metabolism, stimulating protein syn-
lation of the day–night rhythm. These are measured by thesis and carbohydrate metabolism. In general, thyroid
testicular weight, which can vary up to ten-fold under hormones increase basal metabolic rate, heat production
extreme conditions. and growth processes within the body.

Vet Histology.indb 167 16/07/2019 14:59