The hypothalamicpituitarygonadal axis

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The hypothalamic-pituitary-testis axis is responsible for the control of reproduction in the stallion. The system works in a cascade fashion, with appropriate negative feedback loops. The axis is also controlled by secretions of the pineal gland.

Pineal gland

The pineal gland lies above and behind the hypothalamus, held within a fibrous capsule. Septa extend into the gland from separate masses of round epithelial cells. These cells produce the hormone melatonin from tryptophan; melatonin has an antigonadotrophic effect on the hypothalamus (Sharp and Clever, 1993). The function of these cells is affected by daylength: increasing daylength inhibits pineal function, i.e. the production of melatonin (Wesson et al., 1979). The means by which daylength takes its effect is unclear in the horse but it is believed that impulses from the retina in the eye travel via the rostral accessory optic tract to the thoracic spinal cord and thence to the cranial cervical ganglion. Nerve fibres from the cervical ganglion then follow the path of the arteries to the pineal gland epithelial cells, where they take effect (Venzke, 1975; Moore, 1978; Reiter, 1981).

Hypothalamus

The hypothalamus forms part of the diencephalon of the brain. It is served by the CNS and connects, via this, to several areas of the brain and spinal cord. Various sections of the hypothalamus govern hormones which are released by the pituitary. It consists largely of neural tissue containing the nuclei of nerve cell bodies. These are termed the supraoptic, paraventricular, preoptic and rostral nuclei. As such they are involved not only in reproductive activity via sexual behaviour and the release of trophic hormones, but also in the

Thirst Axis
Fig. 3.24. The hypothalamic-pituitary-testis axis, which controls reproductive function in the stallion.

regulation of appetite, thirst, body temperature, vasomotor activity, emotion, use of the body's nutrient reserves and the states of sleep and wakefulness (Amann, 1993b). The nucleus of particular concern in reproduction is the pre-optic nucleus, containing the cell bodies of the parvicellular neurosecretory cells, the source of gonadotrophin-releasing hormone (GnRH) (Kainer, 1993).

Pituitary

The pituitary or hypophysis is an oval mass contained within a fibrous capsule, weighing in the region of 1.8 g (Thompson, 1992). It lies below, and is connected to, the hypothalamus. The pituitary is divided into the anterior pituitary (adenohypophysis) and the posterior pituitary (neurohypophysis). The posterior pituitary, containing neurological cells, is largely involved in the storage of hormones produced as a result of neural stimulation. It is the anterior pituitary, made up of follicles or cords of cells, which is involved in reproductive control (Kainer, 1993). The anterior pituitary secretes the following hormones: somatotrophin, prolactin, adrenocorticotrophin hormone, FSH, LH, thyrotrophin and lipoproteins. The connection of the anterior pituitary to the hypothalamus is via parvicellular neurosecretory cells and hypophyseal portal vessels. The parvicellular neurosecretory cells terminate in the medial eminence, a neurohaemal organ that acts as a site of transfer or release of inhibitory or releasing hormones into the blood of the hypophyseal portal vessels which pass to the secondary plexus in the anterior pituitary. This in turn passes the hormones on to the target cells in the anterior pituitary. In response the pituitary produces (among other hormones) FSH and LH, which are of prime importance in the control of reproduction in the stallion (Cupps, 1991; Kainer, 1993).

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