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The morphology of the avian pineal glands shows large interspecies variability. Considering the anatomic structure, six types of the pineal organs (I-VI) are distinguished in birds. They differ in the proportion between the distal and proximal parts of the gland as well as in the attachment to the intercommisural region. According to the histological structure, the avian pineal glands are classified into three types: the saccular, tubulo-follicular and solid type. The pineal parenchyma consists of pinealocytes, supporting cells and neurons. Among pinealocytes there are receptor pinealocytes (predominant in the saccular pineals), rudimentary-receptor pinealocytes (predominant in the tubulo-follicular organs) and secretory pinealocytes (most numerous in pineals of the solid type). The population of supporting cells consists of ependymal-like and astrocyte-like cells. Neurons are represented by afferent ganglion cells, present mainly in the saccular pineals. The pineals of tubulo-follicular and solid types possess well developed sympathetic innervation.
The level of melatonin in plasma was investigated in gilts reared during long summer days (sunrise 430, sunset 2030). Lighting in the animal's rooms was provided by windows and fluorescent lamps, which were automatically turned on at 545 and turned off at 1945. The light intensity during photophase was 500 lux at the level of the animal's eyes. Blood samples were taken from twelve gilts over a 24 hour period and further sampling was continued in six pigs over the following 3 days. The effect of exposure to a 500 lux light during the night on plasma melatonin levels was studied in the remaining six gilts. The concentration of melatonin in the plasma was measured by direct radioimmunoassay employing an R/R/19540-16876 antibody and iodinated tracer. Concentrations of the investigated hormone were low during day and rapidly increased at the onset of night during which the plasma melatonin level remained at an elevated plateau and then declined at the onset of day. The mean concentration of plasma melatonin was about three times higher during the night compared to daytime. Monitoring of the pineal hormone level in blood plasma over four consecutive days indicated regular melatonin rhythms in 5 of the 6 investigated pigs. A 500 lux light (turned on for one night) did not prevent a nocturnal increase in plasma melatonin levels.
The article presents the most important mechanisms related to the functioning of the retina, the suprachiasmatic nucleus (SCN) and the pineal gland as components of the mammalian biological clock. Environmental light influences the biological clock in mammals via light-sensitive, retinal ganglion cells containing a photo pigment - melanopsin. The axons of these neurons form the retionohypothalamic tract, which terminates in SCN. Neurons located in SCN generate cyclic, circadian changes in their activity due to a system of clock genes, the transcription of which is mutually controlled by an auto regulatory feedback loop. Glutamate and pituitary adenylate cyclase activating peptide (PACAP) - neurotransmitters released from terminals of the retionohypothalamic tract, synchronize the activity of the SCN neurons with environmental light conditions. The neuronal cells located in SCN influence the pineal activity via a paired, mulisynaptic pathway, composed of neurons of the paraventricular nucleus, the intermediolateral nuclei of the spinal cord and the cranial cervical ganglion, which supply the pineal gland with sympathetic nerve fibers. Norepinephrine, released from these fibers at night, stimulates melatonin secretion. The intracellular mechanisms controlling melatonin synthesis in the pinealocytes have significant variations between species, which accounts for differences in the diurnal patterns of pineal hormone secretion.
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