Pars Distalis

During hibernation (mid February), the number of secretory vesicles in the pars tuberalis cells of the garden door mouse was considerably reduced (Dellmann et al. 1974). The profiles of the rough endoplasmic reticulum, after having lost their ribosomes, tended to become closely apposed. 

Functional receptors for melatonin have been localised and characterised on the pars tuberalis of a number of mammalian species (Morgan etal. 1991, Bockers et al. 1995, Bockmann et al. 1996). Using forskolin (1 [aM), as a non-specific stimulant of adenylate cyclase, melatonin (10 nM) was shown to inhibit the formation of cyclic AMP by 80-90 % in ovine pars tuberalis cells both before and after PercoU centrifugation (Morgan et al. 1991). 

Sequential appUcation of reverse haemolytic plaque assays for growth hormone and prolactin revealed the presence of individual pituitary cells that release both hormones (Frawley et al. 1985). These dual cells accounted for approximately one third of all growth hormone and/or prolactin secre-tors in 24-h pituitary adtures from male rats. 

Kikuta etal. (1993) showed that 34% of cells expressing oestrogen receptors were somatotropes. 

In vivo studies in rodents confirmed that MIF is secreted from the pituitary during stress of hpopo-lysaccharide stimulation and resulted in increased serum levels with an accompanying decrease in pituitary MIF (Calandra et al. 1995). Hypophysec-tomized mice injected with lipopolysaccharide have no detectable serum MIF at a time that MIF levels are highest in control mice, indicating that the pituitary is the major source of serum MIF during systemic inflammatory response. 

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Pro-opiomelanocortin, pro-opiocortin a protein (M, 31,000) from the pars distalis and pars intermedia of the pituitary. P. contains the sequences of ACTH and p-LPH (see Peptides, Fig. 3). In the pars distalis, ACTH is released from P. to regulate adrenocortical function. In the pars intermedia, P. is cleaved to unprocessed a-MSH, which then undergoes amidation and acetylation to a-MSH (see Melanotropin). 

The pituitary gland Figure 4.1) is a bilobed structure located in the sella turcica, immediately below the median eminence area of the hypothalamus. The 2 lobes are derived embryologically from different sources. The anterior lobe or adeno-hypohysis, which is differentiated into 3 distinct zones, the pars distalis, pars intermedia and the pars tuberalis, is derived from ectodermal tissue of the oral... 

The pathway by which MSH-RIF and MSH-RF reach the pituitary gland is unclear. Unlike the pars distalis, the pars intermedia is well supplied with neurones from the hypothalamus but its blood supply is poor. Accordingly it has been suggested that the hypothalamic hormones are released from nerve endings within the pars intermedia, but further experimental evidence is needed to substantiate such a hypothesis. 

Lariviere, N., Seidah, N. G., DeSerres, G., Rochmont, J., and Chretien, M., 1980, Two glycosylation sites on the N-terminal segment of porcine pars distalis pro-opiomelano-cortin, FEBS Lett. 122 279-282. 

Fig. 263. An agranular cell in the pars distalis of the pituitary gland (block 1240) of a rat (No. 8) medicated for 7 consecutive days with intraperitoneal injection of 15 mg carbocro-mene per kg body weight x day. On the last 4 days of experimentation the animal was exposed to an atmosphere containing only 5 % oxygen for 30 min. On August 4, 1976, half an hour after the last exposure under pentobarbital anaesthesia (30 mg/kg), the animal was perfused from the abdominal aorta with 2.5% glutaraldehyde in 0.1 M sodium cacodylate buffer (pH 7.4). Postfixation with 1 % osmium tetroxide in sodium cacodylate buffer. Embedded in Epon 812 and sectioned at 50 nm. Lead citrate and uranyl acetate. Film 633-34...

The GH cells are distributed throughout the pars distalis. They are round to oval and contain evenly distributed immunoreactive GH in the cytoplasm. 

The distribution of lysosomal enzyme including /3-glucuronidase has been studied in the pars distalis of the amphibian pituitary and the reaction is mainly found in tlie lobules of globular basophils (Waechtler and Pearce, 1966). 

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