Wittkowski W, Bockmann J, Kreutz M R, Böckers T M
Institute of Anatomy, AG Molecular Neuroendocrinology, Westfälische Wilhelms-Universität, Münster, Germany.
Int Rev Cytol. 1999;185:157-94. doi: 10.1016/s0074-7696(08)60151-5.
The pars tuberalis of the adenohypophysis is mainly composed of a special type of endocrine cells, pars tuberalis-specific cells, lining the primary capillary plexus of the hypophysial portal system. Dense expression of melatonin receptors and marked changes in morphological appearance, production pattern, and secretory activity during annual cycle show that these cells are highly sensitive to changes in photoperiod. This leads to the hypothesis that the pars tuberalis is involved in the transmission of photoperiodic stimuli to endocrine targets. Several investigations support the theory that pars tuberalis-specific cells are multipotential cells exerting a modulatory influence on the secretory activity of the pars distalis. Specifically, there is accumulating evidence that seasonal modulation of prolactin secretion, independent of hypothalamic input, is due to melatonin-regulated activity of pars tuberalis-specific cells. The exact nature of secretory products and their effects within neuroendocrine regulation, however, remain rather enigmatic. Accordingly, molecular mechanisms regulating gene expression under the influence of photoperiod, respectively, circulating melatonin levels are still incomplete. Recent cloning of melatonin receptor genes and new data on intracellular signal transduction will probably lead to new insights on melatonin action and pars tuberalis-specific cell physiology.
腺垂体结节部主要由一种特殊类型的内分泌细胞，即结节部特异性细胞组成，这些细胞排列在垂体门脉系统的初级毛细血管丛周围。褪黑素受体的密集表达以及在年度周期中形态外观、产生模式和分泌活性的显著变化表明，这些细胞对光周期变化高度敏感。这就引出了一个假说，即结节部参与将光周期刺激传递给内分泌靶标。多项研究支持这样一种理论，即结节部特异性细胞是多能细胞，对远侧部的分泌活动具有调节作用。具体而言，越来越多的证据表明，催乳素分泌的季节性调节独立于下丘脑输入，是由于结节部特异性细胞的褪黑素调节活性所致。然而，分泌产物的确切性质及其在神经内分泌调节中的作用仍然相当神秘。因此，分别调节光周期影响下基因表达和循环褪黑素水平的分子机制仍然不完整。最近褪黑素受体基因的克隆以及细胞内信号转导的新数据可能会为褪黑素作用和结节部特异性细胞生理学带来新的见解。
