Paniagua R, Fraile B, Sáez F J
Department of Cell Biology and Genetics, University of Alcalá de Henares, Madrid, Spain.
Histol Histopathol. 1990 Jul;5(3):365-78.
Most amphibians present an annual testicular cycle characterized by a quiescent period (late autumn-winter) and a spermatogenic period (spring and summer). At the end of the period of spermatogenesis undifferentiated interstitial cells transform into steroid-secreting Leydig cells which regress in spring at the beginning of the new spermatogenetic cycle. The testicular cycle is controlled by the pituitary gonadotropin levels which are high in autumn and winter, low in spring and increase temporarily in the middle of summer. Photoperiod and temperature seem to be the most important external factors involved in the regulation of this cycle in many amphibian species since the colder the geographic area, the longer the quiescent period and the shorter the spermatogenic period. This suggests the occurrence of a potentially continuous cycle in these species, in contrast with that which occurs in other species having an endogenous rhythm of testicular function which is much less sensitive to environmental factors. Although the specific response to temperature can vary widely between species, the most frequent observation in amphibians with a potentially continuous cycle is that exposure to mild temperatures (15-20 degrees C, according to the spring temperatures of the different geographic areas) stimulates spermatogenesis even during the period of testicular quiescence. If this mild temperature is combined with a long photoperiod, complete spermatogenesis is attained. Experiments performed during the period of germ-cell proliferation (development from spermatogonia to round spermatids) indicated that low temperatures (below 11 degrees C) as well as short photoperiods (less than 8 h of light) hinder germ-cell proliferation. Moderately high temperatures (about 30 degrees C) do not impair this proliferation. In the newt Triturus marmoratus, it has been shown that an excessively long photoperiod (over 16 h) has the same effect as a short photoperiod. In this species eyes are not required for the testicular photoperiodic response. Photoperiod appears to have no effect on spermiogenesis (differentiation of round spermatids into spermatozoa), because once round spermatids are formed, spermiogenesis will occur even in total darkness. Mild temperatures seem to be necessary for spermiogenesis as well as for androgen biosynthesis because neither process will take place at extreme temperatures. Results on the effect of photoperiod in steroidogenesis differ between species.
大多数两栖动物呈现出一年一度的睾丸周期，其特点是有一个静止期（秋末至冬季）和一个生精期（春季和夏季）。在生精期结束时，未分化的间质细胞转变为分泌类固醇的莱迪希细胞，这些细胞在新的生精周期开始时的春季退化。睾丸周期受垂体促性腺激素水平控制，该水平在秋冬季节较高，春季较低，在夏季中期会暂时升高。光周期和温度似乎是许多两栖动物物种中调节此周期的最重要外部因素，因为地理区域越寒冷，静止期越长而生精期越短。这表明这些物种中可能存在潜在的连续周期，这与其他具有睾丸功能内源性节律且对环境因素不太敏感的物种不同。尽管不同物种对温度的具体反应差异很大，但在具有潜在连续周期的两栖动物中最常见的观察结果是，即使在睾丸静止期，暴露于温和温度（根据不同地理区域的春季温度，为15 - 20摄氏度）也会刺激生精。如果这种温和温度与长光周期相结合，就能实现完全生精。在生殖细胞增殖期（从精原细胞发育为圆形精子细胞）进行的实验表明，低温（低于11摄氏度）以及短光周期（光照少于8小时）会阻碍生殖细胞增殖。适度高温（约30摄氏度）不会损害这种增殖。在真螈Triturus marmoratus中，已表明过长的光周期（超过16小时）与短光周期具有相同的效果。在该物种中，睾丸光周期反应不需要眼睛。光周期似乎对精子形成（圆形精子细胞分化为精子）没有影响，因为一旦形成圆形精子细胞，即使在完全黑暗中也会发生精子形成。温和温度似乎对精子形成以及雄激素生物合成都是必要的，因为这两个过程在极端温度下都不会发生。关于光周期对类固醇生成影响的结果因物种而异。
