Ahnelt P K, Hokoç J N, Röhlich P
Department of General & Comparative Physiology, School of Medicine, University of Vienna, Austria.
Rev Bras Biol. 1996 Dec;56 Su 1 Pt 2:199-207.
The topography and spectral characteristics of mammalian photoreceptors correlate with both, the present ecological demands and the evolutionary history. The South American Opossum is a marsupial mammal with unspecialized habitus and crepuscular lifestyle. A sparse population of cones (max. = 3000/mm2) can be differentiated into four subtypes by morphological, topographical and immunocytochemical criteria. In spite of this unusual diversity the cone types can be split into two functional groups: The population of single cones labeled by antibody OS-2 for short wavelength sensitive pigments was ubiquitous but at very low densities (200/mm2). The single cones labeled by antibody (COS-1) against long wavelength sensitive pigments constitute the dominant population in the area centralis (2300/mm2). These two single cone types correlate with the pair typically present in placental mammals. Discrimination of spatial and color contrast may be provided by this "modern" set. The COS-1 labeled double and single cones bearing an oil droplet, display a different pattern by being restricted to the inferior (non-tapetal) half of the retina (max = 800/mm2). This additional set of cones with oil droplets and long wavelength pigments is a conservative feature of the opossum retina and other marsupials. As an accessory cone system it is possibly providing enhanced sensitivity at mesopic conditions. During the early evolution of nocturnal mammals with its prominent expansion of rod vision these cone types were conserved but then were lost in placental mammals. Thus the unique features of mammalian retinas are the result of two evolutionary steps: first a reduction of cone based vision, followed by a secondary differentiation of photopic vision and behaviour relying on the remaining set of cones.
哺乳动物光感受器的地形和光谱特征与当前的生态需求和进化历史都相关。南美负鼠是一种有袋类哺乳动物，习性不特化，具有黄昏活动的生活方式。稀疏的视锥细胞群体（最多 = 3000/mm²）可通过形态、地形和免疫细胞化学标准分为四种亚型。尽管存在这种不寻常的多样性，但视锥细胞类型可分为两个功能组：被抗体OS - 2标记的对短波长敏感色素的单视锥细胞群体普遍存在，但密度非常低（200/mm²）。被针对长波长敏感色素的抗体（COS - 1）标记的单视锥细胞在中央凹区域构成优势群体（2300/mm²）。这两种单视锥细胞类型与胎盘哺乳动物中典型存在的一对相对应。这种“现代”组合可能提供空间和颜色对比度的辨别。带有油滴的被COS - 1标记的双视锥细胞和单视锥细胞，通过局限于视网膜的下半部（非反光）呈现出不同的模式（最多 = 800/mm²）。这种额外的带有油滴和长波长色素的视锥细胞群体是负鼠视网膜和其他有袋类动物的保守特征。作为辅助视锥系统，它可能在中间视觉条件下提供增强的敏感性。在夜间哺乳动物早期进化过程中，随着视杆视觉的显著扩展，这些视锥细胞类型得以保留，但后来在胎盘哺乳动物中消失了。因此，哺乳动物视网膜的独特特征是两个进化步骤的结果：首先是基于视锥细胞的视觉减少，随后是明视觉和行为的二次分化，依赖于剩余的视锥细胞群体。
