Roth G, Naujoks-Manteuffel C, Grunwald W
Brain Research Institute, University of Bremen, F.R.G.
J Comp Neurol. 1990 Jan 1;291(1):27-42. doi: 10.1002/cne.902910104.
The tectum mesencephali of salamanders shows a morphology that has long been considered primitive when compared with that of frogs. The alternative hypothesis is that the salamander brain is secondarily simplified. In order to test these two hypotheses, the cytoarchitecture of the tectum and the projections of tectal neurons were studied in 11 species of salamanders. Application of the Golgi method reveals three major morphological types. Type 1 has a very wide dendritic arborization mostly confined to the deep fiber layers, and somata are always located within the most superficial part of the periventricular gray matter. Type 2 possesses a wide to medium-size dendritic arborization. In subtype 2a the somata are located in the uppermost part of the gray, and dendrites always reach the uppermost layer of retinal afferents; in subtype 2b the somata are found in deeper parts of the gray, and dendrites arborize in the deeper layers of retinal afferents; and in subtype 2c the somata are also located in deeper parts, but the wide dendritic arborization is confined to deep fiber layers. Type 3 shows the narrowest dendritic arbors that always reach the upper two tectal fiber layers. The somata are found at any depth of the gray matter. HRP experiments reveal a correlation between morphological differences and the projections of tectal neurons. Type 1- and type 2c-like cells constitute the uncrossed tecto-bulbo-spinal tract, whereas type 1- and type 2a-like cells and migrated large spindle-shaped cells (Salamandra) constitute the crossed tecto-bulbo-spinal tract. Type 3-like neurons project to thalamic, pretectal, and isthmic termination sites. The HRP experiments also demonstrate the existence of two classes of mesencephalic trigeminal cells. A comparison shows that salamanders and frogs possess very similar functional and morphological types of tectal cells. However, tectal cells of salamanders show a "juvenile" morphology, and the number of migrated cells is about 10 times higher in frogs compared to salamanders. Both phenomena are seen as the result of secondary simplification of brain structures in the context of paedomorphosis.
与蛙类相比，蝾螈中脑顶盖的形态长期以来一直被认为是原始的。另一种假说是蝾螈的大脑是次生简化的。为了验证这两种假说，对11种蝾螈的顶盖细胞结构和顶盖神经元的投射进行了研究。应用高尔基方法揭示了三种主要的形态类型。1型具有非常广泛的树突分支，主要局限于深层纤维层，胞体总是位于室周灰质的最表层部分。2型具有广泛到中等大小的树突分支。在2a亚型中，胞体位于灰质的最上部，树突总是到达视网膜传入纤维的最上层；在2b亚型中，胞体位于灰质的较深部分，树突在视网膜传入纤维的较深层分支；在2c亚型中，胞体也位于较深部分，但广泛的树突分支局限于深层纤维层。3型显示出最窄的树突分支，总是到达顶盖的上两层纤维层。胞体见于灰质的任何深度。辣根过氧化物酶（HRP）实验揭示了形态差异与顶盖神经元投射之间的相关性。1型和2c型细胞构成不交叉的顶盖-延髓-脊髓束，而1型和2a型细胞以及迁移的大梭形细胞（火蝾螈属）构成交叉的顶盖-延髓-脊髓束。3型神经元投射到丘脑、顶盖前区和峡核终止部位。HRP实验还证明了存在两类中脑三叉神经细胞。比较表明，蝾螈和蛙类具有非常相似的顶盖细胞功能和形态类型。然而，蝾螈的顶盖细胞呈现出“幼年”形态，与蝾螈相比，蛙类中迁移细胞的数量大约高10倍。这两种现象都被视为幼态持续背景下脑结构次生简化的结果。
