Ashwell K W, Zhang L L
School of Anatomy, University of NSW, Sydney, Australia.
Brain Behav Evol. 1997;49(5):276-94. doi: 10.1159/000112998.
We have studied the cyto- and myeloarchitectural organisation of the spinal cord of an echidna (Tachyglossus aculeatus) with the aid of Nisst staining, darkfield examination and p-phenylenediamine staining. We have also examined the distribution of unmyelinated afferents by labelling with a peroxidase-conjugated lectin derived from Griffonia simplicifolia (B1 isolectin). The cytoarchitectural features characterising the laminar organisation of the spinal cord in eutherian mammals were broadly applicable to the spinal cord of this monotreme. In addition, we identified a distinct group of large neurons in the ventral part of lamina X, extending into the ventral funiculus, that we have called the median nuclear group. We were unable to identify a central cervical nucleus in this echidna on the basis of cytoarchitectural criteria, although all other spinal cord nuclei found in eutherians could be found in this monotreme. Lectin labelling with the Griffonia simplicifolia isolectin B4 revealed a subpopulation of dorsal root ganglion cells similar to those labelled in Eutheria. In this echidna, labelling of unmyelinated fibres was found in Lissauer's zone and laminae I and II, as seen in rats (Rattus norvegicus); there were also deeper patches extending into laminae III to V and what appeared to be commissural axons approaching the dorsal grey commissure, which have not been seen in Eutheria. Fibre calibre in the dorsal and ventral roots of this echidna was similar to that reported in Eutheria, suggesting similar proportion of afferent fibre classes and alpha and gamma motoneurons. In the echidna, mean diameter of myelinated dorsal root axons was 4.65 microns at T1 and 5.22 microns at L3, with a clear bimodal distribution in the L3 dorsal root showing distinct groups at 1 to 5 microns and 6 to 12 microns. These made up approximately 45 and 55% of the total myelinated axon population, respectively. Myelinated fibres in the ventral root at L3 showed two major peaks in distribution. These were at 1 to 4 microns (approximately 32% of the total myelinated fibre population) and at 7 to 14 microns (approximately 58% of the total myelinated fibre population). The cross-sectional area of the dorsal columns of this monotreme was comparable to that of a eutherian mammal of similar body weight, and myelinated axon calibre was similar to that seen in the domestic cat. Our findings indicated that spinal cord cytoarchitectural organisation is highly conserved across class Mammalia, although the lectin labelling findings suggested that termination of unmyelinated afferents in echidnas may differ from that found in Eutheria. The dorsal column system appears to be as anatomically well developed in this monotreme as in those eutherian mammals considered to have a pronounced discriminative tactile sense.
我们借助尼氏染色、暗视野检查和对苯二胺染色，研究了针鼹（短吻针鼹）脊髓的细胞结构和髓鞘结构组织。我们还通过用源自西非豆（B1 同工凝集素）的过氧化物酶偶联凝集素标记，检查了无髓传入纤维的分布。在真兽类哺乳动物中表征脊髓分层组织的细胞结构特征广泛适用于这种单孔类动物的脊髓。此外，我们在 X 层腹侧部分发现了一组独特的大神经元，延伸至腹侧索，我们将其称为正中核群。基于细胞结构标准，我们在这只针鼹中未能识别出中央颈核，尽管在真兽类中发现的所有其他脊髓核在这种单孔类动物中都能找到。用西非豆同工凝集素 B4 进行的凝集素标记揭示了一群背根神经节细胞，类似于在真兽类中标记的那些细胞。在这只针鼹中，在背外侧索以及 I 层和 II 层发现了无髓纤维的标记，如在大鼠（褐家鼠）中所见；还有更深的斑块延伸至 III 层至 V 层，以及似乎是接近背侧灰质连合的连合轴突，这在真兽类中未见。这只针鼹背根和腹根中的纤维直径与在真兽类中报道的相似，表明传入纤维类别以及α和γ运动神经元的比例相似。在针鼹中，T1 水平有髓背根轴突的平均直径为 4.65 微米，L3 水平为 5.22 微米，L3 背根中有明显的双峰分布，在 1 至 5 微米和 6 至 12 微米处有明显的分组。这些分别约占总髓鞘化轴突群体的 45%和 55%。L3 腹根中的髓鞘化纤维在分布上有两个主要峰值。分别在 1 至 4 微米（约占总髓鞘化纤维群体的 32%）和 7 至 14 微米（约占总髓鞘化纤维群体的 58%）。这种单孔类动物背柱的横截面积与体重相似的真兽类哺乳动物相当，有髓轴突直径与家猫中所见相似。我们的研究结果表明，尽管凝集素标记结果表明针鼹中无髓传入纤维的终止可能与在真兽类中发现的不同，但脊髓细胞结构组织在整个哺乳纲中高度保守。背柱系统在这种单孔类动物中似乎在解剖学上与那些被认为具有明显辨别触觉的真兽类哺乳动物一样发达。
