CYTOARCHITECTONIC AND MYELOARCHITECTONIC ORGANIZATION OF THE SPINAL-CORD OF AN ECHIDNA (TACHYGLOSSUS-ACULEATUS)

We have studied the cyto- and myeloarchitectural organisation of the s pinal cord of an echidna (Tachyglossus aculeatus) with the aid of Niss l staining, darkfield examination and p-phenylenediamine staining. We have also examined the distribution of unmyelinated afferents by label ling with a peroxidase-conjugated lectin derived from Griffonia simpli cifolia (B-4 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 Dart o f lamina X, extending into the ventral funiculus, that we have called the median nuclear group. We were unable to identify a central cervica l nucleus in this echidna on the basis of cytoarchitectural criteria, although all other spinal cord nuclei found in eutherians could be fou nd in this monotreme. Lectin labelling with the Griffonia simplicifoli a isolectin B-4 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 e xtending into laminae III to V and what appeared to be commissural axo ns approaching the dorsal grey commissure, which have not been seen in Eutheria. Fibre calibre in the dorsal and ventral roots of this echid na was similar to that reported in Eutheria, suggesting similar propor tions of afferent fibre classes and alpha and gamma motoneurons. In th e echidna, mean diameter of myelinated dorsal root axons was 4.65 mu m at T-1 and 5.22 mu m at L-3, with a clear bimodal distribution in the L-3 dorsal root showing distinct groups at 1 to 5 mu m and 6 to 12 mu m. These made up approximately 45 and 55% of the total myelinated axo n population, respectively. Myelinated fibres in the ventral root at L -3 showed two major peaks in distribution. These were at 1 to 4 pm (ap proximately 32% of the total myelinated fibre population) and at 7 to 14 mu m (approximately 58% of the total myelinated fibre population). The cross-sectional area of the dorsal columns of this monotreme was c omparable to that of a eutherian mammal of similar body weight and mye linated 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 echi dnas may differ from that found in Eutheria. The dorsal column system appears to be as anatomically well developed in this monotreme as in t hose eutherian mammals considered to have a pronounced discriminative tactile sense.