Cyto- and Myeloarchitectonic Organisation of the Spinal Cord of an Echidna (Tachyglossus aculeatus)

Abstract

We have studied the cyto- and myeloarchitectural organisation of the spinal cord of an echidna (Tachyglossus aculeatus) with the aid of Nissl 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 (B₄ 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 proportions of afferent fibre classes and α and γ motoneurons. In the echidna, mean diameter of myelinated dorsal root axons was 4.65 µm at T₁ and 5.22 µm at L₃, with a clear bimodal distribution in the L₃ dorsal root showing distinct groups at 1 to 5 µm and 6 to 12 µm. 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 µm (approximately 32% of the total myelinated fibre population) and at 7 to 14 µm (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.

Analysis of the distribution of binding sites for the plant lectin Bandeiraea simplicifolia I‐isolectin B4 on primary sensory neurones in seven mammalian species

“…The results of this study confirm previous reports of BSI-B 4 binding in the rat, mouse, and rabbit (Streit et al, 1985;Kruger, 1988, 1990;Plenderleith et al, 1992;Plenderleith and Snow, 1993;Kitchener et al, 1994;Wang et al, 1994), and are entirely consistent with studies reporting BSI-B 4 binding in dorsal root ganglia and spinal cord of the chicken (Scott et al, 1990), echidna (Ashwell and Zhang, 1997), opossum (Knott et al, 1999), and rhesus monkey (Tong et al, 1999). Furthermore, recent studies have reported BSI-B 4 binding sites on human primary sensory neurones collected either post-mortem or from patients undergoing surgery (Yiangou et al, 2000;Alavi et al, 2001).…”

Cyto- and chemoarchitecture of the sensory trigeminal nuclei of the echidna, platypus and rat

“…This is similar to the discontinuous distribution of CB immunoreactivity seen in the Sp5C of the echidna and suggests that at least the unmyelinated C fibre afferents are irregularly distributed in the caudal parts of the Sp5 of the echidna. Our study of the echidna spinal cord (Ashwell and Zhang, 1997) showed a similar distribution of lectin labelled afferents in patches deep within the dorsal horn rather than being confined to the gelatinosus layer of the dorsal horn as described for rodents (Plenderleith et al, 1989). This anatomical difference may point to dissimilarities between the echidna and rodents in the processing of information carried by C fibres at both the trigeminal and spinal cord level, but further study would be necessary to confirm this.…”

“…In the brainstem of rodents, this lectin gives terminallike staining in the inner part of lamina II of Sp5C, binding unmyelinated C-fibre primary afferents (Ambalavanar and Morris, 1993). We have previously used this lectin in our study of unmyelinated afferents to the spinal cord of the echidna (Ashwell and Zhang, 1997) and showed that this lectin labels small unmyelinated primary afferents (presumably comprising the C type fibres) arising from small ganglion cells, similar to that seen in rodents (Streit et al, 1985;Plenderleith et al, 1989).…”

“…We applied the same technique used in our previous study of the echidna spinal cord (Ashwell and Zhang, 1997). Sections were incubated at 4 8C for 4 h in a solution of a horseradish peroxidase conjugated B4 isolectin derived from Griffonia simplicifolia (Sigma, 0.5 mg/100 ml Trisbuffered saline with 1% Triton X-100).…”

“…The B4 isolectin derived from Griffonia simplicifolia binds to small unmyelinated afferents in the dorsal roots and trigeminal nerve of both eutherians (Streit et al, 1985;Plenderleith et al, 1989) and monotremes (Ashwell and Zhang, 1997). When we applied peroxidase-conjugated lectin to sections through the trigeminal sensory nuclei of the echidna (Fig.…”