On the problem of lamination in the superficial dorsal horn of mammals: A reappraisal of the substantia gelatinosa in postnatal life

Although it is one of the most distinctive and earliest recognized features in the spinal cord, the substantia gelatinosa (SG) remains among the most enigmatic of central nervous regions. The present neuroanatomical studies e mployed transganglionic transport of horseradish peroxidase conjugates of c holeragenoid (B-HRP) and the B-4 isolectin of Bandeiraea simplicifolia (IB4 -HRP) on opposite sides to compare the projection patterns of myelinated an d unmyelinated cutaneous primary afferents, respectively, within the superf icial dorsal horn of the spinal cord in postnatal mice, from shortly afterb irth to adulthood. Putative unmyelinated afferents labeled with IB4-HRP gav e rise to a dense sheet of terminal-like labeling restricted to the outer h alf of the SG. In contrast, myelinated inputs labeled with B-HRP ga;ve rise to a similarly dense sheet of terminal-like labeling that occupied the inn er half of the SG. This adult organization, with two dense sheets nf termin al labeling in the superficial dorsal horn, was clearly evident shortly alt er birth using these markers, prior to the emergence of the SG. Furthermore , the location of the SG proper varied considerably within the dorsoventral plant of the dorsal horn according to mediolateral and segmental locations , a finding that was also seen in comparative studies in rat and cat. These findings caution against equating the SG in particular, and the superficia l dorsal horn in general, with nociceptive processing; at minimum the SG su bserves a clear duality of function, with only a thin portion of its outerm ost aspect devoted to pain. J. Comp, Neurol. 417: 88-102, 2000. (C) 2000 Wi ley-Liss, Inc.