THE JOURNAL zyxwvu OF COMPARATIVE NEUROLOGY 348225-233 (1994) Developmental Changes in the Laminar Termination of A Fibre Cutaneous Sensory Afferents in the Rat Spinal Cord Dorsal Horn M. FITZGERALD, T. BUTCHER, AND P. SHORTLAND Department of Anatomy and Developmental Biology, University College London, London WClE 6BT, United Kingdom ABSTRACT In order to establish the specificity of growth and termination of dorsal root afferents within the developing spinal cord, the central dorsal horn terminals of myelinated sensory afferents were labelled at various stages in the rat from embryonic day (E)18 through to postnatal day (P) 35 using horseradish peroxidase conjugated to choleragenoid (B-HRP). The preferential labelling of A fibre afferents with this tracer was found to be as clear in the neonate as has been reported for the adult. The results show that while the somatotopic arrangement of A fibre afferent terminals in the dorsal horn is established early in development, the laminar projections are not. Following peripheral nerve or local skin injections of B-HRP, A fibre terminals were found to project throughout laminae I to V, including lamina I1 (substantia gelatinosa). This widespread termination was observed consistently until the end of the third postnatal week. After P22 the terminal field becomes restricted to the normal laminae I11 to v. zyxwvut D 1994 Wiley-Liss, Inc. Key words: primary afferent, primary sensory neuron, substantia gelatinosa, dorsal root ganglion, axon terminals The growth of the central processes of primary sensory neurons into the spinal cord during development has been described in rat (Windle and Baxter, 1936; Smith, 1983; Fitzgerald, 1987; Kudo and Yamada, 1987; Fitzgerald et al., 1991; Ruit et al., 1992) and chick (Davis et al., 1989) by means of a variety of techniques. In the rat, dorsal roots first reach the lumbar cord at embryonic day 12 (E)12, travel rostrocaudally in the bundle of His and then begin to send collaterals into the dorsal grey matter at El5 (Fitzger- ald et al., 1991). The first sensory fibres to grow in are the larger diameter myelinated afferents, some of which are la muscle afferents heading towards the motoneurone pool in the ventral horn, and some of which are AP cutaneous afferents which remain in the dorsal horn (Smith, 1983; Kudo and Yamada, 1987; Ziskind-Conhaim, 1990, Fitzger- ald et al., 1991). These are followed some days later by smaller diameter unmyelinated C fibres growing into the superficial laminae of the dorsal horn, substantia gelatinosa (SG)(Fitzgerald, 1987). Although the pattern of ingrowth of C fibres has been studied in the rat using selective labelling of small diameter afferents with transganglionic wheat germ agglutinin- horseradish peroxidase (WGA-HRP) (Fitzgerald, 1987), enzymes (Coimbra et al., 1986) or neuropeptides (Marti et al., 1987) no such study of the ingrowth of cutaneous A fibres has been undertaken. Nonselective axonal labelling methods such as Golgi staining (Bed et al., 1988), HRP transport or DiI labelling of dorsal roots (Smith, 1983; Kudo and Yamada, 1987; Ruit et al., 1992) or immunostain- ing for the growth associated protein GAP-43 (Fitzgerald et z al., 1991) have provided only the general pattern. So, while a clear picture has emerged of the specificity of growth of afferent C fibres into the spinal cord in terms of somato- topic pattern (Fitzgerald and Swett, 1983) and laminar location (Fitzgerald, 19871,similar information is not avail- able for cutaneous A fibres. The B subunit of cholera toxin conjugated with HRP, when applied to a peripheral nerve, is a selective marker of large myelinated afferents. Transganglionic labelling of skin and cutaneous nerves with B-HRP in adult rats shows terminals restricted to laminae I and 111-V of the dorsal Accepted April 20, 1994. P. Shortland’s present address is Department of Anatomy and Neurobiol- zy ogy, St. Louis University Medical Center, 1402 S. Grand, St. Louis, MO 63104. Address reprint requests to Dr. M. Fitzgerald, Dept. of Anatomy and Developmental Biology, University College London, Gower St., London, WClEGBT, United Kingdom. o 1994 WILEY-LISS, INC.