Jackman, Alana Jane; (1997) Factors determining the development of mammalian sensory neurons and cutaneous innervation in vivo. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The developing innervation of rat hindlimb skin and spinal cord was investigated from E13-E21 using general and phenotype-specific markers. The timing, pattern and density of axon bundles were elucidated using PGP 9.5, GAP 43 and peripherin as pan-neuronal markers, RT97 for A-fibres and CGRP, trkA and IB4 for small A and C-fibres. RT97 and trkA positive fibres innervated skin at E14, RT97 was also found in the dorsal horn at E14 but no trkA labelling was detected until E18. CGRP expression appears simultaneously in the both targets at E19, suggesting it is expressed subsequent to target innervation. The developmental series implies that large and small diameter fibres innervate the peripheral target together while the central target is innervated by A-fibres prior to C-fibres. The selectivity of RT97 as an A-fibre label was confirmed by construction of size-frequency histograms at E18. Using in situ hybridisation temporal expression of CGRP mRNA was consistent with peptide expression. Quantitative analysis of epidermal innervation shows all subpopulations transiently grow to the skin surface at E17-E18 before retraction of fibres to a sub-epidermal plexus. Administration of anti-NGF in utero results in a selective loss of small diameter DRG cells (Ruit et al, 1992). This treatment was used to examine the consequences upon the remaining DRG subpopulation and the pattern of cutaneous innervation. DRG counts at E21 showed a cell loss of 43% restricted to the small cell population yet this treatment resulted in total abolition of all epidermal innervation by E21, while sub-epidermal innervation was relatively unchanged. The experimental group showed an increase in the number and size of larger cells, with a population of cells sized 600-800μm2. These results indicate that DRG subpopulations have different developmental patterns of innervation but that anti-NGF treatment renders the epidermis non-permissive to innervation by all fibre types.

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