We have investigated the time course and extent to which peripheral nerve lesions cause a morphological reorganization of the central terminals of choleragenoid-horseradish peroxidase (B-HRP)-labelled primary afferent fibers in the mammalian dorsal horn. Choleragenoid-horseradish peroxidase is retrogradely transported by myelinated (A) sensory axons to laminae I, III, IV and V of the normal dorsal horn of the spinal cord, leaving lamina II unlabelled. We previously showed that peripheral axotomy results in the sprouting of numerous B-HRP-labelled large myelinated sensory axons into lamina II. We show here that this spread of B-HRP-labelled axons into lamina II is detectable at 1 week, maximal by 2 weeks and persists for over 6 months postlesion. By 9 months, however, B-HRP fibers no longer appear in lamina II. The sprouting into lamina II occurs whether regeneration is allowed (crush) or prevented (section with ligation), and does not reverse at times when peripheral fibers reinnervate the periphery. We also show that 15 times more synaptic terminals in lamina II are labelled by B-HRP 2 weeks after axotomy than in the normal. We interpret this as indicating that the sprouting fibers are making synaptic contacts with postsynaptic targets. This implies that A-fiber terminal reorganization is a prominent and long-lasting but not permanent feature of peripheral axotomy. We also provide evidence that this sprouting is the consequence of a combination of an atrophic loss of central synaptic terminals and the conditioning of the sensory neurons by peripheral axotomy. The sprouting of large sensory fibers into the spinal territory where postsynaptic targets usually receive only small afferent fiber input may bear on the intractable touch-evoked pain that can follow nerve injury.

1. The development of the blood‐brain and blood‐c.s.f barriers to lipid insoluble substances of different molecular radii has been studied in fetal sheep, early (60 days) and late (125 days) in gestation, using labelled erythritol (C14), sucrose (3H or 14C), inulin (3H or 14C) and albumin (125I), or albumin and IgG detected by immunoassay. 2. Morphological studies of fetal brain and choroid plexus at the same gestational stages were carried out using thin section electron microscopy and the freeze fracture techniques. 3. Penetration of markers into c.s.f. was substantially greater at 60 days than at 125 days, but at both ages the steady‐state level achieved appeared to be related to molecular size. 4. A simple model describing penetration from blood into c.s.f. at 60 days is proposed. It involves the assumption that c.s.f. and brain extracellular fluid are effectively separate compartments; morphological and permeability data which supports this assumption is presented. The data for c.s.f. at 60 days are consistent with the suggestion that the markers penetrate into c.s.f. by diffusion and are not restricted by small pores in the interface between blood and c.s.f. 5. The reduction in penetration which occurred by 125 days for all markers except erythritol appears to be accounted for by an increase in the sink effect and a decrease in the effective surface area for exchange between blood and c.s.f. 6. Intercellular tight junctions of both cerebral endothelial cells and choroid plexus epithelial cells were well formed at 60 days gestation. There was no change in junctional characteristics previously thought to correlate with transepithelial permeability (tight junction depth and strand number) between the two ages studied, although there were marked changes in permeability. 7. Evidence is advanced in support of the hypothesis that in the fetus much of the penetration of lipid insoluble non‐polar substances across the blood‐c.s.f. barrier and perhaps across the blood‐brain barrier occurs via a transcellular route consisting of a system of tubulo‐cisternal endoplasmic reticulum. Penetration via the choroid plexus appears to be the dominant route for penetration from blood into c.s.f. in the 60 day fetus.

Terminal Schwann cells, when stained for S100 (a calcium binding protein), can be seen to cap motor axons at the neuromuscular junction. Within days of denervation the Schwann cells begin to stain for the low affinity nerve growth factor receptor, but remain Thy-1 negative, and elaborate fine processes. These processes become longer and more disorganized over weeks, and cells positive for S100 and nerve growth factor receptor migrate into the perisynaptic area. Reinnervation results in a withdrawal of the processes. The morphology and location of terminal Schwann cells seems to depend on axonal contact. The spread of Schwann cells and their processes away from the synaptic zone following denervation, implies that these cells do not target axons directly to the endplate.

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Skin innervation during wound healing was investigated using immunocytochemical staining with the panneuronal marker antiprotein gene product (PGP) 9.5, which labels the entire innervation of the skin throughout development and in the adult. Full-thickness skin wounds in the hairy skin of the foot in neonatal rats result in pronounced hyperinnervation of the tissue that persists long after the wound has healed (at least 12 weeks). Quantification of this hyperinnervation by image analysis indicates that skin innervation density in the wounded area can increase up to 300%. The effect is greatest when wounds are performed at postnatal day (P) 0 or 7, declining when performed at P14 and P21 to resemble the weaker and transient effect in the adult. Staining with selective markers for different neuronal populations innervating skin (monoclonal anti-RT97 staining the myelinated axons of large light sensory ganglion cells; anticalcitonin gene-related peptide staining unmyelinated C axons, thinly myelinated A delta axons, and a subpopulation of large A fibres) reveal that both A- and C-fibre sensory axons contribute to this response. Destruction of the majority of the C-fibre population with neonatal capsaicin pretreatment, which leaves large A fibres intact, significantly reduces the hyperinnervation response at 14 days, confirming a major contribution from both A and C fibres. Sympathetic axons, stained with anti-tyrosine hydroxylase, do not sprout into the wounded area. Furthermore, pretreatment of neonates with 6-hydroxydopamine, which destroys the sympathetic innervation, does not significantly reduce the overall sprouting response, as identified by anti-PGP9.5 staining. Behavioural sensory testing revealed a 50% drop in the mechanical threshold in the wounded area after 3 weeks. These remarkably long-term and specific effects on sensory terminal axons following neonatal skin wounding indicate the plasticity of cutaneous innervation density following alterations in the target tissue at a critical stage of development.