Recovery of non-specific cholinesterase activity in sensory corpuscles of mouse toe skin after irreversible inhibition of this enzyme and cold injury

Degeneration and regeneration of cutaneous sensory nerve formations

Auxiliary structures of the cutaneous sensory nerve formations (SNF) are dependent on sensory innervation during their critical period of development. Denervation of mature cutaneous corpuscles results in survival of the terminal Schwann cells and the capsular structures which are probably responsible for successful reinnervation of the cutaneous SNF. In addition, the basal lamina tubes of Schwann cells are connected with the terminal Schwann cells and play an important role in the guidance of regrowing axons to their original targets. Long-lasting denervation causes atrophic changes of the terminal Schwann cells and alterations of their molecular equipment. These atrophic changes in the terminal Schwann cells may be responsible for erroneous reinnervation of cutaneous SNF. A population of the cutaneous Merkel cells surviving denervation may also serve as targets for regrowing sensory axons. The basal laminae of terminal Schwann cells are produced under control of the sensory terminals during maturation of cutaneous SNF. In adult animals, the basal laminae are capable of stimulating differentiation of migrated Schwann cells to the terminal Schwann cells without the presence of the sensory terminals. Nonspecific cholinesterase (nChE) is secreted by the terminal Schwann cells and is attached to their extracellular matrix. The synthesis of these molecules in adult animals is not influenced by the sensory terminals. However, the presence of nChE molecules is associated with living terminal Schwann cells. Fetal orthotopically grafted dorsal root ganglion (DRG) neurons have the ability to reinnervate cutaneous SNF of adult hosts. When cutaneous areas are denervated, axons from adjacent sensory nerves may extend collateral branches into this area. The capacity for such extension is dependent on: (1) type of sensory nerve ending, C and A delta fibers having significantly greater capacity than sensory axons of larger caliber; (2) age of the animal, immature animals generally showing a greater capacity for collateral sprouting; (3) the state of the adjacent axons, those already in a growth mode being more capable than "resting" ones; and (4) the regional and mechanical conditions at the site of denervation, hindpaw skin being much less extensively reinnervated by collateral fibers than that of the trunk.

Enzyme histochemistry of cutaneous sensory nerve formations

The cutaneous sensory nerve formations belong to the structures which are studied intensely by the enzyme activity histochemistry since the early history of this technique. The histochemical localization of the activities of nonspecific cholinesterase, alkaline phosphatases, acid phosphatase, adenosine tri- and diphosphatases, adenylate cyclase, and dipeptidylpeptidase-IV in the cutaneous sensory nerve formations, mainly sensory corpuscles, is reviewed. The histochemical approach has brought new knowledge of both morphological building of these unique structures and their biochemical constituents. Taken together, the present results of enzyme histochemistry provide insight into the function of enzymes, and disclose new relationships between the sensory terminals and auxiliary structures in the cutaneous sensory nerve formations.

Peripheral target reinnervation following orthotopic grafting of fetal allogeneic and xenogeneic dorsal root ganglia

The sensory reinnervation of dermal papillae and epidermis of glabrous skin, interosseal Pacinian corpuscles, and muscle spindles of the soleus and extensor digitorum longus muscles has been examined 1, 3, and 8 months (allografts) or 3 and 5 weeks (xenografts) following orthotopic grafting of fetal allogeneic or xenogeneic (mouse) dorsal root ganglia (DRG) into ganglionectomized adult rats. Sensory axons in target tissues were identified immunohistochemically by monoclonal antibodies against growth-associated peptide (GAP-43), heavy neurofilament protein (RT-97), anti-mouse-specific membrane glycoprotein Thy-1.2, and polyclonal antibody to calcitonin gene-related peptide (CGRP). Absence of axonal marker staining in target structures of control animals 10 days or 3 months following ipsilateral enucleation of the L3-L6 DRG without grafting indicated an elimination of host normal (intact), regenerating, or collaterally sprouting nerve fibers. The consistent finding of immunolabeled axons ending free and in encapsulated structures in the target tissues of both allo- and xenografted rats indicates that grafted primary sensory neurons can survive and send axonal processes down the full length of the hind limb, to terminate in host target tissues. Axons of xenografted fetal mouse sensory neurons grow in adult rat hosts for distances of 4 cm or more, attaining lengths far greater than called for by their normal developmental programs.

Combination of non-specific cholinesterase histochemistry and immunofluorescence staining for the study of the sensory innervation of skin and muscle

In the present study we describe the application of the non-specific cholinesterase (nChE) histochemical method for the detection of encapsulated sensory nerve endings prior to immunofluorescence staining of the sensory nerve fibres. The nChE staining of Schwann-derived structures surrounding sensory terminals allowed us to identify unequivocally the sensory corpuscles in the skin and the muscle proprioceptors (muscle spindles and Golgi tendon organs) in longitudinal sections of muscle tissue. The nChE staining of sensory nerve endings and immunofluorescence-labelled nerve fibres and their terminals could be viewed and photographed in the same section using appropriate filters. Since nChE activity persists in terminal Schwann cells for a long time after loss of the sensory axons, this combined enzyme- and immunohistochemical approach is also useful for experimental studies involving denervation and re-innervation of sensory nerve endings.

Migration of Schwann cells from the distal stump of the sciatic nerve 1 week after transection: the effects of insulin and cytosine arabinoside

We have examined the migratory capacity of Schwann cells from the distal stump of a 1-week transected sciatic nerve of adult rat for a distance of 10 mm. The distal stump was introduced into the open end of a silicone chamber packed with artificial fibrin sponge (Gelaspon) soaked in phosphate-buffered saline (control chambers), cytosine arabinoside (Ara-C) (0.05 mM), or insulin (40 U/ml). Migrating Schwann cells were distinguished from fibroblasts by the presence of non-specific cholinesterase (nChE) activity and glial fibrillary acidic protein (GFAP). The cells of distal stumps including Schwann cells accepted Gelaspon as a suitable adhesive substratum. In the chambers filled with Gelaspon soaked in phosphate-buffered saline alone Schwann cells were outnumbered by fibroblasts. The addition of Ara-C resulted in greater numbers of Schwann cells, which migrate longer distances into the chambers. The application of insulin enhanced Schwann cell migration as well. These morphologic observations were further supported by biochemical measurements of nChE activity. The results suggest an influence on Schwann cell migration by fibroblasts of connective tissue sheaths and a stimulation of Schwann cell migration by insulin.