THE DEVELOPMENT OF CUTANEOUS AFFERENT PATHWAYS IN FETAL SHEEP - A STRUCTURAL AND FUNCTIONAL-STUDY

In this study we have examined the functional and structural developme nt of cutaneous afferent pathways in the fetal sheep hindlimb from 67 to 143 days of gestation. The earliest age at which extracellular disc harges could be evoked in dorsal root ganglia and in dorsal horn cells by natural cutaneous stimulation was 75 days. The majority of cells r esponded to light stroking or indentation of the skin (low threshold) although some cells responded to intense squeezing (high threshold). W ith increasing gestational age, the majority of cells continued to res pond to low threshold stimuli with cells responding to intense mechani cal stimuli being recorded less frequently. Dorsal root ganglion cells responding to the cutaneous application of noxious heat (> 45 degrees C) were first observed at 107 days. We have also shown that fibres pr ojecting from dorsal horn cells and presumably entering the spinothala mic tract carry signals at least as far as the mid thoracic spinal cor d by 104 days. Ultrastructural studies of the skin revealed bundles of unmyelinated axons located adjacent to blood vessels throughout the d ermis at 68 days. Innervation of the skin was first observed by silver staining at 75 days when fibres could be seen running along blood ves sels in the dermis. With increasing gestational age fibres were seen t raversing the dermis to innervate the epidermis as free nerve endings. Wool/hair follicle innervation was first evident at about 100-106 day s with only a few follicles being innervated at this age. By 115 days, nerve endings had begun to form circumferential wrappings around foll icles and by 134 days lanceolate terminals were observed around the ba se of the follicles. Biocytin-labelled afferent fibres entered the dor sal horn at 56 days. This initial innervation was sparse but by 76 day s there was a marked increase in both the number of afferent fibres en tering the dorsal horn and in the extent of their arborisation. Ultras tructural studies revealed that terminals immunoreactive for calcitoni n gene-related peptide were present in lamina I as early as 61 days. T he period of maximal synaptogenesis and synaptic maturation of this gr oup of terminals appeared to occur between 87-128 days. Therefore the commencement of activation of dorsal horn cells by natural cutaneous s timulation occurs by mid gestation (75 days) in the fetal sheep. This is at the same time or just after peripheral nerves first innervate th e skin and about 2 weeks after primary afferent terminals can be ident ified in the dorsal horn.