TIMECOURSE OF DEVELOPMENT OF THE WALLABY TRIGEMINAL PATHWAY .1. PERIPHERY TO BRAIN-STEM

The development of the vibrissae and their innervation and the maturat ion of the brainstem trigeminal sensory nuclei have been studied in th e wallaby, Macropus eugenii, from birth to adulthood. At birth, develo ping vibrissal follicles consist of solid epidermal pegs surrounded by dermal condensations. The developing follicles and adjacent skin are innervated by trigeminal afferents. Ten days after birth the follicle contains a dermal papilla and the deep vibrissal nerve can be recognis ed. A hair cone is present at postnatal day (P) 30 and hairs are appar ent on the skin surface by P35. By P63 the deep vibrissal nerve can be seen innervating Merkel cells in the outer root sheath; in addition, the first signs of the blood sinus can be recognised. Innervation of t he inner conical body and lanceolate and lamellated receptors supplyin g the mesenchymal sheath and waist region are not seen until P119, whe n the follicle resembles that seen in the adult. At birth, central pro cesses of the trigeminal ganglion cells have entered the trigeminal tr act and extend from the rostral pens to the upper cervical cord. Label ling with a carbocyanine dye at PO shows afferents extending medially from the tract into the trigeminal subnuclei at all levels. At this st age the trigeminal nuclei appear as areas of increased cell density in the lateral brainstem. By P30-40 the four subnuclei can be distinguis hed on the basis of shape, cytoarchitecture, and succinic dehydrogenas e reactivity. Adult morphology is not fully established until P210. In mature animals, nucleus principalis contains closely packed, polymorp hic cells, frequently aligned parallel to thick fibre bundles that tra verse the nucleus obliquely. Subnuclei oralis and interpolaris contain sparsely distributed, medium to large cells, randomly oriented, as we ll as prominent rostrocaudally directed fibre bundles. Subnucleus caud alis consists of the marginal layer, substantia gelatinosa, and magnoc ellular layers as described in other species. Patches of increased suc cinic dehydrogenase or cytochrome oxidase reactivity, presumably corre sponding to the vibrissae, are present in subnuclei principalis, inter polaris, and caudalis in developing and adult animals, although the pa ttern is less clear than in rats. The brainstem patches are first seen at P40, approximately 6 weeks before the corresponding vibrissal-rela ted pattern develops in the cortex. This suggests that the onset of pa tch formation may be regulated independently at different levels of th e pathway. (C) 1994 Wiley-Liss, Inc.