Cell death organizes the postnatal development of the trigeminal innervation of the cerebral vasculature.

In the adult trigeminal ganglion single cell bodies that innervate the middle cerebral artery (MCA) are different from but situated near to one or more cell bodies that innervate the forehead (O'Connor and Van der Kooy, submitted). Multiple fluorescent retrograde axonal tracing in postnatal day 3-90 rats was employed to describe the development of this adult pattern of trigeminal projections. We found that close to 90% of the cells that innervate the MCA at postnatal day 5 (PND 5) are eliminated by PND 90. Less than 20% of the ganglion cells innervating the forehead die over the same postnatal period. Subpopulations of cells in the ganglion were observed to have a maximal rate of death during different postnatal periods. First, 15-20% of the cells throughout the ophthalmic division die between PND 5 and PND 10. Second, a small population of cells that had early projections to the contralateral MCA die out completely by PND 22. Third, cells with a projection only to the MCA die primarily between PND 10 and PND 54. Fourth, during the first postnatal week there are many cells that project to both the MCA and the forehead; however, 90% of this population dies by PND 90. This elimination is observed latest in the postnatal period, with these cells exhibiting their greatest rate of cell death between PND 22 and PND 90. Thus, cell death is the primary postnatal mechanism that produces this organization in the ophthalmic division of the trigeminal ganglion and retraction of axonal collaterals is a minor mechanism. We suggest that the latest period of death in cells with divergent artery and forehead projections as well as the ultimate persistence of some artery projecting cells beyond PND 90, may be due to the larger peripheral fields of innervation of these trigeminal ganglion cells.