Target influences on the development of leech neurons

A pair of Retzius neurons is found in each segmental midbody ganglion of the CNS of the leech Hirudo medicinalis. Although all Retzius neurons appear to have the same cell lineage and are indistinguishable from one another through the initial phases of axonogenesis, later in development two pairs of Retzius neurons--those in the segments containing the male and female reproductive ducts--acquire distinctive morphological and physiological characteristics. Experimental manipulations of the reproductive ducts in early embryos have indicated that the outgrowing Retzius axons receive a signal from these peripheral targets that triggers major changes in their developmental program. Such 'end-organ specification' has been shown to contribute to the differentiation of neurons in other nervous systems as well, and the mechanisms underlying such control can be investigated in great detail in the relatively simple, segmented nervous system of the leech.