Development of identified glia that ensheathe axons in Hirudo medicinalis.

Interaction between neurons and glia may contribute to the formation of characteristic nerve bundles formed by axon elongation along stereotypic pathways. This study reports the temporal and spatial distribution of identified ensheathing glia during embryonic development in the leech. The development of connective glia was followed 1) using an immunohistochemical probe (monoclonal antibody Lan3-13), which recognized connective glia, and 2) using electron microscopy. Embryonic glia were initially located in the medial region of the lateral connectives and contained intermediate filaments. Glia cells continued to develop throughout embryogenesis; the number and size of glial processes increased, and they ensheathed smaller bundles of axons. The glial cell recognized by Lan3-13 first appeared after axons had already begun to form the connectives. This suggests that these particular glial cells may not function in the initial guidance of axons along stereotypic pathways. However, another cell that contained small bundles of intermediate filaments and glycogen granules was present at early stages of connective formation. These cells may be undifferentiated or transient glia, which could contribute to the formation of characteristic nerve bundles.