The maturation of the end bulb of Held in the rat anteroventral cochlear nucleus.

The maturation of the end bulb of Held was studied in serial thin sections through the rostral pole of the rat anteroventral cochlear nucleus (AVCN) at 2-day intervals from birth through 16 days of age. In the neuropil of newborn rats primary auditory terminals occasionally synapse with large dendritic processes of spherical cells. Between 6 and 10 days of age, the spherical cell has numerous, long, fingerlike appendages at the base and proximal part of its primary dendrite which extend into the neuropil and envelope a single primary terminal. Small processes from the terminal extend along the appendages toward the cell body. At 10 days, these processes reach the dendritic pole of the soma and fill the interstices between the appendages. Synaptic contacts are present between the end bulb processes and the cell body. At 12 days, the end bulb covers most of the dendritic pole of the spherical cell. The remaining somatic appendages are small and are clustered beneath the end bulb. Synapses between the end bulb and soma are frequent. At 16 days, the end bulb resembles that seen in the mature rat. Nonprimary terminals appear on the soma at 10 days. These terminals are randomly distributed, with fine processes which extend from the end bulb over the remaining regions of the cell body.