An ultrastructural study of glomeruli associated with vomeronasal organs transplanted into the rat CNS

Rat neonate vomeronasal organs were transplanted into the parietal cortex of littermates to examine their survival and the behavior of axon growth into the surrounding host brain parenchyma. After survival times of 10-100 days the brains were processed for ultrastructural examination. The transplanted vomeronasal organs (VNO) formed several vesicles lined with a sensory epithelium. From these sensory epithelia, VNO neurons leave the epithelium and enter the host brain. Transplant neurons grew axons that fasciculated into bundles surrounded by sheath cell processes and formed one or more fiber plexuses containing distinct globose or spherical-shaped glomerular-like structures. The glomeruli consisted of nerve terminals between which existed asymmetric synaptic contacts. Rarely did we observe clear reciprocal synapses. The glomeruli also contained terminals that showed signs of degeneration, such as increased density of the terminals, clumping of mitochondria and multivesicular bodies. The glomeruli were not partitioned or subdivided by glial septa; however, glial profiles were interspersed among the sensory terminals. Transplant glomeruli also lacked periglomerular cells and had no definitive glial envelope. These results suggest that glomerular formation is not dependent on dendrite contribution of second order neurons or glial support, but rather on a complementary population of receptor neurons.