Morphogenesis of the trigeminal mesencephalic nucleus in the hamster: cytogenesis and neurone death

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Abstract
The chronology of cellular development in the trigeminal mesencephalic nucleus of the hamster has been studied by light and electron microscopy. Major developmental features are compressed into a two-week period that spans pre- and postnatal maturation. Special emphasis is focused on the sequence of developmental events, with the following points analyzed in this report: cell proliferation, cytological and axonal development, cell death, and cluster formation.These centrally located primary sensory neurones seem to arise from a separate germinal population found in the alar plate of the midbrain and pons. The neurones are remarkable for their precocious formation and characteristic appearance. In hamster embryos they are first detected when the neural tube is only a couple of cell layers thick. Prenatally, the neurones grow rapidly and have doubled in diameter by birth. Axons form shortly after the cells are formed. This is marked by a sudden upsurge in cytoplasmic tubules and filaments in one end of the soma. Postnatally, cellular growth continues at a reduced rate. However, shortly after birth many small spines protrude from the cell body, thus increasing the surface area available for nutritive exchange. Myelination of the axon commences 6 days after birth.Cell counts on a series of pre- and postnatal animals detected a considerable overproduction of neurones in the mesencephalic nucleus. Prior to birth there is a rapid elimination of almost 50% of these cells. Degenerating cells were identified in both light and electron microscopes. The temporal relationship between peripheral innervation, cell death and synaptogenesis suggests a developmental mechanism that provides some flexibility in the formation of synaptic connexions in the mesencephalic nucleus.Clusters of 2 –4 tightly packed neurones are a characteristic feature of the developing and adult mesencephalic nucleus. Specialized junctions connect adjacent neurones and are thought to provide the basis for electrotonic coupling within the nucleus. Macula adhaerens plaques were present in both pre- and postnatal hamsters. However, close appositions were only identified postnatally.