ABSTRACT
Both invertebrate and vertebrate animal models share many of the same basic mechanisms for patterning along the D/V axis. In general, the signals that pattern neuronal types along this axis are concentrated in nonneural tissues located at or near the dorsal and ventral halves of the neural tube. These signals regulate the expression of specific transcription factors in progenitor cells located at various distances from the site of the signal. Neuronal progenitors or precursors are cells that have the capacity to develop into a restricted number of neuron types but have not yet taken on all of the characteristics of a specialized cell type. Each neuronal type that develops along the D/V axis is exposed to a particular concentration of signals that helps establish the unique transcription factor code in a given cell type. The expression of specific transcription factors regulates, in turn, the expression or repression of the genes that determine the morphological and behavioral characteristics of each neuronal type. Much more is discussed about cell fate options and cell differentiation in Chapter 6. Here in Chapter 4, the focus is on the initial patterning of progenitor cells along the D/V axis and how scientists identify the cell populations that give rise to different neuronal subtypes. Therefore, exploring the mechanisms that govern D/V axis organization is also an opportunity to begin to examine how gene expression patterns regulate neuronal specificity, the processes by which precursor cells adopt specific neural characteristics.
