Neurons originate from the stem cells (neuroblasts) of the neuroepithelium. Understanding of the process of neurogenesis was enhanced by studies on relatively simple nervous systems, such as those ofthe nematode, leech, and insects. During embryonic development of the CNS, an enormous diversity of cellular types are arranged and interconnected in a remarkably precise pattern. For example, within the neuroepithelium in each segment of the grasshopper embryo, 61 neuronal precursor cells (neuroblasts) are arranged in two

Weeks Months

and also from the highly stereotyped family of neurons it produces.

As a neuroblast appears in the neuroepithelium, it divides repeatedly to generate a chain of ganglion mother cells. Each ganglion mother cell then divides once more to produce two ganglion cells in a chain of cell doublets, which then differentiate into a family of neurons. The families of neurons originating from different precursor neuroblast cells differ by their unique morphology, physiology, biochemistry, and function.

Thus, each neuroblast appears to generate a specific family of neurons, and a neuroblast's position in the epithelium is so specific that no other cell, including the ectodermal cell adjacent to it, can take its place, nor can the progeny of one neuroblast be replaced by the progeny of the neighboring neuroblasts. The final differentiation of the individual neurons depends on their mitotic ancestry and their later cell-specific interactions with other neurons in the environment.

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