Cell Birth Lineage And Death

Cells being born in the developing cerebellum. All nuclei are labeled in red; the green cells are dividing and migrating into internal layers of the neural tissue. [Courtesy of Tal Raveh, Matthew Scott, and Jane Johnson.]

During the evolution of multicellular organisms, new mechanisms arose to diversify cell types, to coordinate their production, to regulate their size and number, to organize them into functioning tissues, and to eliminate extraneous or aged cells. Signaling between cells became even more important than it is for single-celled organisms. The mode of reproduction also changed, with some cells becoming specialized as germ cells (e.g., eggs, sperm), which give rise to new organisms, as distinct from all other body cells, called somatic cells. Under normal conditions somatic cells will never be part of a new individual.

The formation of working tissues and organs during development of multicellular organisms depends in part on specific patterns of mitotic cell division. A series of such cell divisions akin to a family tree is called a cell lineage, which traces the progressive determination of cells, restricting their developmental potential and their differentiation into specialized cell types. Cell lineages are controlled by intrinsic (internal) factors—cells acting according to their history and internal regulators—as well as by extrinsic (external) factors such as cell-cell signals and environmental inputs (Figure 22-1). A cell lineage begins with stem cells, unspecialized cells that can potentially reproduce themselves and generate more-specialized cells indefinitely. Their name comes from the image of a plant stem, which grows upward, continuing to form more stem, while sending off leaves and branches to the side. A cell lineage ultimately culminates in formation of terminally differentiated cells such as skin cells, neurons, or muscle cells. Terminal differentiation generally is irreversible, and the resulting highly specialized cells often cannot divide; they survive, carry out their functions for varying lengths of time, and then die.

Many cell lineages contain intermediate cells, referred to as precursor cells or progenitor cells^ whose potential to form different kinds of differentiated cells is more limited than that of the stem cells from which they arise. (Although some researchers distinguish between precursor and progenitor cells, we will use these terms interchangeably.) Once a new precursor cell type is created, it often produces transcription factors characteristic of its fate. These transcription factors

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