Key Concepts Of Section 221

The Birth of Cells

■ In asymmetric cell division, two different types of daughter cells are formed from one mother cell. In contrast, both daughter cells formed in symmetric divisions are identical but may have different fates if they are exposed to different external signals (see Figure 22-1).

■ Pluripotent stem cells can produce more than one type of descendant cell, including in some cases a stem cell with a more-restricted potential to produce differentiated cell types (see Figure 22-2).

■ Cultured embryonic stem cells (ES cells) are capable of giving rise to many kinds of differentiated cell types. They are useful in production of genetically altered mice and offer potential for therapeutic uses.

■ Populations of stem cells associated with most tissues (e.g., skin, intestinal epithelium, blood) regenerate differentiated tissue cells that are damaged or sloughed or become aged (see Figures 22-4 and 22-5).

■ Stem cells are prevented from differentiating by specific controls. A high level of p-catenin, a component of the Wnt signaling pathway, has been implicated in preserving stem cells in the skin and intestine by directing cells toward division rather than differentiation states.

■ Plant stem cells persist for the life of the plant in the meristem. Meristem cells can give rise to a broad spectrum of cell types and structures.

■ During development, precursor cells generally lose potential; that is, they become progressively restricted in the number of different cell types they can form.

■ Early in animal development, the three germ layers— ectoderm, mesoderm, and endoderm—form. Each gives rise to specific tissues and organs (see Figure 22-7).

■ Germ-line cells give rise to eggs or sperm. By definition, all other cells are somatic cells.

■ Embryonic development of C. elegans begins with asymmetric division of the fertilized egg (zygote). The lineage of all the cells in adult worms is known and is highly reproducible (see Figure 22-9).

■ Short regulatory RNAs control the timing of developmental cell divisions by preventing translation of mRNAs whose encoded proteins control cell lineages (see Figure 22-10).

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