Stomach Colon Liver Pancreas Urinary bladder Epithelial parts of trachea lungs pharynx thyroid intestine

▲ FIGURE 22-7 Fates of the germ layers in animals. During the early period of mammalian development, cells migrate to create the three germ layers: ectoderm, mesoderm, and endoderm. This process is called gastrulation. Cells in the different layers have largely distinct fates and therefore represent distinct cell lineages. The detailed cell lineage, however, is not exactly the same in different individuals. (a) Shown here is a sketch of a human embryo about 16 days after fertilization of an egg. The first cells to move from the epiblast into the interior form endoderm, and they are followed by invaginating cells that become mesoderm. The remaining epiblast cells become ectoderm. Hypoblast = covering layer. Amnioblasts = cells that will line amniotic cavity. Yolk sac = reminder of our egg-bound ancestors. (b) Scanning electron micrograph of a cross section of a similar-stage embryo. (c) Some of the tissue derivatives of the three germ layers are listed. [Part (a) after T W. Sadler, 2000, Langman's Medical Embryology, 8th ed. (Baltimore: Lippincott Williams and Wilkins), pp. 64-65, Figure 4.3). Part (b) courtesy of Kathy Sulik, University of North Carolina-Chapel Hill.]

questions about how many adult somatic cells do in fact have a complete functional genome. Even the successes, like the famous cloned sheep "Dolly," appear to have some medical problems. Even if differentiated cells have a physically complete genome, clearly only portions of it are transcrip-tionally active (Chapter 10). Whether the genome of a differentiated cell can revert to having the full developmental potential characteristic of an embryonic cell is a matter of considerable debate. A cell could, for example, have an intact genome, but be unable to properly reactivate it due to inherited chromatin states.

These observations raise two important questions: How are cell fates progressively restricted during development? Are these restrictions irreversible? In addressing these questions, it is important to remember that what a cell in its normal in vivo location will do may differ from what a cell is capable of doing if it is manipulated experimentally. Thus the observed limits to what a cell can do may result from natural regulatory mechanisms or may reflect a failure to find conditions that reveal the cell's full potential.

Although our focus in this chapter is on how cells become different, their ability to remain the same also is critical to the functioning of tissues and the whole organism. Nondividing differentiated cells with particular characteristics must retain them, sometimes for many decades. Stem cells that divide regularly, such as a skin stem cell, must produce one daughter cell with the properties of the parental cell, retaining its characteristic composition, shape, behavior, and responses to specific external signals. Meanwhile, the other daughter cell with its own distinct inheritance, as the result of asymmetric cell division, embarks on a particular differentiation pathway, which may be determined both by the signals the cell receives and by intrinsic bias in the cell's potential, such as the previous activation of certain genes.

The Complete Cell Lineage of C. elegans Is Known

In the development of some organisms, cell lineages are under tight genetic control and thus are identical in all individuals of a species. In other organisms the exact number and arrangement of cells vary substantially among different individuals. The best-documented example of a reproducible pattern of cell divisions comes from the nematode C. elegans. Scientists have traced the lineage of all the somatic cells in C. elegans from the fertilized egg to the mature worm by following the development of live worms using Nomarski interference microscopy (Figure 22-8).

About 10 rounds of cell division, or fewer, create the adult worm, which is about 1 mm long and 70 ^m in diameter. The adult worm has 959 somatic cell nuclei (hermaphrodite form) or 1031 (male). The number of somatic cells is somewhat fewer than the number of nuclei because some cells contain multiple nuclei (i.e., they are syncytia). Remarkably, the pattern of cell divisions starting from a C. elegans fertilized egg is nearly always the same. As we discuss later in the chapter, many cells that are generated during development undergo programmed cell death and are missing in the adult worm. The consistency of the C. elegans cell lineage does not result entirely from each newly born cell inheriting specific information about its destiny. That is, at their birth cells are not necessarily "hard wired" by their own internal inherited instructions to follow a particular path of differentiation. In some cases, various signals direct initially identical cells to different fates, and the outcomes of these signals are consistent from one animal to the next.

The first few cell divisions in C. elegans produce six different founder cells, each with a separate fate as shown in Figure 22-9a, b. The initial division is asymmetric, giving rise to P1 and the AB founder cell. Further divisions in the P lineage form the other five founder cells. Some of the signals controlling division and fate asymmetry are known. For example, Wnt signals from the P2 precursor control the asymmetric division of the EMS cell into E and MS founder cells. Wnt signaling (see Figure 15-32) is also used in other asymmetric divisions in worms.

The bilateral symmetry of the worm implies duplication of lineages on the two sides; curiously, though, functionally equivalent cells on each side can arise from a pattern of division that is different on the two sides. Some of the embryonic cells function as stem cells, dividing repeatedly to form more of themselves or another type of stem cell, while

▲ FIGURE 22-8 Newly hatched larva of C. elegans. Many of in this micrograph obtained by Nomarski interference microscopy.

the 959 somatic cell nuclei in this hermaphrodite form are visualized [From J. E. Sulston and H. R. Horvitz, 1977, Devel. Biol. 56:110.]

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