It has long been suspected that all the differentiated cell types in the lining of the intestine derive from a single type of stem cell. But firm proof was lacking, and the precise nature and location of the stem cells were disputed.
To solve the problem, and indeed to understand the organization of any stem-cell system, we need to discover how its cells are related to one another—who
Figure 22–3 The definition of a stem cell.
each daughter produced when a stem cell divides can either remain a stem cell or go on to become terminally differentiated. in many cases, the daughter that opts for terminal differentiation undergoes additional cell divisions before terminal differentiation is completed; such cells are called transit amplifying cells.
Figure 22–4 An adenoma in the human colon, compared with normal tissue from an adjacent region of the same person’s colon. the specimen is from a patient with an inherited mutation in one of his two copies of the Apc gene. a mutation in the other Apc gene copy, occurring in a colon epithelial cell during adult life, has given rise to a clone of cells that behave as though the wnt signaling pathway is permanently activated. as a result, the cells of this clone form an adenoma—an enormous, steadily expanding mass of giant cryptlike structures.
Figure 22–5 Clonal analysis using a genetic marker. a modern method for tracking cell lineage uses transgenic animals containing two transgenes, which together drive expression of a readily detected and heritable marker protein in a small subset of stem cells. the first transgene (top) carries two adjacent protein-coding sequences, GFP and CreERT2, both expressed under the control of the Lgr promoter that is active only in stem cells and not in their differentiated progeny. GFP encodes green fluorescent protein (see Chapter 9), which is used here simply to confirm expression in the entire stem-cell population. the CreERT2 gene encodes a chimeric form of the Cre recombinase called CreeRt, which consists of Cre recombinase linked to the estrogen receptor protein; this enzyme becomes active as a recombinase only when it binds the artificial estrogen analog tamoxifen.
the second transgene (bottom) carries a marker gene, LacZ, under the control of a promoter that is active in all cells. the LacZ gene encodes β-galactosidase, an enzyme that can be detected histochemically in tissues (see Figure 7–28). however, LacZ expression in the transgene shown here is prevented by a blocking sequence (red) that is flanked by LoxP sites (pink; see Figure 5–66). when tamoxifen is provided, CreeRt becomes active––leading to a recombination event that removes the blocking dna sequence (and leaves one LoxP site behind). as a result, the lacZ marker is expressed. because this change is heritable, the marker continues to be expressed in all cells descended from those in which a recombination event has occurred. with a low dose of the inducer molecule tamoxifen, it is possible to activate the marker at random in just a few widely spaced cells, which, in the course of time, give rise to widely separated and easily distinguished clones of progeny (see Figure 22–6).
is descended from whom, or, equivalently, what progeny will be produced from any given cell. This can best be done using a heritable marker that can be activated in an individual cell, thus allowing the identification of the clone of progeny descended from that cell. A modern method uses transgenic animals to create a visible genetic mark in just a few widely spaced cells, which, in the course of time, give rise to widely separated and easily distinguished clones of progeny, as explained in Figure 22–5.