Estimates of the number of clonogenic cells in crypts of murine small intestine

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Abstract
There is a proliferative cell hierarchy in the mouse intestinal crypt with ancestral stem cells which can regenerate all components of the lineage after injury (clonogenic cells). The number of these clonogenic or regenerative cells per crypt can be estimated from radiobiological experiments where doses of radiation are used to kill cells and ablate crypts. Various approaches can be adopted which provide different estimates of this number of cells. One of the conventional approaches used in the past provided estimates of about 70–80 clonogenic cells per crypt (i.e. about 50% of the proliferative or 30% of all crypt cells). A technically simpler approach has recently been suggested. This has been used here to provide many independent estimates of the number of crypt clonogenic cells. These suggest about 32 clonogenic cells exist per crypt i.e. about half the previous estimate and about twice the number of putative “functional” stem cells (those which operate as stem cells in the normal steady-state crypt). The reasons for the differences are discussed. The new estimates are compatible with the hypothesis that the crypt contains a ring of about 16 functional stem cells which are expected to be clonogenic, besides which there is a second ring of 16 clonogenic cells which represent early transit cells (the immediate daughters of the stem cells) which can act as clonogenic cells if required after radiation injury.