Hazel Cheng

Abstract The previous articles of this series provided presumptive evidence that the four main differentiated cell types in the epithelium of the mouse small intestine: villus columnar, mucous, entero‐endocrine, and Paneth cells, originate from the same precursor, the crypt‐base columnar cell. In the present work, direct evidence was obtained in support of this view. It was first found that crypt‐base columnar cells phagocytose non‐viable cells in their vicinity, with the result that a large phagosome appears in the cytoplasm. Such phagosomes were then used as markers to follow the evolution of crypt‐base columnar cells. In normal control animals, a rare crypt‐base columnar cell includes a large phagosome containing Paneth cell remnants. By six hours after injection of two μCi 3 H‐thymidine per g body weight, a fair number of crypt‐base columnar cells include a different type of phagosome containing labeled nucleus and granulefree cytoplasm, which is attributed to phagocytosis of a labeled crypt‐base columnar cell killed by beta‐radiation from the incorporated 3 H‐thymidine. By 12 hours after 3 H‐thymidine injection, phagosomes have appeared in partly differentiated mid‐crypt columnar cells and oligomucous cells; by 18–24 hours, in fully differentiated columnar cells and in Paneth cells; and by 30 hours, in an entero‐endocrine cell. Since phagosomes are first found in crypt‐base columnar cells and only later in the four differentiated cell types, it is concluded that crypt‐base columnar cells transform into cells of these four types and, therefore, behave as the stem cells of the epithelium. The finding of rare epithelial cells containing two different types of secretory material (either mucous globules and entero‐endocrine granules, or mucous globules and Paneth cell granules) confirms that the stem cells are multipotential. These findings support the Unitarian Theory of epithelial cell formation in the small intestine.

Abstract The mucous cell population of duodenum, jejunum and ileum was investigated in the light and electron microscopes with the help of radioautography in mice sacrificed at various times after single injection or continuous infusion of 3 H‐thymidine. Mucous cells are characterized by globules of mucus and by dilated cisternae of rough endoplasmic reticulum. Two subgroups of mucous cells, one called common and the other granular , may be identified. The granular mucous cells differ from the common ones by the presence of small dense granules embedded within the mucous globules. Each subgroup is further divided into immature oligomucous cells containing few mucous globules, and mature goblet cells with a large accumulation of mucous globules. Common and granular oligomucous cells are found exclusively in the crypt, mainly within the lower mid‐crypt, whereas the corresponding two types of goblet cells are present in the upper part of the crypts and in the lower part of the villi. Only common mucous cells are observed in the upper part of the villi. The two types of oligomucous cells, but not goblet cells, have the ability to take up 3 H‐thymidine and divide. Electron microscopic radioautography demonstrates that, as oligomucous cells migrate upwards, they transform into goblet cells. The latter then migrate to the villus epithelium. In the case of granular mucous cells, this migration is associated with a gradual loss of the characteristic dense granules, so that the granular goblet cells reaching the upper part of the villi become common goblet cells. The goblet cells in the villus epithelium, regardless of their origin, ascend towards the villus tips where they are lost through the extrusion zones. The turnover time of common mucous cells is about three days, as for columnar cells; and that of granular mucous cells, somewhat shorter. In both cases, the divisions of oligomucous cells account only for the production of about half the mucous cells present. Hence, the other half must be derived from precursors other than oligomucous cells. Since a few crypt‐base columnar cells contain the odd mucous globule, they are suspected of being the precursors of the two types of oligomucous cells and, through them, of the entire mucous cell population.

Abstract The columnar cells, i.e., the cells which contain neither mucous globules, nor entero‐endocrine granules, nor Paneth type secretion, were investigated independently of other cells in duodenum, jejunum and ileum using light and electron microscopy as well as radioautography in mice sacrificed at various times after single injection or continuous infusion of 3 H‐thymidine. The columnar cells exhibit local differences allowing their classification into four sub‐groups: (a) The crypt‐base columnar cells are immature proliferative cells occupying the nine lowest cell positions of the crypt on the average. (b) The mid‐crypt columnar cells occupy the next positions up to 19 in ileum and 27 elsewhere; they proliferate and show features gradually changing from undifferentiated ones near the base to partially differentiated ones higher up. (c) The crypt‐top columnar cells occupy the rest of the crypt; they do not divide but they continue to differentiate. (d) Finally, the villus columnar cells are fully differentiated absorptive cells. Cryp‐base and mid‐crypt columnar cells take up 3 H‐thymidine label prior to division. With time after a 3 H‐thymidine injection, the intensity of their labeling decreases, while heavily labeled columnar cells appear in crypt‐top by six hours and on the villus by 12 and more hours. Hence, columnar cells migrate. The migration is associated with gradual differentiation from the immature crypt‐base columnar cells to the mature villus columnar cells. The latter eventually reach the villus tips where they drop into the lumen. The columnar cells constitute a large majority of the epithelial cells (95, 94 and 89% in duodenum, jejunum and ileum, respectively). Hence, they are likely to play a key role in the renewal of the epithelium in the three regions of the small intestine. The turnover time of columnar cells estimated from results of continuous 3 H‐thymidine infusion is 3.3 days in duodenum and 3.4 days in jejunum. Evidence from turnover time data indicates that the mitoses of columnar cells produce more cells than required for their own renewal. Presumably some of the mitoses give rise to cells of a type other than columnar.

The proglucagon-derived peptide glucagon-like peptide 2 (GLP-2), a product of a subset of gut epithelial cells, is pursued clinically for its ability to stimulate gut epithelial growth and repair. Here we show that although specific epithelial progenitors respond to GLP-2 administration, the epithelium does not express the GLP-2 receptor. Rather, enteric neurons express the receptor, respond to GLP-2, and transmit a signal (which can be blocked by the voltage-gated sodium channel inhibitor tetrodotoxin) back to the epithelium. Thus the nervous system is a key component of a feedback loop regulating epithelial growth and repair.