Abstract
The restriction of Paneth cell formation to the top of the Paneth cell distribution in the adult was suggested to be due either to the existence of a stem‐cell zone or to the influence of a Paneth cell population‐density gradient (Bjerkness and Cheng, 1981). To distinguish between the two possible mechanisms, the development of the Paneth cell distribution in neonatal mice (0–10 days old) was studied. If restricted formation were due to the presence of a population‐density gradient of Paneth cells, then in neonatal animals, in the absence of a Paneth cell population‐density gradient, Paneth cell formation would occur throughout the crypt base. If, on the other hand, restricted formation were due to the presence of a stem‐cell zone, and if this mechanism were operative in the newborn, Paneth cell formation in the newborn would be restricted to the region above the stem‐cell zone. The position of each Paneth cell within the crypt, and the size of its largest granule, were recorded. On day 0, Paneth cells were present, but crypts were poorly developed and positional assignment was not possible. On day 1, immature crypts developed. All Paneth cells found in immature crypts on day 1 were at the crypt‐surface junction (approximately position 5). On day 2, most Paneth cells were at the crypt‐surface junction. Thereafter, Paneth cells began to appear at lower positions. On day 3, there were 15 times more Paneth cells in position 5 than in position 1. On day 4, there were still three times more Paneth cells in position 5 than in position 1. With age, the proportion of Paneth cells in position 1 increased while that in position 5 decreased. On day 10 there were more Paneth cells in position 1 than in 5. At all time intervals, granules of Paneth cells in position 1 were significantly larger than those in position 5, indicating that Paneth cells in position 1 were older than those in position 5. It was concluded that in the neonate, before the establishement of a Paneth cell population‐density gradient, Paneth cell formation was restricted to positions 5 and above. This supports the existence of a stem‐cell zone, not a Paneth cell population‐density gradient, as the underlying mechanism of restricted Paneth cell formation in the adult.