Acinar cell proliferation in the parotid and submandibular salivary glands of the neonatal rat.

Although the rat salivary glands are deficient in acini at birth, acinar cells proliferate rapidly during the early post-natal period. The pattern of acinar cell proliferation was analysed in the parotid and submandibular glands of neonatal rats from day to birth until day 34. Mitotic and [3H]thymidine ([3H]TdR) labelling indices of the two glands show distinctly different patterns. Analysis of cell division in the rat parotid gland demonstrated a peak of mitotic index at 14 days (2.9 +/- 0.4%) and labelling index at 16 days (25.2 +/- 2.1%). Submandibular gland acinar cell proliferation reaches a zenith between 7-8 days; labelling index (14.2 +/- 1.1%) and mitotic index (2.3 +/- 0.3%). Cell proliferation decreases rapidly in both glands after reaching a peak in activity. Gland size increases more rapidly in both glands after gland which correlates with the earlier shift from cell proliferation to differentiation which occurs in this organ. Circadian rhythms of [3H]TdR incorporation were also investigated in this study. A circadian rhythm of [3H]TdR incorporation into DNA occurs at 15 days after birth with a peak at 06.00 hours in both glands and a trough occurring at 15.00 hours in parotid gland and 18.00 hours in the submandibular gland. Determination of specific activity of DNA (ct/min per microgram DNA) on days 8, 10, 12, 13, 14, 15, and 16 after birth at 06.00 and 15.00 hours indicated that a circadian rhythm in [3H]TdR incorporation into DNA began on day 14. The developmental switch from suckling to solid food may be an initiating factor in the sychronization of the circadian rhythm in cell proliferation.