Do Secretory‐Stage Ameloblasts Divide? Experimental Evidence

Abstract

Our objective is to test the dictum that secretory ameloblasts cannot divide. This idea is often mentioned in dental biology textbooks without citation or evidence; its source seems to be studies of rodent incisors from the 1960's. However, rodent incisors are ever‐growing, and not acceptable models for enamel epithelia function in other mammals. Also, there are two reasons to expect that mature ameloblasts divide. First, recent studies show that tensile forces induce mitosis in epithelia, and it is likely that the enamel epithelium undergoes tension as enamel is deposited and the surface area of the growing crown expands. Second, we now know that ameloblasts are subject to the same biorhythms that synchronize proliferation of osteoblasts. If ameloblast division is similarly coordinated by the nervous system and occurs at night, it is unlikely that it will have been observed since most experimental protocols are carried out during daylight hours.To test the hypothesis, we employed two methods. First, 3–6 day old outbred ICR mouse pups (Crl:CD1) used for another research purpose were euthanized at 4 hour intervals that spanned the entire day/night cycle. Specimens euthanized during the day were utilized as controls, while those specimens euthanized at night were considered as experimental tissues for the purpose of the study. Whole heads were thin‐sectioned in a coronal plane and stained using standard H&E protocols to identify mitotic figures in secretory‐stage ameloblast cells. Second, we labeled juvenile primate specimens for immunoreactivity to Proliferating Cell Nuclear Antigen (PCNA), which is expressed in cells that are undergoing DNA repair and/or replication in preparation for mitosis. Animal protocols were approved by respective Institutional Animal Care and Use Committees (IACUC) at Slippery Rock University and Florida Atlantic University.In our mouse specimens, we have not found signs of mitotic figures; however in one juvenile collared lemur we did see signs of strong PCNA expression in cervical loop ameloblasts that had just entered the secretory stage. This weakly supports our hypothesis, and warrants searches for other patterns of gene expression and mitosis.