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David W Dempster, Tethering Formation to Resorption: Reversal Revisited, Journal of Bone and Mineral Research, Volume 32, Issue 7, 1 July 2017, Pages 1389–1390, https://doi.org/10.1002/jbmr.3169
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In 1965, Harold Frost and colleagues(1) deduced the sequence of cellular events in the bone remodeling cycle based on the seminal observation that the vast majority (97%) of osteons in adult human bone are formed over scalloped cement lines, indicating that resorption precedes formation. The cement lines joining the newly formed bone to the older bone were, therefore, correctly designated as reversal lines.
The reversal phase of the remodeling cycle is critical to the maintenance of bone balance. Yet, until recently, our understanding of the cellular and molecular events that transpire during reversal remained elusive. There are several reasons for this, not least of which is that the reversal phase is short‐lived, lasting only 7 to 14 days in humans.(2) Because observation of events in tissue sections depends on the frequency and the duration of the event, the chances of visualizing cancellous bone remodeling units in the reversal phase are slim. In the late 1970s, Roland Baron and colleagues increased the odds of observing bone remodeling units in reversal by synchronizing them in the rat jaw in a tooth movement model. By studying the bone remodeling units under a variety of experimental conditions, Baron and colleagues observed an intermediate phase between resorption and formation, which they termed the reversal phase (Fig. 1).(3) This phase was characterized “by the presence of large mononucleated cells with a low nucleo‐cytoplasmic ratio within Howship's lacunae without remaining osteoclast, without osteoid tissue, and without a cementing reversal line buried in the bone localized in this area. It is not known if the cells are post‐osteoclasts or pre‐osteoblasts.” This essentially was the sum state of our knowledge of the reversal phase for the next 35 years.
