Different mechanisms are used to specify new segments during, for example, somitogenesis in vertebrates and Drosophila body segmentation. Other segmented animals as some annelids keep adding segments as they grow. In a recent paper the laboratory of Shigeo Hayashi describes how new segments are added during the regeneration of the tail in the polychaete Perinereis nuntia (http://www.ncbi.nlm.nih.gov/pubmed/23608458). In many models the new posterior segments are added from a segment-addition zone.
In this study the authors work on the species P. nuntia that, in laboratory conditions, adds a new posterior segment every 4 days. This new segment is added between the last posterior segment and the terminal end, called pygidium. During tail regeneration, a new pygidium is formed and then new segments are added initially at a rate of 1 segment per day. First, the authors show how during normal growth of segments it appears a single row of cells with high PCNA (proliferating nuclear antigen) expression at the border between the last segment and the pygidium. Because PCNA expression is high in late G1-S phase these observations suggest that this single row of cells are in synchronous G1-S phase and has been named by the authors as “zone of cell-cycle synchronization” (ZCS). In addition, and in contrast to the quite random orientation of the mitoses in more anterior regions of the segment, the cells in the ZCS divide along the AP axis with the division axis almost perpendicular to the segmental furrow. The authors also show how changes in PCNA staining are related to changes in chromatin structure based on labelling of the histone modifications H3K9me2 and H3K9, K14ac. Blastema cells are characterized by high H3K9me2 and proliferation rate, whereas when cells differentiate exit the cell cycle and display high level of acetylated histone H3.
Next, the authors follow the regeneration of the new segments by analysing the expression of wingless (wg) and hedgehog (hh). These two genes are expressed in rows of cells at the anterior and posterior side of the segmental furrow, respectively. These cells also overlap with the ZCS of cells with high PCNA expression. Remarkably, the expression of tcf, a nuclear effector of Wg signalling is expressed in a segmental pattern peaking at the hh stripe and decreases in a posterior gradient, suggesting an important role of Wg signalling during segmentation. In fact, treatment with LiCl, which mimics overexpression of Wg signalling produces various regeneration defects. Thus, the row of cells with high PCNA expression at the posterior border of the newly formed segment is disorganized and the animals show a decrease in segment number and segments wider than in controls.
From these results the author suggest a model in which the last row of the most posterior segment serves as the source of Wg that coordinates cell-cycle entry and recruitment of anterior pygidium cells that will initiate the formation of a new segment in the ZCS. Thus, and in contrast to other models, it appears that in P. nuntia segmentation is initiated by cell cycle synchronization and row-by-row addition of cells to the posterior end of the new segment.