Development of the lateral line system in Xenopus laevis. II. Cell multiplication and organ formation in the supraorbital system.

Cell multiplication was studied during development of the supraorbital lateral line system in Xenopus laevis. The increase in cell number is biphasic. The first phase extends from the beginning of primordial elongation to the end of primary organ formation. Cell number increases linearly during this interval. Throughout this phase, a constant number of cells is in S phase of the cell cycle at a given time, despite a more than 10-fold increase in total cell number. After their formation, the number of the primary organs remains essentially constant. The individual primary organs are not clones of cells. Different organs grow at different rates, and become more and more heterogeneous in size. The second phase which is correlated with accessory organ formation is characterized by an elevated growth rate. This phase was not studied in detail. If developing larvae are starved, growth is normal up to completion of the first growth phase but is arrested at this point. The frequency distribution of the sizes of such growth-arrested organs approximates a binominal distribution. From its characteristics, a detailed model of cell proliferation and organ formation can be deduced: cell multiplication occurs through asymmetrically dividing stem cells, which become allocated to the forming organs at random and go through a fixed number of cell divisions.