Stage‐ and tissue‐specific patterns of cell division in embryonic and larval tissues of amphioxus during normal development

SUMMARY The distribution of dividing cells is described for embryos and larvae of amphioxus ( Branchiostoma floridae ) pulse labeled with bromodeoxyuridine. Because cell division is assessed for all of the developing tissues, this is the first comprehensive study of developmental cell proliferation for an animal lacking a stereotyped cell lineage. In amphioxus, cell divisions are virtually synchronous during cleavage, but become asynchronous at the blastula stage. Starting at the neurula stage, after the origin of the mesoderm, the proportion of dividing cells progressively declines in the somitic mesoderm and notochord. Other tissues, however, deviate from this pattern. For example, in the mid‐neurula, there is a brief, intense burst of mitosis at the anterior end of the neural plate. Also, from the neurula through the early larval stage, all of the ectoderm cells cease dividing and develop cilia that propel the animal through the water; subsequently, in the epidermis of later larvae, mitosis resumes and the proportion of ciliated cells declines as muscular undulation gradually replaces ciliation for swimming. Finally, in the early larvae, there is a terminal arrest of cell division in three cell types that differentiate early to participate in feeding as soon as the mouth opens—namely the ciliated pharyngeal cells that produce the feeding current and the secretory cells of the club‐shaped gland and endostyle that export food‐trapping mucus into the pharynx. In sum, these stage‐ and tissue‐specific changes in cell proliferation intensity illustrate how the requirements of embryonic and larval natural history can shape developmental programs.