Effects of the curly tail genotype on neuroepithelial integrity and cell proliferation during late stages of primary neurulation

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

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.

Cell death in early neural life

Programmed cell death is a relevant process in the physiology and pathology of the nervous system. Neuronal cell death during development is well characterized, and studies of this process have provided valuable information regarding the regulatory mechanisms of cell death in the nervous system. In the last few years, cell death occurring at earlier developmental stages and affecting proliferating neuroepithelial cells and recently born neuroblasts has been recognized. In this review we cover the observations on cell death in the early, proliferating stages of vertebrate neural development. Genetically modified mouse model systems and complementary in vivo approaches in other vertebrates have provided a solid basis for its relevance and contribution to normal neural development, as well as for the pathological consequences of its deregulation. However, the precise functional role of cell death remains a topic of debate.

Concordia discors: duality in the origin of the vertebrate tail

The vertebrate tail is an extension of the main body axis caudal to the anus. The developmental origin of this structure has been a source of debate amongst embryologists for the past century. Some view tail development as a continuation of the morphogenetic processes that shape the head and trunk (i.e. gastrulation). The alternative view, secondary development, holds that the tail forms in a manner similar to limb development, i.e. by secondary induction. Previous developmental studies have provided support for both views. Here I revisit these studies, describing caudal morphogenesis in select vertebrates, the associated genes and developmental defects, and, as a relevant aside, consider the developmental and evolutionary relationships of primary and secondary neurulation. I conclude that caudal development enlists both gastrulation and secondary induction, and that the application of recent high-resolution cell labelling technology may clarify how these discordant programmes interact in building the vertebrate tail.