Do cells become homeless during neural tube closure?

of apoptosis at sites of morphogenetic tissue fusion may not necessarily indicate a functional role, but rather a secondary outcome. The next question is: why do cells die during fusion of embryonic epithelia? Our study of the spatio-temporal distribution of dying cells during mouse neurulation revealed apoptosis predominantly associated with three main events: bending and fusion of the neural folds, post-fusion remodelling of the dorsal neural tube and surface ectoderm, and emigration of neural crest cells. In each of these embryonic events, cells undergo marked changes in shape and alteration in their association with the underlying extracellular matrix (ECM). For example, fusion and remodelling of the neural folds, to create tissue continuity across the dorsal midline, implies that some cells at the fold tips will alter their contacts with the adjacent cells and/or with ECM. Moreover, subsequent initiation of neural crest cell migration requires an epithelium to mesenchyme transition as cells detach from the neural folds and migrate away. In the early 1990s, a new Greek-derived term—anoikis, i.e., “homelessness”—was coined to indicate apoptosis induced by lack of correct cell/cell or cell/ECM attachment. Signals from the ECM were found to be fundamental in preventing cells from starting the apoptotic intracellular program. Once initiated, however, anoikis did not differ from apoptosis either biochemically or morphologically, the term simply emphasising a particular stimulus for cell death.8 During neurulation, the basal surfaces of neuroepithelial cells contact extracellular matrix, which is interposed between neural plate and surface ectoderm dorsally, and between neuroepithelium and paraxial mesoderm or notochord, more ventrally. As the dorsal neuroepithelium bends inwards, to bring the neural folds together, a primitive basement membrane containing type IV collagen, fibronectin, Programmed cell death is a physiological event of animal development, first described in 1951 by Glücksmann1 as a normal component of developmental processes. Since then, the morphological events and signaling pathways that characterize apoptotic cell death, and distinguish it from inflammation-associated necrosis, have been investigated in great detail.2 While apoptosis is the most studied and best characterized form of programmed cell death, a role in development for cell death with autophagy has also been suggested,3 originally stimulated by studies in yeast. The requirement of apoptosis for cell removal in the developing interdigital region and in mammary tissue of male embryos is well established. Similarly, during nervous system development, programmed cell death is fundamental for regulating the number of differentiated neurons, with elimination of axonal misconnections. What is less clear is whether programmed cell death also plays a crucial role in morphogenetic tissue fusion events such as closure of the neural tube and fusion of the palatal shelves. Building on previous work in chick neurulation,4 we recently confirmed that dying cells are associated spatio-temporally with closure of the mouse neural tube (Fig. 1). We examined genetic mutants in which apoptosis is severely diminished and found that neural tube closure occurs apparently normally in the forebrain and spinal neural tube, although the hindbrain and caudal midbrain remain open. Most strikingly, when we inhibited apoptosis chemically in intact, cultured mouse embryos, we found that closure of the entire neural tube, including midand hindbrain, proceeded to completion.5 Our findings indicate that apoptosis, while plentiful and strategically placed to participate in neurulation, is not actually required for completion of neural tube closure in mice. In this context, it is interesting that controversy still surrounds a putative role for programmed cell death during fusion of palatal shelves. While cell death is abundant during this morphogenetic process, experimental inhibition of apoptosis during palatal shelf fusion has given conflicting results.6,7 We suggest, therefore, that the occurrence Cell Cycle Features: