Lateral line placodes of aquatic vertebrates are evolutionarily conserved in mammals.

Placodes are focal thickenings of the surface ectoderm which, together with neural crest, generate the peripheral nervous system of the vertebrate head. Here we examine how, in embryonic mice, apoptosis contributes to the remodelling of the primordial posterior placodal area (PPA) into physically separated otic and epibranchial placodes. Using pharmacological inhibition of apoptosis-associated caspases, we find evidence that apoptosis eliminates hitherto undiscovered rudiments of the lateral line sensory system which, in fish and aquatic amphibia, serves to detect movements, pressure changes or electric fields in the surrounding water. Our results refute the evolutionary theory, valid for more than a century that the whole lateral line was completely lost in amniotes. Instead, those parts of the PPA which, under experimental conditions, escape apoptosis have retained the developmental potential to produce lateral line placodes and the primordia of neuromasts that represent the major functional units of the mechanosensory lateral line system.

Expression patterns of erythropoietin and its receptor in the developing spinal cord and dorsal root ganglia.

Recombinant human erythropoietin (EPO) is neuroprotective in animal models of adult spinal cord injury, and reduces apoptosis in adult dorsal root ganglia after spinal nerve crush. The present work demonstrates that spinal cord and dorsal root ganglia share dynamic expression patterns of EPO and its receptor (EPOR) during development. C57Bl mice from embryonic days (E) 8 (E8) to E19 were studied. In spinal cord and dorsal root ganglia, EPOR expression in all precursor cells preceded the expression of EPO in subsets of neurons. On E11, EPO-immunoreactive spinal motoneurons and ganglionic sensory neurons resided adjacent to EPOR-expressing radial glial cells and satellite cells, respectively. From E12 onwards, EPOR-immunoreactivity decreased in radial glial cells and, transiently, in satellite cells. Simultaneously, large-scale apoptosis of motoneurons and sensory neurons started, and subsets of neurons were labelled by antibodies against EPOR. Viable neurons expressed EPO and EPOR. Up to E12.5, apoptotic cells were EPOR-immunopositive, but variably EPO-immunonegative or EPO-immunopositive. Thereafter, EPO-immunonegative and EPOR-immunopositive apoptotic cells predominated. Our findings suggest that EPO-mediated neuron-glial and, later, neuron-neuronal interactions promote the differentiation and/or the survival of subsets of neurons and glial cells in central as well as in peripheral parts of the embryonic nervous system. Correspondingly, expression of phospho-Akt-1/protein-kinase B extensively overlapped expression sites of EPO and EPOR, but was absent from apoptotic cells. Identified other sites of EPO and/or EPOR expression include radial glial cells that transform to astrocytes, cells of the floor plate and notochord as well as neural crest-derived boundary cap cells at motor exit points and cells of the primary sympathetic chain.

High resolution scanning and three-dimensional reconstruction of cellular events in large objects during brain development.

Detailed knowledge of the spatial and temporal interactions of distinct cellular events and of the genes involved in their regulation is a precondition for the understanding of morphogenetic and pathogenetic processes. Here, how patterns of cellular events in large objects can be visualized with the help of the image acquisition system 'Huge Image' is demonstrated. Huge images are composed of a multitude of small images scanned with the highest light microscopical resolution. The system is equipped with a programmable autofocus device and permits precise and rapid cytological diagnosis. A vector-based three-dimensional (3-D) reconstruction method which, in future projects, will be combined with 'Huge Image', is applied to visualize dynamic interactions between macrophages and the occurrence of apoptotic neuroepithelial cells in the early developing forebrain of Tupaia belangeri (Scandentia). Proportionally correct meshwire surfaces of small and large objects are generated independently of each other. The combined reconstruction of cellular events and large embryonic surfaces can be carried out from only subsets of histological serial sections, and, compared with volume-based systems, with a much lower need for memory. The practicability of our approach is compared with recent other methods used to demonstrate apoptotic patterns.