Understanding axial progenitor biology in vivo and in vitro.

The generation of the components that make up the embryonic body axis, such as the spinal cord and vertebral column, takes place in an anterior-to-posterior (head-to-tail) direction. This process is driven by the coordinated production of various cell types from a pool of posteriorly-located axial progenitors. Here, we review the key features of this process and the biology of axial progenitors, including neuromesodermal progenitors, the common precursors of the spinal cord and trunk musculature. We discuss recent developments in the in vitro production of axial progenitors and their potential implications in disease modelling and regenerative medicine.

Immunomodulation stimulates the innervation of engineered tooth organ.

The sensory innervation of the dental mesenchyme is essential for tooth function and protection. Sensory innervation of the dental pulp is mediated by axons originating from the trigeminal ganglia and is strictly regulated in time. Teeth can develop from cultured re-associations between dissociated dental epithelial and mesenchymal cells from Embryonic Day 14 mouse molars, after implantation under the skin of adult ICR mice. In these conditions however, the innervation of the dental mesenchyme did not occur spontaneously. In order to go further with this question, complementary experimental approaches were designed. Cultured cell re-associations were implanted together with trigeminal ganglia for one or two weeks. Although axonal growth was regularly observed extending from the trigeminal ganglia to all around the forming teeth, the presence of axons in the dental mesenchyme was detected in less than 2.5% of samples after two weeks, demonstrating a specific impairment of their entering the dental mesenchyme. In clinical context, immunosuppressive therapy using cyclosporin A was found to accelerate the innervation of transplanted tissues. Indeed, when cultured cell re-associations and trigeminal ganglia were co-implanted in cyclosporin A-treated ICR mice, nerve fibers were detected in the dental pulp, even reaching odontoblasts after one week. However, cyclosporin A shows multiple effects, including direct ones on nerve growth. To test whether there may be a direct functional relationship between immunomodulation and innervation, cell re-associations and trigeminal ganglia were co-implanted in immunocompromised Nude mice. In these conditions as well, the innervation of the dental mesenchyme was observed already after one week of implantation, but axons reached the odontoblast layer after two weeks only. This study demonstrated that immunodepression per se does stimulate the innervation of the dental mesenchyme.

Analysis of mesenchyme in the developing hind limb of Rana pipiens larvae with implications for neural development.

Mesenchyme in the hind limbs of Rana pipiens tadpoles may serve as an important influence on the development of specific neural structures involved in limb innervation. Thus a histological quantification of mesenchyme was undertaken to identify landmark stages with respect to mesenchyme presence and neural events. Mesenchyme remained as a high percentage of the limb tissue until stage V (Taylor-Kollros stages, '46), after which it declined dramatically until its virtual absence after stage XI. The volume of mesenchyme, however, was greatest at stages VIII-IX. Periods of high and low mesenchyme content were correlated in time with potential limb involvement in regulating limb innervation and motor neuron loss from the lateral motor columns. This provides additional evidence for developmental relationships between events of the limb and neural tissues.

Grafting of additional periphery reduces embryonic loss of neurons.

Effects of increased peripheral field of innervation on the magnitude of the spontaneously occurring embryonic cell death in the chick trochlear and isthmo-optic nuclei were examined. Grafting of an additional optic primordium and the surrounding mesoderm (which forms extraocular muscles) was performed at stage 11 (HH stage series). The grafted tissue is innervated by the appropriate neuron pool as revealed by the retrograde axonal flow of HRP. Cell counts of the trochlear nucleus on day 19 indicate a mean increase of 37% (range 19--62%) in the number of cells on the experimental side (contralateral to the graft) as compared to the ipsilateral control nucleus of the same embryos. Cell counts of the isthmo-optic nucleus on day 19 show an average increase of 35% (range 27--41%) on the experimental (contralateral) side over the ipsilateral control side. This increase in cell survival is not due to a stimulatory effect of grafting on cellular proliferation as revealed by the cell counts of trochlear and isthmo-optic nuclei on day 9 and 11 respectively. Thus, the increased cell survival is attributed solely to the reduction in the magnitude of the embryonic cell death. Whether this reduction in embryonic cell death is due to increased number of synaptic sites or increased amounts of trophic substances remains uncertain.

Scanning electron microscopic observations of the development of the somites and their innervation in anuran larvae.

The development of the paraxial mesoderm in tadpoles of Xenopus, Bufo and Rana was observed with a scanning electron microscope. In addition to examination of the differentiation of the surface and the interior of the somites, some attention was also paid to the transformation of the material of the neural crests and to the innervation of the developing myotome.