The hindbrain neural crest and the development of the enteric nervous system

The wonder of things is the beginning of knowledge, as was already stated by Aristotle, the fIrst embryologist known to history. Embryology has remained a source of wonder ever since. It all starts with the fusion of the female egg and the male sperm. Sperm cells were first described by Antonie van Leeuwenhoek (1632-1723) in 1678, who believed them to be parasitic animals present in the male semen, that had nothing to do with reproduction. Nicolas Hartsoeker (1656-1725), the other discoverer of sperm believed that the entire embryonic individual lay preformed within the head of the sperm, as depicted in his famous homunculus (Fig. 1). The fIrst evidence for the existence of the female egg was presented by Reinier de Graaf (1641-1673), although the egg itself was only described in 1827 by Karl von Raer (1792-1876). The actual fertilization process was observed only a century ago by Herman Fol, a Swiss zoologist

Ultimate toughness of amorphous polymers

The deformation and toughness of amorphous glassy polymers is discusses in terms of both the molecular network structure and the microscopic structure at length scales of 50-300 nm. Two model systems were used: polystyrene-poly(2,6-dimethyl-1,4-phenylene ether) blends (PS-PPE; where PS possesses a low entanglement density and PPE a relatively high entanglement density) and epoxides based on diglycidyl ether of bisphenol A (DGEBA) with crosslink densities comparable with up to values much higher than the thermoplastic model system. The microscopic structure was controlled by the addition of different amounts of non-adhering core-shell-rubber particles. Toughness is mainly determined by the maximum macroscopic draw ratio since the yield stress of most polymers approximately is identical (50-80 MPa). It is shown that the theoretical maximum draw ratio, derived from the maximum (entanglement or crosslink) network deformation, is obtained macroscopically when the characteristic length scale of the microstructure of the material is below a certain dimension; i.e. the critical matrix ligament thickness between added non-adhering rubbery particles ('holes'). The value of the critical matrix ligament thickness (IDc) uniquely depends on the molecular structure: at an increasing network density, IDc increases indepent of the nature of the network structure (entanglements or crosslinks). A simple model is presented based on an energy criterion to account for the phenomenon of a critical ligament thickness and to describe its strain-rate and temperature dependency