SMITH, Kathleen K; VAGLIA, Janet: Early development of the cranial neural crest, neural tube and paraxial mesoderm in marsupials.

Marsupial mammals are highly altricial at birth when compared to most other amniotes, yet the neonate must possess adaptations to allow it to independently travel to, identify and attach to the teat, suckle and process food at this overall embryonic state. It has long been known that the morphological configuration of neonatal marsupials differs from that seen in even the most altricial placental mammal, and in previous work we have shown that this is reflected in part by a delay in the development of the central nervous system and an advancement of cranial skeletal and muscular systems. Here we discuss the relations of the neural crest, the development of regional segmentation within the neural tube, and the development and differentiation of the paraxial mesoderm. We show that neural crest initiates migration at the early neural plate stage at a time when no morphological boundaries exist between the forebrain, midbrain, or hindbrain. Most neural crest migration is complete in the cranial region before the anterior neuropore closes. We also show that at the time of early crest migration, the paraxial mesoderm exists as a thin plate of cells, with no apparent subdivision or segmentation. We discuss these differences in the context of a number of specific hypotheses on the existence of regional specification of pre-migratory crest cells, and the interaction of the neural plate, paraxial mesoderm and neural crest in the patterning of the craniofacial region. Finally, we discuss the differences as well as a number of underlying similarities in the morphogenetic movements in the neural crest and facial structures in marsupials and other amniotes.