Early development of the neural plate, neural crest and facial region of marsupials

Marsupial mammals have a distinctive reproductive strategy. The young are b orn after an exceptionally short period of organogenesis and are consequent ly extremely altricial. Yet because they must be functionally independent i n an essentially embryonic condition, the marsupial neonate exhibits a uniq ue suite of adaptations. In particular, certain bones of the facial region, most cranial musculature and a few additional structures are accelerated i n their development. In contrast, central nervous system structures, especi ally the forebrain, are markedly premature at birth, resembling an embryoni c d 11 or 12 mouse. This review examines the developmental processes that a re modified to produce these evolutionary changes. The focus is on the earl y development of the neural plate, neural crest and facial region in the ma rsupial, Monodelphis domestica, compared with patterns reported for rodents . Neural crest begins differentiation and migration at the neural plate sta ge, which results in large accumulations of neural crest in the facial regi on at an early stage of development. The early accumulation of neural crest provides the material for the accelerated development of oral and facial s tructures. The first arch region is massive in the early embryo, and the de velopment of the olfactory placode and frontonasal region is advanced relat ive to the forebrain region. The development of the forebrain is delayed in marsupials relative to the hindbrain or facial region. These observations illustrate how development may be modified to produce evolutionary changes that distinguish taxa. Further, they suggest that development is not necess arily highly conserved, but instead may be quite plastic.