COMPARATIVE MORPHOLOGY AND CYTOKINETICS OF SKELETAL GROWTH IN HATCHLINGS OF ALTRICIAL AND PRECOCIAL BIRDS

This study addresses the development of the skeleton in birds and its relationship to the altricial-precocial spectrum. In particular, it ai ms at: (1) providing a broad basis of anatomical information on avian hatchling skeletons, (2) clarifying the topographic organization of gr owth zones in neonates of altricial and precocial birds, (3) providing quantitative data on the skeletal tissue compartmentalization of spec ies in the altricial-precocial spectrum and relating these to life-his tory traits, and (4) studying the dynamics of cartilage cell prolifera tion in wing and leg skeletal elements in an altricial, a semiprecocia l, and a precocial bird. Enzyme clearing techniques are used to study developmental stages of skeletons in neonate birds. Serial sections th rough wing and legs of neonates were used to show the cytological orga nization of growth zones, and computer aided morphometry of tissue com partments provided data about tissue compartmentalization. Labelling t echniques using tritium-labelled thymidine were employed to study the cytokinetic parameters of cartilaginous cell proliferation. The major results show that: (1) Even though altricial and precocial hatchlings differ in many aspects of their anatomy, the number and topography of ossifications in their skeletons are very similar. Topographic variati ons of a few elements are not related to the altricial-precocial spect rum; (2) in contrast, the quantitative design of the avian neonate ske leton is highly variable. A high proportion of cartilage in the skelet on relates to high postnatal growth rates; (3) the cytological organiz ation of growth plates in epiphyses of long bones is identical in altr icial, semiprecocial, and precocial neonates; (4) cytokinetic paramete rs vary among species, with the altricial starling showing the highest proliferation index, the semiprecocial tern intermediate, and the pre cocial quail lowest. The paper discusses the relationship of tissue co mpartmentalization of the neonate skeleton to the rate of postnatal gr owth. The size of the cartilage proliferation zone in the epiphyses an d the proliferation index of cartilage cells contribute to the variati on of postnatal growth in birds.