THE CANALICULI OF WILLIAMSON IN HOLOSTEAN BONE (OSTEICHTHYES, ACTINOPTERYGII) - A STRUCTURAL AND ULTRASTRUCTURAL-STUDY

The organization and structure of the canaliculi of Williamson and the ir associated cells are studied in the bone of various elements of the dermal skeleton and of the endoskeleton in extinct and living Holoste i, and in regenerated scales of Lepisosteus oculatus. Light microscopi cal observations of ground and semi-thin sections of various skeletal elements confirm and complete previous data on the organization of the canaliculi. Transmission electron microscopy enables original descrip tions of the cells of Williamson, of their cytoplasmic processes occup ying the canaliculi of Williamson and of their environment. The cytopl asmic content of the cells of Williamson and their processes is compos ed mainly of microfilaments, microtubules, small vesicles and mitochon dria; this does not resemble what is known for osteoblasts and odontob lasts. Moreover, as an alternative for the lack of ontogenetic series, regenerated scales are used to describe recently formed cells of Will iamson at the upper scale surface and within vascular canals. Our cyto logical observations support a nutritive role for the cells of William son and their processes. The phylogenetical origins of the cells of Wi lliamson are discussed.