Characterization of the major parathyroid hormone target cell in the endosteal metaphysis of rat long bones.
The majority of in vivo competitive binding of parathyroid hormone (PTH) in the endosteal metaphysis of rat long bones was recently shown to be localized in the intertrabecular tissue to a cell that is distinct from a differentiated osteoblast. In the present report we have further characterized this cell, termed a parathyroid hormone target (PT) cell, by light and electron microscopy using radioautography and histochemical techniques. These studies demonstrate that the PT cell is a mononuclear cell with a large cell body located at times between clusters of differentiated osteoblasts, as well as in other regions of the intertrabecular tissue. Its long cytoplasmic processes extend from the bone matrix through the intertrabecular region toward vascular structures, interdigitating with various cells of the endosteum. A distinctive tubular structure originating in the Golgi system and often associated with long mitochondria and glycogen particles extends throughout the cytoplasmic processes of the PT cell. Based on its capacity to incorporate [3H]thymidine, the PT cell appears to divide rather slowly. The identification of occasional hybrid cells with ultrastructural features of both the PT cell and the differentiated osteoblast and the presence of histochemical evidence for alkaline phosphatase activity suggest that the PT cell is of the osteoblast lineage. These studies therefore morphologically define a major osseous target cell for PTH that, although of the osteoblast lineage, is not a differentiated osteoblast and provide in vivo evidence that characteristics of the "osteoblast phenotype" are not restricted to a sole osseous cell type.