Sự khác biệt của hợp tử totipotent thành quần thể đa năng hạn chế phát triển tế bào gốc, và cam kết tiếp theo và biểu hiện của kiểu hình cụ thể của tế bào, được cho là được quy định bởi nhiều yếu tố | 70 Bruder and Scaduto 66-kDa homolog of mammalian osteopontin 10 . As the vasculature penetrates the first collar of bone formed along the diaphysis phagocytic cells remove the hypertrophic cartilage core and allow its replacement by stromal and hematopoietic marrow elements. In this way the cartilage core precisely defines the geometric boundaries of the eventual marrow cavity. CELL LINEAGE AND THE ORIGIN OF OSTEOBLASTS Differentiation of the totipotent zygote into developmentally restricted pluripotent stem cell populations and the subsequent commitment and expression of specific cellular phenotypes is believed to be regulated by a variety of factors including inherent genomic potential cell-cell interactions and environmental cues. In considering the mechanisms involved the concept of cell lineage is fundamentally relevant. Our logic in part is based on the cellular relationships proven to exist in the hematopoietic lineage pathways. As it is generally understood the term lineage refers to the progression of particular cell precursors as they mature and give rise to differentiated cells tissues and organs. The accurate description of such a cell lineage depends on the ability to identify a particular feature or collection of features which can be traced from the differentiated cell type back through its precursors. Because the formation of specialized tissues is a progressive phenomenon many generations of cells lie between the stem cell and the fully differentiated phenotype which forms the mature tissue. Our hypothesis was that a discrete series of steps or transitions exists between osteoprogenitor cells and the fully expressive osteoblast comparable to that documented during hematopoiesis 11 or development of the nematode Caenorhabditus elegans 12 . Analysis of these lineage steps is paradoxically both facilitated and complicated by the variety of tissues containing osteoblast progenitors. Embryonic limb bud mesenchyme developing and mature periosteum and .
