Identification of Bone Cell Activity

Abstract

Ultimately the validity of any of the methods of quantitative bone morphology depend on accurate identification of the bone cell activity. The cells and structures that are evaluated vary from one investigator to another, but they nearly always include bone formation which is distinguished either by osteoblasts, by osteoid covered by osteoblasts, by the presence of low density bone in a microradiograph, or by a double (or less satisfactorily, a single) tetracycline band. The active osteoblasts involved in new matrix deposition are generally cuboidal and lined up in sheets on the bone surface and are relatively easy to recognize (Figure 38). They are invariably associated with unmineralized osteoid tissue, with tetracycline uptake and, except in osteomalacia, with a mineralization front. Measurements of the length of surface covered by osteoid tissue which is covered with a layer or sheet of osteoblasts, often termed active osteoid, can also be used rather than a count of osteoblasts. The osteoid tissue can be seen most readily in undemineralized, unstained sections where it appears as a translucent layer containing osteocytes (Figure 39). A double label of tetracycline, resulting from two or more injections of tetracycline, given at an interval of a few days, also produces evidence of new matrix production and is probably the most certain method to evaluate the presence of new bone deposition (Figure 40). However, so many patients and animals have received previous doses of tetracycline that the majority of biopsy samples are labeled many times before the intended two labels are administered, and it is often difficult to distinguish the previous labels from those given for the purpose of measuring bone formation.