ate ce entaletadeli ct,ia 122 the tissues had sequestered radioproline during the numbers of osteoblasts increase in rachitie rat this point, it should be noted first that Tom periosteum, endosteum, and the endosteal surface in the area of the proximal tibial metaphysis was averaged for 30-50 cells to obtain a quantitative estimate of the utility of the tracer for bone matrix synthesis (vide infra). With the staming method em- genic cells of mice within 15-30 min after a injection. Secondly, in concert with results from les using radioglycine as a tracer for collagen mation, 1% 12) it is known that osteogenic cell port a large porportion of their tracer content cell with an eccentric nucleus, a prominent nucleolus and a juxtanuclear vacuole (Golgi apparatus) located on or close to a bone surface. The relative number of osteoblasts and their precursor cells, variously called reticular, mesenchymal, or osteoprogenitor cells, was not estimated in the primary spongiosa, thesized collagen molecules by 4 hr. However, p: is also an active metabolite and can be transfo via glutamic acid into other amino acids whic! initial 4 hr of the experiment. The number of silver grains over the cytoplasm of labeled osteoblasts in the ployed, an osteoblast was defined as a basophilic although Rohr‘) has indicated that the absolute found peak uptake of radioproline in the ¢ the surfaces of bone as an integral part of newly utilized for a number of other compounds, pro! and mucopolysaccharides. Thus the early grain c« per se cannot be depended upon to measure onl: rate of collagen synthesis by labeled osteoblasts. The reliability of the initial gram counts wa: sayed independently by measuring the position o labeled matrical (collagen and mucopolysacchar! band of silver grains buried within the corte days after radioproline administration (Figure The thickness of lamellar bone deposited by osteoblasts on the growing surfaces of the (endosteal, .periosteal) and transverse epiphy bone during this interval was calculated by mex ing the total distance from the leading edge of continuous band of labeled matrix to the anaton surfaces. This was done in the center of approxim: 60 adjacent high powerfields (400 x) with an oc micrometer. This value was divided by 3 so that rate of appositional bone growth could be expre: in microns per day, An estimation of the thickne- the band of labeled matrix was also attempted, fo lieu of definitive grain count data early after tr: administration, this measure should reflect accura' the rate at which the osteogenic cells on these faces were producing newstruetural collagen. T! two sets of data were also expressed as an Osi blast Activity Index (QAI), which is defined as observed thickness of the ?3H-proline label divided Fic. 95Autoradiographs of the articular surface of the transverse epiphyseal bone from rats sacrificed at two intervals of time after a single injection of *H-proline. A. a band of silver grains at a-b representing collagen newly formed byosteoblasts (ob!) 4 hr after injection. The evtoplasm of the osteoblasts is lightly labeled. B, the position of the silver grains over labeled collagen lamellae 3 days postinjection. Interval a-c, thickness of lamellar bone formed in 3 davs. Interval b~c, thickness of the bandof silver grains. Note the trail of silver grains (interval a-h) due to reutilization of radioproline. Hematoxylin and eosin. Original magnification 250 X. the observed apposition rate (microns/microns/da It is unlikely that these data would be complica at the light microscope level by any change in catabolic rate of newly synthesized collagen m« cules or differential packing of collagen fibers b into the skeleton.{?- 15) A ratio of 1 would suge that the osteoblasts formed the labeled bone mat in exactly 24 hours’ time. Other values would inversely proportional to the pace at which | osteogenic cells were performing. It was difficult apply this type of analysis to trabecular bone, fir beeause it is less well-oriented than compact bo and second, because our animals were not sacrific at narrowtime intervals.