Cartilage Calcification

Nineteenth century histologist Ranvier discovered the point of ossification and the calcification of cartilage as observed in embryos (Ranvier, 1875). By skillfully using simple methods such as hand-cutting with a razor, or with a lead-screw microtome of his own invention (still marketed as the Ranvier hand microtome), and chromic acid and carmine to stain, he observed the deposition of calcareous salts around cartilage capsules. Ranvier described chondrocytes and their distinctive arrangement in series to yield larger capsules. He observed how calcified capsules open into one another to form anfractuous cavities that become the earliest marrow spaces, and wondered what determines the resorption of the walls. After injecting Prussian blue to mark vessels in growing animals, he described wall destruction as proceeding selectively in the direction of vessel growth (Figure 1a). Soon after, Schäfer described how to prepare fresh or fixed cartilage sections, and recommended the use of osmic acid, silver nitrate and gold chloride (Schäfer, 1897). He then (Schäfer, 1907) published a series of four colour drawings comprising all the stages of ossification (Figure 1b); these drawings have been a source of inspiration for all later histology textbooks (see, for instance, Figure 1 c). Subsequent methodological advances in microscopy and microtomy allowed confirmation of all the early observations. In the first third of the twentieth century histology textbooks by Cajal, Bouin, Möllendorff, Maximow & Bloom, Cajal & Tello, Levi and Di Fiori, described endochondral ossification, the ossification centre, and the growth plate as we know them today. Interestingly, some doubts persisted about hypertrophic chondrocytes. Using silver reducing methods, Cajal and Tello described the well-developed Golgi apparatus of hypertrophic chondrocytes, and how it is reduced and fragmented in the last row of cells (Figure 1d), as well as the vacuolization of chondrocytes that makes these cells hypertrophic (Figure 1e).