Endochondral ossification up close

Metaphyseal side of the physis, 1-day-old mouse (C57BL/6), nondecalcified section. Three events are necessary for orderly endochondral ossification: (1) mineralization of chondrocyte matrix, (2) apoptosis of chondrocytes with further mineralization, and (3) ingrowth of blood capillaries. Mineralized longitudinal septa (black arrow) will soon be covered by a thin layer of osteoid forming the primary spongiosa. Following hypertrophy, chondrocytes die (apoptotic morphology, red arrow), leaving empty lacunae for migrating blood capillaries (yellow dashed lines). Failure of one of these three processes will result in failure of or delayed endochondral ossification (vitamin D deficiency, traumatic physitis, and septic embolism are examples of pathologic processes affecting endochondral ossification).