Bone development. A. Intramembranous ossification in a cross-section through a canine embryonic limb bone. At the top of the image a row of osteoblasts, which arose from osteoprogenitor cells in the mesenchyme, is actively adding material to the outer surface (appositional growth). The patches of primary bone are interspersed between islands of mesenchymal connective tissue; some may remain to become Haversian canals of forming osteons. The marrow cavity, filled with small, dark hematopoietic cells, is seen at the bottom of the image. Osteoclasts are frequently seen at the marrow side of the bone. B. Endochondral ossification at the epiphyseal growth plate in a human embryonic long bone. Five zones of cartilage are labeled from top to bottom: resting cartilage zone, proliferating zone, hypertrophy zone, calcification zone, and ossification zone. The organized columns of proliferating and hypertrophic chondrocytes produce interstitial expansion that moves the epiphyseal plate toward the top, away from the midpoint of the diaphysis. Hypertrophic chondrocytes are intermixed with those undergoing calcification (dark blue areas). In the pink regions, the calcified cartilage has been converted to bone, which is then pared away by osteoclasts on the marrow surfaces. C. Long bone development by endochondral and intramembranous ossification. The diagram illustrates how a hyaline cartilage model of a long bone is converted into a growing bone, beginning with the formation of a collar around the middle of the diaphysis. The complete ossification (closure) of the epiphyseal growth plate occurs sometime in the second decade of life and ends the lengthening process. Source: BONE, The Big Picture: Histology Citation: Ash R, Morton DA, Scott SA. The Big Picture: Histology; 2017 Available at: https://accessphysiotherapy.mhmedical.com/DownloadImage.aspx?image=/data/books/2058/ashhist_ch4_f003.png&sec=155783992&BookID=2058&ChapterSecID=155783955&imagename= Accessed: October 17, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved