Zebrafish bone cells and ossification types. (A) Bone is formed by osteoblasts and osteocytes, while cartilage is formed by chondroblasts and chondrocytes, and both bone and cartilage are degraded by osteoclasts. All bone cell types develop from progenitors similar to the mammalian counterpart and share similar gene expression profiles (genes are indicated above arrows). Note however that HSCs in zebrafish are not present in the bone marrow but in the head kidney. In addition, the genes for collagen X, encoded by col10, and SRY-box transcription factor 9 (indicated by*), encoded by sox9, are expressed during osteoblasts differentiation in zebrafish, but not in humans. (B) Three types of ossification are present in zebrafish: (i) intramembranous ossification, (ii) perichondral ossification, present in teleosts but not in humans, and (iii) endochondral ossification. (i) During intramembranous ossification mesenchymal stem cells condensate and differentiate into pre-osteoblasts and finally into mature osteoblasts that deposit bone matrix (osteoid) that subsequently mineralizes. (ii) Perichondral ossification starts at the surface of a cartilaginous template where osteoblasts deposit bone matrix without replacing the cartilage. (iii) Endochondral ossification is the process by which growing cartilage is replaced by bone to allow the skeleton to grow. For ossification to start, matrix surrounding the chondrocytes must calcify so that osteoclasts can break down the cartilage. In teleost two types of endochondral ossification exist. Type I endochondral ossification, typical in the ceratohyal, resembles the mammalian endochondral ossification process. This is characterized by a hypertrophic zone, where the cartilage matrix calcifies, followed by a degradation zone where osteoclasts (also referred to as chondroclasts) degrade the cartilaginous matrix, and a bone formation zone. Type II ossification, in the hypurals, is characterized by a lack of the calcification and ossification zones, leading to tubular concave bones filled with adipose tissue. Schematics based on detail description in Weigele and Franz-Odendaal (36). A, adipose zone; C, calcification zone; CB, chondroblasts; CC, chondrocytes; D, degradation zone; H, hypertrophic zone; HSC, hematopoietic stem cell; M, maturation zone; MSC, mesenchymal stem cell; O, ossification zone; OB, osteoblasts; OC, osteoclasts; OT, osteocytes; P, proliferation zone; R, rest zone.
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