A New Mechanism of Cartilage Growth in mammals “Involvement of CD117 Positive Undifferentiated Cells in Interstitial Growth”

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Two types of cartilages growth are commonly described; the appositional growth and interstitial growth. The manner of production of new cartilage matrix is different in both types. Appositional type relies on addition of peripheral matrix upon activation of the perichondrial stem cells and interstitial growth depends on secretion of new cartilage matrix by chondrocytes progeny. The current study described a different type of cartilage growth in different skeletal elements of camel embryos. Camel embryos were collected, fixed, dehydrated, and embedded in paraffin. Paraffin sections of the whole embryos were stained with H&E, Crossman trichrome and Mallory trichrome. Immunohistochemical staining for CD117 and type II collagen were used in embryonic cartilage. Other samples were processed to be examined by scanning electron microscopy. 

An Early Embryonic skeleton represented by cartilaginous elements. Undifferentiated cells continuing with the perichondrium penetrated the growing cartilage of ribs, pelvic bone, scapula, and sternum. The cells were CD117 positive and morphologically resembled the undifferentiated mesenchymal cells; they appeared spindle or flattened in shape with an oval nucleus and were connected by cytoplasmic processes. The orientation of the invaded cells could be as low or high cellular densities or as individual cells. The invaded mesenchymal cells transformed to chondrocytes and produce new cartilage matrix. They were immune-stained for Type II collagen The cellular penetration aimed to supply the developing cartilage by undifferentiated cells destined to become chondrocytes and involved in the interstitial growth of the fetal cartilage.

Cartilages growth occurs by secretion of new cartilage matrix. Appositional growth depends on activation of the perichondrial cells to secrete additional cartilage matrix encompassing the growing cartilage. Chondrocytes propagation and subsequent interstitial secretion of new cartilage matrix by chondrocyte progenies result in interstitial growth. Appositional growth commits growth of the cartilage width while the increase in cartilage length relies on interstitial growth. An Unusual mode of cartilage growth has been described in the cartilage of quail embryos and the air breathing organ of catfish in which the undifferentiated mesenchymal cells participate in the interstitial type of cartilage growth. The mesenchymal cells derived from the perichondrium invade the interior of the growing cartilage and transformed chondrocytes to secrete new cartilage matrix. In femur and tibia of quail embryos, the mesenchymal invasion is limited to the central hypertrophic zone to provide the cartilage templates by a large population of chondrogenic potential cells. In air-breathing organ of catfish, mesenchymal invasion occurs in multiple sites and involved in growth, renewal, and replacement of the existing cartilage.