  The flat bone develops through intramembranous ossification, in which the
mesenchymal cells are directly driven towards osteogenic lineage without the
formation of cartilage template. While long bone develops through endochondral
ossification, where cartilage template act as an intermediate stage between
mesenchymal and bone tissues. Although the avian sternum is a flat bone, some
studies describe formation of a cartilage template during its development. The
aim of the current study was to observe the mechanism of ossification in quail
sternum during embryonic development. Thirty quail embryos were collected for
the current study (5 embryos/ day) during the period between Day (D) 5 and D10
of embryonic development and processed for light microscopy. The
differentiation of mesenchymal condensation in to the chondrogenic cells was
observed at D5 whereas the secretion of extracellular matrix could be evident
at D6. The cartilage primordia were observed by D7 which were consisted of
chondrocytes, embedded in matrix and surrounded by perichondrium. Later these
primordia were developed in to cartilage template by D8 where the chondrocytes
were present in their lacuna. This template attained the shape of future
sternum by D9, which was more distinct at D10. These preliminary observations
suggested that the quail sternum grows through endochondral ossification. The
future study will further explore the histological changes of quail sternum
during post-hatching development.
