A synchondrosis is characterised by a hyalin* cartilaginous intermediary substance. This interposed layer is thin and acts as as an epiphyseal cartilage whose both surfaces would be fertile. Such an union can only exist between two bones of endochondral origin.
Endochondral ossification is one of the two essential processes during fetal development of the mammalian skeletal system by which bone tissue is created. Unlike intramembranous ossification, which is the other process by which bone tissue is created, cartilage is present during endochondral ossification
The hyalin tissue of some synchondrosis can be more or less mixed with fibrous tissue, doing the transition with the symphysis.
*a clear substance produced especially by the degeneration of epithelial or connective tissues.