Atelosteogenesis

Atelosteogenesis is a process of bone formation that occurs in certain fish species, particularly those belonging to the order Osteoglossiformes, which includes the garfish and bowfin. Unlike the typical osteogenesis seen in most vertebrates, where bone tissue is derived from mesenchymal cells, atelosteogenesis involves the direct transformation of cartilage into bone without the intermediate formation of a bone marrow cavity.

This unique process begins with the formation of a cartilage template, which is then invaded by blood vessels and osteoblasts. These osteoblasts differentiate directly from the cartilage cells, leading to the deposition of bone matrix. The resulting bone lacks a medullary cavity, which is characteristic of most vertebrate bones. Instead, the bone tissue is directly surrounded by connective tissue.

Atelosteogenesis is considered a primitive trait, as it is more similar to the bone formation process seen in early vertebrates and invertebrates. It is thought to have been retained in certain fish species due to evolutionary constraints or selective advantages in specific ecological niches. The study of atelosteogenesis provides valuable insights into the evolution of bone formation and the diversity of skeletal structures in vertebrates.