Dermal bone

Type of bone that forms in the skin

Dermal bone

Latin	os dermale
Precursor	Mesenchyme
System	Skeletal system

Dermal bone is a type of bone that forms directly within the dermis of the skin, without a cartilaginous precursor. This type of bone is distinct from endochondral bone, which forms from a cartilage template. Dermal bones are primarily found in the exoskeletons of vertebrates, such as the bony plates of turtles, the scales of fish, and the osteoderms of some reptiles.

Formation

Dermal bone formation is a process known as intramembranous ossification. During this process, mesenchymal stem cells within the dermis differentiate directly into osteoblasts, which are bone-forming cells. These osteoblasts secrete osteoid, an unmineralized bone matrix, which then becomes mineralized to form mature bone.

Functions

Dermal bones serve various functions depending on their location and the species in which they are found. Common functions include:

• Protection: Dermal bones often form protective armor, such as the bony plates in the shells of turtles or the scales of certain fish.
• Support: In some species, dermal bones provide structural support, as seen in the skulls of vertebrates where they contribute to the cranial structure.
• Mineral storage: Like other bones, dermal bones can serve as reservoirs for minerals such as calcium and phosphorus.

Examples in Vertebrates

• Fish: Many fish have dermal bones in the form of scales, which provide protection and reduce water resistance.
• Amphibians: Some amphibians, like the axolotl, have dermal bones that contribute to their cranial structure.
• Reptiles: Reptiles such as crocodiles and turtles have extensive dermal bone structures, including osteoderms and shells.
• Mammals: In mammals, dermal bones are less common but can be found in the form of antlers and some cranial bones.

Evolutionary Significance

Dermal bones are considered to be one of the earliest forms of bone to evolve in vertebrates. They are thought to have originated as a form of external armor in early jawless fish, providing protection from predators and environmental hazards. Over time, the role and complexity of dermal bones have evolved, contributing to the diverse array of vertebrate forms seen today.

Also see

• Endochondral ossification
• Intramembranous ossification
• Osteoblast
• Osteoclast
• Bone remodeling