a) Non-lamellar bone, also called primary, or immature bone;
b) Lamellar bone, also known as secondary, or mature bone.
The non-lamellar immature bone is always the first to be formed in the body during fetal life, during bone growth, bone remodeling, healing of bone fractures. In these situations the non-lamellar bone is gradually replaced by the mature one that is the predominant type in healthy adults. The non-lamellar type remains in only a few places, one example being the dental alveoli that hold the teeth (where bone remodeling is quite constant).
Main differences in the arrangement of cells and collagen fibers in both types:
Immature, non-lamellar bone or primary bobe: the collagen fibers are arranged in many directions. The osteocytes are not regularly arranged. This type contains more cells per bone volume than the lamellar bone.
Mature, lamellar or secondary bone: the collagen fibers are organized in parallel forming sheets called bone lamellae. The osteocytes are regularly disposed between the lamellae. The collagen fibers that form each lamella are arranged in parallel to each other. However, when observing adjacent lamellae, the collagen fibers can be seen to maintain an angle in relation to the collagen fibers of their neighbor lamellae (similarly as the fibers in wood plates are ordered in plywood). This type of bone has fewer osteocytes per bone volume.
Top left: non-lamellar bone. Top right: an isolated lamella of lamellar bone. Lower row left: a stack of two bone lamellae. Lower row right: a stack of three bone lamellae. Notice different angles of collagen fibers between neighbor lamellae.
