The manners in which the osteocytes and collagen fibers are organized in the bone result in two kinds of bone tissue:
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 form in the body, during fetal life, during bone growth, bone remodeling, healing of bone fractures.
The mature lamellar bone normally replaces the immature bone and becomes the predominant type in adults. In healthy conditions, non-lamellar bone exists only in a few places in adults, one example being the dental alveoli that hold the teeth.
Main histological differences between both types:
Immature or non-lamellar bone: the the collagen fibers of its extracellular matrix are in many directions, and the osteocytes are not regularly arranged. It contains more cells per bone volume than the lamellar bone.
Mature or lamellar bone: its parallel collagen fibers are organized in sheets called bone lamellae. The collagen fibers that form each lamella are arranged in parallel to each other. However, in adjacent lamellae, the collagen fibers maintain an angle in relation to the collagen fibers of their neighbor lamellae as wood plates are ordered in plywood. This bone type has less osteocytes per bone volume and the cells have a very organized distribution between the bone lamellae.
More information on lamellar and non-lamellar bone in the following pages.
