Content Reviewers:Rishi Desai, MD, MPH
Bones are composed primarily of an extracellular calcified material called the bone matrix or collagen matrix.
It also has three main types of cells: osteocytes, osteoblasts, and osteoclasts.
Osteocytes are found in cavities or lacunae between the layers of the bone matrix and assist with the nutrition of the bone.
Osteoblasts are found mostly along the surface of bones, but also within the bone matrix itself.
They’re responsible for both synthesizing and mediating the mineralization of the bone matrix.
Osteoclasts are large multinucleated cells that are responsible for removing calcified bone matrix and allow for the constant turnover and remodelling of bones.
Morphologically, there are two main types of bone.
The compact or cortical bone is the dense portion that’s found closer to the surface of bones.
The deeper portion of bones have interconnecting cavities and is called the cancellous or trabecular bone.
In this electron microscopy or EM image of the tibia, the compact bone is also blue, and the more central trabecular bone is yellow. Microscopically, both compact and trabecular bones will be organized or arranged in two forms.
Layered or lamellar bone has a bone matrix that’s arranged in sheets.
80% of lamellar bone is found within the compact bone and only 20% is found within the trabecular bone. The more immature woven bone, has collagen fibers that are arranged randomly.
This form of bone is found mostly within developing and growing bones, as well as bones that have healed after being fractured.
Alright, let’s compare sections of a long bone and a flat bone.
This long bone is a partial cross-section of the head of a long bone, and the flat bone is a cross-section from the skull.
Both sections have been decalcified in order to make it easier to cut the bone into thin sections, but the collagen is still present in the slides.
Since the denser compact bone is typically found closer to the surface, it’s seen in the long bone along the outer edge, whereas the flat bone has compact bone on both of its outer flat surfaces.
“Trabecula” means “little beam” in latin, which is why the trabecular bone has small beam-like structures that are also called spicules.
The spicules connect with each other to form a network of interconnecting spaces that contain bone marrow.
The vertebrae is another example of a bone that has both types of morphology, but its compact bone is only a very thin outer layer, with the majority of the bone consisting of trabecular bone.