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.
In this image, we can also see that the bone marrow is still present within the spaces
formed by the spicules.
The osteoclasts can be found within the bone marrow.
In this image, the osteoclast is sitting within a resorption cavity it formed on the surface
of the bone matrix.
Identifying the multiple nuclei within the osteoclast can help differentiate it from
other similar-looking cells such as megakaryocytes, which only have a single multi-lobed nucleus
instead.
Okay, let’s take a closer look at compact bones, which have structural units called
the Haversian system or osteons.
An osteon consists of concentric lamellae or layers, with a diameter of about 100 to
250 µm and a central canal called the Haversian canal.
This canal contains small blood vessels and nerves, as we can see in this image.
we can see a small blood vessel and remnants of nerve fibers as well.
The concentric layers of mineralized bone matrix radiate out from the canal and are
called Haversian Lamellae.
Along the border of each osteon is a thin purple or basophilic line that’s called
the cement line.
The white spaces throughout the osteon are the lacunae, which contain remnants of osteocytes.
In this image of a haversian canal, the osteoblasts can be seen lining the inner surface of the
Haversian canal.
Osteoblasts are large spindle-shaped or cuboidal cells that have a basophilic cytoplasm.
Alright, as a quick recap…
Morphologically, bones have two main types.
The denser compact or cortical bone, which is found closer to the surface, and the cancellous
or trabecular bone, which is the spongy bone that consists of trabecula or spicules that
form interconnecting cavities.
Microscopically, the lamellar bone is arranged in sheets of collagen matrix and woven bone
has collagen fibers that are arranged randomly.
There are also three main cell types that are part of the bone.
Osteocytes are found in cavities or lacunae between the layers of collagen matrix.
Osteoblasts are found along the surface of bones and also lining the Haversian canal
within the bone.
And osteoclasts are large multinucleated cells that can be found on the surface of the bone
matrix.