There are two types of osseous tissue; cortical and
proportion cortical and cancellous bone varies between different bones. For
example, the vertebral bodies are mostly cancellous bone and the shafts of the
long bone are mostly cortical. The variation in bone type is related to the
functions of the bone.
Cortical bone forms the external surface of bones. The functional unit
of cortical bone is the osteon and it is a highly organized tissue. The bone
matrix is organized in thin plates known as lamellae, which are organized in
circumferential layers at the periosteal and endosteal surfaces, and
concentrically at the osteons, forming Haversian systems. The bone matrix is
very compact, which gives the bone greater density, stiffness and compressive
strength. Whilst cortical bone is very strong, it’s structure means that it is
only able to resist stresses from a limited number of direction, which is why
it is found in shafts of long bones. Cortical bone is thickest where it is
under the greatest force. It functions to support the weight of the body, protect
organs, and also plays a critical role in calcium homeostasis. Cellular
activities take place through resorbing channels.
Cancellous bone forms the interior structure of bones. In cancellous
bone the lamellae are arranged in opposition in order to form structures known
as trabeculae. These trabeculae are the functional unit of cancellous bone.
They are positioned along lines of stress in a manner known as cross bracing. This
structure maximizes the ability of the bone to withstand forces from multiple
directions but only adds minimal mass to the bone, keeping the skeleton as
light-weight as possible. As cancellous bone is more capable of resisting
forces applied from multiple directions, it is found in the epiphysis of long
bones. As the interior structure of bone, cancellous bone also functions to
protect the bone marrow. Its porosity allows for storage of the bone marrow,
high vascularization and its function as the site of erythropoiesis. It is also
the site of mineral storage and high metabolic activity, with cancellous bone
containing more metabolically active surfaces in a given volume than cortical
bone. Cellular activities take place on these surfaces. Cancellous bone is
softer and weaker than cortical bone, which maintains bone flexibility,
allowing for movement of the skeleton.