Compact Bone, Spongy Bone, and Other Bone Components

Information

The hard mineral component of bones is also known as osseous tissue. Osseous tissue comes in two forms, both of which are present in every bone in the body: compact bone and spongy bone. The two forms mainly differ in how the bone mineral is organized and in how much empty space there is among the solidified extracellular matrix. Compact bone appears solid and spongy bone consists of a web- or sponge-like arrangement of solidified extracelluar matrix.

While compact bone appears at first glance to be solid and uninterrupted, closer inspections reveals that the osseous tissue only makes up from 70-95% of the available volume. There are pores and spaces even in compact bone. Except at its edge, the osseous tissue of compact bone is arranged in cylindrical osteons. Each osteon is a compact cylinder of concentric lamellae. The only cells in an osteon are the osteocytes that are found on the edges of each lamella. Osteocytes are found in lacunae, which are the cell-shaped empty spaces that prevent the solid, mineralized extracellular material of bone from crushing the osteocytes. Cytoplasmic extensions branch out from each osteocyte and are housed in the canaliculi, the thin empty channels that again prevent the solid, mineralized extracellular material from crushing the osteocyte extensions. At the center of each osteon is a central canal (also known as a Haversian canal) through which blood vessels, lymph vessels, and nerves can travel to service and signal the cells throughout the compact bone. At the base of individual osteons are perforating canals (also called Volkmann’s canals), which are empty spaces that allow blood vessels, lymph vessels, and nerves to travel across bone, linking up with the vessels and nerves in the central canals. Compact bone is sometimes called cortical bone.

At the outer edges of compact bone, rather than being arranged in osteons, the osseous tissue is arranged in circumferential lamellae. These travel parallel to the outer edge of the bone and are usually only a few lamellae deep before the osteons start up.

The bones of the body only have compact bone on their outermost surfaces and never very deep. The bulk of most bone tissue is made of spongy bone. In spongy bone there far more uncompacted space. The osseous tissue only makes up somewhere between 10-70% of the available volume, depending on how spongy it is.  The rest of the volume is made up of mostly bone marrow, although there are also blood vessels, lymph vessels, nerves traveling through the spaces. In spongy bone the osseous tissue is arranged into trabeculae, which are the interconnected columns of osseous tissue which create the sponge-like grid of spongy bone. Within a single trabecular, there are concentric lamellae, with osteocytes in lacunae connected to one another via canaliculi, similar to the tissue arrangement in the osteons of compact bone. However, unlike osteons, trabeculae do not have central canals or perforating canals containing blood vessels, lymph vessels, and nerves. The vessels and nerves of spongy bone travel through the spaces between trabeculae and do not need separate passageways. Spongy bone is sometimes called cancellous bone or trabecular bone.

The outsides of all the bones of the body are covered with a layer of irregular dense connective tissue proper called the periosteum. There is a different layer of connective tissue proper that lines all the internal cavities of bones – the central canals of osteons in compact bone, and the exterior surface of trabeculae in spongy bone – called the endosteum.

The longs bones of the body, found in the arms, legs, hands, and feet of the body, have an additional feature unique to their long shape. In the diaphysis, or shaft, of each long bone, the is a central hollow cavity, called the medullary cavity. Having no heavy osseous tissue in the center of the long bones makes them lighter. The non-long bones just rely on having spongy bones in their interior to reduce their overall mass. The medullary cavity, live the spaces in spongy bone, is filled with bone marrow.

compact-bone-features

Figure 6.2. The structural features of compact bone.

 

spongy_bone

Figure 6.3 The structural features of spongy bone.

 

Lab 6 Exercises 6.2

The instructor will provide you with a plastic model of enlarged bone tissue. Find all the following items in the model and be prepared to point out any three to the instructor on their request.

compact bone             spongy bone               lacuna                         canaliculum

central canal               perforating canal        lamella                        circumferential lamella

trabecular                   periosteum                  endosteum                  bone marrow

bone-model

Figure 6-4 The model of enlarged bone tissue you will use to identify the histological features listed above.