The histological study of compact bone usually uses slices that have been dried, cut with a saw, and ground to translucent thinness. This procedure destroys the cells and most other organic content but reveals fine details of the inorganic matrix (fig. 7.4). Such sections show onionlike concentric lamellae—layers of matrix concentrically arranged around a central (haversian) canal and connected with each other by canaliculi. A central canal and its lamellae constitute an osteon (haversian system)—the basic structural unit of compact bone. In longitudinal views and three-dimensional reconstructions, we can see that an osteon is actually a cylinder of tissue surrounding a central canal. Along their length, central canals are joined by transverse or diagonal passages.
Collagen fibers "corkscrew" down the matrix of a given lamella in a helical arrangement like the threads of a screw. In the adjacent lamella, they angle in the opposite direction—alternating between right- and left-handed helices from lamella to lamella (fig. 7.4b). This enhances the strength of bone on the same principle as plywood, made of thin layers of wood with the grain running in different directions from one layer to the next. The helices tend to be more stretched out along the longitudinal axis of bones that must resist tension (bending), but are tighter and run more nearly across the bone in bones that must resist compression.
The skeleton receives about half a liter of blood per minute. Blood vessels, along with nerves, enter the bone tissue through nutrient foramina on the surface. These open into narrow perforating (Volkmann16) canals that cross the matrix and feed into the central canals. The innermost osteocytes around each central canal receive nutrients from these blood vessels and pass them along through their gap junctions to neighboring osteocytes. They also receive wastes from their neighbors and convey them to the central canal for removal by the bloodstream. Thus, the cytoplasmic processes of the osteocytes maintain a two-way flow of nutrients and wastes between the central canal and the outermost cells of the osteon.
Not all of the matrix is organized into osteons. The inner and outer boundaries of dense bone are arranged in circumferential lamellae that run parallel to the bone surface. Between osteons, we can find irregular regions called interstitial lamellae, the remains of old osteons that broke down as the bone grew and remodeled itself.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.