2. Microscopic features

Fig. 116:   Cross-section, in the occlusal plane, through the maxillary premolar region.   The alveolar bone proper (AB) can be seen surrounding each tooth as a continuous thin plate of compact bone supported by the trabeculae of the adjacent spongy bone (S).   It becomes fused with and indistinguishable from the cortical plate (CP) of the alveolar process.  The periodontal ligament attaches the tooth (T) to the alveolar bone.                               

Fig. 117:   Magnified view of Fig. 116.  Note the bony trabeculae (BT) that help support the thin alveolar bone layer (AB).  The histological structure of the alveolar bone reflects the remodeling that takes place to accomodate mesial drifting of the dentition.   As the dentition  wears, the teeth tend to move through the bone in a mesial direction to maintain tight contacts between the teeth.  This means that the bone mesial to a tooth must resorb to allow the tooth to move, while the bone distal to it is undergoing new bone apposition to maintain the width of the periodontal ligament.  In this section, mesial is to the left.   

Fig. 118:   Periapical radiograph of maxillary posterior region.  The radiograph represents a summed image of all the structures between the x-ray source and the film.   Dense structures like teeth and bone appear light, while non-mineralized tissues are dark.  The image that corresponds to the alveolar bone proper is the thin, white line that parallels the outline of the roots of the teeth.  The radiographic term for this image is the lamina dura (LD).  The periodontal ligament space (PDL) appears as a dark line between the lamina dura and the root surface.  The trabecular pattern of the cancellous bone (S) can also be readily detected.         

Fig. 119 (From Lindhe, J., 1983): Bone is produced by osteoblasts (OB) that are found in the periosteum, endosteum and periodontal ligament adjacent to bone-forming surfaces.  These specialized cells originate from less differentiated precursor cells close to the bone.   These cells are in turn derived from undifferentiated ectomesenchymal cells found in the periosteum, endosteum and the periodontal ligament.  During bone formation, osteoblasts become incorporated into bone as osteocytes (OC) that are completely surrounded by bone.  The chamber in which they are trapped is called a lacuna (plur. lacunae). Osteocytes remain connected to osteoblasts and other osteocytes (see Fig. 120)  by cytoplasmic processes that run through small canals in the bone, or canaliculi (C).            



Fig. 120 (From Lindhe, J., 1983):  Diagram illustrating canaliculi (C) connecting adjacent osteocytes in their lacunae (OC) to one another.    

Fig. 121:   Histologic section through compact bone.  The osteocytes (OC) in their lacunae are distributed throughout the entire tissue.  In stained sections, such as this one, the dense array of canaliculi that connect adjacent lacunae is readily observed.
 



Fig. 122:  Cortical plate of compact bone in the mandible.  The mandible is enveloped by a well-developed cortex of compact bone.  The bulk of the compact bone consists of cylindrical units of bone, the osteons or Haversian systems (HS).  Each osteon has a central canal, the Haversian canal that houses a blood vessel.  Haversian canals are linked to one another and the periphery of the cortex by Volkman canals that course perpendicularly to the Haversian canals.  The outer and inner layers of the cortex consist of parallel lamellae of compact bone, called the external (ECL) and internal circumferential lamellae.  The bone that fills the spaces between adjacent osteons is the interstitial bone.



Fig. 123:  Section of mandibular cortex through the external circumferential lamellae (ECL) and periosteum (P).  The cortical plate undergoes continuous remodeling.   Dark blue stained osteons (HS1) with wide Haversian canals are relatively young, while the pink-staining osteons (HS2) with small Haversian canals are more mature.   Interstitial bone (IB) fills the spaces between the osteons.