BONE HISTOLOGY AND ALVEOLAR BONE STRUCTURE
Author: Sanketh DS, MDS
Bone is a specialised form of connective tissue that consists of cells and intercellular material. Osteoblasts, osteocytes and osteoclasts make up the cells of the bone, while the inter-cellular material is made of organic and inorganic material. By dry weight, bone comprises of 33% organic matrix, with 28% representing type I collagen, and non-collagenous proteins like bone sialoprotein, osteocalcin,osteopontin, osteonectin and proteoglycans making up 5%. The organic matrix is mineralized by hydroxyapatite (inorganic material), which comprises the rest of the 67% of bone by dry weight.
Though bone can be classified into different types based on its shape as long (eg.tibia, humerus), short (eg.carpals, tarsals), flat (eg.sternum,scapula) or irregular(vertebrae, maxilla); most bones are essentially made of two tissue patterns called compact or cortical bone and a fine network of cancellous bone or spongy/trabecular bone.
Let’s have a look at the histology of bone with the mandible as an example. The mandible has a rigid outer cortical bone or cortex forming an outer shell surrounding a medullary cavity or a marrow cavity. The medullary cavity apart from housing the bone marrow is interrupted by a network of numerous bony spicules or trabeculae called the cancellous bone or the spongy bone. The cortical and cancellous bones are histologically identical in that, they are made of concentric sheets of collagen called lamellae.
Compact bone histology
Histologically three types of lamellae can be identified in the compact bone, them being circumferential, concentric and interstitial lamellae.
Bulk of the compact bone is made of concentric lamellae that are actually bony layers wrapped around a central cylindrical tunnel or canal. The central canal is called the Haversian canal and houses blood vessels and nerves. The Haversian canal and the concentric lamellae together form the Haversian system. The concentric lamellae or the Haversian system form the basic metabolic unit of bone called the osteon. Circumferential lamellae surround the outer most and the inner most layers of the compact bone. Lamellae found in between the concentric lamellae are called the interstitial lamellae. The interstitial lamellae actually represent concentric lamellae that have been partly resorbed and remodelled and no longer surround a Haversian canal.
During bone formation when the organic matrix is laid down by osteoblasts, they often get buried within the lamellae and are housed in small cavities called lacunae. Osteoblasts housed in lacunae are called osteocytes and they have delicate cytoplasmic processes radiating out that are housed in micro canals called canaliculi. These canaliculi are interconnecting and help osteocytes contact adjacent osteocytes as well as the osteoblasts lining the bone surface.
The outer most layer of the compact bone is surrounded by a periosteum having two layers; an outer fibrous layer and an inner layer facing the bone, having precursor bone cells and a rich blood supply. The haversian canals communicate with each other as well as the periosteum and the marrow space via canals called Volkmann canals. Volkmann canals like haversion canals also contain blood vessels. This system of interconnected Haversian and Volkmann canals, supplies nutrients and nourishes the bone.
Cancellous/Trabecular/Spongy bone histology
The innermost layer of the compact bone is called the endosteum and is covered by inner circumferential lamellae. The innermost layer faces the medullary cavity that houses a fine network of bony spicules called the cancellous bone or trabecular bone. These spicules of trabecular bone are interconnecting and their ends are attached to the endosteum. Like the compact bone, the trabecular bone is also made of lamellae that are concentric. However, trabecular bone does not have osteons like in the compact bone.
STRUCTURE OF ALVEOLAR BONE/PROCESS
The alveolar bone also called the alveolar process is the elevated ridge of the maxilla and the mandible where the teeth are housed. Strictly speaking, there is no defined boundary between the body of the jaws and the alveolar process. The alveolar bone or process can be divided into an alveolar bone proper and a supporting alveolar bone.
The alveolar bone proper (ABP) is the bone forming the tooth socket or the alveolus that surrounds the roots of tooth. Histologically it is a compact bone having osteons. It is also called bundle bone since bundles of principle periodontal ligament fibres called Sharpey’s fibres attach to the bone from the tooth. The ABP or the alveolar sockets have numerous perforations of small canals that carry nerves and blood vessels. Hence the ABP is also called cribriform plate. The alveolar sockets are separated by an interdental septum and have inter-radicular septa separating roots of the tooth. The interdental and the inter-radicular septa are also cribriform plates and house numerous canals called canals of Zuckerkandl and Hirshfeld. In radiographs, the ABP is more radio-dense than the surrounding bone and can be seen as thin radio-opaque bone surrounding the roots of teeth. Radiographically, the ABP is called lamina dura. Its increased radio density is not because of high mineral content but due to the bone being thick and not having trabeculations.
The rest of the alveolar process gives support to the ABP and is called supporting alveolar bone (SAB). The SAB consists of the buccal and lingual cortical plates and trabecular or cancellous bone in between the cortical plates as well as the ABP. The cortical plates of the mandible are thicker than the maxilla since maxillary cortex has numerous pores where many blood vessels and nerves traverse. Also the maxilla has more trabecular bone than the mandible.
The trabecular bone according to its arrangement in the alveolar process is divided into two types called type I and type II. Type I trabecular bone has a horizontal or a ladder like arrangement and is usually more prominent in the mandible. Whereas the type II arrangement is an irregularly arranged trabecular bone and is more common in the maxilla.
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OTHER (HACKDENTISTRY) PRACTICE/STUDY RESOURCES