Types of bone
Bone is classified according to its type into:
1. Lamellar bone (mature or secondary)
a- Compact (cortical) bone.
b- Spongy ( Cancellous, trabecular ) bone. Compact and spongy bones are microscopically similar and are formed in the same way.
2. Non lamellar bone (woven, primary or immature bone).
3. Bundle bone
 Lamellar bone
Lamellar bone is either dense ivory compact bone or spongy bone . Bone is arranged in layers called lamellae. ( a ) Compact bone It is very dense and does not have marrow spaces. In compact bone, there are three kinds of lamellae.
Circumferential lamellae: These lamellae form the external and internal lamination of the cortical bone and are adjacent to periosteum and endosteum.
– Haversian ( concentric ) lamellae :
These are in Haversion system and also called osteons. The lamellae are arranged concentrically from (4-20 lamellae) around a central canal ( Haversian canal ) which contains blood vessels, nerves and some connective tissue. The canals are lined by endosteum. Lacunae with osteocytes are arranged concentrically with their canaliculi radiating and connecting the Haversian canal. The Haversian canals are connected to the periosteum, marrow cavity and with each other by transverse canals (Volkman’s canals).
The purpose of the Haversian system is to distribute nutrients to osteocytes in compact bone.
– Interstitial lamellae
These are parallel lamellae randomly scattered between Haversian and circumferential lamellae. They represent remnants of Haversian and circumferential lamellae left over after bone remodeling.
( b ) Spongy bone
It has a sponge – like appearance due to bone trabeculae and numerous marrow spaces.
Spongy bone is formed of network of bony trabeculae with many intervening thin irregular marrow spaces. These trabeculae are composed of layers of thin irregular bone lamellae with lacunae containing osteocytes. The trabeculae are lined by a delicate layer of connective tissue called endosteum which contains osteoprogenitor cells, osteoblasts and osteoclasts. The marrow spaces contain the blood supply of the spongy bone.
In both types of lamellar bone, the collagen fibers are oriented in parallel way forming an angle of 45° with that of the succeeded and preceded lamellae.
The incremental lines of lamellar bone
These lines can be identified in decalcified sections. 1-Faint lines:
Appear in sections stained with silver impregnation as faint black lines, formed due to change of the direction of the collagen fibers of the matrix from one layer to another. The angle is about 45°.
2-Resting lines :
They appear in sections stained with (H&E) as dark blue lines which are either straight or gently undulated. They demonstrate the incremental pattern of bone formation. The resting lines correspond to the rest period between the successive layers of bone. 3-ReversaI lines :
They appear in sections stained with (H&E) as dark blue scalloped lines. They demonstrate the osteoclastic activity on the
bone undergoing resorption, followed by osteoblastic new bone formation over the old bone. Both old bone and new bone are separated by scalloped line, in which the convexities are towards the old resorbed bone.
[21 Non lamellar bone
: (Woven, primary, immature bone, embryonic bone and bone of emergency). This bone is present in: a- Fetal life.
b- In the extraction socket, c- In the fracture site.
It is characterized by the high concentration of osteocytes, irregular arrangement of collagen fibers and the smaller content of minerals. So it appears more radiolucent than mature bone. Woven bone is resorbed and replaced by lamellar bone.
T31 Bundle bone
It is the third type of bone. It is called bone of attachment because it is present at sites of attachment of muscles and ligaments. It lines the socket of the teeth in which the periodontal ligament fibers are anchored (Sharpey’s fibers ). Its histological structure consists of bundles of woven bone with the fibers running parallel to the socket wall.
The Alveolar process of Maxilla and Mandible
The alveolar process is that bone of the jaws which contains and supports the socket (alveoli) of the erupted teeth. It consists of: (1) Alveolar bone proper
a- Bundle bone
b- Haversian (lamellar) bone
(2) Supporting alveolar bone a- Cortical plates b- Spongy bone
Development of the alveolar bone
The alveolar bone starts to develop near the end of the second month of fetal life. Both the maxilla and mandible form a groove at their free surface (towards the oral cavity). The tooth germs of the deciduous teeth are contained in this groove. Gradually, bony septa develop between the adjacent tooth germs.
The alveolar process develops during eruption of the teeth.
In fetal life, the developing bone is a non lamellar type of bone surrounded by a thick periosteum. Areas of secondary cartilages may appear at the growing alveolar margins during the rapid growth of alveolar bone.
After eruption of teeth, the alveolar bone gradually takes its adult form.