The BonesLectured by Bien Nillos, MD

Comparative Vertebrate Anatomy

Dermal bone consists of bony structures (plates and scales) that develop in the skin. Dermal bone does not form from cartilage first and then calcify, but endoskeletal bone does; in fact, in cartilaginous fishes it may never form true bone.

VERTEBRATEBONES

DERMAL BONE ENDOSKELETON

All vertebrates have cartilage in addition to bone, or instead of bone.

Function of the Bones1. Support. Provide a hard framework.

2. Protection of many vital organs.

3. Movement - act as levers with skeletal muscles moving them. Joints control possible movements.

4. Mineral storage.

5. Blood cell formation – hematopoiesis

Types of Bones

According to Development1. Membranous bones2. Cartilaginous bones3. Membro-cartilaginous bones

Ossification is the formation of bone by the activity of osteoblasts and osteoclasts and the addition of minerals and salts. Calcium compounds must be present for ossification to take place. Osteoblasts do not make these minerals, but must take them from the blood and deposit them in the bone. By the time we are born, many of the bones have been at least partly ossified.

BONE FORMATION

• Formed either by direct ossification of embryonic connective tissue (intramembranous ossification) or by replacement of hyaline cartilage (intracartilaginous or endochondral ossification).

• Intramembranous ossification takes place in the so-called membrane bones of the skull

• Endochondral ossification is characteristic of the bones of the trunk and extremities.

Intramembranous Ossification

a. Increased vascularity of tissue. b. Active proliferation of mesenchymal cells. The mesenchymal cells give rise to

osteogenic cells, which develop into osteoblasts. c. Osteoblasts begin to lay down osteoid. Osteoid is the organic part of bone without the

inorganic constituent.d. Osteoblasts either retreat or become entrapped as osteocytes in the osteoid.e. The osteoid calcifies to form spicules of spongy bone. The spicules unite to form

trabeculae. The inorganic salts carried in by the blood vessels supposedly bring about calcification. The salts are deposited in an orderly fashion as fine crystals (hydroxyapatite crystals) intimately associated with the collagenous fibers. These crystals are only visible with the electron microscope.

f. Bone remodeling occurs. Periosteum and compact bone are formed.

Endochondral Ossification

This type of ossification involves the replacement of a cartilaginous model by bone and is best observed in long bones, such as the humerus or femur

a. Primary ossification center.

b. Bone collar

c. Periosteal buds

d. Secondary ossification centers

5 zones of Cartilage

1. Zone of reserve cartilage.

2. Zone of cell proliferation

3. Zone of cell and lacunar maturation and hypertrophy enlargement

4. Zone of calcification5. Zone of cartilage

removal and bone deposition

Types of Bone

• On the basis of Structure1. Compact

2. Spongy

Compact or cortical bone, is made up many rod-like units called osteons or Haversian systems which run longitudinally within the bone. Haversian systems have a central Haversian canal which carries blood and lymphatic vessels and nerve branches.

The osteon, or Haversian system, is the fundamental functional unit of much compact bone

Each osteon consists of concentric layers, or lamellae, of compact bone tissue that surround a central canal, the Haversian canal. The Haversian canal contains the bone's nerve and blood supplies. The boundary of an osteon is the cement line

Osteons are separated from each other by interstitial lamellae between systems.Some of the osteoblasts develop into osteocytes, each living within its own small

space, or lacuna. Osteocytes make contact with the cytoplasmic processes of their counterparts via a network of small canals, or canaliculi. This network facilitates the exchange of nutrients and metabolic waste.

Spongy or cancellous bone consists of a lattice of thin threads of bone called trabeculae and is less dense than compact bone. The orientation of the trabeculae is affected by the mechanical stress to which the bone is exposed .

END OF PART ONE