Bone Histology Types of Bone Cells 1 Osteocyte

Bone Histology

Types of Bone Cells 1) Osteocyte: Mature bone cell 2) Osteoblast: Bone-forming cells 3) Osteocast: Bone-destroying cells Remember, bone is a living tissue!

Microscopic Structure of a Compact Bone Osteon System: A central (Haversian) canal with concentric rings (lamellae) of bone matrix running lengthwise. Very strong!

• Lacunae: tiny cavities inside the lamellae rings. This is where the osteocytes are found.

• The central canal carries blood vessels and nerves to all areas of the bone.

So how do all bone cells get nourishment and contact the body outside the bone? Canaliculi (kan” ah-lik’-u-li): tiny canals that radiate outward from the central canals to each lacunae space. Volkmann’s Canals: canals that run at right angles to the central canals and perforate the shaft of the bone.

Spongy bone (cancellous bone) Trabeculae: Loosely organized lamellae rings with osteocytes (no central canal). Canaliculi connect the osteocytes. filled with red and yellow bone marrow - osteocytes get nutrients directly from circulating blood. Works like struts along lines of stress in bone to offer strength, yet lightweight. Grrrr!

• • Long bone anatomy Diaphysis: long shaft of bone Epiphysis: ends of bone Epiphyseal plate: growth plate Metaphysis: b/w epiphysis and diaphysis Articular cartilage: covers epiphysis Periosteum: bone covering (pain sensitive) Sharpey’s fibers: periosteum attaches to underlying bone • Medullary cavity: Hollow chamber in bone - red marrow produces blood cells - yellow marrow is adipose. • Endosteum: thin layer lining the medullary cavity

Diaphysis

Epiphysis

Epiphyseal (growth) plate

Medullary cavity

• Histology of bone tissue Cells are surrounded by matrix. - 25% water - 25% protein - 50% mineral salts 4 cell types make up osseous tissue Osteoprogenitor cells Osteoblasts Osteocytes Osteoclasts

• Osteoprogenitor cells: - derived from mesenchyme - all connective tissue is derived - unspecialized stem cells - undergo mitosis and develop into osteoblasts - found on inner surface of periosteum and endosteum.

• Osteoblasts: - bone forming cells - found on surface of bone - no ability to mitotically divide - collagen secretors § Osteocytes: - mature bone cells - derived form osteoblasts - do not secrete matrix material - cellular duties include exchange of nutrients and waste with blood.

• Osteoclasts - bone resorbing cells - bone surface - growth, maintenance and bone repair Abundant inorganic mineral salts: - Tricalcium phosphate in crystalline form called hydroxyapatite Ca 3(PO 4)2(OH)2 - Calcium Carbonate: Ca. CO 3 - Magnesium Hydroxide: Mg(OH)2 - Fluoride and Sulfate

• Bone formation (ossification) occurs in two ways 1 - Intramembranous ossification 2 - Endochondral ossification Both methods above lead to the same bone formation but are different methods of getting there. Ossification (osteogenesis) begins around the 6 th 7 th week of embryonic life. At this time the embryonic skeleton is made of fibrous membranes and hyaline cartilage.

Hyaline Cartilage Review a. Most abundant b. Provides support, flexibility and resilience c. Located: a. forming nearly all the fetal skeleton b. articular cartilage: ends of moving bones c. costal cartilage: ribs to sternum d. tip of nose e. respiratory cartilage

• Skeletal Cartilage: 1. Chondrocytes: cartilage producing cells. 2. Lacunae: small cavities where the chondrocytes are encased. 3. Extracellular matrix: jellylike ground substance. 4. Perichondrium: layer of dense irregular connective tissue that surrounds the cartilage. 5. No blood vessels or nerves

• Intramembranous (within the membrane) ossification: Bone develops from a fibrous membrane. - flat bones of skull - mandible - clavicles -mesenchymal cells become vascularized and become osteoprogenitor cells and then osteoblasts. - organic matrix of bone is secreted - osteocytes are formed - calcium and mineral salts are deposited and bone tissue hardens. - trabeculae develop and spongy bone is formed - red marrow fills spaces

Replacement of hyaline cartilage with bone is called. Endochondral (intracartilaginous) ossification Most bones are formed this way (i. e. long bones). Where bone is going to form: 1 - mesenchymal cells differentiate into chondroblasts (immature cartilage cells) which produces hyaline cartilage. Perichondrium develop around new cartilage 2 - Chondrocytes (mature) mitotically divide increasing in length This pattern of growth: interstitial growth. - growth from within

Growth of cartilage in thickness occurs from the deposition of new matrix to the periphery formed by chondroblasts within the perichondrium. Appositional growth. Chondrocytes undergo hypertrophy, swell and burst. p. H of the matrix changes and calcification is triggered. Ultimately, cartilage cells die. Lacunae are now empty.

Nutrients are supplied by way of the nutrient artery passing through the perichondrium through the nutrient foramen. Osteoprogenitor cells are stimulated in the perichondrium to produce osteoblasts. A thin layer of compact bone is laid down under the perichondrium called the periosteal bone collar. Perichondrium becomes periosteum

Osteoblasts begin to deposit bone matrix forming spongy bone trabeculae. In the middle of the bone, osteoclasts break down spongy bone trabeculae and form a hollowed out cavity called the medullary cavity. This cavity will be filled with red bone marrow for hemopoiesis. The shaft of the bone is replaced (was hyaline cartilage) with compact bone.

• Physiology of bone growth: - epiphyseal plate (bone length) - 4 zones of bone growth under h. GH. 1 - Zone of resting cartilage: - no bone growth - located near the epiphyseal plate - scattered chondrocytes - anchors plate to bone 2 - Zone of proliferating cartilage - chondrocytes stacked like coins - chondrocytes divide

3 - Zone of hypertrophic (maturing) cartilage - large chondrocytes arranged in columns - lengthwise expansion of epiphyseal plate 4 - Zone of calcified cartilage - few cell layers thick - occupied by osteoblasts and osteoclasts and capillaries from the diaphysis - cells lay down bone - dead chondrocytes surrounded by a calcified matrix. Matrix resembles long spicules of calcified cartilage. Spicules are partly eroded by osteoclasts and then covered in bone matrix from osteoblasts: spongy bone is formed.

Age 18 -21: Longitudinal bone growth ends when epiphysis fuses with the diaphysis. - epiphyseal plate closure - epiphyseal line is remnant of this - last bone to stop growing: clavicle

• Bone width: increase in diameter of bone occurs through appositional growth. - Osteoblasts located beneath the periosteum secrete bone matrix and build bone on the surface - Osteoclasts located in the endosteum resorbs (breakdown) bone.

• Bone Remodeling: - bone continually renews itself - never metabolically at rest - enables Ca to be pulled from bone when blood levels are low - osteoclasts are responsible for matrix destruction - produce lysosomal enzymes and acids - spongy bone replaced every 3 -4 years - compact bone every 10 years

- Blood calcium levels signal release of either parathyroid hormone (PTH, secreted by parathyroid gland) and calcitonin (secreted by thyroid). PTH causes calcium release from bone matrix by stimulating osteoclast activity and bone resorption. Calcitonin inhibits bone resorption and causes calcium salts to be deposited in bone matrix. Vitamins A, C, D and B 12 help in bone remodeling

• Fractures: Any bone break. - blood clot will form around break - fracture hematoma - inflammatory process begins - blood capillaries grow into clot - phagocytes and osteoclasts remove damaged tissue - procallus forms and is invaded by osteoprogenitor cells and fibroblasts - collagen and fibrocartilage turns procallus to fibrocartilagenous (soft) callus

- broken ends of bone are bridged by callus - Osteoprogenitor cells are replaced by osteoblasts and spongy bone is formed - bony (hard) callus is formed - callus is resorbed by osteoclasts and compact bone replaces spongy bone. Remodeling : the shaft is reconstructed to resemble original unbroken bone. Closed reduction - bone ends coaxed back into place by manipulation Open reduction - surgery, bone ends secured together with pins or wires