Module 6.2 Microscopic Structure of Bone Tissue

bone cell type: Osteocytes

Osteocytes maintain the bone extracellular matrix (ECM). Osteoblasts become surrounded and eventually trapped by secreted bone matrix in a small cavity known as a lacuna At this point, these cells are called osteocytes. Osteocytes are mostly "re- tired" from their jobs and are relatively inactive. They do, however, secrete chemicals that are required for maintaining the ECM They also appear to recruit osteoblasts to build up areas of the bone under tension.

lamellae

The "rings" of our miniature trees are very thin layers of bone termed lamellae

bone matrix

The extracellular material of bone tissue, including ground substance and collagen fibers. Excludes the inorganic calcium salts.

organic matrix

The remaining 35% of total bone weight consists of an organic matrix, which contains the usual ECM "ingredients," including collagen fibers.

trabeculae of spongy bone

Trabeculae are covered with endosteum and usually do not contain osteons. However, they do contain concentric lamellae, within which we find canaliculi and lacunae housing osteocytes. No central or perforating canals are present within trabeculae, and the cells obtain their oxygen and nutrients from the blood vessels in the bone marrow.

lamellae (Compact Bone)

Concentric rings made up of groups of hollow tubes of bone matrix

central canal (haversian canal) (Compact Bone)

In the center of each osteon is a hole called the central canal contains blood vessels and nerves that supply the cells of the osteon. This canal, like all internal canals of bone, is lined by endosteum.

canaliculi (Compact Bone)

Lacunae are connected to one another by tiny canals called canaliculi

circumferential lamellae

Layers of bone matrix that go all the way around the bone.

bone cell type: Osteoclasts

Osteoclasts are large, multinucleated cells derived from the fusion of cells formed in the bone marrow. They reside in shallow depressions on the internal or external surfaces of bone. Osteoclasts are responsible for the process of bone resorption, during which they break down the bone ECM.

inorganic matrix

65% of the total weight of bone consists of an inorganic matrix composed of minerals (The predominant ingredient of the inorganic matrix is calcium salts (in fact, bone stores about 85% of the total calcium ions in the body), with a good amount of phosphorus as well. Most of the calcium and phosphorus salts exist as large molecules of a mineral called hydroxyapatite crystals)

lacunae (Compact Bone)

small cavities that are filled with extracellular fluid and located between lamellae. small cavities in bone that contain osteocytes

bone cell type: Osteoblasts

are cuboidal to columnar cells found in the inner periosteum and endosteum. Osteoblasts are derived from flattened cells called osteogenic cells, which differentiate into osteoblasts when stimulated by certain chemical signals. Osteoblasts are bone-building cells that perform the process of bone deposition, during which they build up the bone ECM. In this process osteoblasts secrete the organic matrix and aid in formation of the inorganic matrix.

what. is the primary tissue type found in bones?

bones tissue or osseous tissue

perforating canal

connects central canals perforating canals run perpendicular to the osteons. They carry blood vessels from the periosteum that merge with vessels in the central canals.

Function of osteoclasts

degrade bone to initiate normal bone remodeling and mediate bone loss in pathologic conditions by increasing their resorptive activity.

interstitial lamellae

fill spaces between osteons

What makes bone one of the hardest substances in the body?

hydroxyapatite crystals [Ca10(PO4)6(OH)2] (gives bone its strength and the ability to resist compression, allowing it to perform its functions of support andprotection)

Osteon structure components:

lamellae central canal lacunae canaliculi

bone cell types:

osteoblasts, osteocytes, osteoclasts

osteon

resembles a forest of small, tightly packed trees Each "tree" is a unit called an osteon

osteoid

unmineralized bone matrix The organic matrix of bone is known as osteoid (Osteoid consists of protein fibers, proteoglycans, glycosaminoglycans, glycoproteins, and bone-specific proteins)

Apply What You Learned: 1. The disease osteogenesis imperfecta is characterized by defective collagen in the organic matrix of bone. Predict the effects of such a disease. 2. What would probably happen to bone with more osteoclast than osteoblast activity? 3. How does the structure of compact bone follow its function?

1. The disease osteogenesis imperfecta is characterized by defective collagen in the organic matrix of bone. Predict the effects of such a disease. 1. Bones affected by osteogenesis imperfecta are extremely brittle, as they lack the collagen fibers to give them tensile strength. This makes them much more likely to fracture when subjected to even normal physical forces. 2. What would probably happen to bone with more osteoclast than osteoblast activity? 2. Excess osteoclast activity would mean that more ECM would be broken down than built, which would lead to progressive weakening of the bone. 3. How does the structure of compact bone follow its function? 3. The function of compact bone is to provide a hard outer shell that resists all types of forces and protects the inner spongy bone and marrow. The concentric rings of bone matrix that characterize compact bone make it extremely strong and hard and enable it to perform these functions.

Functions of osteoblasts and osteocytes

1. osteogenic cells differentiate into osteoblasts 2. osteoblasts deposit bone until they are trapped and become osteocytes 3. Osteocytes maintain the bone extracellular matrix (ECM)

Match the following terms with the correct definition. _____ Lacunae _____ Lamellae _____ Central canal _____ Canaliculi _____ Perforating canal a. Tiny canals that connect lacunae b. Runs perpendicular to the osteons, contains blood vessels c. Cavities that house osteocytes d. Rings of bone matrix e. Runs down the center of the osteon, contains blood vessels

_____ Lacunae: c. Cavities that house osteocytes _____ Lamellae: d. Rings of bone matrix _____ Central canal: e. Runs down the center of the osteon, contains blood vessels _____ Canaliculi: a. Tiny canals that connect lacunae _____ Perforating canal: b. Runs perpendicular to the osteons, contains blood vessels a. Tiny canals that connect lacunae b. Runs perpendicular to the osteons, contains blood vessels c. Cavities that house osteocytes d. Rings of bone matrix e. Runs down the center of the osteon, contains blood vessels

What happens if a) remove organic matrix b) remove inorganic matrix from bones?

a) Bone without its organic matrix (collagen) is brittle and shatters easily. b) Bone without its inorganic matrix (minerals) cannot resist compression.