Bone 1 Flashcards
What are bones?
- specialized connective tissue composed of cells and calcified extracellular matrix (ECM)
- The cells and calcified ECM form a rigid organ
What is the function of bones?
- Hematopoiesis= the formation of blood cells from hematopoietic stem cells found in the bone marrow
- Lipid & mineral storage= bone is a reservoir holding adipose tissue within the bone marrow and calcium within the hydroxyapatite crystals
- Support= bone forms scaffolding for skeletal muscle and other soft tissues, it creates a framework that gives the general shape of the body
- Protection= particularly the axial skeleton which surrounds the major organs of the body
What are components of bone?
- Extracellular matrix
2. Cells of bone tissues
Describe the extracellular matrix bone
Refers to the molecules that provide biochemical and structural support to the cells that populate bone
Describe the organic portion of ECM in bone
-Organic portion is predominantly (90% of total weight of matrix proteins) composed of collagen one proteins
-Mainly type 1 collagen (also type V, III, XI, XIII)
-Non-collagenous proteins (about 10%) constitute the ground substance and include proteoglycans, glycoproteins, bone specific proteins, growth factors and cytokines
Essential to bone development, growth, remodel8ng, and repair
Both collagen and ground substance become mineralized to form bone tissue
Describe the inorganic portion of ECM in bone
Represents about 50% of the dry weight of the matrix and is composed mainly of calcium phosphate in the form of hydroxyapatite crystals
-Contains trace minerals such as bicarbonate, citrate, magnesium, potassium and sodium
Describe the cells that are components of bone
-Osteoprogenitor(osteogenetic): the stem cells of bone, they are derived from primitive mesenchymal cells
Forms a population of stem cells that can differentiate into osteoblasts which secrete the organic unmineralized matrix or osteoid
- Osteoblasts: synthesize uncalcified/unmineralized extracellular matrix called osteoid. This will later become calcified/mineralized to form bone.
- Osteocytes: As the osteoid mineralizes, the osteoblasts become entombed between lamellae in lacunae where they mature into osteocytes. They then monitor the minerals and proteins to regulate/maintain bone mass
-Osteoclasts: Multinucleated giant cells derived from monocyte progenitor cells responsible for resorbing and remodeling bone by releasing H+ ions and lysosomalenzymes
The balance of osteoblast to osteoclast activity is crucial in the maintenance of the tissue’s structural integrity. It also plays a role in conditions such as osteoporosis
Bone-lining cells: found on the inner and outer surface of bone; the endosteum & periosteum
What is the function of osteoclasts?
Functions in resorption, the breakdown of bone matrix
Describe osteocytes formation
Osteoggenic cell develops into an osteoblast
Osteoblast forms bone matrix and develops into osteocytes
Osteocyte maintains bone tissue
What are osteoblasts?
Cuboidal cells located on the periphery of bone. Displays euchromatic nuclei with a basophilic cytoplasm
What are osteocytes?
Mature osteocytes: mature osteoblasts entrapped in bones matrix
What are osteoclasts?
Large, multinucleated macrophages located along the endosteum
What are the shape and location of osteogenic cells?
- Are spindle shaped connective tissue cells (derived from embryonic mesenchyme stem cells) in the periosteum and endosteum that can differentiate into osteoblasts
- In mature bone in which there is no active new bone formation or remodeling, the osteoprogenitor cells become flattened spindle shaped cells closely applied to the bone surface, when they are sometimes called “inactive osteoblasts”
What are osteoblasts derived from?
Osteoprogenator cells; are basophilic
What are the shapes of osteoblasts?
Morphologically May appear as a cuboidal layer(high secreting activity) or flattened (declining secretions)
Cytoplasmic processes contact them with other osteoblasts and osteocytes
Describe the significant chemical composition and secretions of osteoblasts
- During synthesis osteoblasts have well developed rER and Golgi apparatus
- PAS positive granules in the cytoplasm are most likely the precursors of glycosaminoglycans (of the osteoid matrix)
- Eventually matrix secretion entraps the osteoblast in a lacunae with it’s cytoplasmic processes extending into canaliculi
- Can secrete osteoclast stimulating factor
- Osteoclast eventually ceases its secretory function, undergoes changes to its morphology and becomes an osteocyte
Explain the structure-function of osteocytes
- Mature bone cells, derived from osteoblasts
- Housed in their own lacunae, maintain communication and exchange nutrients and metabolites via gap junctions between their narrow cytoplasmic processes (filopodia) extending through the canaculi
- Increased amounts of condensed nuclear chromatin, reduced amounts of rER, and a smaller Golgi apparatus vs osteoblasts