37 Questions
What is the primary function of bone in the body?
To support the body and protect internal organs
Which type of collagen is an important component of osteoid?
Type I collagen
What is the role of osteocalcin in bone development?
To bind Ca2+ ions
How do osteocytes communicate with adjacent cells?
Through a network of long dendritic processes
What is the role of osteoclasts in bone development?
To locally erode bone matrix
What is the result of high concentrations of Ca2+ and PO4- ions in the bone matrix?
Formation of hydroxyapatite crystals
What is the primary function of the periosteum?
To mediate bone growth and remodeling
What type of fibers bind the periosteum to the bone matrix?
Type I collagen fibers
Which cells are present in regions of periosteum adjacent to bone?
Osteoprogenitor cells and osteoblasts
What is the endosteum composed of?
A thin layer of active and inactive osteoblasts
Where are osteoblasts required for bone growth?
In both the periosteum and endosteum
What is the purpose of the canaliculi in the osteon?
To allow all cells to communicate with the central canal
What type of bone tissue is characterized by small bony trabeculae or spicules?
Cancellous bone
What is the function of osteoclasts and osteoblasts in bone remodeling?
To remodeled woven bone into lamellar bone
What is the organization of lamellae in compact bone?
Concentrically around small central canals
What is the term for the small, knobby, bulbous ends of long bones?
Epiphyses
What is the term for the immature bone tissue formed during osteogenesis or repair?
Woven bone
Which type of ossification is responsible for the formation of bones in the skull and jaws?
Intramembranous ossification
What is the initial cavity in the diaphyses of fetal long bones formed from?
Enclosure of the cartilage within a collar of woven bone
What is the role of periosteal osteoblasts and vasculature in primary ossification centers?
To enter the initial cavity and promote bone growth
What is the location of the cartilage in the epiphyseal growth plate?
Between the primary and secondary ossification centers
What is the characteristic of bone formation in all bones except those of the skull and jaws?
Endochondral ossification
What is the function of the zone of proliferation in the growth plate?
Chondrocytes undergo mitosis
What occurs in the zone of hypertrophy?
The most mature chondrocytes swell up and undergo apoptosis
What is the result of the events in the zone of cartilage calcification?
Spaces are created in the matrix that are invaded by osteoblasts and vasculature
What is the role of osteoblasts in the zone of ossification?
Lay down woven bone
What is the result of appositional bone growth?
The bone circumference increases
What is the main function of osteoclasts and osteoblasts in bone remodeling?
To change the size and shape of bones according to mechanical stress
What is the initial stage of bone repair after fracture or other injury?
Activation of periosteal fibroblasts to produce a soft callus of fibrocartilage-like tissue
What is the purpose of bone remodeling?
To change the size and shape of bones according to mechanical stress
What is the final stage of bone repair after fracture or other injury?
Remodeling of lamellar bone to produce stronger bone
What is the function of periosteal fibroblasts in bone repair?
To produce an initial soft callus of fibrocartilage-like tissue
What is the primary function of calcium in the body?
To maintain proper blood calcium levels
Which hormone stimulates osteoclasts to elevate calcium levels in blood?
Parathyroid hormone (PTH)
What is the result of inadequate dietary calcium intake?
Mobilization of calcium from bone
Which cells are involved in maintaining proper blood calcium levels?
All three major bone cells
What is the effect of calcitonin on osteoclast activity?
Inhibition of osteoclast activity
Study Notes
Bone Structure and Function
- Bone is a type of connective tissue that provides support to the body, protects internal organs, and acts as the body's calcium reservoir.
Cells and Matrix Components of Bone
- Osteoblasts differentiate from osteoprogenitor cells and secrete osteoid, which allows matrix mineralization to occur.
- Osteoid components include type I collagen, osteocalcin, and matrix vesicles with enzymes generating PO4-.
- High concentrations of Ca2+ and PO4- ions cause formation of hydroxyapatite crystals, which gradually calcify the entire matrix.
- Osteocytes differentiate from osteoblasts and become enclosed within matrix lacunae, maintaining the matrix and detecting mechanical stresses on bone.
- Osteocytes communicate with adjacent cells via a network of long dendritic processes that extend through the matrix via narrow canaliculi radiating from each lacuna.
- Osteoclasts are formed by fusion of several blood monocytes and locally erode bone matrix during osteogenesis and bone remodeling.
Periosteum
- A layer of dense connective tissue on the outer surface of bone
- Bound to bone matrix by bundles of type I collagen called perforating (or Sharpey) fibers
- Regions adjacent to bone are rich in osteoprogenitor cells and osteoblasts
- Mediates much bone growth and remodeling through osteoprogenitor cells and osteoblasts
Endosteum
- A thin layer of active and inactive osteoblasts
- Lines all internal surfaces within bone
- Osteoblasts in endosteum are required for bone growth
Types of Bone
- Dense bone beneath the periosteum is called compact bone
- Deep to compact bone are small bony trabeculae or spicules of cancellous (or spongy) bone
Bone Structure
- Long bones of the limbs have both compact and cancellous bone in epiphyses (knobby, bulbous ends) and diaphysis (intervening shaft)
Immature Bone
- Woven bone is formed during osteogenesis or repair and has a calcified matrix with randomly arranged collagen fibers
Bone Remodeling
- Osteoclasts and osteoblasts remodel woven bone into lamellar bone with new matrix deposited in distinct layers with parallel collagen bundles
Lamellar Bone
- Compact and cancellous bone are both types of lamellar bone
- Most lamellar bone consists of lamellae organized concentrically around small central canals containing blood vessels and nerves, called an osteon or Haversian system
- Osteocytic lacunae occur between the lamellae, with canaliculi radiating through the lamellae, allowing all cells to communicate with the central canal
Osteogenesis
- Bones of the skull and jaws develop through intramembranous ossification, where osteoblasts differentiate directly from mesenchyme progenitor cells.
- In contrast, all other bones form through endochondral ossification, involving osteoprogenitor cells surrounding and invading hyaline cartilage models in the embryo.
- Primary ossification centers in fetal long bones develop when chondrocytes die after being enclosed by a collar of woven bone, creating an initial cavity.
- Periosteal osteoblasts and vasculature then enter this cavity, marking the beginning of primary ossification.
- Secondary ossification centers develop later, within the epiphyses, with the cartilage of the epiphyseal growth plate situated between the primary and secondary ossification sites.
Bone Elongation During Childhood
- Growth plates consist of an interrelated series of developing zones that enable bone elongation during childhood
Zones of Growth Plate Development
- Reserve Zone: most distal zone, composed of typical hyaline cartilage that serves as a "resting" zone
- Proliferation Zone: adjacent to reserve zone, chondrocytes undergo mitosis and appear stacked within elongated lacunae
- Hypertrophy Zone: most mature chondrocytes swell up, compress the matrix, and undergo apoptosis, located closer to the primary ossification center
- Zone of Cartilage Calcification: spaces created in the matrix by apoptosis are invaded by osteoblasts, osteoclasts, and vasculature from the primary center
- Zone of Ossification: woven bone is initially laid down by osteoblasts and remodeled into lamellae bone
Bone Growth and Development
- Appositional bone growth increases the circumference of a bone through osteoblast activity at the periosteum
- Enlargement of the medullary marrow cavity accompanies appositional bone growth
Bone Growth and Remodeling
- Bone growth occurs throughout life, with continuous turnover of cells and matrix through the activities of osteoblasts and osteoclasts.
- Lamellae and osteons are temporary structures that are constantly replaced and rebuilt in a process of bone remodeling.
- Bone remodeling allows bones to change size and shape according to changes in mechanical stress.
Bone Repair
- Bone repair after fracture or injury involves the activation of periosteal fibroblasts to produce an initial soft callus of fibrocartilage-like tissue.
- The soft callus is gradually replaced by a hard callus of woven bone.
- The hard callus is soon remodeled to produce stronger lamellar bone.
Calcium Homeostasis
- Calcium (Ca2+) is a crucial ion for all cells, and its levels are maintained through bone storage and mobilization.
- When dietary calcium is sufficient, Ca2+ is stored in bone.
- When dietary calcium is deficient, Ca2+ is mobilized from bone.
Bone Cell Activity
- Maintenance of proper blood calcium levels involves the activity of osteoblasts, osteoclasts, and osteocytes.
- Subtle paracrine interactions among bone cells and other cells regulate this process.
Hormonal Regulation
- Parathyroid hormone (PTH) indirectly stimulates osteoclasts, increasing calcium levels in the blood.
- Calcitonin inhibits osteoclast activity, decreasing blood calcium levels.
Learn about bone as a type of connective tissue, its components, and its role in the body. Explore osteoblasts, osteoid, and matrix mineralization.
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