WEEK 1: Histology of bone Flashcards
State the functions of bone.
- Provides support
- Protects vital organs
- Reservoir of calcium and other ions
Outline main cells of the bone tissue and their functions.
- Osteoprogenitor cells that differentiate into osteoblasts
- Osteoblasts are growing cells which synthesize and secrete the organic components of the matrix
- Osteocytes are found in cavities (lacunae) between bone matrix layers (lamellae), with cytoplasmic processes in small canaliculi (L. canalis, canal) that extend into the matrix
- Osteoclasts are giant, multinucleated cells involved in removing calcified bone matrix and remodeling bone tissue
Osteoblasts secrete the matrix that then hardens by calcification, trapping the differentiating cells now called osteocytes in individual lacunae.
Osteocytes maintain the calcified matrix and receive nutrients from microvasculature in the central canals of the osteons via very small channels called canaliculi that interconnect the lacunae.
Osteoclasts are monocyte-derived cells in bone required for bone remodeling.
State the origin of the following cell.
Osteogenic cells
Osteoblasts
Osteoclasts
Osteocytes
Osteogenic cells:
Form: Undifferentiated mesenchymal cells.
Osteoblasts:
Form: Mononucleated cells derived from osteogenic cells.
Osteoclasts:
Form: Large, multinucleated cells formed by the fusion of monocytes or macrophages within the bone marrow.
Osteocytes:
Form: Mature bone cells derived from osteoblasts.
Describe the process of Mineralization in bone matrix.
From their ends adjacent to the matrix, osteoblasts secrete type I collagen, several glycoproteins, and proteoglycans.
Some of these factors, notably osteocalcin and certain glycoproteins, bind Ca2+ with high affinity, thus raising the local concentration of these ions.
Osteoblasts also release very small membrane-enclosed matrix vesicles with which alkaline phosphatase and other enzymes are associated.
These enzymes hydrolyze PO4 ions from various
macromolecules, creating a high concentration of these ions locally.
The high ion concentrations cause crystals of
CaPO4 to form on the matrix vesicles.
The crystals grow and mineralize further with formation of small growing masses of hydroxyapatite [Ca10(PO4)6(OH)2] which surround the collagen fibers and all other macromolecules.
Eventually the masses of hydroxyapatite merge as a confluent solid bony matrix as calcification of the matrix is completed.
Are very large, motile cells with multiple.
nuclei that are essential for matrix resorption during bone growth and remodeling.
Name the cells.
Why are they large in size?
In areas of bone undergoing resorption,
They lie on the bone surface within enzymatically etched depressions or cavities in the matrix known ________________.
Osteoclasts
The large size and multinucleated condition of osteoclasts are due to their origin from the fusion of bone marrow-derived monocytes.
In areas of bone undergoing resorption, osteoclasts on the bone surface lie within enzymatically etched depressions or cavities in the matrix known as resorption lacunae (or Howship lacunae).
Describe the bone resorption process by osteoclasts.
In an active osteoclast, the membrane domain that
contacts the bone forms a circular sealing zone that binds the cell tightly to the bone matrix and surrounds an area with many surface projections, called the ruffled border.
This circumferential sealing zone allows the formation of a specialized microenvironment between the osteoclast and the matrix in which bone resorption occurs.
Into this subcellular pocket the osteoclast pumps protons.
The sealed space between the cell and the matrix is acidified to ~pH 4.5 by proton pumps in the ruffled part of the cell membrane and receives secreted matrix metalloproteases and other hydrolytic enzymes.
Acidification of the sealed space promotes dissolution of hydroxyapatite from bone and stimulates activity of the protein hydrolases, producing localized matrix resorption.
The breakdown products of collagen fibers
and other polypeptides are endocytosed by the
osteoclast and further degraded in lysosomes, while Ca2+ and other ions are released directly and taken up by the blood.
Define PERIOSTEUM AND ENDOSTEUM.
All bones are lined on their internal and external surfaces by layers of connective tissue containing osteogenic cells.
Endosteum on the internal surface surrounding the marrow cavity and periosteum on the external surface.
State the 2 main types of bone.
Woven bone: Immature bone; primary bone;
bundle bone.
Lamellar bone: Mature bone; secondary bone
Differentiate between the 2 main types of bone.
Woven bone
*Newly calcified
*Developing and growing bones; hard callus of bone fractures
*Irregular and random arrangement of cells and
collagen
*Lightly calcified
Lamellar bone
*Remodeled from woven bone
*All normal regions of adult bone
*Parallel bundles of collagen in thin layers (lamellae), with regularly spaced cells between.
*Heavily calcified
The lamellar bone is made up of 2 types. State them and their percentages.
Compact bone, ~80% of all lamellar bone
*Cortical bone
Cancellous bone, ~20% of all lamellar bone
*Spongy bone; trabecular bone; medullary bone
Differentiate between compact bone and cancellous bone.
COMPACT BONE
*Thick, outer region (beneath periosteum) of bones
*Parallel lamellae or densely packed osteons, with
interstitial lamellae
*The compact bone is normally covered externally with periosteum and all trabecular surfaces of the cancellous bone are covered with endosteum.
*Compact bone forms the outer layer of bones and provides strength and support.
CANCELLOUS BONE
*Inner region of bones, adjacent to marrow Cavities
*Interconnected thin spicules or trabeculae covered by endosteum
*The small trabeculae that make up highly porous
cancellous bone serves as supportive struts, collectively providing considerable strength, without greatly increasing the bone’s weight.
*A higher proportion of osteocytes than mature
lamellar bone.
*Immature woven bone forms more quickly but has less strength than lamellar bone
What is an osteon?
An osteon, also known as a Haversian system, is the basic structural and functional unit of compact bone tissue.
Describe the structure oof an osteon.
The osteon is a cylindrical structure that consists of concentric rings of bone tissue called lamellae. These lamellae surround a central canal known as the Haversian canal. The Haversian canal contains blood vessels, nerves, and lymphatic vessels that nourish and innervate the bone.
- Concentric lamella:
*Lamellae are concentric rings or layers of mineralized matrix (bone tissue) that surround the central Haversian canal.
*They provide structural support and rigidity to the bone, contributing to its strength. The lamellar arrangement helps distribute mechanical stresses evenly throughout the bone. - Haversian Canal or Central canal:
Function:
*It contains blood vessels, nerves, and lymphatic vessels. The primary function of the Haversian canal is to provide a passageway for the exchange of nutrients, oxygen, and waste products between the bone cells (osteocytes) and the surrounding tissues.
*It ensures that bone cells receive the necessary resources for their metabolic activities. - Osteocytes:
*Osteocytes are mature bone cells embedded within small spaces called lacunae, found between the lamellae.
*Osteocytes maintain the bone tissue and play a crucial role in sensing mechanical stresses.
*They also contribute to bone remodeling by signaling for the activation of osteoclasts (bone-resorbing cells) or osteoblasts (bone-forming cells) in response to changes in the mechanical environment. - Canaliculi:
*Canaliculi are tiny channels or canals that connect adjacent lacunae and allow communication between osteocytes.
*These channels facilitate the exchange of nutrients and signaling molecules, enabling osteocytes to communicate with each other and with the central Haversian canal.
* Canaliculi form a network that helps coordinate the activities of osteocytes within the osteon. - ) Transverse perforating (Volkmann) canals (P) connecting adjacent osteons of compact lamellar bone.
*Such canals “perforate” lamellae and provide another source of microvasculature for the central canals of osteons. - Circumferential lamellae: Prevent twisting of bone. Circumferential lamellae contribute to the overall strength and rigidity of compact bone. By encircling the entire bone, they provide additional support and reinforcement, helping to resist bending and torsional forces.
- Interstitial lamellae: Found in spaces between osteons. It connects adjacent osteons and integrate the overall structure of compact bone. They play a role in maintaining the cohesion and continuity of the bone tissue.
Differentiate between intramembranous and endochondral ossification.
OUTLINE BONES FORMED VIA EACH PROCESS.
Intramembranous Ossification: It involves mesenchymal (undifferentiated embryonic connective tissue) condensation directly transforming into bone tissue without a cartilage intermediate.
Location: Intramembranous ossification occurs in flat bones of the skull, facial bones, and clavicles.
Endochondral Ossification: It involves the initial formation of a hyaline cartilage model, which is later replaced by bone tissue.
Location: Endochondral ossification occurs in long bones, such as the femur and humerus, and in the development of most of the skeleton.
Describe the process of endochondral ossification.
- Formation of Hyaline Cartilage Model:
The process begins with the formation of a hyaline cartilage model, which represents the future shape of the bone.
Mesenchymal cells differentiate into chondroblasts, which secrete the cartilage matrix. - Cartilage Model Enlargement:
Chondrocytes (cartilage cells) in the center of the cartilage model undergo hypertrophy (enlarge).
This hypertrophy leads to an increase in the size of the cartilage model.
- Calcification of Cartilage Matrix:
The cartilage matrix around the hypertrophic chondrocytes begins to calcify (mineralization).
Calcium salts are deposited in the cartilage matrix, making it more rigid. - Blood Vessel Invasion:
Blood vessels and capillaries invade the calcified cartilage, bringing with them osteoprogenitor cells (precursors to osteoblasts) and other cells, including osteoclasts. - Formation of Primary Ossification Center:
Osteoprogenitor cells differentiate into osteoblasts, which start depositing bone matrix (osteoid) on the calcified cartilage.
The primary ossification center is established in the diaphysis (shaft) of the bone.
- Development of Trabeculae:
Trabeculae, or spongy bone, is formed as osteoblasts deposit bone matrix.
Osteoclasts may also be involved in the remodeling of bone tissue during this process.
- Secondary Ossification Centers (In Long Bones):
In long bones, such as the femur and humerus, secondary ossification centers form in the epiphyses (ends) of the bone.
Similar processes of cartilage hypertrophy, calcification, and invasion of blood vessels occur in the epiphyses.
- Formation of Articular Cartilage and Epiphyseal Plate:
The cartilage at the ends of the bone persists as articular cartilage, allowing for smooth joint movement.
The region between the diaphysis and epiphysis, known as the epiphyseal plate (or growth plate), contains cartilage that continues to divide and contribute to bone lengthening during growth.
- Completion of Ossification:
Over time, the bone replaces most of the cartilage model, except for the articular cartilage at the joints and the epiphyseal plates in growing bones. - Epiphyseal Plate Closure:
During adolescence, the epiphyseal plates gradually close as cartilage cells stop dividing and are replaced by bone tissue.
After closure, the bone reaches its final adult length.
