Chapter 6: Bone Tissue
Bone Tissue: Comprehensive Study Notes
Introduction and Functions of the Skeletal System
The skeletal system performs six crucial functions:
Support: It serves as a structural framework for the body and provides attachment points for tendons and ligaments, which are essential for muscle connectivity.
Protection: It safeguards vital internal organs, such as the brain (within the skull) and organs within the chest cavity (rib cage).
Assistance in Body Movements: Working in conjunction with muscles, bones act as levers to facilitate movement.
Mineral Homeostasis (Storage and Release): Bones are the primary reservoir for essential mineral salts like calcium and phosphorus, releasing them into the bloodstream as needed to maintain mineral balance.
Blood Cell Production (Hematopoiesis): Red bone marrow, located within certain bones, is responsible for the production of red blood cells, white blood cells, and platelets.
Triglyceride Storage: Yellow bone marrow, found in the medullary cavity of adult long bones, consists mainly of adipose cells that store triglycerides, serving as a potential chemical energy reserve.
Bone as a Dynamic Tissue
Bone is an exceptionally dynamic tissue, constantly undergoing a process called remodeling, which involves a continuous cycle of building up (deposition) and breaking down (resorption) of bone tissue.
As a component of the larger musculoskeletal organ system, the skeletal system is composed of several different tissues.
The two primary tissues are bone (osseous tissue) and cartilage.
Tissues of the Skeletal System (General)
The skeletal system, beyond bone and cartilage, incorporates other vital tissues:
Epithelium (Endothelium): Forms the walls of capillaries, facilitating nutrient and waste exchange within bone tissue.
Nerves: Highly innervated, particularly in the periosteum, which explains its sensitivity to pain.
Red Marrow: Involved in hematopoiesis (blood cell production).
Yellow Marrow: Primarily for fat storage (triglycerides).
Compact vs. Spongy Bone
Bone is characterized as a highly vascularized connective tissue possessing a hard, mineralized extracellular matrix.
It exists in the body in two distinct structural arrangements:
Compact Bone:
Constitutes the majority of bone mass in many areas.
Provides robust protection and support.
Forms the diaphysis (shaft) of long bones.
Comprises the external layer of all bones.
Spongy Bone (Trabecular Bone):
Is a less organized tissue compared to compact bone, characterized by a network of bony spikes called trabeculae.
It is lightweight and provides tissue support.
Forms much of the epiphysis (ends) and the internal cavity of long bones.
Periosteum
The periosteum is a tough, dense, irregular connective tissue sheath that envelops the external surface of bone, excluding articular surfaces.
It contains osteoblasts, which are bone-building cells, crucial for bone growth in thickness (appositional growth) but not in length.
The periosteum plays a vital role in fracture repair by providing a source of osteoblasts.
It also serves as a crucial attachment point for tendons (connecting muscle to bone) and ligaments (connecting bone to bone).
Medullary Cavity and Endosteum
The medullary cavity is a significant space located within the diaphysis of long bones.
In adults, this cavity is primarily filled with fatty yellow bone marrow, which stores triglycerides.
The endosteum is a thin membrane that specifically lines the internal surfaces of the medullary cavity and the trabeculae of spongy bone.
It is composed of osteoclasts (bone-resorbing cells), osteoblasts (bone-building cells), and a layer of connective tissue.
Skeletal System Cells
Osseous tissue contains various specialized cells, originating from osteogenic (stem) cells derived from mesenchyme.
Osteogenic Cells: These are undifferentiated stem cells found in the periosteum, endosteum, and central canals of osteons. They divide to produce osteoblasts.
Osteoblasts: These are bone-building cells responsible for synthesizing and secreting collagen fibers and other organic components that constitute the bone's extracellular matrix. They initiate calcification.
Osteocytes: Mature osteoblasts that have become trapped within the calcified matrix. Their primary function is for the daily maintenance of bone tissue, including nutrient and waste exchange.
Osteoclasts: These are large, multinucleated cells (formed from the fusion of approximately 50 monocytes) responsible for bone breakdown (resorption). As specialized white blood cells, they migrate from the bone marrow to become 'fixed macrophages' within the bone substance, where they attach to the bone surface and release powerful lysosomal enzymes and acids to digest the organic matrix and dissolve the mineral salts.
Chemical Constituents of Bone
Bone tissue is precisely composed of:
25\% water
25\% organic proteins
50\% mineral salts (primarily hydroxyapatite crystals)
Organic Constituents:
Collagen fibers are the most abundant organic component, providing bone with its characteristic flexibility and impressive tensile strength (resistance to stretching or tearing).
Inorganic Hydroxyapatite Crystals (Mineral Salts):
These crystalline calcium phosphate salts are responsible for the bone's hardness and rigidity.
The main mineral salt is calcium phosphate ((Ca3PO4)_2).
Other significant mineral salts include calcium carbonate (CaCO_3 - similar to marble).
Trace elements such as magnesium, fluoride, and sulfate are also incorporated into the mineral matrix.
Cartilage
Cartilage is a poorly vascularized connective tissue, meaning it has a limited blood supply, which impacts its repair capabilities.
Its matrix is predominantly composed of chondroitin sulfate and various types of fibers.
The specific fiber types (e.g., collagen, elastic) determine the classification of cartilage into types such as hyaline cartilage, fibrocartilage, or elastic cartilage.
Hyaline Cartilage:
A common type, often found as articular cartilage.
Articular cartilage is a thin layer of hyaline cartilage that covers the epiphysis of long bones where they form an articular (joint) surface.
Its smooth, resilient surface reduces friction and absorbs shock where one bone moves against another bone within a joint.
The perichondrium is a dense irregular connective tissue membrane that surrounds the surface of most cartilage.
Chondrocytes are the mature cells responsible for forming and maintaining the cartilage matrix.
Many major bones in the body initially form from a cartilaginous model during embryonic development (endochondral ossification), while others develop directly from mesenchymal tissue without a cartilaginous stage (intramembranous ossification).