Skeletal System Overview

Chapter 7 Lecture Outline: Skeletal System Overview

• The skeletal system includes bones, cartilage, ligaments, and other connective tissues.
• Bones are the primary organs and create a rigid framework for the body, serving various functions.

Types of Bone Tissue

• Compact bone is dense or cortical bone and makes up 80% of bone mass.
• Spongy bone, also called cancellous or trabecular bone, is porous, located internally to compact bone, and accounts for 20% of bone mass.

Cartilage

• Cartilage serves as semirigid connective tissue, more flexible than bone.
• Hyaline cartilage attaches ribs to the sternum, covers bone ends, exists within growth plates, and is a model for bone formation.
• Fibrocartilage is weight-bearing and withstands compression, found in intervertebral discs, pubic symphysis, and menisci of the knee.
• Ligaments connect bone to bone.

General Functions of Bones

• Bones offer support and protection.
• Bones act as levers for movement.
• Bones facilitate hemopoiesis (blood cell production) in red bone marrow CT.
• Bones store minerals (calcium and phosphate) and energy reserves.

Classification of Bones

• Bone classification is based on shape, with four main classes: long, short, flat, and irregular.
• Long bones have a greater length than width, like the femur and humerus.
• Short bones have a length nearly equal to their width, exemplified by carpals and tarsals.
• Flat bones have flat, thin surfaces and may be slightly curved, such as cranial bones.

Gross Anatomy of Long Bones

• The diaphysis is an elongated, typically cylindrical shaft that provides leverage and weight support.
• The medullary cavity is a hollow space within the diaphysis that contains red bone marrow in children and yellow bone marrow in adults.
• The epiphysis is a knobby region at each end of long bones.
• The proximal epiphysis is the end of the bone closest to the body trunk.
• The distal epiphysis is the end farthest from the trunk.
• The epiphysis is composed of an outer thin layer of compact bone and an inner region of spongy bone.

Articular Cartilage of Bone

• Articular cartilage covers the joint surface.
• Articular cartilage consists of a thin layer of hyaline cartilage.
• Articular cartilage reduces friction and absorbs shock in moveable joints.

Metaphysis and Epiphyseal Plate

• The metaphysis is the region of mature bone between the diaphysis and epiphysis.
• The epiphyseal plate, located within the metaphysis, is a growth plate composed of a thin layer of hyaline cartilage.
• Epiphyseal plate provides for lengthwise bone growth.
• The epiphyseal line is the remnant of the epiphyseal plate in adults.

Periosteum of Bone

• The periosteum is a tough sheath that covers the outer surface of bone.
• The outer fibrous layer of dense irregular CT protects the bone from surrounding structures and anchors blood vessels and nerves.
• Periosteum serves as an attachment site for ligaments and tendons.
• The inner cellular layer includes osteoprogenitor cells, osteoblasts, and osteoclasts.
• The periosteum is attached to the bone by numerous collagen fibers called perforating fibers.

Endosteum of Bone

• The endosteum covers all internal surfaces of bone within the medullary cavity.
• The endosteum is an incomplete layer of cells that contains osteoprogenitor cells, osteoblasts, and osteoclasts.

Gross Anatomy of Other Bone Classes

• Short, flat, and irregular bones differ from long bones in their gross anatomy.
• An external surface composed of compact bone and an interior composed of spongy bone.
• Diploë refers to spongy bone in the flat bone of the skull.
• These bone classes lack a medullary cavity.

Blood Supply

• Bones are highly vascularized, especially in regions of spongy bone.
• Vessels enter from the periosteum.
• Nutrient foramen, a small opening or hole provides artery entrance and vein exit.
• Nerves accompany blood vessels through the foramen.
• Nerves innervate bone, periosteum, endosteum, and marrow cavity.

Bone Marrow Overview and Types

• Bone marrow is soft CT of bone.
• Red bone marrow and yellow bone marrow are two types of bone marrow.

Red Bone Marrow

• Red bone marrow, or myeloid tissue, is hemopoietic (blood cell forming).
• It consists of reticular CT, immature blood cells, and fat.
• In children, it is located in spongy bone and the medullary cavity of long bones.
• In adults, it's located only in selected areas of the axial skeleton: skull, vertebrae, ribs, sternum, ossa coxae, proximal epiphyses of humerus and femur.

Yellow Bone Marrow

• Yellow bone marrow is a product of red bone marrow degeneration as children mature.
• It is a fatty substance that may convert back to red bone marrow during severe anemia, facilitating production of additional erythrocytes.

Microscopic Anatomy of Bone Tissue

• Bone CT (osseous CT) is the primary component of bone composed of cells and extracellular matrix.
• Four types of cells are found in bone CT: osteoprogenitor cells, osteoblasts, osteocytes, and osteoclasts

Osteoprogenitor Cells

• Osteoprogenitor cells are stem cells derived from mesenchyme.
• Cellular division yields another stem cell and a "committed cell," which matures to become an osteoblast.
• Osteoprogenitor cells are located in the periosteum and endosteum.

Osteoblasts

• Osteoblasts form from osteoprogenitor stem cells.
• Osteoblasts synthesize and secrete osteoid, an initial semisolid organic form of bone matrix that later calcifies.
• Osteoblasts become entrapped within the matrix and differentiate into osteocytes.

Osteocytes of Bone

• Osteocytes are mature bone cells derived from osteoblasts.
• Osteocytes detect stress on bone and trigger new bone formation.

Osteoclasts

• Osteoclasts are large, multinuclear, phagocytic (cells that eat other cells) cells.
• Osteoclasts are derived from fused bone marrow cells and have a ruffled border that increases surface area exposed to bone.
• Osteoclasts are located within/adjacent to a depression/pit on the bone surface known as a resorption lacuna.
• Osteoclasts are involved in bone resorption.

Bone Matrix Components

• Bone matrix has organic and inorganic components.
• The organic components consist of osteoid produced by osteoblasts, which includes collagen protein and semisolid ground substance of proteoglycans and glycoproteins.
• It gives bone tensile strength by resisting stretching and contributes to bone flexibility.
• Inorganic components include salt crystals of calcium phosphate, Ca3(PO4)2, which interacts with calcium hydroxide to form hydroxyapatite, Ca10(PO4)6(OH)2
• Other substances incorporated into crystals include calcium carbonate, sodium, magnesium, sulfate, and fluoride.
• Crystals deposit around collagen fibers.
• The crystals harden the matrix and account for relative rigidity of bones.

Bone Formation and Resorption

• Bone formation begins with the secretion of osteoid.
• Calcification occurs through the deposition of hydroxyapatite crystals.
• It requires both Vitamin D to enhances calcium absorption and Vitamin C which is required for collagen formation.
• Bone matrix is destroyed by substances released from osteoclasts during bone resorption.
• Proteolytic enzymes released from lysosomes chemically digest organic matrix components.
• Hydrochloric acid dissolves calcium and phosphate.
• Freed calcium and phosphate ions enter the blood.

Compact Bone

• Compact bone is composed of small cylindrical structures called osteons (Haversian systems) which are its basic functional and structural units.
• Osteons are oriented parallel to the bone diaphysis.
• Osteons appear as a bull's-eye target.

Osteon Components

• The central canal is a cylindrical channel at the center of the osteon and parallel to it containing blood vessels and nerves.
• Concentric lamellae are rings of bone CT that surround the central canal comprised of collagen fibers.
• Collagen fibers that is in 90 degrees from previous and next lamellae
• Osteocytes are mature bone cells found in small spaces between concentric lamellae (lacunae).
• Canaliculi are tiny, interconnecting channels within bone CT that extend from each lacuna to connect to other lacunae and the central canal.
• Canaliculi house osteocyte projections that allow intercellular contact and allow exchange of nutrients, minerals, gases, and wastes.

Bone Structures

• Perforating canals (Volkmann canals) are perpendicular to central canals and connect them within different osteons.
• Circumferential lamellae in long bones, external rings of bone run immediately internal to periosteum, and internal rings of bone run internal to the endosteum. Both run the entire circumference of the bone. The components of compact bone between osteons or leftover parts of osteons that have been partially resorbed. Interstitial lamellae are components of compact bone between osteons or leftover parts of osteons that have been partially resorbed.

Spongy Bone

• Trabeculae are open lattices of narrow rods and plates of bones.
• Bone marrow fills spaces that create a meshwork of crisscrossing bars that provide resistance to stresses.
• Parallel lamellae that contains bone matrix an osteocytes between lamellae.
• Canaliculi radiate from lacunae.