Skeletal System: Functions and Cartilage

• The skeletal system includes bones, joints, and connective tissues.

Skeletal System Functions

• Provides support and a framework for the body, such as legs supporting the trunk.
• Protects from compression, tension, and friction, exemplified by the skull protecting the brain.
• Allows movement by providing attachment points for skeletal muscles and forming joints.
• Stores fat, calcium ions, and phosphate ions.
• Makes blood in bone marrow.

Major Problems for the Skeletal System

• Movement causes friction and reduces stability.
• Providing support and protection requires resistance to compression, tension, and friction.
• Structures needed for one function can interfere with others; a stable synovial joint may not be mobile, and vice versa.

About Cartilage

• Cartilage is a thin connective tissue made by chondroblasts and maintained by chondrocytes.
• The extracellular matrix includes a firm ground substance to resist compression and collagen 1 and collagen 2 fibers to resist tension.

Types of Cartilage

• Hyaline cartilage mainly contains collagen 2 fibers, has more ground substance, and is water-rich, making it compression-resistant; examples include costal and articular cartilage.
• Fibrocartilage contains collagen 1 fibers packed in layers to resist compression; an example is the menisci of the knee.
• Elastic cartilage contains elastic fibers in a web and collagen 2 fibers; an example is the epiglottis.

Perichondrium

• Covers most cartilage and is made of DICT to resist tension, also resisting compression when cartilage is compressed.
• Blood vessels and nerves run through the perichondrium, but not into the cartilage itself.
• Ligaments and tendons fuse with the perichondrium
• Cartilage is avascular and lacks nerves.

About Bone

• Made by osteoblasts, maintained by osteocytes, and broken down by osteoclasts.
• Bone cells reside in tiny spaces within the extracellular matrix, connected by tiny canals.
• Bone cells extend their membranes into canals, forming gap junctions with other cells.
• Gap junctions facilitate direct and rapid communication between cells.
• Ions and molecules travel through gap junctions quickly.

Extracellular Matrix

• Contains many collagen 1 fibers.
• The ground substance is hard due to mineral crystals like calcium and phosphate, which resist compression.

Types of Bone

• Compact bone is on the bone's surface.
• Osteons, shaped like poles, make up compact bone and resist compression.
• Osteons consist of layered tissue with collagen fibers running in different directions to resist tension in any direction.
• The central canal connects to tiny canals.
• Spongy bone is in the bone's interior.
• Bone is arranged in webs ("trabeculae") with bone marrow.
• Spongy bone resists compression by distributing compressive forces throughout the web.

Periosteum

• Covers most of the bone and is made of DICT to resist tension.
• Blood vessels and nerves run through the periosteum and into the bone.
• Ligaments and tendons fuse with the periosteum.

Cardiovascular System Integration

• Bone is richly vascularized.
• Blood is made in bone marrow and it supplies oxygen and nutrients to cells in spongy bone and cells in compact bone through the central canals.

Nervous System Integration

• Nerves run through the central canals of osteons.
• Sensory receptors in joints and bones gather data about pain, pressure, body movement, and body position.

Bone Remodeling

• Bone remodeling is the continuous process of adding and subtracting bone tissue, influenced by mechanical stress and hormones.
• Osteocytes sense pressure and tell osteoblasts to become more active in stressed areas, secreting fibers and ground substance.
• Osteocytes signal osteoclasts to become more active in areas with less stress, secreting HCl and enzymes.
• Osteoclast plasma membranes are wavy, which increases the membrane's surface area.
• Hormones influence remodeling.
• Low calcium levels trigger the release of hormones which increases osteoclast activity.
• High calcium levels reduce hormone release, making osteoclasts less active and osteoblasts more active, storing calcium in the bone.

About Joints

• Joints attach bones, cartilage, or teeth.
• The primary function of joints is to provide stability to the skeleton, limiting movement.
• Fibrous joints are joined by collagen fibers, such as sutures and gomphoses.
• Cartilaginous joints are joined by cartilage, such as symphyses.
• Synovial joints stabilize the skeleton and allow movement.

Synovial Joint Components

• The articular capsule has two layers: a fibrous capsule (DICT) that grows into the periosteum and a synovial membrane (specialized CT) with blood vessels that makes synovial fluid.
• Joint cavity: the space between bones is filled with synovial fluid and lined by the synovial membrane
• Articular cartilage : hyaline cartilage covers the parts of bones that meet, has no perichondrium, and is supplied by synovial fluid so the surface is slipper and doesn't compress.
• Ligaments connect bone to bone across a joint, improving stability by resisting tension (DRCT).
• Bursae are flexible sacs of the synovial membrane filled with synovial fluid.
• Bursae provide lubrication between structures like ligaments and bones to reduce friction.
• Fibrocartilage and fat pads improve bone fit and provide cushioning by resisting compression; examples include knee menisci.

How the Skeletal System Solves Problems

• Compression is addressed with osteons, mineral crystals, trabeculae, fat pads, fibrocartilage pads, synovial fluid, perichondrium, and articular cartilage.
• Tension is addressed with osteons, periosteum, fibrous capsules, ligaments, and perichondrium.
• Friction in synovial joints is addressed with articular cartilage, synovial fluid, and bursae.