Skeletal Systems: Hydrostatic, Exo, and Endoskeletons

• Skeletal systems provide support, protection, and movement to organisms

Types of Skeletal Systems

• There are three main types of skeletal systems: hydrostatic skeletons, exoskeletons, and endoskeletons

Hydrostatic Skeletons

• Hydrostatic skeletons rely on fluid pressure for support
• Found mainly in soft-bodied invertebrates such as earthworms, nematodes, and jellyfish
• Muscles contract to change the shape of fluid-filled compartments, enabling movement
• The fluid within the compartment maintains a constant volume
• Longitudinal and circular muscles work antagonistically against the fluid to produce movement
• Contraction of circular muscles elongates the body
• Contraction of longitudinal muscles shortens the body
• Advantages include flexibility and the ability to squeeze into small spaces
• Disadvantages include limited protection and support compared to other skeletal types

Exoskeletons

• Exoskeletons are external, hard coverings that protect and support the body
• Found in arthropods such as insects, crustaceans, and arachnids
• Made of chitin, a polysaccharide material
• May be coated with calcium carbonate for added hardness in some species
• Provide physical protection from predators and environmental hazards
• Reduce water loss in terrestrial environments
• Exoskeletons do not grow, so they must be shed and replaced periodically through a process called molting or ecdysis
• During molting, the animal secretes a new, larger exoskeleton beneath the old one
• The old exoskeleton splits open, and the animal emerges
• The new exoskeleton is soft and vulnerable initially, hardening over time
• Arthropods are particularly vulnerable to predation during and immediately after molting
• Advantages: strong protection, support, and leverage for muscles
• Disadvantages: weight limits size, molting is risky, and restricts growth to intermittent stages

Endoskeletons

• Endoskeletons are internal support structures
• Found in vertebrates such as fish, amphibians, reptiles, birds, and mammals, as well as some invertebrates like echinoderms (e.g., starfish)
• Provide support and protection for internal organs
• Allow for greater flexibility and range of motion than exoskeletons
• Can grow continuously with the animal, unlike exoskeletons
• Vertebrate endoskeletons are composed of bone and cartilage
• Cartilage is a flexible connective tissue composed of cells called chondrocytes embedded in a matrix of collagen and other materials
• Bone is a rigid connective tissue composed of cells called osteocytes embedded in a mineralized matrix of calcium phosphate
• Bone provides strong support, protection, and mineral storage
• Bones are connected at joints, which allow for movement
• Ligaments are tough connective tissues that connect bones to each other at joints
• Tendons are connective tissues that connect muscles to bones, enabling movement

The Vertebrate Skeleton

• The vertebrate skeleton is divided into two main parts: the axial skeleton and the appendicular skeleton

Axial Skeleton

• The axial skeleton forms the central axis of the body
• Includes the skull, vertebral column, and rib cage
• The skull protects the brain and sensory organs
• The vertebral column (spine) protects the spinal cord and provides support for the body
• The rib cage protects the heart and lungs
• The vertebral column consists of individual vertebrae separated by intervertebral discs
• Intervertebral discs are made of cartilage and provide cushioning and flexibility to the spine

Appendicular Skeleton

• The appendicular skeleton includes the bones of the limbs and the girdles that attach them to the axial skeleton
• Pectoral girdle (shoulder girdle) connects the upper limbs to the axial skeleton
• Pelvic girdle (hip girdle) connects the lower limbs to the axial skeleton
• The bones of the limbs are adapted for different functions, such as walking, running, swimming, and flying
• The forelimbs of mammals include the humerus, radius, ulna, carpals, metacarpals, and phalanges
• The hindlimbs of mammals include the femur, tibia, fibula, tarsals, metatarsals, and phalanges

Bone Structure

• Bone is a complex tissue composed of both organic and inorganic components
• Organic components include cells (osteocytes, osteoblasts, osteoclasts) and collagen fibers
• Inorganic components include mineral salts, primarily calcium phosphate, which provide rigidity
• There are two main types of bone tissue: compact bone and spongy bone
• Compact bone is dense and strong, found in the outer layers of bones
• Spongy bone is less dense and contains many spaces, found in the interior of bones
• Spongy bone is filled with red bone marrow, which produces blood cells
• Bones are covered by a membrane called the periosteum, which contains blood vessels and nerves

Bone Growth and Development

• Bone growth occurs through two main processes: intramembranous ossification and endochondral ossification
• Intramembranous ossification occurs when bone develops directly from sheets of mesenchymal connective tissue
• Endochondral ossification occurs when bone develops from a cartilage template
• Most bones in the body develop through endochondral ossification
• Bone remodeling is a continuous process in which bone tissue is broken down and rebuilt
• Osteoclasts break down bone tissue, releasing minerals into the bloodstream
• Osteoblasts build new bone tissue, incorporating minerals from the bloodstream
• Bone remodeling is important for maintaining bone strength, repairing damage, and regulating calcium levels in the blood

Joints

• Joints are the connections between bones that allow for movement
• There are three main types of joints: fibrous joints, cartilaginous joints, and synovial joints
• Fibrous joints are immovable or only slightly movable
• Cartilaginous joints allow for limited movement
• Synovial joints allow for a wide range of motion
• Synovial joints are characterized by a joint cavity filled with synovial fluid, which lubricates the joint and reduces friction
• The ends of bones in synovial joints are covered with articular cartilage, which provides a smooth surface for movement
• Synovial joints are reinforced by ligaments, which stabilize the joint and prevent excessive movement

Muscle-Skeleton Interactions

• Skeletal muscles are attached to bones by tendons
• Muscles work in pairs to produce movement
• When one muscle contracts, its antagonist muscle relaxes
• Contraction of a muscle pulls on the bone to which it is attached, causing movement at the joint
• Levers are rigid structures (bones) that pivot around a fixed point (joints)
• Muscles provide the force to move levers, allowing for efficient movement
• The musculoskeletal system is a complex and integrated system that allows for movement, support, and protection

Overview of the three main types of skeletal systems: hydrostatic skeletons, exoskeletons, and endoskeletons. Hydrostatic skeletons rely on fluid pressure for support and are found in soft-bodied invertebrates. Exoskeletons are external, hard coverings that protect and support the body.