Tissues Living Communities Lecture Part 3 Study Notes
Overview of Connective Tissue
Definition and Function: Connective tissue is diverse, serving to support, bind, and protect other tissues and organs within the body. It also stores energy (fat), transports substances (blood), and provides structural integrity.
Extracellular Matrix (ECM): This non-living component is a distinguishing feature of connective tissue. It is composed of three main parts:
Ground substance: An amorphous, gel-like material (e.g., water, proteoglycans, glycoproteins) that fills the space between cells and fibers, allowing for nutrient and waste exchange.
Protein fibers: Provides strength and elasticity. These include collagen fibers (for tensile strength), elastic fibers (for stretch and recoil), and reticular fibers (for delicate support networks).
Cells: Various cell types, such as fibroblasts, adipocytes, macrophages, and mast cells, are embedded within the ECM.
Types of Connective Tissues
Tendons: Strong, fibrous connective tissue that connects muscle to bone.
Ligaments: Connective tissue that connects bone to bone, providing stability to joints.
Bone: Hard and rigid tissue forming the skeleton, providing support and protection.
Fat (Adipose tissue): Specializes in storing energy, insulating, and cushioning organs.
Cartilage: Flexible and resilient tissue found in joints, nose, ears, and trachea.
Blood: A fluid connective tissue involved in transport of gases, nutrients, and waste products.
Cartilage
Also Known As: Gristle (a less formal term).
Characteristics:
Possesses a firm yet flexible matrix, making it more rigid than dense connective tissue but more pliable than bone.
Unique for its avascular nature (lacks blood vessels) and aneural nature (lacks nerves), which means it receives nutrients via diffusion and is less prone to bleeding or direct pain sensation when injured.
Cellular Structure:
Primarily composed of chondrocytes, which are mature cartilage cells.
Chondrocytes reside in small, fluid-filled spaces within the matrix called lacunae. These cells are responsible for maintaining the cartilage matrix.
Matrix Components:
Composed of a rich ground substance (containing high concentrations of proteoglycans and hyaluronic acid), tissue fluid, and a network of collagen and/or elastic fibers that contribute to its specific properties.
Locations in the Body:
Found in articular surfaces of joints (e.g., knee, hip) to reduce friction, in the ear, nose, trachea and bronchi (for structural support), vocal cords, and importantly, as a framework for bone formation during embryonic development and in the epiphyseal plates (growth plates) of juvenile bones.
Nutrition:
Due to its avascular nature, cartilage receives its essential nutrients from the surrounding perichondrium, a dense irregular connective tissue sheath that is rich in small blood vessels. Nutrients diffuse from the perichondrium through the matrix to reach the chondrocytes.
Healing and Repair:
Its avascular nature significantly slows down healing and repair processes, as nutrients flow slowly via diffusion, making cartilage injuries challenging to recover from.
Types of Cartilage:
Hyaline Cartilage:
The most common and most rigid type of cartilage. It has a smooth, glassy appearance.
Composed primarily of tightly packed, fine collagen fibers within its matrix, giving it strong resistance to compression.
Found extensively in the embryonic skeleton, lining the trachea and bronchi, forming the nasal septum, present in the articular cartilage of synovial joints, in the growth plates of long bones (epiphyseal plates), and connecting the ribs to the sternum (costal cartilage).
Elastic Cartilage:
Characterized by the presence of a large number of elastic fibers in addition to collagen, providing exceptional flexibility and the ability to return to its original shape after deformation.
Found in structures requiring considerable flexibility, such as the epiglottis (which covers the trachea during swallowing) and the external ears (auricle).
Fibrocartilage:
A unique blend, often considered a transitional tissue, as it is merged with both hyaline cartilage and dense connective tissue. It contains thick, coarse bundles of collagen fibers arranged in parallel rows.
Distinctively lacks a perichondrium, which differentiates it from hyaline and elastic cartilage.
Highly specialized for withstanding strong compression forces and absorbing shock.
Found in locations subject to significant mechanical stress, including the intervertebral discs of the spinal column, the menisci of the knee joints, and the symphysis pubis (pelvic joints).
Bone
Definition: The hardest and most rigid form of connective tissue, forming the primary structure of the skeleton, providing support, protection for internal organs, and acting as a reservoir for minerals like calcium and phosphate, and housing bone marrow.
Matrix Composition:
Composed of both organic collagen fibers (which provide flexibility and tensile strength) and inorganic calcium salts (primarily calcium phosphate in the form of hydroxyapatite crystals, responsible for hardness and rigidity).
Vascularization:
Bone is exceptionally well-vascularized, meaning it has numerous blood vessels running throughout it, crucial for its metabolic activity, growth, repair, and mineral homeostasis.
Structure:
The fundamental structural unit of compact bone is the osteon or Haversian system. Each osteon consists of concentric rings of bone matrix (lamellae) surrounding a central Haversian canal, which contains blood vessels, lymphatic vessels, and nerves.
Bone cells include:
Osteoblasts: Bone-forming cells responsible for synthesizing and secreting the organic components of the matrix.
Osteoclasts: Large, multinucleated cells that resorb, or break down, bone tissue, playing a critical role in bone remodeling and calcium homeostasis.
Osteocytes: Mature bone cells that reside in lacunae within the matrix. They maintain the bone tissue and communicate with other bone cells.
Cellular Organization:
Mature osteocytes reside in small cavities called lacunae and communicate with each other and with blood vessels via tiny channels called canaliculi. This network allows for nutrient and waste exchange.
Bone undergoes constant remodeling throughout life in response to mechanical stress (e.g., Wolff's Law), hormonal signals, and calcium levels, ensuring its strength and adaptability.
Blood
Classification: Blood is uniquely classified as a fluid connective tissue, despite its liquid nature, because it originates from mesenchyme, contains cells suspended in an extracellular matrix, and performs binding and transport functions.
Matrix Composition:
The ground substance of blood is plasma, a yellowish fluid consisting mostly of water (92\%), with dissolved proteins (e.g., albumin, globulins, fibrinogen), electrolytes, hormones, nutrients, and waste products.
The fibrous components are soluble proteins, primarily fibrinogen, which polymerizes into insoluble fibrin strands during blood clotting, forming the structural framework of a clot.
Cell Types:
Includes formed elements:
Thrombocytes (platelets): Cell fragments essential for blood clotting and hemostasis.
Lymphocytes (a type of white blood cell): Key immune cells involved in specific immunity.
Eosinophils, Neutrophils, Basophils, Monocytes (other types of white blood cells): Involved in various immune responses, fighting infection and inflammation.
Erythrocytes (red blood cells), previously called reticulocytes in their immature form: Biconcave discs that lack a nucleus and are filled with hemoglobin, primarily responsible for oxygen and carbon dioxide transport.
Membranes
Mucous Membranes (Mucosa):
These membranes line all body cavities and hollow organs that have connections to the outside environment, such as the digestive, respiratory, urinary, and reproductive tracts.
Typically composed of a layer of epithelium (which can be stratified squamous in areas of abrasion like the mouth or simple columnar in areas of secretion/absorption like the stomach) overlying a layer of loose connective tissue called the lamina propria. A deeper layer of smooth muscle, the muscularis mucosae, may also be present.
The submucosa is a deeper, denser connective tissue layer that connects the mucosa to underlying structures.
Function:
Produce mucus, a viscous, slippery secretion that is a mixture of water, electrolytes, and the complex glycoprotein mucin (produced by specialized goblet cells within the epithelium).
Mucus serves multiple vital functions: lubrication, protection against pathogens, trapping foreign particles, and providing a moist surface.
Facilitate the absorption of substances (e.g., medications administered sublingually or nasally).
Crucial in clinical assessments; inspection of mucous membranes can reveal information about hydration status, inflammation, anemia, and capillary refill time (CRT).
Introduction to Muscle Tissue
General Definition: Composed of specialized elongated cells, called muscle fibers or myocytes, that are highly organized and designed for contraction, allowing for movement, posture maintenance, and heat production.
Key Proteins: The contractile properties of muscle tissue are due to the interaction of two primary proteins: actin (thin filaments) and myosin (thick filaments), which slide past each other during contraction (sliding filament theory).
Types of Muscle Tissue:
Skeletal Muscle:
Voluntary muscle (under conscious control) that appears striated (striped) due to the orderly arrangement of actin and myosin filaments.
Made of very large, elongated, cylindrical cells called muscle fibers that are multinucleated.
Primarily attached to bones via tendons, facilitating body movement. Collagen fibers surround individual muscle cells (endomysium), bundles of cells (perimysium), and the entire muscle (epimysium), merging to form the tendons that connect to bones.
Smooth Muscle:
Involuntary muscle (not under conscious control) and non-striated (lacks the striped appearance).
Composed of small, spindle-shaped cells with a single, centrally located nucleus.
Found in the walls of hollow internal organs (e.g., digestive tract, blood vessels, bladder, uterus), where it is responsible for slow, sustained contractions, such as peristalsis (rhythmic waves of contraction that propel substances through tubes like the digestive tract) and regulation of blood pressure.
Cardiac Muscle:
Found exclusively in the heart wall; it is striated like skeletal muscle but is involuntary.
Composed of branching cells that are typically uninucleated and interconnected by specialized junctions called intercalated discs, which contain gap junctions and desmosomes, allowing for rapid propagation of electrical signals and strong cell-to-cell adhesion.
Characteristically contains pacemaker cells that generate their own electrical impulses, regulating the rhythmic heartbeat without external nervous stimulation.
Nervous Tissue
Function: The primary tissue type responsible for integrating and coordinating body functions by rapidly receiving, processing, and transmitting electrical signals (nerve impulses or action potentials) throughout the body.
Locations: The main component of the brain, spinal cord (forming the central nervous system), and the peripheral nerves that extend to all parts of the body.
Cell Types: Comprises two main types of cells:
Neurons: The excitable cells that transmit electrical signals. Each neuron consists of:
The perikaryon (cell body): Contains the nucleus and most organelles.
Dendrites: Branching extensions that receive incoming signals from other neurons.
Axon: A single, long extension that transmits signals away from the cell body to other cells.
Neuroglia (glial cells): Non-excitable supporting cells that nourish, protect, insulate, and support neurons, playing crucial roles in maintaining the neural environment.
Closing Remarks
This lecture provides a foundation for understanding tissue organization. The next lecture will transition to the phases of healing and repair processes within these tissues.
Remember to prioritize self-care, including staying hydrated, for optimal learning and well-being. Farewell for now.