💀Anatomy and Physiology I Unit 4 – The Tissue Level of Organization

Key Concepts and Definitions

• Tissues consist of cells and extracellular matrix that work together to perform specific functions
• Histology studies the microscopic anatomy of cells and tissues using a microscope
• Extracellular matrix (ECM) includes the fluid, ground substance, and fibers surrounding the cells
  • Ground substance consists of proteoglycans, multiadhesive glycoproteins, and other molecules
  • Fibers include collagen, elastic, and reticular fibers
• Cell junctions connect cells to each other or to the ECM
  • Anchoring junctions (desmosomes, hemidesmosomes) provide mechanical attachment
  • Occluding junctions (tight junctions) seal adjacent cells together
  • Communicating junctions (gap junctions) allow chemical and electrical signals to pass between cells
• Tissue membranes cover body surfaces, line body cavities, and form protective sheets around organs
  • Cutaneous membrane (skin) covers the exterior body surface
  • Mucous membranes line cavities open to the exterior (gastrointestinal, respiratory, urinary, and reproductive tracts)
  • Serous membranes line body cavities closed to the exterior (peritoneal, pleural, and pericardial cavities)

Types of Tissues

• Epithelial tissue covers body surfaces, lines cavities and ducts, and forms glands
  • Characteristics include polarity, specialized contacts, avascularity, and regenerative capacity
• Connective tissue binds, supports, and protects organs and tissues
  • Consists of a ground substance, fibers, and various cells (fibroblasts, macrophages, mast cells, and others)
  • Subtypes include loose connective tissue, dense connective tissue, cartilage, bone, blood, and adipose tissue
• Muscle tissue generates force and movement through contraction
  • Includes skeletal muscle, cardiac muscle, and smooth muscle
• Nervous tissue receives stimuli and conducts electrical impulses to coordinate body functions
  • Composed of neurons and glial cells (astrocytes, oligodendrocytes, microglia, and ependymal cells)

Epithelial Tissue

• Epithelial tissue consists of closely packed cells with little intercellular matrix
• Classification based on the number of cell layers and cell shape
  • Simple epithelium has a single layer of cells (simple squamous, simple cuboidal, simple columnar)
  • Stratified epithelium has multiple layers of cells (stratified squamous, stratified cuboidal, stratified columnar, transitional)
• Specialized structures include microvilli, cilia, and stereocilia
  • Microvilli increase surface area for absorption (intestinal lining)
  • Cilia move substances over the cell surface (respiratory tract)
  • Stereocilia are long, non-motile projections (epididymis)
• Glandular epithelium secretes substances into ducts or directly into the bloodstream
  • Exocrine glands secrete into ducts (sweat glands, salivary glands)
  • Endocrine glands secrete into the bloodstream (thyroid gland, adrenal glands)

Connective Tissue

• Connective tissue proper includes loose and dense connective tissues
  • Loose connective tissue has a loose arrangement of fibers (areolar, adipose, reticular)
  • Dense connective tissue has a high proportion of collagen fibers (dense regular, dense irregular, elastic)
• Specialized connective tissues include cartilage, bone, blood, and adipose tissue
  • Cartilage provides support and flexibility (hyaline, elastic, fibrocartilage)
  • Bone provides structural support and protection (compact bone, spongy bone)
  • Blood transports gases, nutrients, and waste products (plasma, erythrocytes, leukocytes, platelets)
  • Adipose tissue stores energy and provides insulation (white adipose tissue, brown adipose tissue)
• Connective tissue fibers include collagen, elastic, and reticular fibers
  • Collagen fibers provide tensile strength and resistance to stretching (tendons, ligaments)
  • Elastic fibers allow tissues to stretch and recoil (skin, blood vessels)
  • Reticular fibers form a supportive framework (lymph nodes, spleen)

Muscle Tissue

• Skeletal muscle attaches to bones and is responsible for voluntary movements
  • Consists of long, cylindrical, multinucleated cells called muscle fibers
  • Exhibits cross-striations and voluntary control
• Cardiac muscle is found in the heart and pumps blood through the circulatory system
  • Consists of branched, striated cells with a single nucleus
  • Exhibits intercalated discs and autorhythmicity
• Smooth muscle is found in the walls of hollow organs and blood vessels
  • Consists of spindle-shaped cells with a single nucleus
  • Lacks striations and is involuntarily controlled
• Muscle contraction is triggered by the release of calcium ions and the sliding of myosin and actin filaments
  • Sarcomeres are the basic functional units of muscle fibers
  • The sarcoplasmic reticulum stores and releases calcium ions during contraction and relaxation

Nervous Tissue

• Nervous tissue is specialized for the conduction of electrical impulses
• Neurons are the functional units of the nervous system
  • Consist of a cell body, dendrites, and an axon
  • Classified as sensory neurons, motor neurons, or interneurons
• Neuroglia (glial cells) support and protect neurons
  • Astrocytes provide structural support and regulate the extracellular environment
  • Oligodendrocytes form myelin sheaths around axons in the central nervous system
  • Schwann cells form myelin sheaths around axons in the peripheral nervous system
  • Microglia are the immune cells of the nervous system
• Synapses are the junctions between neurons where neurotransmitters are released
  • Chemical synapses use neurotransmitters to transmit signals
  • Electrical synapses allow direct ion flow between cells through gap junctions

Tissue Repair and Regeneration

• Tissue repair involves the replacement of damaged or lost cells and extracellular matrix
• The inflammatory response is the initial stage of tissue repair
  • Characterized by redness, swelling, heat, pain, and loss of function
  • Involves the recruitment of immune cells to the site of injury
• Regeneration is the replacement of damaged tissue with new cells of the same type
  • Occurs in tissues with high mitotic activity (epithelial tissue, bone marrow)
• Fibrosis is the formation of scar tissue in response to injury
  • Occurs in tissues with limited regenerative capacity (cardiac muscle, nervous tissue)
• Factors affecting tissue repair include age, nutritional status, and the presence of infection
  • Older individuals have a slower healing process due to reduced cellular activity
  • Adequate nutrition is essential for the synthesis of new cells and extracellular matrix components

Clinical Applications

• Tissue engineering involves the development of biological substitutes to restore, maintain, or improve tissue function
  • Utilizes scaffolds, cells, and growth factors to create functional tissues
  • Applications include skin grafts, cartilage repair, and organ regeneration
• Stem cell therapy uses undifferentiated cells to replace damaged or diseased tissues
  • Embryonic stem cells are pluripotent and can differentiate into any cell type
  • Adult stem cells are multipotent and have a more limited differentiation potential
• Cancer is characterized by the uncontrolled growth and spread of abnormal cells
  • Carcinomas arise from epithelial tissue (lung, breast, colon)
  • Sarcomas arise from connective tissue (bone, muscle, cartilage)
• Tissue biopsy is the removal of a small sample of tissue for microscopic examination
  • Used to diagnose diseases such as cancer, infections, and autoimmune disorders
  • Can be performed using a needle (fine-needle aspiration) or through surgical excision