Tissue level organization

• Tissues are collections of specialized cells and cell products that perform specific functions

  Tissues in combinations form organs, e.g. heart or liver

  Histology is the study of tissues
• Four types of tissue

  • Epithelial
  • Connective
  • Muscle
  • Nervous
• Epithelial Tissue

  • covers exposed surfaces
  • lines internal passageways
  • forms glands
• Connective tissue

  • fills internal spaces
  • supports other tissues
  • transports materials
  • stores energy
• Muscle tissue

  • specialized for contraction
  • skeletal muscles, heart muscles, and muscular walls of hollow organs such as blood vessels
• Nervous tissue

  • carriers electrical signals from one part of the body to the other
• Epithelial tissues includes:

  – Epithelia (singular, epithelium)

  • Layers of cells covering internal (lines organs) or external surfaces so protect things from entering or leaving (barrier)

  – Glands 

  • Structures that produce fluid secretions
• Functions of epithelial tissue

  1. Provide physical protection

  2. Control permeability

  3. Provide sensation

  4. Produce specialized secretions
• Characteristics of epithelia

  – Polarity (apical and basal surfaces)

  – Cellularity (cell junctions)

  – Attachment (basement membrane) 

  – Avascularity (avascular)

  – Regeneration
• Classifying epithelia

  • Simple (single) vs stratified (layered)
  • Squamous, cuboidal, and columnar
• Squamous epithelia

  – Simple squamous epithelia

  • Absorption and diffusion 

  – Mesothelium

  • Lines body cavities 

  – Endothelium

  • Forms inner lining of heart and blood vessels 

  – Stratified squamous epithelia

  • Protect against mechanical stresses

  • Keratin adds strength and water resistance
• Cuboidal epithelia

  – Simple cuboidal epithelia

  • Secretion and absorption

  • Glands and portions of kidney tubules 

  – Stratified cuboidal epithelia 

  • Relatively rare

  • Ducts of sweat glands and mammary glands
• Transitional epithelia

  – Tolerate repeated cycles of stretching without damage 

  – Found in urinary bladder

  – Appearance changes as stretching occurs

  – changes from squamous to cuboidal
• Columnar epithelia

  – Simple columnar epithelia

  • Absorption and secretion

  • Found in stomach, small intestine, large intestine 

  – Pseudostratified columnar epithelia

  • Typically have cilia

  • Found in nasal cavity, trachea, bronchi

  – Stratified columnar epithelia 

  • Relatively rare

  • Provide protection in pharynx, anus, urethra
• Glandular epithelia

  – Glands are collections of epithelial cells that produce secretions

  – Endocrine glands

  • Release hormones that enter bloodstream 

  • No ducts

  – Exocrine glands

  • Produce exocrine secretions

  • Discharge secretions through ducts onto epithelial surfaces
• Gland structure

  – Unicellular glands 

  – Multicellular glands
• Unicellular glands

  – Goblet cells are unicellular exocrine glands

  • In epithelia of intestines

  • Secrete mucin, which mixes with water to form mucus
• Multicellular exocrine glands are classified by 

  1. Structure of the duct

  • Simple (undivided)

  • Compound (divided)

  2. Shape of secretory portion of the gland

  • Tubular (tube shaped)

  • Alveolar or acinar (blind pockets)

  3. Relationship between ducts and glandular areas

  • Branched (several secretory areas sharing one duct)
• Simple gland types

  
• Compound gland types

  
• Types of secretions produced by exocrine glands 

  – Serous glands

  • Watery secretions 

  – Mucous glands

  • Secrete mucins

  – Mixed exocrine glands

  • Both serous and mucous
• Three components of connective tissues 

  1. Specialized cells

  2. Extra cellular protein fibers

  3. Fluid called ground substance
• Matrix consists of extracellular components of connective tissue (fibers and ground substance)

  – Majority of tissue volume

  – Determines specialized function
• Functions of connective tissues

  • Establishing a structural framework for the body
  • Transporting fluids and dissolved materials
  • Protecting delicate organs
  • Supporting, surrounding, and interconnecting other types of tissue
  • Storing energy reserves, especially triglycerides
  • Defending the body from invading microorganisms
• Categories of connective tissues

  1.  Connective tissue proper

  • Connect and protect

  2. Fluid connective tissues

  • Transport

  3. Supporting connective tissues

  • Structural strength
• Categories of connective tissue proper 

  – Loose connective tissue

  • More ground substance, fewer fibers

  • Example: fat (adipose tissue) 

  – Dense connective tissue

  • More fibers, less ground substance 

  • Example: tendons
• Cells of connective tissue proper 

  – Fibroblast, Fibrocytes, Adipocytes, Mesenchymal cells, Macrophages, Mast cells, Lymphocytes, Microphages, Melanocytes
• Connective tissue fibers 

  1. Collagen fibers

  2. Reticular fibers

  3. Elastic fibers
• Collagen fibers

  – Most common fibers in connective tissue proper 

  – Long, straight, and unbranched

  – Strong and flexible

  – Resist force in one direction

  – Abundant in tendons and ligaments
• Reticular fibers

  – Form a network of interwoven fibers (stroma)

  – Strong and flexible

  – Resist forces in many directions

  – Stabilize functional cells (parenchyma) and structures 

  – Example: sheaths around organs
• Elastic fibers

  – Contain elastin

  – Branched and wavy

  – Return to original length after stretching 

  – Example: elastic ligaments of vertebrae
• Ground substance

  – Is clear, colorless, and viscous

  – Fills spaces between cells and slows pathogen movement
• Loose connective tissues

  – “Packing materials”

  – Fill spaces between organs, cushion cells, and support epithelia
• Types of loose connective tissues in adults 

  1. Areolar tissue

  2. Adipose tissue

  3. Reticular tissue
• Areolar tissue

  • Least specialized
  • Open framework
  • Viscous ground substance
  • Elastic fibers make it resilient, can distort without damage
  • Holds capillary beds
  • Example: under skin (subcutaneous layer)
• Adipose tissue

  – Contains many adipocytes (fat cells)

  • Adipocytes in adults do not divide 

  – Shrink as fats are released

  – Expand to store fat

  • Mesenchymal cells divide and differentiate 

  – When more storage is needed

  – To produce more fat cells

  – May be removed (temporarily) via liposuction in cosmetic surgery
• White fat

  – Most common

  – Stores fat and absorbs shocks 

  – Slows heat loss (insulation)
• Brown fat

  – Found in babies and young children

  – More vascularized

  – Adipocytes have many mitochondria

  – Breakdown of lipids releases energy and warms body
• Reticular tissue

  • Provides support
  • Reticular fibers form a complex, three-dimensional stroma
  • Support functional cells of organs
  • Found in liver, kidney, spleen, lymph nodes, and bone marrow
• Dense connective tissues

  • Contain many collagen fibers

  – Three types of dense connective tissues

  • Dense regular 

  • Dense irregular 

  • Elastic