tissue

• Major types of tissues

  • Epithelial tissue
  • Connective tissue
  • Muscle tissue
  • Nervous tissue
• Epithelial tissue

  • Form protective coverings and function in secretion and absorption
  • Are found throughout the body
  • Lack blood vessels but nutrients diffuse to epithelium from underlying connective tissues, which have abundant blood vessels
  • Epithelial cells readily divide
  • Epithelial cells are tightly packed

  
• Classifications of epithelial tissue

  • Squamous
  • Cuboidal
  • Columnar

  
• Simple squamous epithelium


  • Consists of a single layer of thin, flattened cells
  • Substances pass rather easily
  • Easily damaged due to being thin and delicate
• Stratified squamous epithelium

  • Has many cell layers resulting in being thick
  • Forms the outer layer of the skin (epidermis)
  • Lines the oral cavity, esophagus, vagina, and anal canal

  
• Simple cuboidal epithelium


  • Consists of a single layer of cube-shaped cells
  • Usually have centrally located, spherical nuclei
• Stratified cuboidal epithelium


  • Consists of two or three layers of cuboidal cells that form the lining of a lumen
  • Lines the larger ducts of the mammary glands, sweat glands, salivary glands, and pancreas
  • Also forms the lining of developing ovarian follicles and seminiferous tubules, which are parts of the female and male reproductive systems

  
• Transitional epithelium


  • Is specialized to change in response to increased tension
  • Forms the inner lining of the urinary bladder and lines the ureters and the superior urethra
• Simple columnar epithelium


  • Are elongated
  • Composed of a single layer of cells with elongated nuclei usually located at about the same level, near the basement membrane
  • Can be ciliated or nonciliated

  
• Simple columnar epithelium


  • Have microvilli
  • Have goblet cells
• Stratified columnar epithelium


  • Consists of several layers of cells
  • The superficial cells are columnar, whereas the basal layers consist of cuboidal cells

  
• Pseudostratified columnar epithelium


  • Appear to be stratified or layered, but they are not
  • The nuclei lie at two or more levels in the row of aligned cells
  • Lines the passages of the respiratory system
• Glandular epithelium


  • Is composed of cells specialized to produce and secrete substances into ducts or into body fluids
  • Usually found within columnar or cuboidal epithelium, and one or more of these cells constitute a gland

  
• Classification of exocrine glands (according to how they secrete)

  • Merocrine glands
  • Apocrine glands
  • Holocrine glands

  
• Merocrine glands


  Glands that release fluid by exocytosis
• Serous fluid


  Typically watery, and has a high concentration of enzymes
• Mucus


  Thick and is rich in the glycoprotein mucin and abundantly secreted by cells, for protection, in the inner linings of the digestive, respiratory, and reproductive systems
• Apocrine glands


  Glands that lose small portions of their glandular cell bodies during secretion
• Holocrine glands


  Glands that release entire cells that disintegrate to release cell secretions
• Connective tissue


  • Support soft body parts and bind structures together
  • Bind structures, provide support and protection, serve as frameworks, fill spaces, store fat, produce blood cells, protect against infections, and help repair tissue damage
  • Are farther apart than epithelial cells, and they have an abundance of extracellular matrix between them
  • Most connective tissue cells can divide
  • Most cases they have good blood supplies and are well nourished
  • Some are quite rigid
• Major cell types of connective tissue

  • Fibroblasts
  • Mast cells
  • Wandering cells
• Fibroblasts


  • Large, star-shaped cells
  • They produce fibers by secreting proteins into the extracellular matrix of connective tissues
  • Produce 3 types of connective tissues: collagenous fibers, elastic fibers, and reticular fibers
• Collagenous fibers


  • Are thick threads of the protein collagen
  • They are grouped in long, parallel bundles, and are flexible but only slightly elastic
  • They have great tensile strength
• Dense connective tissue


  Tissue containing abundant collagenous fibers
• Loose connective tissue


  Has fewer collagenous fibers
• Elastic fibers


  • Are composed of a springlike protein called elastin
  • Are weaker than collagenous fibers, but they are easily stretched or deformed and will resume their original lengths and shapes when the force acting on them is removed
  • They are sometimes called yellow fibers because tissues well supplied with them appear yellowish
• Reticular fibers


  • Thin collagenous fibers
  • They are highly branched and form delicate supporting networks in a variety of tissues
• Categories of connective tissue

  • Connective tissue proper
  • Loose connective tissue
  • Dense connective tissue
  • Specialized connective tissue
• Areolar tissue


  • Forms delicate, thin membranes throughout the body
  • Binds the skin to the underlying organs and fills spaces between muscles
  • Lies beneath most layers of epithelium, where its many blood vessels nourish nearby epithelial cells

  
• Adipose tissue


  • Develops when certain cells (adipocytes) store fat as droplets in their cytoplasm and enlarge
  • Cushions joints and some organs, such as the kidneys
  • Insulates beneath the skin, and it stores energy in fat molecules
• Reticular connective tissue


  • Composed of thin, collagenous fibers in a three-dimensional network
  • Helps provide the framework of certain internal organs

  
• Dense connective tissue


  • Consists of many closely packed, thick, collagenous fibers and a fine network of elastic fibers
  • Has few cells, most of which are fibroblasts
  • The blood supply to dense connective tissue is poor, slowing tissue repair
• Specialized connective tissue


  • Is a rigid connective tissue
  • Provides support, frameworks, and attachments, protects underlying tissues, and forms structural models for many developing bones
  • Cartilage extracellular matrix is abundant and is largely composed of collagenous fibers embedded in a gel-like ground substance
  • Cartilage cells, or chondrocytes occupy small chambers called lacunae and lie completely within the extracellular matrix
• Types of cartilage

  • Hyaline cartilage
  • Elastic cartilage
  • Fibrocartilage
• Hyaline cartilage


  • The most common type
  • Has very fine collagenous fibers in its extracellular matrix and looks somewhat like white glass
  • It is found on the ends of bones in many joints, in the soft part of the nose, and in the supporting rings of the respiratory passages

  
• Elastic cartilage


  • Has a dense network of elastic fibers and thus is more flexible than hyaline cartilage
• Fibrocartilage


  • A very tough tissue
  • Has many collagenous fibers
  • It is a shock absorber for structures that are subjected to pressure
• Bone


  • Is the most rigid connective tissue
  • Its hardness is largely due to mineral salts, such as calcium phosphate and calcium carbonate, between cells
• Blood


  • Transports a variety of materials between interior body cells and those that exchange substances with the external environment
  • Is composed of formed elements suspended in a fluid extracellular matrix called blood plasma
  • Most blood cells form in red marrow within the hollow parts of certain long bones
• Mast cells


  • Are large and widely distributed in connective tissues
  • They are usually near blood vessels
  • Release heparin, which prevents blood clotting, and histamine, which promotes some of the reactions associated with inflammation and allergies