Biology 🧫

• Exocrine glands


  Secrete into a duct or onto a surface
• Endocrine glands


  Secrete into the bloodstream
• Unicellular glands


  • Composed of a single cell (e.g. goblet cells in tracheal epithelium)
• Multicellular glands


  • Classified according to duct branching as simple (duct does not branch) or compound (duct branches)
  • Further classified according to the shape of the secretory unit

  
• Simple tubular glands


  Secretory cells are arranged in straight tubules

  
• Simple branched tubular glands


  Secretory cells are split into two or more tubules

  
• Simple coiled tubular glands


  Have a long duct, and secretory cells are formed by coiled tubules
• Simple acinar glands


  Have a short, unbranched duct; the secretory cells are arranged in acini form

  
• Simple branched acinar glands


  Have a short, unbranched duct, and their secretory cells are formed into branched acini

  
• Compound tubular glands


  Have branched ducts, and their secretory cells are formed into branched tubules

  
• Compound acinar glands


  Have branched ducts, and the secretory units are branched acini

  
• Compound tubuloacinar glands


  Have branched ducts, and the secretory units are formed by both an acinar component and a tubular component

  
• Types of secretory products of exocrine glands


  • Serous (non-viscous, watery fluid)
  • Mucous (mucins that form mucus)
  • Mixed (serous and mucous)

  
• Holocrine glands


  Formed from cells that accumulate a product and then the entire cell disintegrates
• Merocrine glands


  Secretory cells release their contents by exocytosis, packaging secretions in secretory vesicles

  
• Apocrine glands


  Cells accumulate secretory products within the apical portion of their cytoplasm, which then decapitates and is transported to the skin surface
• Endocrine glands


  • May be unicellular or multicellular, lack a duct system, and secrete hormones into the bloodstream

  
• Malignant tumors of epithelial origin are called carcinomas, and malignant tumors derived from glandular epithelial tissue are called adenocarcinomas
• Connective tissue


  Responsible for providing and maintaining the form of organs throughout the body. Functioning in a mechanical role, they provide a matrix that connects and binds other tissues and cells in organs and gives metabolic support to cells as the medium for diffusion of nutrients and waste products.
• Connective tissue


  • Structurally formed by three classes of components: cells, fibers, and ground substance. The major constituent is the extracellular matrix (ECM).

  
• Components of extracellular matrix (ECM)


  • Protein fibers (collagen, reticular, and elastic fibers)
  • Ground substance
• Cells found in connective tissue

  • Fibroblasts
  • Mast cells
  • Macrophages
  • Plasma cells
  • Leukocytes (lymphocytes, neutrophils, eosinophils, basophils)
  • Adipocytes (fat cells)
• Fibroblasts


  The most common cells in connective tissue, responsible for the synthesis of all components of the extracellular matrix

  
• Macrophages


  Highly phagocytic cells derived from blood monocytes
• Mast cells


  Contain and release heparin, histamines, and various chemotactic mediators involved in inflammatory responses
• Plasma cells


  Derived from B-lymphocytes, able to secrete antibodies that are antigen specific
• Leukocytes


  Transient cells of connective tissue that migrate from blood vessels into connective tissue
• Adipocytes (fat cells)


  Arise from undifferentiated mesenchymal cells, store energy in the form of triglycerides and synthesize hormones such as leptin
• Ground substance


  Highly hydrophilic, viscous complex of ionic macromolecules (glycosaminoglycan and proteoglycans) and multiadhesive glycoproteins (laminin, fibronectin, and others) that stabilizes the ECM
• Fibers


  Long, slender protein polymers present in different proportions in different types of connective tissue. The three main types are collagen, reticular, and elastic fibers.

  
• Collagen fibers


  • The most common and widespread fibers in connective tissue, composed primarily of type I collagen. They add strength to the connective tissue.

  
• Reticular fibers


  • Very delicate fibers that form fine networks, composed of type III collagen. They provide a supportive, meshlike framework for organs composed mostly of cells.

  
• Elastic fibers


  • Composed of elastin and fibrillin, have a very resilient nature (stretch and recoil) important in areas like the lungs, aorta, and skin.
• The regenerative capacity of connective tissue is clearly observed in organs damaged by ischemia, inflammation, or traumatic injury. Spaces left after such injuries are filled by connective tissue, forming dense irregular scar tissue.
• The healing of surgical incisions and other wounds depends on the reparative capacity of connective tissue, particularly on activity and growth of fibroblasts.
• In some rapidly closing wounds, a cell called the myofibroblast, with features of both fibroblasts and smooth muscle cells, is also observed. Their activity is important for the phase of tissue repair called wound contraction.