TISSUES 4

• Epithelial Tissue


  Principal Functions:

  • Covering, lining, and protecting surfaces (e.g. epidermis)
  • Absorption (e.g. intestinal lining)
  • Secretion (e.g. parenchymal cells of glands)
• Epithelial Cells


  • Shapes and dimensions range from columnar, cuboidal, or squamous cells
  • Nuclei also vary in shape: elliptic (oval), spherical, or flattened
  • Most epithelia are adjacent to connective tissue containing blood vessels
  • Generally show polarity, with organelles and membrane proteins distributed unevenly within the cell

  
• Basement Membranes


  Thin extracellular, felt-like sheet of macromolecules; a semipermeable filter for substances reaching epithelial cells from below

  • Basal Lamina – a thin, electron-dense, sheet-like layer of fine fibrils
  • Reticular Lamina – more diffuse and fibrous layer beneath basal lamina
• Functions of Basement Membrane


  • - Filters substances reaching epithelial cells from below
  • Helps provide structural support for epithelial cells
  • Attach epithelia to underlying connective tissue
  • Mediate many cell-to-cell interactions involving epithelia
  • Marks routes for certain cell migrations along epithelia
  • Serves as scaffold that allows rapid epithelial repair and regeneration
• Functions of Basal Lamina


  • - Help organize integrins and other proteins in the PM of epithelial cells
  • Maintains cell polarity
  • Helps localize endocytosis, signal transduction, and other activities

  
• Intercellular Adhesion & Other Junctions

  • Tight / Occluding Junctions (form a seal between adjacent cells)
  • Adherent / Anchoring Junctions (sites of strong cell adhesion)
  • Gap Junctions (channels for communication between adjacent cells)
• Specializations of the Apical Cell Surface

  • Microvilli

  2. Stereocilia

  3. Cilia

  
• Microvilli


  • - Usually of uniform length, densely packed
  • Visible as a brush or striated border projecting into the lumen (e.g. small intestine)
  • 1 µm long, 0.1 µm wide on average; hundreds or thousands present on the end of each absorptive cell; total surface area can be increased by 20-30 fold
  • Contains bundled actin filaments capped and bound to the surrounding PM by actin-binding proteins

  
• Stereocilia


  • - Longer and less motile than microvilli
  • With specialized mechanosensory function in cells (e.g. inner ear) and for absorption in tissues (e.g. male reproductive tract - epididymis)
• Cilia


  • - Long, highly motile, larger than microvilli, contains internal arrays of microtubules (not microfilaments)
  • Most (if not all) epithelial cells have at least one primary cilium (not motile, enriched with receptors and signal transduction complexes for detection of light, odors, motion, and flow of liquid past the cells)
  • Abundant on cuboidal or columnar cells of many epithelia
  • 5-10 µm long, 0.2 µm in diameter
  • Axoneme – consists of 9 peripheral microtubule doublets arrayed around 2 central microtubules (9+2 assembly of microtubules); elastic but relatively stiff; structure is maintained by nexins
  • Basal Bodies – apical cytoplasmic structures below the cell membrane; structurally very similar to centrioles, consisting of 9 microtubular triplets linked in pinwheel-like arrangement

  
• Types of Epithelia


  • Covering or Lining Epithelia

  2. Secretory (Glandular) Epithelia
• Covering (Lining) Epithelia


  • - Simple Squamous Epithelium
  • Simple Cuboidal Epithelium
  • Simple Columnar Epithelium
  • Stratified Epithelium
  • Transitional Epithelium (Urothelium)
  • Pseudostratified Epithelium
• Secretory (Glandular) Epithelia


  Epithelial cells that function mainly to produce and secrete various macromolecules through specialized organs called glands
• Secretions of Glandular Epithelia

  • - Proteins (e.g. pancreas)
  • Lipids (e.g. adrenal, sebaceous glands)
  • Carbohydrate and Protein Complexes (e.g. salivary glands)
  • Proteins, Lipids, CHO (e.g. mammary glands)
  • Water and Electrolytes (e.g. sweat glands)
• Scattered secretory cells (unicellular glands)

  Common in simple cuboidal, simple columnar, and pseudostratified epithelia (e.g. goblet cells in the lining of small intestine and respiratory tract)

  
• Exocrine and Endocrine Glands

  • - Exocrine Glands remain connected with the surface epithelium, the connection forming the tubular ducts lined with epithelium that deliver secreted material where it is used.
  • Endocrine Glands lose the connection to their original epithelium and therefore lack ducts. Capillaries adjacent to endocrine cells absorb their secreted hormone products for transport in blood to target cells throughout the body.
• Connective Tissue


  Provides a matrix that supports and physically connects other tissues and cells together to form the organs of the body

  • Interstitial Fluid of CT – gives metabolic support to cells; medium for diffusion of nutrients and waste products
  • Extracellular Matrix (ECM) – major constituent of CT; combinations of protein fibers (collagen and elastic fibers) and ground substances (proteoglycans, glycosaminoglycans or GAGs, and multiadhesive glycoproteins)
• Embryonic Mesenchyme


  Tissue developing mainly from the mesoderm; consists largely of viscous ground substance with few collagen fibers; produces all types of CT proper and specialized CT

  
• Mesenchymal Cells


  • - Undifferentiated, generally elongated but with many shapes, have large nuclei and prominent nucleoli (indicative of high levels of synthetic activity)
  • "Spindle-shaped" due to their scant cytoplasm extended as two or more thin cytoplasmic processes
  • Surrounded by ECM that they produced and that consists largely of simple ground substance rich in hyaluronan (hyaluronic acid), but with very little collagen
  • Multipotent stem cells

  
• Fibroblasts


  • - Most common; produce and maintain most of the tissues' extracellular components
  • Synthesize and secrete collagen and elastin, as well as the proteoglycans, GAGs, and multiadhesive glycoproteins that comprise the ground substance
  • Distinct levels of fibroblast activity (active and quiescent) can be observed histologically
  • Active Fibroblasts – have large, euchromatic nuclei
  • Inactive/Quiescent Fibroblasts (Fibrocytes) – smaller, with more heterochromatic nuclei

  
• Adipocytes


  • - Large, mesenchymally derived cells specialized for cytoplasmic storage of lipid as neutral fats, or less commonly for the production of heat
  • Adipose Tissue – have large population of adipocytes which serves to cushion and insulate the skin and other organs
• Macrophages


  • - Present in CT of most organs; sometimes referred to as "histiocytes"
  • Typical macrophage measures between 10-30 µm in diameter and has an eccentrically located, oval or kidney-shaped nucleus
  • Have highly developed phagocytic ability and specialize in turnover of protein fibers and removal of apoptotic cells, tissue debris, or other particulate material, being especially abundant at sites of inflammation

  
• Collagen Fiber


  • - Collagen fibers are the most abundant protein fibers found in the extracellular matrix of connective tissues in animals
  • They provide strength, structure, and support to tissues such as skin, tendons, ligaments, bones, and cartilage
  • Collagen fibers are composed of triple helices of protein chains, primarily consisting of glycine, proline, and hydroxyproline amino acids
  • These fibers are synthesized by specialized cells called fibroblasts, chondrocytes, and osteoblasts
  • Collagen fibers undergo cross-linking, forming a network that contributes to tissue integrity and resilience
  • They play a crucial role in wound healing, providing a scaffold for cell migration and tissue regeneration
  • Collagen fibers contribute to the elasticity and flexibility of tissues, allowing them to withstand stretching and mechanical stress
  • Various types of collagen exist, with each type having distinct structural and functional properties tailored to specific tissues and organs
  • Collagen disorders, such as Ehlers-Danlos syndrome and osteogenesis imperfecta, result from genetic mutations affecting collagen synthesis or structure, leading to tissue fragility and other health complications
  • Collagen-based products, such as supplements and topical creams, are marketed for their potential benefits in promoting skin health, joint function, and overall well-being
• Reticulin Fiber


  • - Reticulin fibers are primarily composed of type III collagen
  • Reticulin fibers provide structural support and help maintain the architecture of tissues, particularly in organs such as the liver, spleen, and lymph nodes
  • These fibers form a fine meshwork that supports cells and other tissue components
  • Changes in reticulin fiber distribution and density can indicate pathological conditions, such as fibrosis or certain types of cancer
  • The synthesis and degradation of reticulin fibers are tightly regulated processes influenced by various factors including growth factors, cytokines, and mechanical forces
  • Diseases such as cirrhosis of the liver, myelofibrosis, and certain types of tumors can lead to alterations in reticulin fiber deposition and organization
• Elastic Fiber


  • - Elastic fibers are a type of connective tissue fibers primarily composed of elastin, a highly elastic protein, along with fibrillin microfibrils
  • They provide elasticity and resilience to tissues, allowing them to stretch and recoil without permanent deformation
  • Elastic fibers are abundant in tissues requiring elasticity, such as the skin, lungs, arteries, and ligaments
  • These fibers contribute to the maintenance of tissue structure and function, allowing organs to expand and contract with movement or physiological changes
  • Elastin molecules within elastic fibers are cross-linked to form a network, providing strength and flexibility to the tissue
• Ground Substance


  • Proteoglycans: Proteins with attached glycosaminoglycan (GAG) chains that play a key role in maintaining the hydration and viscosity of the ground substance and regulate cell behavior and signaling within the extracellular matrix
  • Glycosaminoglycans (GAGs): Long unbranched polysaccharide chains composed of repeating disaccharide units that contribute to the gel-like consistency of the ground substance and attract and bind water molecules, maintaining tissue hydration
  • Multiadhesive Proteins: Proteins with multiple adhesive domains that facilitate cell adhesion, migration, and signaling within the extracellular matrix and play roles in tissue development, wound healing, and immune response
  • The ground substance provides a medium for the diffusion of nutrients, gases, and metabolites between cells and blood vessels, and serves as a barrier against pathogens and mechanical stresses, helping to protect the underlying tissues
• Types of Connective Tissue

  • A. Connective Tissue Proper

  1. Loose connective tissue (or areolar tissue)

  2. Dense irregular connective tissue

  3. Dense regular connective tissue

  B. Embryonic Connective Tissue

  • Mesenchyme
  • Mucoid (Mucous) Connective Tissue

  C. Specialized Connective Tissue

  • Reticular Connective Tissue
  • Adipose Tissue
  • Cartilage
• Mesenchyme


  • - Sparse, undifferentiated cells, uniformly distributed in matrix with sparse collagen fibers
  • Contains stem/progenitor cells for all adult connective tissue cells
  • Mesodermal layer of early embryo

  
• Mucoid (Mucous) Connective Tissue

  • Gel-like connective tissue with a few cells found most abundantly around blood vessels in the umbilical cord

  
• Reticular Connective Tissue


  • Consists of delicate networks of type III collagen and is most abundant in certain lymphoid organs where the fibers form attachment sites for lymphocytes and other immune cells
• Adipose Tissue


  • - The defining cells of adipose tissue, adipocytes, are very large cells derived from mesenchyme and specialized for energy storage in lipid droplet(s) with triglycerides
  • Adipocytes store lipids from three sources: from dietary fats packaged as chylomicrons in the intestine; from triglycerides produced in the liver and circulating as VLDLs; and from fatty acids synthesized locally
  • White Adipose Tissue: Found in many organs throughout the body, typically forming about 20% of the body weight in adults
  • Brown Adipose Tissue: Comprises up to 5% of the newborn body weight but smaller amounts in adults, with adipocytes that are typically smaller than those of white fat and contain primarily many small lipid droplets (they are multilocular) in cytoplasm containing many mitochondria and a central nucleus

  
• Cartilage


  • - Cartilage is a tough, resilient type of connective tissue that structurally supports certain soft tissues, notably in the respiratory tract, and provides cushioned, low-friction surfaces in joints
  • Cells of cartilage, chondrocytes, make up a small percentage of the tissue's mass, which is mainly a flexible mass of ECM
  • Chondrocytes are embedded within lacunae surrounded by the ECM
  • Cartilage ECM typically includes collagen as well as abundant proteoglycans, notably aggrecan, which bind a large amount of water
  • Cartilage always lacks blood vessels, lymphatics, and nerves, but it is usually surrounded by a dense connective tissue perichondrium
• Brown Adipose Tissue


  50 to 150 μm
• Brown fat


  • Comprises up to 5% of the newborn body weight but smaller amounts in adults
  • Adipocytes are typically smaller than those of white fat and contain primarily many small lipid droplets (they are multilocular) in cytoplasm containing many mitochondria and a central nucleus
• Cartilage


  A tough, resilient type of connective tissue that structurally supports certain soft tissues, notably in the respiratory tract, and provides cushioned, low-friction surfaces in joints
• Cells of cartilage


  • Chondrocytes make up a small percentage of the tissue's mass, which is mainly a flexible mass of ECM
  • Chondrocytes are embedded within lacunae surrounded by the ECM
  • Cartilage ECM typically includes collagen as well as abundant proteoglycans, notably aggrecan, which bind a large amount of water
  • Cartilage always lacks blood vessels, lymphatics, and nerves, but it is usually surrounded by a dense connective tissue perichondrium that is vascularized

  
• Hyaline Cartilage


  • The ECM is homogenous and glassy, rich in fibrils of type II collagen and aggrecan complexes with bound water
  • The ECM has less collagen and more proteoglycan immediately around the lacunae, producing slight staining differences in this territorial matrix

  
• Elastic Cartilage


  • Resembles hyaline cartilage in its chondrocytes and major ECM components, but its matrix includes abundant elastic fibers, visible with special stains, which increase the tissue's flexibility
  • Provides flexible support for the external ear as well as certain structures of the middle ear and larynx
• Fibrocartilage


  • Contains varying combinations of hyaline cartilage in small amounts of dense connective tissue
  • Histologically it consists of small chondrocytes in a hyaline matrix, usually layered with larger areas of bundled type I collagen with scattered fibroblasts
  • Provides very tough, strong support at tendon insertions and in intervertebral discs and certain other joints
• Bone


  • A type of connective tissue with a calcified extracellular matrix (ECM), specialized to support the body, protect many internal organs, and act as the body's Ca2+ reservoir
  • Primarily composed of cells (Osteoblasts, osteocytes, and osteoclasts), fibers (collagen, type I), and ground substance (mineralized matrix, hydroxyapatite crystals)