Connective Tissue Fibers, Cells, and Classification

Reticular Fibers

  • Structure and Composition:
    • Composed of short, fine fibers made from a distinct type of collagen compared to the thicker, more common collagen fibers.
    • Extensively branched, forming delicate, net-like networks (reticul = network).
  • Location and Functional Roles:
    • Surround small blood vessels and support the soft tissue of organs.
    • Particularly abundant where connective tissue borders other tissue types, such as in the basement membrane of epithelial tissues and around capillaries.
    • Form fuzzy "nets" that offer greater flexibility and stretch than larger collagen fibers.

Connective Tissue Cells

  • Resident Cell Types and Developmental Stages:
    • Each major class of connective tissue contains a resident cell type existing in immature (-blast) and mature (-cyte) forms:
    • Connective Tissue Proper: Fibroblasts become fibrocytes.
    • Cartilage: Chondroblasts (kon'dro-blast") become chondrocytes.
    • Bone: Osteoblasts (os'te-o-blast") become osteocytes.
  • Blast vs. Cyte Dynamics:
    • -blast Cells: Actively mitotic immature cells that secrete the ground substance and specific fibers characteristic of their matrix.
    • -cyte Cells: Mature, less active cells that form after synthesizing the matrix; responsible for maintaining the health of the matrix.
    • Reversion Capability: If the matrix is injured, mature -cyte cells can easily revert to their more active -blast form to repair and regenerate the matrix.
  • Exception of Blood:
    • Blood represents the fourth major connective tissue class and serves as an exception to standard resident cell dynamics.
    • Its immature blood cell-forming type (formerly called a hemocytoblast) is called a hematopoietic stem cell (hem"ah-to-poy-et'ik).
    • Hematopoietic stem cells are not located in blood tissue itself and do not create the fluid matrix (plasma) of blood.

Ancillary Cell Types in Connective Tissue

  • Adipocytes (ad'I-po-sītz):
    • Commonly called adipose or fat cells.
    • Function to store energy as fat.
  • White Blood Cells (WBCs or Leukocytes):
    • Include neutrophils, eosinophils, lymphocytes, and other cell types concerned with tissue response to injury.
  • Mast Cells:
    • Typically cluster along blood vessels.
    • Oval-shaped cells that detect foreign microorganisms (e.g., bacteria, fungi) and initiate local inflammatory responses against them.
    • Cytoplasm contains secretory granules (mast = stuffed full of granules) filled with chemical mediators of inflammation, especially in severe allergies:
    • Heparin (hep'ah-rin): An anticoagulant chemical that prevents blood clotting when free in the bloodstream; in human mast cells, it appears to regulate the action of other mast cell chemicals.
    • Histamine (his'tah-mēn): A substance that makes capillaries leaky.
    • Proteases: Protein-degrading enzymes.
    • Other enzymes: Catalysts assisting in inflammatory responses.
  • Macrophages (mak'ro-fāj"es; macro = large, phago = eat):
    • Large, irregularly shaped cells that avidly devour a broad variety of foreign materials, ranging from foreign molecules to entire bacteria to dust particles.
    • Referred to as "big eaters," they dispose of dead tissue cells and serve as central actors in the immune system.
    • Distribution: Peppered throughout loose connective tissue, bone marrow, and lymphoid tissue.
    • Mobility: May be attached to connective tissue fibers (fixed) or migrate freely through the extracellular matrix.
    • Selective Appetites: Certain populations display target preferences; for example, spleen macrophages primarily dispose of aging red blood cells while remaining capable of ingesting other material.

Embryonic Origin and Classification of Connective Tissue

  • Extracellular Matrix Basis:
    • All classes of connective tissue consist of living cells surrounded by an extracellular matrix.
    • Major structural differences reflect cell type, as well as the types and relative amounts of fibers present.
  • Embryonic Origin (Mesenchyme):
    • All mature connective tissues arise from a common embryonic tissue called mesenchyme (meh'zin-kīm).
    • Structure: Possesses a fluid ground substance containing fine, sparse fibers and star-shaped mesenchymal cells.
    • Development: Arises during the early weeks of embryonic development and eventually differentiates (specializes) into all other connective tissue cells.
    • Persistence: Some mesenchymal cells remain in mature connective tissues to provide a source of new cells.
  • Structural Hierarchy and Classification:
    • All mature connective tissues except for bone, cartilage, and blood are classified as connective tissue proper.
    • Subclasses of Connective Tissue Proper:
    • Loose Connective Tissues: Areolar, Adipose, and Reticular tissues.
    • Dense Connective Tissues: Dense Regular, Dense Irregular, and Elastic tissues.
    • Main Connective Tissue Classes Summary:

Overview of types of connective tissue

  • Connective Tissue divides into:
    • Connective Tissue Proper:
      • Loose (Areolar, Adipose, Reticular)
      • Dense (Regular, Irregular, Elastic)
    • Cartilage (Hyaline, Elastic, Fibrocartilage)
    • Bone
    • Blood