Connective Tissue Cells

Epithelial Tissue: General Characteristics

• Epithelial tissue is composed of densely packed cells with minimal intercellular substance.
• It rests on a basement membrane and the underlying connective tissue.
• Epithelium is avascular, meaning it lacks blood vessels.
• It is rich in nerve endings.
• Epithelium has a high capacity for regeneration.
• Forms sheets that cover surfaces and line all cavities in the body, known as covering and lining epithelium.
• Also forms clusters of cells that form glands. This is called glandular epithelium.
• Modified structures that act as sensory receptors are sensory epithelium.

Basement Membrane

• The basement membrane separates the epithelium from the underlying connective tissue.
• Appears as a thin layer when viewed with a light microscope (LM) and can be stained with PAS and silver.
• Under an electron microscope (EM), the basement membrane is composed of:
  • A cell coat of epithelial cells.
  • A basal lamina: a homogenous layer consisting of glycoproteins.
  • A layer of reticular fibres.
• The function of the basement membrane includes:
  • acting as a barrier against diffusion of macromolecules
  • providing support for the epithelial layer.

Covering & Lining Epithelium: Simple Epithelium

• Simple squamous epithelium consists of a single layer of flat cells with flat nuclei.
• It is found lining blood vessels and body cavities.
• Simple cuboidal epithelium consists of a single layer of cubical cells with central rounded nuclei.
• It lines renal tubules and thyroid follicles.
• Simple columnar epithelium consists of one layer of tall cells with oval basal nuclei.
• It lines the stomach, intestines, and gallbladder.
• Pseudostratified columnar epithelium is composed of one layer of cells, where some cells are tall and reach the surface, while others are shorter and do not reach the surface, but all rest on the basement membrane.
• It appears falsely stratified.
• It lines the trachea as pseudostratified columnar ciliated epithelium.

Stratified Epithelium

• Stratified squamous epithelium has several layers of cells.
• Its basal cells being columnar, the following layers polyhedral, and the superficial cells flattened squamous.
• Stratified squamous epithelium has two types:
  • Non-keratinized, lining the oral cavity and oesophagus.
• Keratinized, covers the skin.
• Transitional epithelium can change its form.
• It lines organs that can expand significantly, such as the urinary bladder.
• When stretched (as in a full bladder), it resembles stratified squamous non-keratinized epithelium. When relaxed (as in an empty bladder):
  • The basal cells are short columnar
  • Intermediate cells are polyhedral
  • Superficial cells are large, rounded with convex upper surfaces and contain one or two nuclei.

Glandular Epithelium: Classification

• Ways of classifying glandular epithelium:
  • Presence or absence of ducts:
    • Exocrine glands: possess ducts that carry their secretion to the surface of the epithelium
    • Endocrine glands: are ductless and secrete hormones directly into the blood.

Exocrine Glands

• Further classified according to the nature of secretion into:
  • Serous glands: secrete watery secretion rich in enzymes.
  • Mucous glands secrete viscid mucous secretion.
  • Muco-serous glands secrete both mucous and serous secretion.
• Classified by the mode of secretion:
  • Merocrine glands: secrete via exocytosis, leaving the cell membrane intact and with no part of the cell being lost.
  • Apocrine glands: secretion involves the loss of the apical part of the cell.
  • Holocrine glands: cells accumulate secretion, then burst and are totally included in the secretion.

Sensory Epithelium (Neuro-Epithelium)

• Epithelial tissue modified to act as a receptor.
• Consists of three types of cells: sensory cells, supporting cells, and basal cells.
• Found in taste buds in the tongue and olfactory epithelium in the nose.

Connective Tissue: Components and Functions

• Connective tissue is a basic tissue comprising of 3 components:
  • Connective tissue cells, connective tissue fibres, ground substance and tissue fluid
• Functions:
  • Connects and holds other tissues together.
  • Nutrients are transported from capillaries to the cells of other tissues.
  • Mechanism for defence and immunity.

Connective Tissue Fibres

• Types: collagenic, elastic, and reticular fibres.
• Collagen (white) fibres:
• Composed of the protein collagen
• Colourless elongated/wavy bundles
• They are non-branching, small fibrils.
• Collagenic fibres are acidophilic, thus staining pink with H&E stain
• Provide strength to the tissue
• Elastic (yellow) fibres:
• Composed of the protein elastin
• Long, thin, branched fibres form irregular networks.
• Elastic fibres are weakly acidophilic with Hx&E stain
• Take a dark brown colour with orcein stain
• Provide elasticity and strength to the tissue
• Reticular fibres:
  • Fine fibres arranged in networks.
  • Mainly formed of the protein collagen with carbohydrate.
  • Not visible in H&E preparations, but with silver stains, they take a black colour.
  • Provide the framework of the organs.

Connective Tissue Cells

• Fixed cells:
  • Stable, long-lived cells.
  • Include fibroblasts, fat (adipose) cells, and mesenchymal cells.
• Free (wandering) cells:
  • Short-lived, changing population.
  • Includes mast cells, plasma cells, macrophages, and leukocytes.
• Fibroblast:
  • The most common cell in connective tissue proper.
  • Originates from mesenchymal cells dividing and differentiating into active young fibroblasts and older fibroblasts are called fibrocytes
  • Active young cells have an oval pale nucleus with a prominent nucleolus and abundant basophilic cytoplasm.
  • Contains numerous rough endoplasmic reticulum, well-developed Golgi stacks, and transfer vesicles.
  • Functions include formation of both components of the intercellular substance, secretion of mucopolysaccharide (GAG) of the matrix, secretion of tropocollagen to form collagenic fibers, secretion of elastin to give elastic fibers, and repair of wounds.
• Plasma cells:
  • Origin: from B – lymphocytes that enter the loose connective tissue from the blood stream, and are not developed from mesenchymal cells.
  • Spherical or ovoid cells with an eccentric nucleus which has coarse heterochromatin (clock – face chromatin).
  • The cytoplasm is intensely basophilic except near the nucleus, where it is pale (negative Golgi area).
    • EM: numerous RER and free ribosomes, Golgi located near the nucleus, and secretory vesicles containing immunoglobulin. The cytoplasm contains rounded acidophilic Russell bodies.
  • Function: the formation of antibodies that circulate into the blood (humoral antibodies).
• Mast cells are rounded/spindle shaped with a rounded central nucleus.
  • The cytoplasm is studded with basophilic granules that stain metachromatically (purple to red) with Toluidine blue.
  • Function: secretion of heparin and histamine.
• Macrophages (histiocytes): derived from mesenchymal cells in prenatal life and from monocytes in postnatal life.
  • Oval shaped with eccentric condensed indented nucleus
  • The cytoplasm contains lysosomes.
  • Function: phagocytosis of foreign materials.

Ground Substances of Connective Tissue

• Composed of hyaluronic acid (a non-sulphated acid mucopolysaccharide glucosaminoglycan, GAG).
• Sulphated acid mucopolysaccharids and Glycoprotein, functions in attaching cells to fibres.
• Function: transport of nutrients and oxygen from capillaries to other tissues.

Classification of Connective Tissues

• Types of connective tissues:
  • Connective tissue proper:
    • Loose connective tissue
    • Dense white fibrous connective tissue
    • Dense elastic connective tissue
    • Reticular tissue
    • Adipose tissue
    • Mucous (embryonic) connective tissue
  • Specialized connective tissue:
    • Cartilage
    • Bone
    • Blood

Test your knowledge of connective tissues. Questions cover roles like cell attachment, staining properties, specialized types, and antibody production. Focus on plasma cells, ground substance, and key functions.