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>>Refer to Foundations/Pracs/Introduction to Histology - Cells and Tissues for images

Epithelium

  • Epithelium forms continuous layers of cells that cover surfaces and line cavities of the body.
  • These cavities include
    • Closed peritoneal, pleural and pericardial cavities, where the epithelium is called mesothelium
    • Open cavities (digestive, respiratory and urogenital organs) which connect with the outside.
  • Epithelium lines the cardiovascular and lymph passageways as endothelium.
    • The parenchymal (secretory) cells of glands (e.g. sweat, salivary) are also epithelium.

Components

  • Epithelial cells are always in close apposition to each other, with a space between membranes of only about 20nm.
  • A small amount of intercellular material, called cement substance (glycosaminoglycan) allows cells to glide over each other and offers only minimal resistance to the migration of leucocytes and other connective tissue cells through the epithelial layers.
  • Epithelial attachment points (junctional complexes) occur between neighbouring epithelial cells to hold adjacent membranes in close apposition.
    • They also serve as anchoring sites for the fine filaments of the cytosokeleton, which assists in stablising the cell shape.
  • Epithelial cells rest on a basal lamina separating them from underlying connective tissue (CT). Epithelium is avascular and for its nutrition depends on diffusion of substances across the basement membrane.

Classification of epithelia

  • Based on 3 morphological characteristics
    • The number of cell layers: a single layer = simple epithelium; more than one layer = stratified epithelium.
    • The shape of the component cells when seen in sections taken at right angles to the epithelial surface: the shape may be squamous (flattened), cuboidal (about equal dimensions), or columnar (taller than it is wide).
    • The presence of surface specialisations e.g. cilia, microvilli and keratin.

Main functions of epithelium

  • Protection of underlying tissues of the body from abrasion, radiation, dehydration, bacterial invasion e.g. the epidermis of the skin
  • Regeneration e.g. in skin wound healing and epidermal replacement (approx. every 28 days), in the renewal of the lining cells of the uterus following menstruation and in the replacement of the cells lining the gastro-intestinal (HI) tract (every 4 to 6 days). The turnover rate is related to their location and function.
  • Secretion by glandular epithelial cells of products that are expelled into the blood stream (e.g. hormones by thyroid follicles), into ducts and hollow organs (e.g. stomach acids and pancreatic enzymes), or onto the skin (sweat from sweat glands and sebum from sebaceous glands).
  • Absorption between body compartments via selective permeability of intercellular junctions between epithelial cells. Transcellular transport of molecules across epithelial layers e.g. lipids in the small intestine and selective re-absorption in the kidney tubules (e.g. sodium).
  • Detection of sensations via the neuro-epithelium of the taste buds, specialised hair cells in the ear and tactile endings in the skin.
  • Lubrication by various types of glandular epithelial cells, which secrete copious amounts of mucous, a viscous product rich in mucopolysaccharides and mucoproteins. Mucous is an excellent lubricant and aids the movement of food along the alimentary tract. Mesothelial cells, lining the closed body cavities, secrete a thin serous fluid that prevents friction of organs rubbing against each other.
  • Excretion by epithelial cells (kidney tubules, sweat glands), which filter waste products from the blood, and then excreted as urine or sweat.
  • Diffusion of gases (O2/CO2) by endothelium of capillaries or lungs.

Glands

  • Glands are invaginations of epithelial surfaces that are formed during embryonic development by proliferation of epithelium into the underlying connective tissues to form secretory units.
    • Parenchyma = {secretory unit, their glands}
    • Stroma = {elements of the connective tissue that support the parenchyma}
  • Endocrine glands lose the ducts and secrete directly into the blood (e.g. hormones)

Connective tissue (CT)

  • Connective tissues (CT) are the supporting framework for all tissues and organs of the body.
  • They provide the means of anchoring and binding organs together as well as providing the packing tissue between them.
  • CT varies in structure and character from loose (subcutaneous areas), to dense irregular (dermis of skin), to dense regular (ligaments and tendons), to firm and flexible (cartilage in the trachea, intervertebral discs, external ear), to rigid (calcified bone), to circulating (blood and lymph). Despite this wide range, all CT have:
    • An intercellular matrix composed of an amorphous ground substance in which are embedded
    • Cells and one or more types of
    • Extracellular fibres (elastic, collagen, or reticular).
  • In blood and lymph, the fibres are strands of fibrin, seen only during clotting.
  • The predominant cell type of CT is the fibroblast, which makes fibres. Other types are wandering cells such as mast cells, plasma cells, various leucocytes and macrophages.

Functions of connective tissue

  • Transport: blood, as a fluid CT, transports O2 and nutrients in the body and removes CO2 and metabolites from the tissues
  • Support: ligaments, tendons (connecting) and rigid forms e.g. bone, cartilage. Cartilage forms the temporary skeleton of the foetus.
  • Repair: scar tissue formation after injury e.g. sprains, wounds, fractures
  • Defense: phagocytosis (macrophages), antibodies (plasma cells), inflammatory response (mast cells which release histamine)
  • Storage: fats (lipids), bone (calcium)
  • Packing: space between epithelium, muscle, glands.

Cartilage

  • Chondroblasts are the cells that produce cartilage.
    • When they mature, they become embedded in the matrix. Then they are called chondrocytes
    • Chondrocytes live in holes in the matrix called lacunae.
  • Matrix of cartilage is the pink central area.
    • Made of chondroitins (sulphated glycosaminoglycans) and elastin fibres (wiki: mainly collagen and proteoglycans)
    • The matrix is dense surrounding the cells (and hence darker) than the sparse matrix further out (ligh pink)

Poor blood supply (cartilage is avascular), so if you damage it, the repair is terrible.

  • Cartilage:
    • Elastic: ear, larynx, epiglottis
    • Fibrous: IV disc, pubic symphysis
    • Hyaline: avascular, no nerves. Larynx, trachea, bronchi, articular surfaces of bones.

Muscle

  • Muscle cells (fibres; the cell is longer than it is wide) produce force which can be used for movements such as locomotion, contraction of organs (e.g. bladder) and pumping movements of the body.
    • This is achieved by the muscle cells’ contractility state by changing their length and developing tension.
    • The contractile elements of muscle cells are (myofibrils) composed of specific arrays of myofilaments, the proteins (actin and myosin) responsible for the contractile capability of the cell.

Types of muscle

  • Skeletal muscle (striated, voluntary): each skeletal muscle fibre is long, cylindrical and multinucleated (diameter range 10 to 100 um). Much of the skeletal muscle cell is composed of longitudinal arrays of cylinder-shaped myofibrils (each 1 to 2 um in diameter). This strictly ordered parallel arrangement of the myofibrils is responsible for the cross-striations of light (I, actin) and dark (A, myosin) banding that is characteristic of skeletal muscle viewed in longitudinal section.
    • Locations: attached to bones or skin e.g. biceps, hamstrings, buccinators etc.
  • Cardiac muscle (striated, involuntary): heart muscle differs from skeletal and smooth muscle in that it possesses an inherent rhythmicity as well as the ability to contract spontaneously. Each cell has a single, large, oval, centrally-placed nucleus, though occasionally two nuclei are present. The banding patterns of cardiac muscle fibres are identical to skeletal muscle.
    • Location: found only in the heart walls and septa (myocardium)
  • Smooth muscle (non-striated, involuntary): Smooth muscle cells are fusiform (spindle-like shape that is wide in the middle and tapers at both ends), and elongated (av. Length from 20um to 200um and diameter from 5 to 6 um). The cells taper at either end, with a single centrally placed cigar-shaped nucleus housing two or more nucleoli. There are no cross striations.
    • Locations: areas such as the walls of blood vessels and hollow organs (digestive, respiratory, urinary, etc), dermis of skin (erector pili muscle)

Nervous tissue

  • The brain and spinal cord comprise the central nervous system (CNS).
  • The nerves that emerge from the spinal cord and brain to pass to parts of the body are the peripheral nervous system (PNS).
  • Nervous tissue, with many interconnections, forms a complex system of neuronal communication within the body and is specialised for detecting stimuli, integrating functions, controlling effectors and higher functions. Nervous tissue consists of cell bodies, cell processes (nerves), and neuroglia (supporting cells).

Neurons: Structural and functional units of the nervous system

  • These cells (around 12 billion) are responsible for the receptive, integrative, and motor functions of the nervous system. They can generate nerve impulses (irritability) and can transmit these impulses along their processes (conductivity).
  • They range in diameter from 5 to 150 um and contain 3 parts: a cell body, multiple dendrites, and a single axon.
  1. Cell body (soma, perikaryon) is the region of the neuron containing a large, pale-staining spherical, nucleus with a conspicuous nucleolus and perinuclear cytoplasm.
  2. Dendrites project from the cell body and are specialised for receiving (afferent) stimuli from sensory cells, axons and other neurons, which are then transmitted towards the soma.
  3. Axons arise as a single thin process extending longer distances from the cell body than the dendrite. As with dendrites, the terminals of the axon are branching and terminate in end bulbs (terminal boutons), which come close to another cell and form a synapse.
Structure of a Neuron
  • Peripheral nerve fibres: Peripheral nerves are bundles (fascicles) of nerve fibres (axons) surrounded by several CT sheaths. Each bundle contains sensory and motor components.
  • Myelinated fibres (1-20 um diameter): Myelin (rich in lipid) is the membrane of the Schwann cell organised into a spiral sheath that is wrapped several times around the axon. Schwann cells are cells whose cytoplasm contains a flattened nucleus, a small Golgi apparatus, and a few mirochondria. Myelinated fibres are capable of rapid transfer of impulses (touch sensory pathways).
  • Unmyelinated fibres (less than 2um in diameter): Some axons in the PNS are surrounded by Schwann cells but not wrapped with layers of myelin. They are found in pain and temperature sensory pathways and motor paths to the viscera.
  • Peripheral nerve endings: nerve fibres convey impulses to and from non nervous structures, such as the skin and muscles, where they terminate in peripheral nerve endings. These terminations may be sensory receptors responding to the sensation of touch (e.g. Meissner, Pacinian corpuscles), pain, temperature and so forth, or motor endings such as a complex ending (motor end plate or neuromuscular junction – NMJ), which lies at the junction between the motor nerve fibre and the skeletal muscle fibre.

See also