Histology

Tissues: collections of similar cells, usually, that work together.

1. Fundamental tissues – consist of and / or give rise to other more specialized tissues; derived from one or more of the three embryonic germ layers: endoderm, mesoderm, Ectoderm

   1. Epithelial – coverings and linings

   2. Connective – widely spaced cells with lots of extracellular matrix between them; range from solid tissues, such as bones, cartilage, ligaments, and tendons to fluid / liquid tissues like blood

   3. Muscular – made up of contractile cells called fibers

   4. Nervous – consist of conductive cells called neurons and non-conductive, supportive cells called neuroglial cells.

• Epithelial tissues

  1. Three types of cellularmorphologies:

     1. Squamous – flat, scale-like cells

     2. Cuboidal – roughly same dimensions in all directions; cube-shaped, roughly; usually possess a spherical nucleus

     3. Columnar – elongated; thin along one axis compared to the axis perpendicular to it; usually possess an oval or ovoid nucleus, elongated as well along the same axis of elongation as the cell.

  2. Origins – ectoderm and endoderm

  3. Morphologies – shaped as described above

  4. Layers

     1. One layer = simple

     2. More than one layer = stratified

  5. Extracellular matrix – basement membrane upon which the basal surface of deepest layer of cells is firmly attached; contains lots of collagen fibers to strengthen the tissue

  6. Avascular – no blood vessels, except for in the cochlear duct of the inner ear

  7. Cell junctions and Cell-matrix junctions

     2. Tight junctions – continuous bands of connective proteins firmly attach adjacent cells to each other; variable in epithelial tissues. E.g., many in tissues like skin; relatively few tight junctions in the mucosal linings of the rectum and vagina

     3. Adherens Junctions – points of attachment between adjacent cells; anchored in the cytoplasm by cytoskeletal proteins called actin filaments

     4. Desmosomes – “spot welds” consisting of connective proteins in Pin-point locations on surfaces of adjacent cells holding them together.

        1. deeply anchored into the cytoplasm by proteins called intermediate filaments

        2. Stronger than adherens junctions

        3. Found where there is more stress and abrasion forces occur such as in the skin and certain wall (tunic) layers in the GI tract.

     5. Gap junctions – holes or pores in the cell membranes interconnecting cell cytoplasm from one cell to another. Good for cell-cell communication and very abundant and common in cardiac muscle fibers of the heart. Allow ion travel from one cell to another to polarize and depolarize cells.

     6. Hemidesmosomes – anchors cells to extracellular matrix; “cell-matrix” junction.

        1. half of a desmosome

        2. anchored into the cell by intermediate filaments

     7. Actin-linked cell-matrix junctions – half of an adherens junction

        1. anchored into the cell by actin filaments

8. General features of epithelial tissues – cover the body, line cavities and organs, etc.

   1. Widely distributed throughout the body

   2. Rely on diffusion for nutrient uptake and waste removal.

• Connective tissues – bone, cartilage, tendons, ligaments, blood, bone marrow, adipose

  1. Origins – Mesenchyme cells (embryonic, fluid-like cells) of the Mesoderm.

  2. Examples and characteristics

     1. Cells are widely spread out.

     2. cell junctions at maturity

     3. Embedded or suspended in extracellular matrix.

        1. Bone matrix

        2. Cartilaginous matrix

        3. Blood plasma

  3. General features of connective tissues – vascularized, except for cartilage and probably some other exceptions

     1. Hold things together.

     2. Support

     3. Protect

     4. Transport / storage

  4. Specific examples

     1. Bone tissues of the skeletal system

     2. Ligaments that connect bone to bone in joints

     3. Tendons – attach muscle to bone or muscle to other tissues, e.g. other muscles or to skin as in many facial muscles

     4. Blood – fluid / liquid connective tissue; extracellular matrix is called blood plasma.

     5. Adipose tissue – fat tissue; adipocytes = fat cells; store fats mostly as oils

• Muscular – cells are called “muscle fibers.”

  1. Specific and general types

     1. Skeletal muscles, e.g., biceps brachii

     2. Smooth muscles, e.g., smooth muscles of the GI tract wall

     3. Cardiac – heart muscle tissue

  2. General characteristics of muscle tissues

     1. Contractile – shorten by force

     2. Extensible – stretch

     3. Excitable – can be stimulated by nerve impulses

     4. Elastic – contract, stretch, and return to original length

• Nervous system

  1. Neurons – nerve cells; conduct nerve impulses.

  2. Neuroglial cells – supportive, connective, and protective cells; non-conductive; do things like make the myelin sheath of many axons of the CNS and PNS

• Histological and cytological staining

  1. Acidic dyes – the pigmented part of the dye is negatively charged and will stick to positively charged structures.

     1. Eosin dye – bright pink, usually; stains cytoplasmic proteins and extracellular proteins

  2. Basic dyes – the pigmented part of the dye is positively charged and will stick to negatively charged structures, like chromosomes (DNA)

     1. Hematoxylin – basic dye that stains nuclei dark blue to purple; binds to negatively charged substances like DNA and RNA

  3. Vital stains - Living

     1. Blood smears – start off with living cells; the dye and the staining technique / procedure will eventually kill the cells, but you can see physiological characteristics of cells.

        1. Wrights stain – useful for differential stains of blood cells

           1. Can differentiate among RBCs WBCs and platelets

           2. Can differentiate among the diverse types of WBCs

     2. Janus green – stains mitochondria

     3. Neutral red – stains lysosomes