Unit 2: Histology Flashcards
Histology
The study of tissues and their cellular components. It is used in the recognition of pathology and in the discovery of how abnormal biochemical and physiological processes result in disease.
Morphology
The examination of cellular arrangements and organization to examine organ structure in terms of form and function.
4 major groups of tissues
Connective, epithelium, muscle, and nervous
Epithelium
Found covering and lining the surfaces of the body and organs.
Structure: sheets of closely packed cells
Function: protection, exchange, secretion
Connective tissue
It’s major function is to hold tissues together. Some have a generalized role in supporting and protecting organs while others have specialized functions (bones provide muscle attachment, blood facilitate the transport of nutrients and removal of wastes).
Structure: spare cells in extracellular matrix
Function: binding and support of other tissues
Muscle tissue
Unique property of being able to move when cells contract. 3 types of muscles:
Skeletal - permits movement of the body
Cardiac - moves blood around the vasculature
Structure: long cells (fingers) with contractile proteins.
Function: movement of body parts
Smooth - facilitates movement of food along the alimentary canal.
Nervous tissue
Highly complex (especially in the brain)
Structure: neurons with branching extensions
Function: transmission of nerve signals
Steps for preparation of specimens for study by standard light microscopy
- Obtaining a sample with the proper orientation
- Fixing or preserving the specimen
- Sectioning the specimen into thin slices
- Staining the specimen for visualization
- Microscopic examination of the specimen
Orientation
First step in light microscopy; orientation helps to visualize the morphology of a structure. This is because improper orientation may cause the tissue to become damaged during later stages of preparation. Most tissues are embedded flat with a margin of embedding medium surrounding them for support. During sectioning the tissue should be oriented in a way that results in least resistance to the knife.
Tubular structures
Tissue that may require a specific orientation in light microscopy preparation; structures with tubular morphology (sweat gland ducts or blood vessels) are usually cut to display the cross-section of the lumen.
Epithelial Biopsies
Biopsies of epithelia, such as the skin or the intestines, are cut in a plane at right angles to the surface, and oriented in a certain way so that the knife cuts from the basement membrane through to the apical epithelial membrane. This way, compression and distortion of the epithelial surface is minimized.
Muscle Biopsies
Sections of muscle may be cut in transverse or longitudinal planes.
Multiple tissue samples
To facilitate comparison, multiple tissue samples are oriented similarly, side by side, with, for example, the apical epithelial surfaces all facing in the same direction.
Fixation
Second step of tissue preparation; helps to prevent the specimen from undergoing physiological changes. Also helps to deactivate internal enzymes within cells to prevent degradation from within. Can also help to increase the mechanical strength of tissue and reduce the growth of microbe colonies.
Methods of fixation
Chemical perfusion, heat fixation, etc.
Embedding
Third step in light microscopy preparation; Embedding involves the following: correctly oriented specimens are fixed and encased within a suitable medium, providing them with elasticity and ensuring they are supported during sectioning and subsequent analysis.
Paraffin wax embedding
The standard method for the preparation of finely sectioned specimens for histological analysis by microscopy. Paraffin wax fills the tissue, supporting all cellular components and resisting distortion during sectioning.
Frozen sections
Alternative to paraffin wax because the wax can be too damaging or time consuming. Specimens are rapidly frozen by immersion in liquid nitrogen so that the tissue hardens into a solid mass.
Sectioning
Fourth step in light microscopy preparation; the slicing of embedded specimen using a microtome and attachment to a surface (usually a slide) for analysis under a microscope.
Stains
Fifth step in light microscopy preparation; Staining is require to see the colorless cell.
Methyl blue
Stain widely used in differential staining of tissue sections, where more than one chemical stain is used, such as trichrome staining.
It stains collagen blue.
Hematoxylin and eosin (H&E)
Staining is used to differentiate nucleic acids in the nucleus from basic proteins within the cytoplasm.
Cell nuclei stain purple/black (hematoxylin) and most components of the cell cytoplasm stain pink/red (eosin).
van Gieson
This stain is used for general staining of connective tissue fibers.
Collagen fibers stain pinkish-red, muscle stains yellow, and elastic fibers stain brown-black.
Trichrome
This method uses three different dyes to differentially stain cellular components. One of these is often methyl blue.
SIlver nitrate
Silver nitrate solution can be used to stain tissues in several different ways. One method, called Golgi’s method, selectively stains the entirety of some neurons black, thus revealing their cellular architecture. A different technique can be used to highlight reticular fibers.
Immunohistochemical methods
utilizes a different approach to visualize cellular contents: rather than dyes, it exploits the highly specific interaction between antibody and antigen to allow precise identification, localization, and in some cases, quantification, of cellular proteins, such as receptors or enzymes.
Light (optical) microscope
How does it work: Uses beams of visible wavelengths of light, focused through glass lenses, to produce magnified images.
Magnification: Up to 2000x.
Lens: Glass.
Uses: ability to view color, relatively low cost makes this device more popular in education and medical sectors.
Stains used: those highlighting tissue/cellular components in different colors. Chromatic dyes used.
Disadvantages: limited magnification and depth field.
Types: simple (one lens), compound (many lenses), and digital light microscope.
Electron microscope
How does it work: Uses particle beams of electrons, focused through magnetic lenses and metal apertures, to produce highly magnified 2D and 3D images.
Magnification: up to 2000000x
Lens: electrostatic/electromagnetic
Uses: Used to visualize smaller structures not visible by light microscopy. Allows 3D visualization of structures. Is used in observation of dry, fixed specimens of micro-organisms, cells, large molecules, biopsy samples, metals, and crystals etc.
Stains used: Stains highlight tissue by increasing electron absorption. Electron-dense heavy metals often used.
Disadvantages:
Expensive to build, maintain, and run.
Very sensitive to vibrations and external magnetic currents.
More elaborate preparation of specimens often needed and specimens must be dry.
Not possible to observe living specimens or color.
Types: Transmission electron microscope (TEM) for 2D images, and scanning electron microscope (SEM) for 3D images.
Characteristics of epithelial tissues
Avascular (no blood cells entering between the cells. The cells are maintained via blood vessels lying deep within the basement membrane.
Highly cellular with little extracellular material.
High regenerative capacity
Functions of epithelial tissue
Selective absorptive barriers regulating the movement of substances into and out of the body.
Secretory surfaces, comprising the functional units of glandular secretion.
Protective surfaces, such as the skin.
What is the classification of covering and lining epithelium based on?
The three-dimensional shape of the epithelial cells comprising it, as observed by light microscopy.
The way in which the epithelial cells are arranged into layers.
Squamos
Epithelial cell; flat, plate-like cells
Cuboidal
Epithelial cell; cube-like cells.
Columnar
Epithelial cell; column-shaped cells. Greater height than width.
Transitional
Epithelial cell; cuboidal or columnar can change shape during stretching and compression.
Simple Epithelium
A single layer of cells.
Stratified Epithelium
Multiple layers of cells.
Pseudostratified Epithelium
Cells appear to be arranged in layers, but careful observation reveals that all cells are anchored to the underlying basement membrane.