Histology: Cells, Tissues, Organs, and Systems

• Histology studies the structure of cells, tissues, and organs, correlating structure with function.
• The human body comprises cells, tissues, organs, and systems.

Cells

• Cells form the simplest structural units in living organisms.
• Cells consist of the nucleus and cytoplasm.
• Cells are small (4 – 200 µm) and examined microscopically.

Tissues

• Tissues consist of similar cells performing specific functions.
• The four basic tissues are: Epithelial, Connective, Muscular, and Nervous.

Organs

• Organs are assemblies of different tissues (e.g., stomach, lung, kidney), each with a special function.

Systems

• Systems involves collections of organs performing related functions like digestion, respiration, and excretion.
• 1 mm = 1000 µm
• 1 µm = 1000 nm

Microtechnique Steps

• A microtechnique involves tissue preparation.

Taking a Specimen

• Small tissue pieces are taken from an anesthetized animal or post-mortem.

Fixation

• Neutral buffered formalin or Bouin's fluid facilitates fixation.
• Fixation retards autolysis and prevents putrefaction.
• It also hardens the tissue and maintains its architecture.

Dehydration

• Tissues are dehydrated using ascending grades of ethyl alcohol (50%, 70%, 90%, 100%).

Clearing

• Xylene clears the tissue to make it transparent and allow light to pass through.

Embedding

• Specimens are embedded in melted Paraffin wax, forming blocks.
• Embedding makes the tissue bulky and hard for thin sectioning by microtome. Tissue is left to harden for about 3 hours.

Sectioning

• Microtomes section paraffin blocks into thin slices (5 – 10 µm).

Mounting and Staining

• Thin paraffin sections are put on clean glass slides smeared with glycerin.
• Staining is necessary as tissues are colorless.
• Hematoxylin and Eosin (H&E) are commonly used stains.

Hematoxylin (H)

• H is a basic stain (blue) that binds to acidic cell components.
• Affinity of acidic structures to basic stains is called basophilia.
• The nucleus (DNA & RNA) stains blue with hematoxylin, indicating it is basophilic.

Eosin (E)

• E is an acidic stain (red) that binds to basic cell components.
• Affinity of basic structures to acidic stains is called acidophilia.
• Cytoplasm usually stains red with eosin, so the cytoplasm is acidophilic.
• Cytoplasm differs according to cell function and the number of organelles.

Ordinary Light Microscope (LM)

• LM uses day or electric light for illumination.
• Light rays are collected and focused by the condenser lens.
• Has an optical system composed of the ocular lens and objective lenses.
• The ocular lens magnifies 2.5, 5, or 10 times
• There are more than two objective lenses
• Objective lenses magnify 40, 60, or 100 times.
• Total magnification is calculated by multiplying the power of the ocular lens by the power of the objective lens.
• Under the light microscope, total magnification is often considered limited.
• Magnification may only be 400, 600, or 1000 times.

Transmission Electron Microscope (TEM)

• TEM uses a beam of electrons for illumination.
• The electron beam passes through the specimen and focuses on a fluorescent screen using electromagnetic fields.
• Electron microscopes magnify at a very high power for studying cell ultrastructure.