histology

• Histology is the study of the tissues of the body and how they are arranged to form organs
• Tissues consist of cells and extracellular matrix (ECM)
• Tissue preparation involves creating thin slices of tissue samples mounted on slides for study
• Fixation is done to preserve tissue structure using fixatives like 37% Formaldehyde for light microscopes and Glutaraldehyde for electron microscopes
• Dehydration involves removing water by soaking tissue samples in different alcohol concentrations ending in 100%
• Clearing is done by soaking in an organic solvent solution where paraffin and alcohol are miscible
• Infiltration involves soaking in melted paraffin or epoxy resins for electron microscopy samples
• Embedding is placing samples in tissue cassettes to allow hardening
• Staining is essential to distinguish histological elements, with Hematoxylin binding to acidic components and Eosin binding to basic components of tissues
• Simple microscopes contain only one magnifying lens, like Leeuwenhoek's with a maximum magnifying power of about x300
• Compound microscopes contain more than one magnifying lens and usually magnify objects about 1,000 times
• Compound microscopes have a resolving power of approximately 0.2 μm
• Total magnification is calculated by multiplying the magnifying power of the ocular lens by the magnifying power of the objective lens being used
• Photographs taken through compound light microscopes are called photomicrographs
• Electron microscopes have a much higher resolving power than compound light microscopes
• Transmission electron microscopes use an electron gun to fire a beam of electrons through a thin specimen, with a resolving power of approximately 0.2 nm
• Scanning electron microscopes bounce electrons off the surface of a specimen and have a resolving power about 100 times less than transmission electron microscopes
• Epithelial tissue classifications and naming:

  • First name of tissue indicates the number of layers: simple (one layer) or stratified (more than one layer)
  • Last name of tissue describes the shape of cells: squamous (wider than tall), cuboidal (as wide as tall), columnar (taller than wide)
  • Naming includes both layers and cell shape followed by 'epithelium' at the end
  • The name may include accessory structures like goblet cells, cilia, or keratin
• Characteristics of epithelial tissues:

  • High cellularity with closely packed cells
  • Avascular, relying on underlying connective tissue for oxygen and nutrition
  • Exhibits polarity with apical, lateral, and basal poles
  • Components include epithelial cells and basement membrane
• General classification of epithelial tissues:

  • Surface epithelium (lining/surface epithelium)
  • Glandular epithelium
• Simple epithelial tissues:

  • Simple squamous epithelium: single layer of flat cells for diffusion and filtration
  • Simple cuboidal epithelium: cube-like cells for secretion and absorption
  • Simple columnar epithelium: column-shaped cells for absorption and secretion, may contain goblet cells
• Stratified epithelial tissues:

  • Stratified squamous epithelium: many layers of squamous cells for protection, can be keratinized or non-keratinized
  • Pseudostratified columnar epithelium: all cells originate at the basement membrane, may contain goblet cells and cilia
  • Transitional epithelium: stretches and permits distension, found in urinary bladder
• Glandular epithelium:

  • Endocrine glands release secretions into blood or lymph ductless
  • Exocrine glands release secretions into surface epithelium, can be unicellular or multicellular
• Goblet cells:

  • Cup-shaped columnar cells that secrete mucin to form mucus
  • Locations include respiratory epithelium and digestive tract
• Multicellular exocrine glands:

  • Classified by complexity of ducts and morphology of secretory unit
  • Types include mucous and serous glands, can be simple or compound
• simple tubular: elongated secretory portion; duct usually short or absent

  • Mucous glands of colon; intestinal glands or crypts (of Lieberkühn)
• simple branched tubular - Several long secretory parts joining to drain into 1 duct

  • glands in the uterus and stomach
• simple coiled tubular - secretory portion is very long and coiled

  • sweat glands
• simple acinar - rounded, saclike secretory portion

  • small mucous glands along the urethra
• simple branched acinar - multiple saclike secretory parts entering the same ducts

  • sebaceous glands of the skin
• compound tubular - several elongated coiled secretory units and their ducts converge to form larger ducts.

  • submucosal mucous glands in the duodenum
• compound acinar - several saclike secretory units with small ducts converge at a larger duct

  • exocrine pancreas
• compound tubuloacinar - ducts of tubular and acinar secretory units converge at larger ducts

  • salivary glands