Intro to Histology - Week 0

The Cell: Fundamentals

• Cells are the basic structural and functional units of all living organisms.
• Approximately 40 trillion cells exist in the human body, which may be eukaryotic or prokaryotic.
• Most cells possess one nucleus; however, some can be multinucleated.
• Protoplasm, the living part of a cell, consists of the nucleus and cytoplasm, surrounded by a plasma membrane.
• Internal cell structure is often modified to reflect specific functions.

Cell Differentiation

• Differentiation begins with the zygote, which undergoes divisions leading to blastomeres and embryonic stem cells.
• Cell differentiation occurs in the fetus and is influenced by gene expression.
• Specific gene sets mediate cytoplasmic activities, positioning cells into specialized tissues.

Basic Tissue Types

• Four fundamental tissue types exist:
  • Epithelial Tissue: Covers surfaces, lines cavities, and forms glands.
  • Connective Tissue: Provides structural and functional support to other tissues, distinguished by its extracellular matrix.
  • Muscle Tissue: Composed of contractile cells, enabling movement.
  • Nervous Tissue: Responsible for receiving, transmitting, and integrating information, controlling body activities.

Visualization of Tissues

• Light microscopy is commonly utilized for visualizing tissue samples.
• Resolving power defines a microscope's ability to distinguish two close points.
• Specimens absorb visible light, which is focused and magnified by glass lenses.

Preparation of Tissues for Study

• Tissue preparation involves creating thin slices or sections for visual examination via transmitted light.
• Key steps in tissue preparation:
  • Fixation: Preserves structure and prevents degradation; commonly involves buffered formaldehyde or glutaraldehyde for electron microscopy.
  • Dehydration: Gradual extraction of water through increasing ethanol solutions.
  • Clearing: Replacement of ethanol with an organic solvent that mixes with both alcohol and embedding media.
  • Infiltration: Cleared tissue is immersed in melted paraffin at 52°-60°C, evaporating the clearing solvent.
  • Embedding: Tissue is allowed to harden in paraffin at room temperature.
  • Trimming: The hardened block is trimmed for microtome sectioning, typically at thicknesses of 3-10 micrometers for light microscopy.