Histology and Cytology Overview

Histology and Cytology

• Cytology is the study of cells, including their structure, function, and division.
• Histology is the study of tissues and organs, including their structure and function.
• Cells form tissues, tissues form organs, and organs form systems, which make up the body.

Tools of Studying Histology

• Microscopes are used to study cells and tissues at different levels of magnification.

The Cell

• Cells are the smallest structural and functional units of living organisms.
• Cells are made of protoplasm, which is divided into cytoplasm and nucleus.
• Cytoplasm includes cytoplasmic organelles, cytoplasmic inclusions, and the cytoplasmic matrix.

Cytoplasmic Organelles

• Membranous Organelles

  • Cell membrane (plasma membrane, plasmalemma): The outer membrane of a cell, acting as a barrier between the internal and external environments. It is too thin to be seen with light microscopes (8-10 nm), but can be stained. At the electron microscope level, it has a trilaminar structure (3 layers).
  • Mitochondria: Involved in cell respiration and energy production. Contain ATP synthase and cristae. The matrix has enzymes, ribosomes, DNA, and RNA. The number varies depending on cell type (e.g., absent in red blood cells, hundreds in muscle and liver cells).
  • Rough Endoplasmic Reticulum (RER): Associated with ribosomes, involved in protein synthesis (secretory proteins, lysosomal proteins, and membrane proteins) and glycosylation (adding carbohydrates to proteins to form glycoproteins). Appears basophilic in light microscopes; tubules, vesicles and flattened cisternae under electron microscope.
  • Smooth Endoplasmic Reticulum (SER): Devoid of ribosomes. Involved in steroid hormone synthesis, drug detoxification (liver cells), lipid synthesis (intestinal cells), calcium ion release/storage (muscle cells), and HCl production (stomach cells). Appears acidophilic in light microscopes, with tubules and vesicles under electron microscope.
  • Golgi apparatus (Golgi complex): Involved in packaging, concentrating, and modifying secretory products, as well as producing primary lysosomes. Composed of flattened cisternae, Golgi vesicles, and a forming and maturing face.

• Non-membranous Organelles

  • Ribosomes: Involved in protein synthesis.
  • Centrioles: Involved in cell division.
  • Cilia and Flagella: Involved in motility.
  • Microfilaments and Microtubules: Contribute to cell structure and function.

Cell Membrane Molecular Structure

• Fluid Mosaic Model: Describes the arrangement of components in the cell membrane.
• Lipids (30%):
  • Phospholipids form a bilayer. Polar heads face outwards; nonpolar tails face inwards.
  • Glycolipids and Cholesterol are also present.
• Proteins (60%):
  • Extrinsic (peripheral) proteins are only partially embedded.
  • Intrinsic (integral) proteins are firmly attached to the lipid bilayer – some are transmembrane proteins (include cell receptors).
• Carbohydrates (10%):
  • Glycocalyx: Carbohydrates are attached to proteins (glycoproteins) and lipids (glycolipids) on the cell surface.
  • Functions: Cell recognition, adhesion, protection from chemical and mechanical damage, and hormone binding.

Cell Membrane Modifications

• Cell junctions (for cell-cell attachment).
• Microvilli and cilia (for motility or surface area increase).

Cell Transportation (Endocytosis and Exocytosis)

• Endocytosis: Engulfing of solid particles (phagocytosis) or fluid droplets (pinocytosis) by forming vesicles.
• Exocytosis: Release of substances from vesicles to the outside of the cell.

Mitochondria

• Involved in cell respiration and energy production.

Endoplasmic Reticulum (ER)

• An irregular network of branching and anastomosing tubules, cisternae, and vesicles within the cytoplasm.

  • Rough ER: Associated with ribosomes; protein synthesis and glycosylation.

  • Smooth ER: Involved in steroid hormone synthesis, drug detoxification, and lipid synthesis.

Golgi Apparatus (Golgi Complex)

• A membranous organelle involved in packaging, concentration, and modification of secretory products, and production of primary lysosomes.

Lysosomes

• Membrane-bounded vesicles containing hydrolytic enzymes that break down macromolecules.

  • Primary lysosomes: Newly formed, containing hydrolytic enzymes.
  • Secondary lysosomes: Formed by fusion with other vesicles; breakdown of material within.
  • Residual bodies: Terminal stage of lysosomal activity; may contain indigestible material.

• These structures are involved in breaking down materials.