Cytology - The Cell Overview

Cytology - The Cell

• Cells are the fundamental, functional units of tissues and the basic unit of life.
• A cell consists of two main parts: cytoplasm and nucleus.

Cytoplasm

• Cytoplasm is composed of:

  • Cell organelles
  • Cell inclusions
  • Cytoskeleton
  • Cytosol

• Organelles are classified as membranous or non-membranous based on the presence or absence of cell membranes.

I. Membranous Organelles

• Cell membrane
• Mitochondria
• Rough endoplasmic reticulum (RER)
• Smooth endoplasmic reticulum (SER)
• Golgi apparatus
  • Lysosomes
• Peroxisomes
• Secretory vesicles

II. Non-Membranous Organelles

• Ribosomes
• Proteasomes
• Cytoskeleton

The Cell Membrane

• The cell membrane separates the cytoplasm from surrounding structures.
• It has selective permeability; regulating the entry and exit of compounds
• Electron microscope (EM) is used to visualize the membrane, showing a trilaminar appearance.
• Membranes are primarily composed of lipids, proteins, and carbohydrates.

Lipids in Cell Membrane

• The arrangement of lipid molecules (primarily phospholipids) forms the basic membrane framework.
• Phospholipid molecules have a hydrophilic polar head and hydrophobic nonpolar tails.
• In an aqueous environment, the hydrophilic heads face outward, and the hydrophobic tails face inward.
• This arrangement creates the trilaminar appearance.
• Lipids regulate the passage of water-soluble and nonpolar substances across the membrane.

Proteins in Cell Membrane

• Proteins are irregularly shaped and some span the entire membrane (transmembrane proteins).
• Proteins have various functions, including:
  • Maintaining cell structure
  • Cell-cell adhesion
  • Transport across the membrane
  • Acting as receptors
  • Enzyme activity

Carbohydrates in Cell Membrane

• Carbohydrates are attached to proteins (glycoproteins) or lipids (glycolipids) on the external surface of the membrane.
• This external carbohydrate layer forms the glycocalyx, which plays a role in cell adhesion and contains blood group antigens (in erythrocytes).

Vesicular Transport

• Macromolecules enter cells via endocytosis, involving membrane folds enclosing material.
• Types of endocytosis include:
  • Phagocytosis ("cell eating")
  • Pinocytosis ("cell drinking")
  • Receptor-mediated endocytosis
• Exocytosis is a vesicular transport process that releases materials from the cell.

Cell Organelles

• The cytoplasm houses various organelles with diverse functions.
• These organelles can be membranous or non-membranous.

1- Ribosomes

• Non-membranous, small particles composed of rRNA and proteins.
• Important in protein synthesis.
• Can be free in the cytoplasm or attached to the rough endoplasmic reticulum (RER).

2-Rough Endoplasmic Reticulum (RER)

• An interconnected network of membranes with ribosomes attached to its surface.
• Involved in protein synthesis and processing of proteins destined for secretion or incorporation into membranes.
• Continuous with the nuclear envelope

3-Smooth Endoplasmic Reticulum (SER)

• An interconnected network of membranes without ribosomes.
• Involved in lipid synthesis, metabolism of carbohydrates, detoxification of chemicals, and storage of calcium.
  • Usually more tubular than RER
• Continuous with the RER

4- Mitochondria

• Bounded by a double membrane (outer and inner); highly folded inner membrane (cristae)
• Important in cellular respiration and energy production (ATP synthesis).
• Contain their own DNA and ribosomes.

5-Golgi Complex

• Stack of flattened, membrane-bound sacs.
• Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

6- Secretory Vesicles

• Membrane-bound sacs that store and transport cellular products outside or within the cell.

7- Lysosomes

• Membrane-bound vesicles containing digestive enzymes.
• Involved in intracellular digestion and degradation of cellular waste.
• Types include primary (containing only digestive enzymes), secondary (containing digested material)
• Heterophagosome & Autophagosome refer to the type of secondary lysosome

8- Peroxisomes

• Membrane-bound vesicles containing enzymes involved in oxidation reactions, including the breakdown of hydrogen peroxide, which is toxic to the cell
• Important in detoxification.

9- Proteasomes

• Protein complexes that break down unwanted or damaged proteins.

The Cytoskeleton

• A network of protein filaments that provides structural support and facilitates cellular movement.
• Types of filaments include:
  • Microtubules: Hollow tubes made of tubulin, involved in cell shape, movement, and intracellular transport, found in centrioles and cilia.
  • Microfilaments (actin filaments): Thin filaments involved in cell movement, shape, and muscle contraction.
  • Intermediate filaments: Present in a variety of cell types, maintaining cell structure and providing strength.

The Nucleus

• The largest organelle in most cells.
• Contains the genetic material (DNA) and controls cellular activities.
• LM shows a central, denser region and
• EM reveals specific components like the nuclear membrane, chromatin, nucleolus, and nucleoplasm.
• Four main components: Nuclear membrane, chromatin, nucleolus, and nucleoplasm

1- Nuclear membrane (or envelope)

- Double-layered membrane surrounding the nucleus.
- Outer membrane is continuous with RER.
- The space between the membranes (perinuclear space) is continuous with the lumen of RER

2- Chromatin

- Composed of DNA and proteins.
- Consists of two types: euchromatin (loosely packed DNA) and heterochromatin (tightly packed DNA).

3- Nucleolus

• Dense structure found in the nucleus.
• Primarily responsible for ribosomal RNA synthesis and ribosome assembly.

4- Nucleoplasm

- Viscous fluid filling the nucleus; containing various components.

Cytoplasmic Inclusions

• Accumulations of metabolites or other substances in the cytoplasm.
• Not organelles; do not perform metabolic activities.
• Examples include lipid droplets, glycogen granules, pigments, crystals etc.