General Histology and Cytology Overview

Histology

• The study of tissues
• Microscopic observations reveal that the body is composed of different tissues
• Tissues are made of cells

General Histology

• Covers the study of cells (cytology) and body tissues

Body Tissues

• Epithelial
• Connective
• Muscular
• Nervous

Cytology

• The study of cells

The Cell

• The basic structural and functional unit of living organisms
• Contains:
  • Protoplasm
  • Nucleus
  • Cytoplasm

Cytoplasm

• Part of the protoplasm located around the nucleus
• Responsible for synthetic and metabolic activities
• Consists of:
  • Cytosol (structure's matrix)
  • Cytoplasmic organelles
  • Inclusions
  • Cytoskeleton

Cytoplasmic Matrix

• Non-organelle component of the cytoplasm
• Occupies the intracellular spaces between organelles and inclusions
• Contains soluble proteins, lipids, carbohydrates, and small ions

Cytoplasmic Organelles

• Permanent and living structures within the cytoplasm
• Perform specific functions
• Two types:
  • Membranous organelles
  • Non-membranous organelles

Membranous Organelles

• Possess a bounding membrane
• Include:
  • Cell membrane
  • Mitochondria
  • Endoplasmic reticulum
  • Golgi apparatus
  • Lysosomes
  • Peroxisomes

Non-Membranous Organelles

• Lack a bounding membrane
• Include:
  • Ribosomes
  • Centrioles

Organelles

• Definition
• Observation methods:
  • Light Microscope (LM)
  • Electron Microscope (EM)
• Functions

Cell Membrane

• Also known as plasmalemma or plasma membrane
• Outer membrane of the cell
• Acts as a barrier between the internal and external environment
• Too thin to be seen with light microscope (LM)
• Appears as a trilaminar structure with electron microscope (EM)
  • Consisting of outer and inner electron-dense layers separated by an intermediate electron-lucent layer

The Cell Boundary

• Cell membrane condensation on the inner aspect of the cell
• Stain condensation on the carbohydrate-rich coat
• Obliquity of sections

Unit Membrane

• Is a trilaminar structure
• The image shows a diagram of a cell membrane. The unit membrane is labelled with an arrow.

Molecular Structure of the Cell Membrane

• Fluid mosaic model
• Composed of:
  • Phospholipids
  • Proteins
  • Carbohydrates

Phospholipids

• Form a central bimolecular layer
• Each molecule has two ends:
  • Polar or hydrophilic (water-loving) end
  • Non-polar or hydrophobic (water-fearing) tail
• Hydrophilic ends are directed outward
• Hydrophobic tails are directed inward toward the center of the membrane

Proteins

• Second major constituent of the cell membrane
• Arranged as globules moving freely within the lipid layer
• Two types:
  • Intrinsic or integral proteins:
    • Firmly attached to the lipid bilayer
    • Some extend throughout the entire thickness of the membrane and constitute transmembrane channels
  • Extrinsic or peripheral proteins:
    • Only partially embedded to either aspect of the membrane

Carbohydrates

• Conjugated with protein (glycoprotein) and lipid (glycolipid) molecules of the cell membrane
• Project from the outer surface of the cell membrane as cell coat or glycocalyx

Functions of the Cell Membrane

• Passive Diffusion
• Facilitated Diffusion
• Active Transport
• Selective transport
• Endocytosis and exocytosis

Passive Diffusion

• Entrance of small molecules
• Depends on a concentration gradient across the plasma membrane
• Diffusion of lipid-soluble substances, oxygen, CO2, water, and small ions

Facilitated Diffusion

• Depends on a concentration gradient across the plasma membrane
• Transport of large water-soluble molecules such as glucose and amino acids
• Requires carriers

Active Transport

• Requires the utilization of energy (ATP)
• Sodium-potassium pump (sodium out and potassium in)

Selective Transport

• Depends on the presence receptors to pick up specific molecules
• Example: hormones

Endocytosis

• Engulfment of substances by the cell membrane

• Two types:

  • Phagocytosis: engulfment of solid particles
  • Pinocytosis: engulfment of minute droplets of fluid

• The engulfed material is surrounded by pseudopodia

• After complete surrounding, the plasma membrane fuses forming a vesicle (phagosome or endocytotic vesicle)

• The phagosome detaches and floats freely

• The image shows a diagram of an endocytosis process. The left side of the diagram shows phagocytosis where a cell consumes a solid particle. The right side of the diagram shows pinocytosis where a cell consumes drops of fluid.

Exocytosis

• Release of substances from the cell

• Some membranous vesicles fuse with the cell membrane

• Release their contents outside the cells

• Occurs in many secretory processes

• The image shows a diagram of exocytosis. A vesicle containing proteins fuses with the cell membrane, releasing the proteins into the extracellular environment.

Other Functions of the Cell Membrane:

• Transmission of nerve impulses in muscle and nerve cells
• Myelin sheath formation (Schwan cell around peripheral nerves)
• Share in the formation of microvilli, cilia, flagella, and cell junctions

Functions of the Cell Coat:

• Mechanical and chemical protection
• Induction of immunological (antigen-antibody) response
• Site for binding of hormones
• Formation of intercellular adhesions
• Formation of the basement membrane
• Cell recognition

Mitochondria:

• Membranous organelles
• Involved in cell respiration and energy production
• With LM:
  • Granules
  • Rod-like
  • Thread-like

Mitochondria Number

• Highly variable based on the energy requirements of the cells
  • Liver cells (active cells) contain as many as 1000 mitochondria
  • Small lymphocytes (inactive cells) contain very few

Mitochondria Location

• Motile organelles
• Localize at intracellular sites of high-energy requirements such as basal regions of ion-transporting cells

Mitochondria with EM

• Ovoid or elongated structures bounded by two membranes

• The inner membrane is covered with tiny spherical projections about 9 µm in diameter supported at narrow stalks, called inner membrane spheres or elementary particles

• The image shows a diagram of a mitochondrion viewed with an electron microscope. It clearly shows an outer membrane and an inner membrane. The inner membrane has several invaginations called cristae. In the center, a matrix is shown which is bounded by the inner membrane.

Mitochondria Matrix

• Contains many electron-dense granules called matrix granules that are the sites for Ca++ ions storage
• Contains DNA and RNA
• The matrix is the site for mitochondrial growth, division, and protein synthesis.

Mitochondria are Semiautonomous:

• Motile
• Divide by binary fission
• Contain DNA and RNA
• Synthesize proteins

Binary Fission

• The image shows a series of diagrams of binary fission. The first diagram shows a mitochondrion with two copies of its DNA molecule. The second diagram shows the mitochondrion getting elongated. The third diagram shows a complete separation of the mitochondrion.

Mitochondria Functions:

• House the chains of enzymes that catalyze reactions that provide the cells with most of its ATP (adenosine triphosphate)
• On demand, the ATP yields its high-energy phosphate bond to another molecule and becomes transformed into ADP
• Within the mitochondrial matrix, ADP is transformed again into ATP

Mitochondria Functions:

• These processes take place within:
  • Mitochondrial matrix
  • Inner mitochondrial membranes
• The matrix contains enzymes of the Krebs cycle and fatty acid oxidation
• The inner membrane contains the cytochromes and the enzymes involved in ATP production
• Participate in the regulation of calcium level within the cytosol

References

• Netter's Essential Histology, Ovalle, 2013
• Functional Histology, Kerr, 2009
• Wheater's Review of Histology & Basic Pathology, Baldwin, 2009
• Color Textbook of Histology 3rd Ed, Hiaat, 2014
• Internet source