Histology Fundamentals

• The notes pertain to Histology I, a guide to the microscopic structure of cells, tissues, and organs, by DVM, MVSc Marwan Taher Abofila.

Overview of Topics Covered

• General/Basic Histology
  • Includes introduction to the topic, cytology, cytogenetics, tissues (epithelial), connective tissue, cartilage, bone and blood
• Special/Systematic Histology
  • Consists of muscular tissue, nervous tissue, cardiovascular, immune lymphatic, and integumentary systems.

Introduction to Histology

• In Latin, "Histo" means tissue, and "Ology" means science.
• Histology is the science of the microscopic study of the components of humans and animals, formed by cells and extracellular material that form tissues, organs, and body systems.
• It involves tissue biology, focusing on how the structure and cell arrangement performs specific functions of organs.

The Study of Histology and its Objectives

• The main objective of a histology course is to understand the microanatomy of cells, tissues, and organs and to correlate morphology (structure & shape) with function.
• It is for identification of tissue and the ability to recognize different tissue types.
• Histology assists in differentiating between normal and abnormal tissue for diagnosis purposes.
• The study of histology and body structure can lead to new scientific discoveries.
• Technical advances improve technology related to biology and medicine

The Importance of Histology

• Histology is essential for diagnosis, as diseases manifest at the cellular and tissue levels.
• Cancers, bone diseases, vascular and liver diseases, and kidney diseases can be definitively diagnosed using cytological and histological techniques.

Principles of Histology Theory

• All living organisms are made of one or more cells.
• The cell is the smallest living structural and functional unit of an organism.
• New cells come from pre-existing cells dividing, except for a fertilized ovum (zygote) that arises from the union of male and female gametes.
• Cells bind to form tissue
  • Tissues combine to form organs
  • Organs with interrelated functions form body systems.

Levels of Organization

• Atom > Molecule > Organelle > Cell > Tissue > Organ > Organ System > Organism

Cells

• Cells form the main structural and functional unit of the body and are the smallest independently-existing unit of living tissues
• They range in size from 4-150 µm, therefore their study needs to be magnified by the microscope.

Cell classifications based on differentiation

• Undifferentiated (Stem) Cells
  • Embryonic stem cells: Zygote (fertilized ova), Totipotential stem cells
  • Non-embryonic stem cells: Germline, somatic, and fetal stem cells
• Progenitor Cells
• Differentiated Cells
  • Germ cells (sperm & ova)
  • Somatic cells (such as hepatocyte, RBC, etc)

Cell classifications based on their structure

• Prokaryotes: Bacteria
  • They are small and have no true nucleus, nuclear membrane or internal membrane
  • They have no organelles other than ribosomes in the cytoplasm
  • Their genetic material contains a single continuous strand forming coils or loops.
• Eukaryotes: Animal cells
  • Large and have a true nucleus with internal and nuclear membranes
  • The cytoplasm contains specialized organelle
  • Genetic material is organized into multiple chromosomes.

Extracellular Matrix (ECM)

• Cells produce the ECM
• The ECM is made of molecules organized to form structures like collagen fibrils and basement membranes.

Tissues

• Tissues consist of cells with similar structures that perform specialized related functions
• Tissues usually have a common embryonic origin.
• Tissue Components: cells and extracellular matrix
• Tissue classification is divided into four basic tissue types based on cell types/structures/functions, composition/characteristics of extracellular matrix, and the space taken up by the cells compared to the matrix.

Four Basic Tissue Types

• Connective
• Muscular
• Nervous
• Epithelial.

Organs and Systems

• Organs: Two or more tissues form larger functional units
  • Examples; kidney, liver, and spleen
• Systems: several organs with interrelated functions form a system
  • Examples; the digestive and urinary system

Types of Microscopes

• The basic type is the light microscope, while others are modifications of it.
• The Light Microscope (LM):
  • Uses daylight or electric light for illumination, focused by a mirror or electric light.
  • The light microscope is made of three parts:
    • Frame that is the mechanical part
    • Magnification that is the optical system
    • Illumination system

The Light Microscopes Composition

• Frame: Supports all parts of the microscope; includes the base, arm, stage, central hole for light, and a optical-system holding body tube .
• Illumination System: Contains the light source (daylight/electric), a condenser, and an iris diaphragm to control light amount
• Magnification System:
  • Ocular lens (eye pieces): near the eyes, with a magnifying power that is 5x, 10x and up to 15x
  • Objective lenses: near the object and have 4 powers
    • 3x or 5x (low power), 10x (medium power), 20x or 40x (high power) and 100x (oil immersion objective)
  • Condenser lens: under the stage and used to concentrate and project the light beam on the object.

Light Microscope (LM)

• The light microscope allows users to examine histological specimens
• Their usefulness stems from magnification and resolution power.
• Magnification power is the degree of enlargement.
• Resolution power is the least distance between 2 separate points that can be distinguished, as opposed to being seen as a single point
  • It measures the ability to see details.
• Resolution Power:
  • Naked eye = 0.2mm
  • Light microscope = 0.2µm.
  • Electron microscope = 0.2nm

Important Notes

• The magnification for histology = (ocular lens power) X (objective lens power).
• The histology image is received directly on the retina or by camera.
• Maximum magnification power of a light microscope = 100 (objective) x 15 (eye)= 1500 times.
• Units of measurement:
  • 1 centimeter (cm) = 10 millimeters (mm)
  • 1 millimeter (mm) = 1000 micrometers (µm)
  • 1 micrometer (µm) = 1000 nanometers (nm)
  • 1 nanometer (nm) = 10 angstroms (Å).

Electron Microscope (EM)

• Electron Microscopes reveal the fine submicroscopic, or ultrastructure
• Includes the transmission and scanning electron microscope.

Test your knowledge of histology with these multiple-choice questions. Explore the core principles, tools, and applications of histology in diagnosing diseases and advancing medical technology. Understand the crucial role of histology in studying tissues and cells.