Detailed note on unit one

   Introduction to Histology

       February 2026

       Tiruzer Bekele (MD, Anatomical pathologist and senior lecturer)

       1. Introduction to histology

HISTOLOGY – The study of tissues (“histo” means tissue or web in Greek)

Ø      Histology is the science of microscopic anatomy of tissue and of how these  tissues are arranged to constitute organs

Ø      This subject involves all aspects of tissue biology, with the focus on how cells’ structure and arrangement optimize functions specific to each organ

Ø      It provides a foundation for understanding the microscopic structure and functions of the organs

Ø      Many diseases profoundly alter the tissues within an affected  organ

o Tissues have two interacting components: cells and extracellular matrix (ECM).

o The ECM consists of (fibers +ground substance + fluids)

o The ECM supports the cells and contains the fluid transporting nutrients to the cells, and carrying away their wastes and secretory products.

o Cells produce the ECM locally and are in turn strongly influenced by matrix molecules. (Bidirectional)

o Many matrix components bind to specific cell surface receptors that span the cell membranes and connect to structural components inside the cells, forming a continuum in which cells and the ECM function together in a well coordinated manner

o The small size of cells and matrix components makes histology dependent on the use of microscopes and molecular methods of study.

o  The most common procedure used in histologic research is the preparation of tissue slices or “sections” that can be examined visually with transmitted light (thin translucent sections are cut from tissues and placed on glass slides for microscopic examination of the internal structures)

o  The ideal microscopic preparation is preserved so that the tissue on the slide has the same structural features it had in the body. However, this is often not feasible because the preparation process can remove cellular lipid, with slight distortions of cell structure.

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CYTOLOGY- The study of cells (Cytos/Kitos means cell in Greek)

Ø      Structural & functional units of life (and of disease  processes) in all tissues, organs & organ systems

Ø      Independent units of life, which is viable and capable of    multiplication

Ø      The human body contains>35 trillion cells each of  which perform a specific function

Ø      In the 19th century, Rudolf Virchow, often referred to as the father of modern pathology, proposed that injury to the smallest living unit of the body, the cell, is the basis of all disease.

Ø      “Think microscopically”

—                Cells vary in size, shape, and function

—                Cells exhibit the following basic functional activities/properties:

Ø Metabolism (absorption, synthesis, respiration and excretion)

Ø Growth and regeneration,

Ø Irritability (excitability)

Ø Movement

Ø Reproduction

Ø Aging

Ø Death

— Constituents of a cell:

Ø Water (60-70%)

Ø Electrolytes - inorganic compounds (5%),

Ø Proteins: enzymes, structural proteins and secretory proteins (15-20%),

Ø Lipids (15%) and

Ø Carbohydrates (1%).

— Components of a cell

Ø Nucleus: Genetic material

Ø Cytoplasm: Limited by cell membrane

—   Contains three structural components

—   Organelles    – metabolically active

—   Cytoskeleton – structural framework (cell support + cell/organelle motility)

—   Inclusions     – cell products, metabolites (metabolically inactive)

—   cytosol – fluid component of a cell

      Cytoplasmic organelles: They can be classified into two types:

      Membranous: Membrane bound

Ø Endoplasmic reticulum, Golgi Apparatus, Mitochondria, Lysosomes

Ø Peroxisomes, Endosomes

      Non- membranous

Ø Ribosomes, Centriole

Ø Microtubuli, Microfilaments and intermediate filaments

o Microtubules are important in maintaining cell shape and as tracks for transport of vesicles and organelles by the motor proteins kinesin and dynein

e.g. Defect in microtubules could result in male sterility and repeated infection from impaired leukocyte migration and phagocytosis

o Intermediate filaments (e.g. cytokeratin, Vimentin, Desmin,Glial Fibrillary acid phosphatase)- Immunohistochemical stains e.g. used in tumor diagnosis

ü Inclusions (Nonliving accumulations of metabolites or cell products)

   e.g. lipid droplets, glycogen granules and pigments like melanin, hemosiderin as well as lipofuscin

ü Cytoplasmic matrix ( cytosol)- viscous fluid medium with dissolved solutes like ions through which diffusion occurs + it supports organelles

Plasma Membrane

—                Outer covering of all eukaryotic cells

—                Composed of a lipid bilayer and associated proteins

—Functions

—As semi permeable membrane separating internal and external environment of a cell or an organelle

—   Preventing intrusion of harmful substances, dispersion of  macromolecules, dilution of enzymes & substrates

—   Maintaining Homeostasis mechanism (maintaining optimal intracellular concentration  of  ions, H₂O, enzymes & substrates – through passive diffusion, facilitated  diffusion and  active transport)

§  A gatekeeper that regulate passage of substances into and out of the cell

§  As a sensory devise that permits the cell to recognize & be  recognized by other cells & macromolecules.

§  Cell-to-cell attachment & communication

—                Endocytosis

—  Phagocytosis (cell-eating): insoluble substances e.g. bacteria/dead cell by neutrophils/macrophages

—  Pinocytosis(cell-drinking): fluid with solutes e.g. Thyroglobulin by thyroid follicular cells

—  Receptor-mediated endocytosis: ligands & surface receptors e.g. LDL

—                Exocytosis e.g. exocrine pancreas and salivary glands

—                Signal transduction

e.g. Nuclear receptors (steroids, retinoids, Vit. D, thyroid hormones)

—                Membrane modification helps to form cellular junction,  cilia, and microvilli

— Signal Reception

—   Communication of cells with one another to regulate their  development into tissues, to control their growth and division,  and to coordinate their functions

—   Endocrine signaling:- hormones

—   Paracrine signaling:- local cells only

—   Synaptic signaling:- neurotransmitters via synapes

—   Autocrine signaling:- self cell