Introduction to Histology PDF

# Introduction into Histology By Dr.Alaa Al-sahlany M.Sc. Dermatology, Boston University, USA Sep 28, 2023 ## Tissues Tissues are groups of cells organized to perform one or more specific functions. There are four basic types of tissue: - **Epithelium (epithelial tissue)** covers body surfaces...

# Introduction into Histology By Dr.Alaa Al-sahlany M.Sc. Dermatology, Boston University, USA Sep 28, 2023 ## Tissues Tissues are groups of cells organized to perform one or more specific functions. There are four basic types of tissue: - **Epithelium (epithelial tissue)** covers body surfaces, lines body cavities, and forms glands - **Muscle tissue** is made up of contractile cells and is responsible for movement - **Nerve tissue** receives and transmits information to control the activities of the body - **Connective tissue** underlies or supports the other three basic tissues ## Epithelial Tissue Epithelium covers body surfaces, lines body cavities, and constitutes glands. The cells that make up epithelium have three principal characteristics: 1. They adhere to one another by cell junctions 2. They exhibit polarity, _apical polarity_, a lateral polarity, and a basal polarity. 3. Their basal surface is attached to an underlying _basement membrane_ ### Basement membrane The basement membrane is a specialized structure located next to the basal part of epithelial cells and the underlying connective tissue stroma. It is composed of: - **Basal lamina** - (amorphous substance) - product of epithelium - **Reticular lamina** - (reticular fibres) - product of connective tissue. ### Classification of epithelial tissue This is a broad classification of epithelial tissue (depicted in a tree diagram): | | | | | | :---- | :--------------------------------------- | :------------------------------------ | :------------------------------------- | | | **Broad classification of epithelial tissue** | | | | | | | | | | **Surface/lining epithelium** | **Glandular epithelium (e.g. glands)** | **Neuroepithelium (e.g. taste buds)** | | | | | **Myoepithelium (e.g. myoepithelial cells)** | | **Functions:** | Protection | Secretion | Sensation | | | Absorption | | Contraction | ### Characteristics of Epithelial tissue - Epithelium is made of either a single layer (simple) or many layers of cells (stratified). - Epithelial cells are adherent to each other by means of _intercellular junctions_ (junctional complexes). - The deep surface (basal) of the epithelium rests on a basement membrane, which separates it from vascular connective tissue. - No blood vessels nor lymphatics are found in the epithelium; nourishment is provided by diffusion from the adjacent supporting connective tissues. - Epithelium has good regenerative capacity. - Its nuclear shape corresponds to cell shape: nuclei are oval in columnar cells, round in cuboidal, and flat in squamous cells. - Epithelium can invaginate and subsequently grow into glands. ### Morphology of Epithelial Tissue This is a table of simple, stratified, and pseudostratified epithelial tissue: | | Simple | Stratified | Pseudostratified | | :-------- | :------------ | :------------- | :--------------- | | Squamous | Simple squamous epithelium | Stratified squamous epithelium | | | Cuboidal | Simple cuboidal epithelium | Stratified cuboidal epithelium | | | Columnar | Simple columnar epithelium | Stratified columnar epithelium | Pseudostratified columnar epithelium | ### Metaplasia Epithelium may undergo morphological and functional changes from one type to another (metaplasia). Epithelium is derived from all three germ layers: - Skin - ectoderm, Respiratory and digestive systems - endoderm, Cardiovascular system - mesoderm **Example of Metaplasia:** Smoking causes chronic irritation of respiratory epithelium and consequently causes metaplasia. Normal columnar epithelium transforms to squamous metaplasia (depicted in a diagram). ### Surface modifications The superficial surface (apical) of the epithelium is free and exposed to air or fluid and often shows modifications (i.e. presence of microvilli or cilia) depending upon the function it is destined to perform. This table lists epithelial cell surface modifications: | Functions | Surface modifications | | :------------------------------------------------------------------------------------ | :---------------------------- | | Surface coat over the absorptive epithelium of small intestine (rich in polysaccharides and hydrolytic enzymes). | 1. Glycocalyx (cell coat) | | Acts as receptor sites for hormones and enzymes. | | | Minute finger-like projections of the cell membrane | 2. Microvilli (brush border) | | Increase the surface area for absorption (intestine) | | | Very long, thick microvilli, non-motile; Increase the surface area (inner ear) | 3. Stereocilia | | Long hair-like projections of cell membrane | 4. Cilia | | Move towards one direction, thereby moving the entangled particles. | | | Beat towards pharynx in respiratory tract and towards uterus in uterine tube. | | ### Comparison of Cilia and Microvilli | | Cilia | Microvilli | | | :-------------- | :-------------- | :---------- | :----------------- | | Motility | Motile | Non-motile | | | Composition | Composed of microtubules | Composed of microfilament | | | Function | Moving particles in one direction | Absorption | | | Example | Respiratory tract, Uterine tube | Intestinal epithelium, Kidney | | ### Primary ciliary dyskinesia (Not included in the exam) This diagram depicts the difference between normal and faulty cilia in the respiratory tract. ### Celiac disease (Gluten sensitivity) (Not included in the exam) This diagram depicts the difference between normal and atrophic microvilli in the small intestine. ## Intercellular junctions (junctional complexes) This table lists the different types of junctions and their functions | Function | Type of junction | Name of junction | | :--------------------------------------------------------------------------------- | :-------------------------------- | :-------------------------------------- | | Seals adjacent cells together | Occluding junction | Zonula occludens (tight junction) | | Couple microfilaments to the basement membrane for cell-cell adhesion | Anchoring junction | Zonula adherens | | Couple the intermediate filaments to basement membrane at the site of cell adhesion | Anchoring junction | Macula adherens (desmosomes) | | Create a conduct for passage of molecules in the intercellular space | Communicating junction | Gap junction (nexus) | This diagram shows how the different types of junctions are arranged in epithelial cells ## Pemphigus (Not included in the exam) Pemphigus is a skin disease due to damage in _macula adherens_.