Epithelium and Histology

• Epithelium is under CLO 1.

Learning Objectives

• Explain the ways epithelia are classified.
• Distinguish between simple, stratified, and pseudostratified epithelia.
• List characteristics of squamous, cuboidal, columnar, and transitional epithelia.
• Identify common surface specializations of epithelia.
• Discuss how organization, thickness, surface specializations, and turnover of an epithelial layer might reflect its function.
• Identify different types of epithelial specimens.

Cells

• Cells represent the smallest and most basic living units.

Tissues and Histology

• A tissue constitutes a collection of similar cells and the extracellular matrix.
• Tissues work together to perform a specific function.
• Histology involves studying the tissues of the body and how they are arranged to form organs.

Classification of Tissues

• The human body consists of four basic tissue types: epithelium, connective, muscular, and nervous.
• Epithelium lines and covers surfaces.
• Connective tissue offers protection, support, and binds structures together.
• Muscular tissue produces movement.
• Nervous tissue responds to stimuli and conducts impulses.

Epithelium

• Epithelium covers the body's surface and serves purposes like protection, adsorption, excretion, secretion, filtration, and sensory reception.
• Epithelium classification is based on cell shape and cell arrangement within the tissue.

Classification Based on Shape

• Squamous (Latin, Squamous-Scale) cells are flat and thin, resembling scales.
• Cuboidal cells possess a basic cube shape, with approximately equal height and width.
• Columnar cells are tall and rectangular, typically taller than they are wide.

Classification Based on Arrangement

• Simple cells exist in a single layer attached to a basement membrane.
• Stratified cells are found in two or more layers stacked.
• Pseudostratified cells appear as multiple layers due to varying cell heights and nucleus positions, even though it's a single layer.
• Transitional cells are rounded and can slide over one another, which allows stretching.

Simple Squamous Epithelium

• This epithelium consists of a single layer of thin, flattened cells that fit tightly together.
• It is located in the serous lining of air sacs in the lungs (alveoli), the lining of the heart, blood vessels, and lymphatic vessels.
• Simple squamous enables materials to pass through via diffusion and filtration and secretes lubricating substances.

Simple Cuboidal Epithelium

• It surrounds the ovary and thyroid.
• It secretes, covers, and absorbs.

Simple Columnar Epithelium

• Ciliated tissues are found in the bronchi, uterine tubes, and uterus.
• Smooth (nonciliated) tissues are in the digestive tract and bladder.
• It provides protection, absorption, secretes mucus (lubrication), and enzymes.

Pseudostratified Columnar Epithelium

• Ciliated tissues of this type line the trachea, nasal cavity, and bronchi.
• It provides protection, secretes mucus and ciliated tissue moves particles trapped in mucus out of the airways.

Stratified Squamous Epithelium

• It lines the esophagus, mouth, and vagina.
• It protects against abrasion.

Squamous Keratinized (Dry)

• This is found in the epidermis.
• It provides protection and prevents water loss.

Stratified Cuboidal Epithelium

• Sweat glands, salivary glands, and mammary glands contain this tissue type.
• It functions as protective tissue, secretion.

Stratified Columnar Epithelium

• It is located in the male urethra, ducts of some glands, and the conjunctiva.
• Functions include secretion and protection .

Transitional Epithelium

• This epithelium lines the bladder, urethra, and ureters.
• It supports protection and distensibility, which allows urinary organs to expand and stretch.

Special Features of Epithelium

• Cilia are hair-like appendages attached to the apical surface of cells, acting as sensory structures or producing movement.
• Goblet cells are specialized to produce mucus to lubricate and protect organ surfaces.
• Villi are finger-like projections from the epithelial layer that increase surface area for faster, more efficient adsorption.
• Microvilli are smaller projections increasing surface area and have a bushy appearance.

Functions of Epithelium

• Epithelial tissue forms boundaries between different environments.
• It provides protection from radiation, desiccation, toxins, and physical trauma.
• It supports the absorption of substances in the digestive tract lining.
• It aids regeneration and excretion of chemicals between underlying tissues and body cavities.
• It assists in secretion of hormones into the blood vascular system, including sweat, mucus, and enzymes.
• It is used in the detection of sensation.

Characteristics of Epithelial Layers

• Epithelial tissue consists of cells laid out in sheets with cell-to-cell attachments, forming a tightly connected, avascular, yet innervated layer.
• Nourishment comes from substances diffusing from blood vessels of underlying connective tissue.
• Epithelial cells feature orientation: one side faces the tissue surface, body cavity, or external environment, and the other joins to a basement membrane.
• The basement membrane is non-cellular and helps cement epithelial tissue to underlying structures.