Anatomy: Epithelial Tissue

• Anatomy & Histology Lecture 2 covers tissues and more specifically, Epithelial Tissue.

Main Types of Tissues

• Tissue is a group of cells working together with similar structure and function.
• Histology studies the microscopic architecture, organization, and function of tissue.
• The four main types of tissues are Epithelium, Connective, Muscle, and Nervous.
• Epithelium covers & lines.
• Connective supports.
• Muscle facilitates movement.
• Nervous controls.

Epithelial Tissue

• Epithelial tissue ("epithe") is laid on and covers surfaces.
• It is composed of specialized cells of one type performing a common bodily function.
• Epithelial tissue covers body surfaces or lines body cavities.
• It forms most glands and always has a free surface.
• Functions include protection from drying, injury, and bacterial invasion, using cilia to sweep away impurities.
• Additional functions are absorption, such as in the small intestine, filtration, and ion transport.
• Secretion also occurs like in the digestive tract.

Special Properties of Epithelial Tissue

• Epithelial tissue is supported by connective tissue and rests on a basement membrane of carbohydrates and proteins.
• It does not have blood vessels (avascular), relying on diffusion and underlying connective tissue for nutrients and oxygen.
• Epithelial tissue has the ability to regenerate and replace lost cells.
• It is innervated, having nervous ends
• Cells are in close contact, forming a continuous sheet with junctions for attachment and communication.
• Epithelial tissue exhibits polarity.
• Polarity means it always has an apical surface, which faces an open space and may have microvilli or cilia.
• Polarity also means it has a basal surface, which is the lower attached surface.

Special Characteristics of Epithelia

• The basal lamina is a thin extracellular layer that supports epithelial cells, separating them from other tissues.
• The basal lamina consists of proteins secreted by the epithelial cells
• Functions include acting as a selective filter for molecules entering the epithelium and providing a scaffolding for cell migration
• The reticular layer is part of the underlying connective tissue and is deep to the basement membrane.

Extracellular Junctions

• A cell junction is a point of cell-to-cell contact connecting one cell to another helping them to function
• Tight junctions form an impermeable barrier between cells.
• Epithelial cells are joined by tight junctions in the small intestine and kidneys, using a zipper-like fastening.
• Gap junctions involve two adjacent plasma membrane channels that join together.
• Gap junctions provide strength and allow ions, sugars, and small molecules to pass between cells
• Gap junctions in heart and smooth muscle ensure synchronized contraction.
• Adhesion junctions hold adjacent plasma membranes together using extracellular filaments attached to cytoplasmic plaques composed of the protein material.

Classifications & Naming of Epithelia

• The first name of a tissue indicates the number of layers.
• Simple epithelium has one layer of cells.
• Stratified epithelium has more than one layer of cells.
• The last name describes the shape of the cells.
• Squamous cells are wider than tall, like plates.
• Cuboidal cells are as wide as they are tall, resembling cubes.
• Columnar cells are taller than they are wide, like columns.

Epithelial Tissue Naming Conventions

• Naming of epithelia includes the number of layers and the shape of the cells.
• For example, stratified cuboidal epithelium.
• The name may include accessory structures like goblet cells, cilia, or keratin.
• Pseudostratified and transitional are special epithelial tissues that do not follow the conventional naming rules
• The cells in epithelial tissues are tightly packed with little space between them.
• The functional and physical properties are derived from the cells themselves.

Simple Squamous Epithelium

• Simple squamous epithelium has a single layer of flat cells with disc-shaped nuclei.
• It is very thin.
• Locations include renal glomeruli, responsible for kidney filtration, and alveoli of lungs.
• It also lines the heart and vessels, forming the structures of capillary walls as endothelium and forms serous membranes (serosa).
• Serous membranes line the ventral body cavity and covers organs.
• Parietal layer of the it lines pleural, pericardial, and peritoneal cavities
• The visceral layer covers visceral organs of those cavities.

Simple Cuboidal Epithelium Description

• A single layer of cube-shaped cells with large, spherical central nuclei.
• Its function is secretion and absorption.
• It can be found mainly lining ducts of endocrine glands.
• It's also present in the thyroid gland, renal tubules, pancreas, ovary surface, and salivary glands.

Simple Columnar Epithelium

• A single layer of columnar shaped cells (rectangular-elongated) with oval nuclei.
• Some have cilia at their apical surface
• May contain goblet cells at mucous membranes, which secrete lubricating mucus.
• Its function is absorption
• It secretes mucus, enzymes, and other substances, and the ciliated type propels mucus or reproductive cells by ciliary action

Simple Columnar Epithelium Location

• Non-ciliated form lines lower digestive tract from stomach, small, and large intestine, and gallbladder.
• Ciliated form lines small bronchi and the uterine tubes.

Stratified Epithelia

• Stratified epithelia contains two or more layers of cells and regenerates from below.
• The main function is protection.
• It is named according to the shape of cells at the apical layer.

Stratified Squamous Epithelium

• It has many layers of cell, and those cells are squamous in shape.
• Deeper layers of cells appear cuboidal or columnar.
• It is the thickest epithelial tissue.
• It is adapted for protection.
• It can withstand abuse and friction and plays a protective role.

Stratified Squamous Epithelium- Specific Types

• Specific type called Keratinized, and this form of the has the protective protein keratin.
• Surface cells are dead and full of keratin (keratinized)
• The non-keratinized (another form, the opposite): Forms moist lining of body openings
• The function is to protect underlying tissues
• Keratinized version forms the epidermis
• Non-keratinized forms lining of esophagus, mouth, and vagina & outer portion of skin lining the various orifices of the body

Stratified Cuboidal Epithelium Description

• Usually is 2 layers
• Mainly lines ducts of large exocrine glands (sweat, mammary, salivary glands).

Stratified Columnar Epithelium Description

• Not very common.
• Thick, waterproof layer.
• Locations include: lining of pharynx & male urethra ducts

Pseudostratified Columnar Epithelium

• The description is such that all cells rest on basement membrane and originate from it.
• Only tall cells reach the apical surface.
• May contains goblet cells and cilia.
• Nuclei lie at varying heights within cells.
• Gives false impression (pseudo of being multi-layered (stratification)
• The function of this is secretion of mucus and propulsion of mucus by cilia as well as absorption
• Locations include non-ciliated type (ducts of male reproductive tubes and other few organs) and ciliated type (lines trachea and most of upper respiratory tract-cilia propels mucus from lungs)

Transitional Epithelium

• Description includes basal cells that are usually cuboidal or columnar.
• Superficial cells are dome-shaped or squamous
• The function is to stretch when filled with urine and permit distension of bladder
• The location of occurs in: hollow urinary organs, ureters, urinary bladder and part of urethra

Glandular Epithelium

• Consists of one or more cells (groups of cells).
• A goblet cell is a gland consisting of a single cell that secretes mucus (makes and secretes a particular product)
• Glands are supported by connective tissue like other epithelial tissues.
• Types of glands- Endocrine glands produce hormones into the intercellular fluid to be picked up and transported to their target organs by blood (pituitary gland and thyroid).
• Endocrine glands helps promote homeostasis. Exocrine glands secrete substances through ducts that open to the skin or to internal body surfaces that lead to the exterior of the body
• E.g. Mucous, sweat, oil, saliva, bile, pancreas, mammary glands.

Exocrine Glands

• Unicellular: Mucus cells or goblet cells are unicellular glands found in columnar epithelium that secretes mucous. Multicellular: the glands have a duct structure/ Holocrine glands, the entire secretory cell ruptures and excreted (becomes part of the secretion)