Epithelial Review

Flashcards covering key vocabulary, definitions, classifications, and functions related to epithelia, tissues, glands, and membranes as presented in the lecture notes from Laurence Borden, Ph.D.'s BIOL 121 Laboratory, Fall 2025.

Tissue

A group of similar cells and cell products working together to perform a specific role in an organ.

Four broad categories of tissues

Epithelial tissue, Connective tissue, Nervous tissue, Muscular tissue.

Organ

A structure with discrete boundaries that is composed of two or more tissue types.

Histology (microscopic anatomy)

The study of tissues and how they are arranged into organs.

Components of all tissues

Cells and Matrix (fibrous proteins and ground substance).

Ground substance

A clear gel also known as tissue fluid, extracellular fluid (ECF), interstitial fluid, or tissue gel.

Fixative (histology)

A substance, such as formalin, used to prevent decay in tissue specimens for microscopic study.

Histological sections

Tissue sliced into thin sections (one or two cells thick) for microscopic examination.

Stains (histological)

Artificially colored substances applied to tissue mounted on slides, which bind to different cellular components to enhance visibility.

Epithelia

Sheets of closely adhering cells, one or more cells thick, that cover body surfaces, line body cavities, constitute most glands, and are avascular.

Avascular

Lacking blood vessels; epithelial tissue is typically nourished by underlying connective tissue.

Functions of Epithelia

Protect deeper tissues, produce and release chemical secretions, excrete wastes, absorb chemicals (including nutrients), selectively filter substances, and sense stimuli.

Basement membrane

A layer between an epithelium and underlying connective tissue, composed of collagen, glycoproteins, and protein-carbohydrate complexes, which anchors the epithelium.

Basal surface

The surface of an epithelial cell that faces the basement membrane.

Apical surface

The surface of an epithelial cell that faces away from the basement membrane, often exposed to the environment or an internal space.

Cilia

Microtubule-composed specializations on the apical surface that use dynein and ATP for a sweeping motion, as seen in breathing passages to push mucous and dirt away from the lungs.

Flagella

Microtubule-composed specializations on the apical surface that enable cells, such as sperm, to swim.

Dynein

A motor protein that enables movement in cilia and flagella, utilizing ATP for energy.

Microvilli

Projections of the apical cell membrane that increase surface area, typically found in tissues involved in absorption.

Brush border

The appearance of cells possessing a great abundance of microvilli, resembling a hair brush, commonly found in absorptive tissues.

Simple epithelia

Epithelia characterized by a single layer of cells, where all cells touch the basement membrane.

Stratified epithelia

Epithelia composed of two or more layers of cells, where only the deepest layer attaches to the basement membrane.

Squamous (cell shape)

Flat-shaped epithelial cells.

Cuboidal (cell shape)

Dice-shaped epithelial cells.

Columnar (cell shape)

Epithelial cells that are taller than they are wide.

Stratum

A term meaning 'layer,' used in the context of stratified epithelia.

Longitudinal section (l.s.)

A tissue specimen cut along its long axis.

Cross section (c.s. or x.s.) / Transverse section (t.s.)

A tissue specimen cut perpendicular to the long axis of an organ.

Oblique section

A tissue specimen cut at an angle between cross and longitudinal sections.

Simple squamous epithelium

A single row of thin, scaly cells that permits rapid diffusion or transport of substances and secretes serous fluid. Locations: alveoli, glomeruli, endothelium, serosa.

Simple cuboidal epithelium

A single layer of square or round cells primarily involved in absorption, secretion, and mucus production/movement. Locations: liver, thyroid, mammary and salivary glands, bronchioles, kidney tubules.

Simple columnar epithelium

A single row of tall, narrow cells with oval nuclei in the basal half, often featuring a brush border of microvilli or cilia, and goblet cells. Functions in absorption, secretion, and mucus secretion. Locations: lining of GI tract, uterus, kidney, and uterine tubes.

Pseudostratified columnar epithelium

An epithelium that appears multilayered due to nuclei being at different levels, though all cells touch the basement membrane. It typically has cilia and goblet cells, secreting and propelling mucus. Locations: respiratory tract and portions of the male urethra.

Goblet cells

Wineglass-shaped mucus-secreting unicellular glands found in simple columnar and pseudostratified epithelia.

Exfoliation (Desquamation)

The process where surface cells of stratified epithelia die and flake off.

Keratinized stratified squamous epithelium

Multiple cell layers where cells become flat and scaly toward the surface, resisting abrasion, retarding water loss through skin, and resisting penetration by pathogenic organisms. Locations: epidermis; palms and soles are heavily keratinized.

Nonkeratinized stratified squamous epithelium

Similar to keratinized epithelium but without a surface layer of dead cells, resisting abrasion and penetration of pathogens. Locations: tongue, oral mucosa, esophagus, and vagina.

Stratified cuboidal epithelium

Two or more cell layers with surface cells that are square or round, functioning in secreting sweat and producing sperm and ovarian hormones. Locations: sweat gland ducts; ovarian follicles and seminiferous tubules.

Transitional epithelium

A multilayered epithelium with surface cells that can change from round to flat when stretched, allowing for the filling of the urinary tract. Locations: ureter and bladder.

Cell junctions

Structures connecting adjacent cells in many epithelia; includes tight junctions, desmosomes, and gap junctions.

Tight junction

A linkage between two adjacent cells by transmembrane cell-adhesion proteins, forming a zone that completely encircles each cell near its apical pole, sealing off intercellular space and preventing substances from passing between cells (water-tight).

Desmosome

A patch that holds cells together, resisting mechanical stress, like a clothing snap. Hook-like, J-shaped proteins anchor the cytoskeleton to a membrane plaque.

Hemidesmosomes

Half desmosomes that anchor basal cells of an epithelium to the underlying basement membrane, preventing the epithelium from easily peeling away from underlying tissues.

Gap (communicating) junction

Formed by ring-like connexons, allowing ions, nutrients, and other small solutes to pass directly between cells. Locations: cardiac and smooth muscle, embryonic tissue.

Connexon

A ring-like structure consisting of six transmembrane proteins arranged around a water-filled pore, forming part of a gap junction.

Gland

A cell or organ that secretes substances for use elsewhere in the body or releases them for elimination from the body, usually composed of epithelial tissue.

Secretion (glandular product)

A product produced by a gland that is useful to the body.

Excretion (glandular product)

A waste product released by a gland for elimination from the body.

Exocrine glands

Glands that maintain contact with the surface of the epithelium by way of a duct, secreting products to external surfaces (e.g., sweat, tear glands) or internal surfaces (e.g., pancreas, salivary glands).

Endocrine glands

Glands that have no ducts, secreting hormones directly into the blood. Examples: thyroid, adrenal, and pituitary glands.

Hormones

Chemical messengers secreted by endocrine glands that stimulate cells elsewhere in the body.

Unicellular glands

Glands found within an epithelium that is predominantly nonsecretory, capable of being exocrine (e.g., mucus-secreting goblet cells) or endocrine.

Capsule (glandular)

A connective tissue covering of an exocrine gland.

Septa (trabeculae)

Extensions of the capsule that divide the interior of an exocrine gland into compartments (lobes and lobules).

Stroma (glandular)

The connective tissue framework of a gland that supports and organizes the glandular tissue.

Parenchyma (glandular)

The cells that perform the tasks of synthesis and secretion in a gland, typically cuboidal or simple columnar epithelium.

Simple duct

An unbranched duct of an exocrine gland.

Compound duct

A branched duct of an exocrine gland.

Tubular gland

An exocrine gland with a narrow, tube-shaped secretory portion.

Acinar gland (alveolus)

An exocrine gland where secretory cells form a dilated sac-like portion.

Tubuloacinar gland

An exocrine gland possessing both tubular and acinar secretory portions.

Serous glands

Glands that produce thin, watery secretions, such as perspiration, milk, tears, and digestive juices.

Mucous glands

Glands that produce glycoprotein, mucin, which absorbs water to form mucus.

Mixed glands

Glands that contain both serous and mucous cell types and produce a mixture of both types of secretions.

Cytogenic glands

Glands that release whole cells, such as sperm and egg cells.

Merocrine secretion

A mode of secretion where vesicles release their product by exocytosis, observed in tear glands, pancreas, gastric glands, and eccrine sweat glands.

Apocrine secretion

A mode of secretion where a lipid droplet covered by membrane and cytoplasm buds from the cell surface, characteristic of milk fat secretion by mammary gland cells.

Holocrine secretion

A mode of secretion where cells accumulate a product until they disintegrate, releasing a mixture of cell fragments and synthesized substances. Examples: oil glands of scalp and skin, glands of eyelids.

Membranes

Sheets of tissues that may be only epithelial, only connective, or a mix of epithelial, connective, and muscular tissues, often covering surfaces or lining cavities.

Cutaneous membrane (the skin)

The largest membrane in the body, consisting of stratified squamous epithelium (epidermis) resting on a layer of connective tissue (dermis), serving as a relatively dry protective layer.

Mucous membrane (mucosa)

A membrane that lines passages opening to the external environment (e.g., digestive tract), composed of an epithelium, lamina propria (areolar tissue), and muscularis mucosae (smooth muscle), with absorptive, secretory, and protective functions, often containing mucus-producing goblet cells.

Lamina propria

The layer of areolar connective tissue found beneath the epithelium in a mucous membrane.

Muscularis mucosae

A thin layer of smooth muscle found in the mucous membrane.

Serous membrane (serosa)

An internal membrane composed of simple squamous epithelium resting on a layer of areolar tissue, which produces serous fluid and covers organs, lining walls of body cavities.

Endothelium

The simple squamous epithelium that lines blood vessels and the heart.

Mesothelium

The simple squamous epithelium that lines body cavities, such as the pericardium, peritoneum, and pleura.