Types of Tissues and Histology

Comprehensive vocabulary flashcards covering the tissue level of organization, including cell junctions, epithelial and connective tissue classification, cells, fibers, matrices, cartilage, bone, liquid connective tissue, and membranes.

Tissue

A group of cells that usually have a common origin in an embryo and function together to carry out specialized functions.

Histology

The microscopic study of tissues.

Pathologist

A physician who examines cells and tissues to help other physicians make diagnoses.

Primary Tissue Types

The four fundamental tissue classes in the human body: epithelial tissue, connective tissue, muscular tissue, and nervous tissue.

Epithelial Tissue

Tissue that covers body surfaces, lines hollow organs, cavities, and ducts, forms glands, and allows the body to interact with internal and external environments.

Connective Tissue

Tissue that protects and supports the body and its organs, binds organs together, stores energy reserves as fat, and helps provide immunity to disease.

Muscular Tissue

Tissue composed of specialized cells for contraction and generation of force, generating heat that warms the body.

Nervous Tissue

Tissue that detects changes in a variety of conditions inside and outside the body and responds by generating electrical signals (nerve impulses).

Cell Junctions

Contact points between the plasma membranes of tissue cells.

Tight Junctions

Strands of transmembrane proteins that seal adjacent cells tightly together to prevent substances from passing between cells and stop organ contents from leaking.

Adhering Junctions

Cell junctions containing plaque and cadherins that hold cells together to help them resist separation during movement.

Desmosomes

Spot weld cell junctions containing plaque, cadherins, and keratin intermediate filaments that provide strength and stability under tension or contraction.

Hemidesmosomes

Cell junctions resembling half of a desmosome that anchor epithelial cells to the basement membrane using integrins attached to laminin.

Gap Junctions

Cell junctions made of connexins forming tiny fluid-filled channels (connexons) that allow ions and small molecules to pass directly between cells for communication.

Epithelial vs Connective Tissue Structure

Epithelial tissue consists of many tightly packed cells with little to no extracellular matrix, whereas connective tissue consists of a few scattered cells surrounded by large amounts of extracellular matrix.

Epithelial Cell Surfaces and Basement Membrane

Structural organization of epithelial cells showing the apical (free) surface, lateral surfaces with cell junctions, basal surface, and the underlying basement membrane.

Apical Surface

The free top surface of an epithelial cell that faces a body surface, body cavity, lumen, or duct, and may contain cilia or microvilli.

Lateral Surfaces

The side surfaces of an epithelial cell that face adjacent cells and contain cell junctions.

Basal Surface

The deepest surface of an epithelial cell that adheres to the underlying basement membrane via hemidesmosomes.

Basement Membrane

A thin extracellular layer that anchors epithelial tissue to underlying connective tissue, composed of a basal lamina and a reticular lamina.

Basal Lamina

The layer of the basement membrane located closest to epithelial cells, which is secreted by the epithelial cells.

Reticular Lamina

The layer of the basement membrane located closest to connective tissue, which is produced by connective tissue cells known as fibroblasts.

Classification of Surface Epithelium

The structural categorization of epithelial tissues based on the number of cell layers (simple, pseudostratified, stratified) and the cell shape (squamous, cuboidal, columnar).

Surface Epithelium

Epithelial tissue arranged in continuous sheets that covers body surfaces and lines internal organs, blood vessels, ducts, body cavities, and tracts.

Glandular Epithelium

Epithelial tissue specialized for secretion that forms the secreting portion of glands.

Simple Epithelium

A single layer of cells functioning in diffusion, osmosis, filtration, secretion, and absorption.

Pseudostratified Epithelium

Epithelium that appears multilayered because nuclei lie at different levels, but is actually a single layer because all cells rest on the basement membrane.

Stratified Epithelium

Epithelium composed of two or more layers of cells, primarily serving a protective function in areas subject to wear and tear.

Squamous Cells

Flat and thin epithelial cells designed for the rapid passage of substances.

Cuboidal Cells

Cube-shaped epithelial cells that are as tall as they are wide, functioning in secretion and absorption.

Columnar Cells

Tall, column-shaped epithelial cells that protect underlying tissues and function in secretion and absorption.

Transitional Epithelium

Stratified epithelium composed of cells that change shape, allowing urinary organs like the bladder to stretch.

Endothelium

Specialized simple squamous epithelium that lines the interior surface of the heart, blood vessels, and lymphatic vessels.

Mesothelium

Specialized simple squamous epithelium that forms the epithelial layer of serous membranes.

Endocrine Glands

Ductless glands that secrete hormones into the interstitial fluid, which then diffuse into the bloodstream to produce widespread effects.

Exocrine Glands

Glands that secrete products into ducts that empty onto an epithelial surface, typically producing local effects.

Mixed Glands

Glands containing both endocrine and exocrine tissue, such as the pancreas, ovaries, and testes.

Unicellular Glands

Single-celled exocrine glands, such as goblet cells that secrete mucus directly onto apical surfaces.

Multicellular Glands

Exocrine glands composed of many cells forming distinct microscopic structures or organs, such as sweat, oil, and salivary glands.

Simple Gland

An exocrine gland whose duct does not branch.

Compound Gland

An exocrine gland whose duct branches.

Merocrine Secretion

Functional classification of exocrine glands in which secretions are released by exocytosis without damaging the cell.

Apocrine Secretion

Functional classification of exocrine glands in which secretory products accumulate at the apical surface, which then pinches off from the rest of the cell.

Holocrine Secretion

Functional classification of exocrine glands in which the entire cell fills with secretory material and ruptures to release the product.

Fibroblasts

Large, flat, abundant connective tissue cells that divide and secrete fibers and extracellular matrix.

Macrophages

Phagocytic cells derived from monocytes that engulf and destroy bacteria, cell debris, and foreign material in connective tissue.

Plasma Cells

Immune cells residing in connective tissue, particularly in digestive and respiratory tracts, that secrete antibodies.

Mast Cells

Cells involved in inflammation and defense that can bind, ingest, and kill bacteria within connective tissue.

Adipocytes

Fat cells specialized for storing triglycerides, located beneath skin and around organs.

Leukocytes

White blood cells (such as neutrophils and eosinophils) that migrate into connective tissues during infection, allergic reactions, or parasitic infestations.

Ground Substance

The fluid, semifluid, gelatinous, or calcified matrix component located between connective tissue cells and fibers.

Glycosaminoglycans (GAGs)

Complex polysaccharides in ground substance that trap water to make the matrix jellylike.

Proteoglycans

Structures consisting of a core protein with attached glycosaminoglycans that help trap water in ground substance.

Hyaluronic Acid

A slippery, viscous GAG that binds cells together, lubricates joints, and helps maintain the shape of the eyeballs.

Hyaluronidase

An enzyme that breaks down hyaluronic acid, making the ground substance more liquid to facilitate cell movement.

Fibronectin

The main adhesion protein of connective tissue that links collagen fibers, ground substance, and cells together.

Collagen Fibers

Strong, flexible fibers composed of collagen (the most abundant protein in the body, representing about $25\%$ of total protein) that resist tensile/pulling forces.

Elastic Fibers

Small, branching fibers composed of elastin and fibrillin that stretch and return to their original shape.

Reticular Fibers

Thin collagen fibers with glycoprotein coatings that form branching networks to support soft organ stroma and basement membranes.

Chondrocytes

Mature cartilage cells located within matrix spaces called lacunae.

Lacunae

Small spaces within the extracellular matrix that enclose chondrocytes in cartilage or osteocytes in bone.

Perichondrium

A dense irregular connective tissue membrane covering cartilage that contains blood vessels, nerves, and chondroblast precursor cells.

Antiangiogenesis Factor

A chemical substance secreted by cartilage that prevents blood vessel growth.

Interstitial Growth

Cartilage growth from within, occurring when chondrocytes divide and secrete new extracellular matrix, expanding the tissue internal content.

Appositional Growth

Cartilage growth at the outer surface, where inner perichondrial cells differentiate into chondroblasts to add matrix beneath the perichondrium.

Osteon

The basic structural unit of compact bone tissue, also called a haversian system.

Bone Lamellae

Concentric rings of mineralized extracellular matrix containing calcium salts and collagen fibers in compact bone.

Canaliculi

Tiny canals extending from bone lacunae containing osteocyte extensions, enabling nutrient exchange and waste removal.

Osteonic Canal

The central channel running through an osteon that houses blood vessels and nerves.

Trabeculae

An irregular latticework of thin bone columns/beams that forms the structural foundation of spongy bone.

Endosteum

A membrane lining internal bone cavities, including yellow bone marrow spaces.

Erythrocytes

Red blood cells responsible for transporting oxygen and removing a portion of carbon dioxide ($\text{CO}_2$).

Thrombocytes

Cell fragments in blood, commonly called platelets, that participate in blood clotting.

Lymph

A clear liquid connective tissue flowing in lymphatic vessels composed of liquid extracellular matrix and variable cellular components.

Mucous Membrane

An epithelial membrane lining cavities that open directly to the exterior, such as the digestive, respiratory, urinary, and reproductive tracts.

Lamina Propria

The supporting areolar connective tissue layer of a mucous membrane that serves as a site for nutrient, waste, and gas exchange.

Serous Membrane

An epithelial membrane lining cavities that do not open to the exterior, consisting of a parietal layer lining cavity walls and a visceral layer covering organs.

Synovial Membrane

A membrane lining joint cavities composed of synoviocytes and connective tissue, lacking an epithelial layer, that secretes lubricating synovial fluid.