Histology: Epithelial, Connective, Muscular, and Nervous Tissues

Comprehensive vocabulary flashcards covering histology concepts, including the classification, structure, function, and locations of epithelial, connective, muscular, and nervous tissues, as well as intercellular junctions, gland types, membranes, tissue growth, repair, and cell death.

Epithelium

A sheet of closely packed cells covering body surfaces or lining cavities; it contains little extracellular material, is avascular (nourished by diffusion from underlying connective tissue), and rests on a basement membrane.

Basement Membrane

A layer of glycoproteins and collagen anchoring an epithelium to underlying connective tissue and regulating exchange between them.

Apical Surface

The surface of an epithelial cell or tissue that faces the lumen or the exterior environment.

Basal Surface

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

Simple Squamous Epithelium

A single layer of thin, flat cells with flattened nuclei that functions in rapid diffusion, filtration, and serous fluid secretion; located in lung alveoli, kidney glomerular capsules, serous membranes (mesothelium), and inner linings of the heart and blood vessels (endothelium).

Simple Cuboidal Epithelium

A single layer of square or round cells with round central nuclei that functions in absorption, secretion, and mucus production/movement; located in the liver, thyroid gland follicles, kidney tubules, many gland ducts, and bronchioles.

Simple Columnar Epithelium

A single layer of tall, narrow cells with oval basal nuclei, often possessing goblet cells and microvilli; functions in absorption, mucus secretion, and movement of egg/embryo by cilia; located in the inner lining of the stomach, intestines, gallbladder, uterus, and uterine tubes.

Pseudostratified Columnar Epithelium

An epithelium that appears multilayered because nuclei are at several levels, but every cell touches the basement membrane (though not all reach the surface); often ciliated with goblet cells; secretes and propels mucus; found in the respiratory tract (nasal cavity to bronchi) and portions of the male urethra.

Goblet Cell

A wineglass-shaped unicellular gland found in columnar epithelia that secretes mucus.

Microvilli

Short, nonmotile membrane extensions that form a brush border to increase absorptive surface area.

Cilia

Longer, motile cell extensions that move mucus or other materials across an epithelial surface.

Keratinized Stratified Squamous Epithelium

A multilayered tissue whose surface cells are dead, flat, filled with keratin, and lack nuclei; functions to resist abrasion, water loss, and infection; forms the epidermis of the skin.

Nonkeratinized Stratified Squamous Epithelium

A multilayered tissue whose surface cells are flat, living, and contain nuclei; provides an abrasion- and penetration-resistant moist surface in the tongue, oral mucosa, esophagus, anal canal, and vagina.

Stratified Cuboidal Epithelium

An epithelium consisting of two or more layers of cells with roughly square or round surface cells; secretes sweat, produces sperm, and produces ovarian hormones; found in sweat gland ducts, egg-producing ovarian follicles, and seminiferous tubules.

Urothelium (Transitional Epithelium)

A multilayered epithelium whose surface cells are dome-shaped when relaxed and flattened when stretched (with some cells being binucleate); stretches to allow filling while protecting underlying tissues from urine; located in kidney calyces, ureters, urinary bladder, and part of the urethra.

Stratified Columnar Epithelium

A rare epithelial type consisting of short basal cells beneath columnar surface cells; located in portions of the male urethra and some large gland ducts.

Connective Tissue

A primary tissue class characterized by cells spaced apart by abundant extracellular matrix (fibers and ground substance); functions include binding organs, support, protection, movement, storage, heat production, and transport; mostly highly vascular (with cartilage as the exception).

Fibroblasts

The most abundant cells of fibrous connective tissue; responsible for producing collagen fibers and ground substance.

Macrophages

Large phagocytic cells in connective tissue that engulf foreign matter and dead cells, as well as activate the immune system.

Plasma Cells

Connective tissue cells derived from B lymphocytes that synthesize and secrete antibodies.

Mast Cells

Connective tissue cells often positioned near blood vessels that secrete heparin (an anticoagulant) and histamine (which dilates blood vessels).

Adipocytes

Fat cells that store triglycerides; they constitute the predominant cell type in adipose tissue.

Collagenous Fibers

Tough, flexible fibers that resist stretch, composed of collagen (the body's most abundant protein); appear white in fresh tissue and dominate tendons and ligaments.

Reticular Fibers

Thin collagen fibers coated with glycoprotein that form a spongelike framework (stroma) in the spleen, lymph nodes, and bone marrow.

Elastic Fibers

Thinner fibers composed of elastin that allow stretching and recoiling; appear yellow in fresh tissue and occur in the skin, lungs, and large arteries.

Ground Substance

Gelatinous to rubbery material found between cells and fibers in connective tissue; consists of glycosaminoglycans (GAGs), proteoglycans, and adhesive glycoproteins.

Glycosaminoglycans (GAGs)

Long polysaccharide chains in ground substance that attract water and regulate its movement; examples include chondroitin sulfate and hyaluronic acid.

Proteoglycan

A large molecule with a protein core and numerous radiating GAG chains resembling a bottlebrush; binds cells to matrix components and provides ground substance with its gel-like consistency.

Adhesive Glycoproteins

Proteins in connective tissue ground substance (e.g., fibronectin) that bind cell surfaces to collagen and other extracellular matrix components.

Areolar Tissue

A loose fibrous connective tissue with loosely arranged fibers, abundant blood vessels, and significant open space filled with ground substance; loosely binds epithelia to deeper tissues and surrounds blood vessels, nerves, esophagus, trachea, and forms the papillary layer of the dermis.

Reticular Tissue

A loose fibrous connective tissue composed of reticular fibers and cells that forms the supportive framework for lymphatic organs (lymph nodes, spleen, thymus, bone marrow).

Dense Regular Connective Tissue

A fibrous connective tissue containing densely packed, parallel collagen fibers, compressed fibroblast nuclei, little open space, and few blood vessels; binds bones together and attaches muscle to bone (forming tendons and ligaments).

Dense Irregular Connective Tissue

A connective tissue containing densely packed collagen fibers arranged in random directions; withstands unpredictable stresses from multiple directions; found in the deeper (reticular) layer of the dermis, periosteum, and organ capsules (e.g., kidneys, spleen).

Brown Fat

A specialized type of adipose tissue rich in mitochondria that generates heat rather than ATP; prominent in infants.

Cartilage

A supportive connective tissue with a flexible rubbery matrix containing chondrocytes within lacunae; avascular and therefore heals slowly.

Chondroblasts

Cartilage-forming cells that secrete extracellular matrix; once entrapped within lacunae, they become chondrocytes.

Perichondrium

A sheath of dense irregular connective tissue surrounding most hyaline and elastic cartilages; absent from fibrocartilage.

Hyaline Cartilage

Cartilage with a clear, glassy matrix containing fine, mostly invisible collagen fibers and clustered chondrocytes in lacunae; found in articular cartilage of joints, costal cartilages, the nose, trachea, larynx, fetal skeleton, and epiphyseal plates.

Elastic Cartilage

Cartilage similar to hyaline cartilage but rich in elastic fibers; provides flexible, elastic support in the external ear (auricle) and epiglottis.

Fibrocartilage

Cartilage featuring coarse, readily visible collagen fiber bundles and lacking a perichondrium; resists compression and absorbs shock; located in the pubic symphysis, intervertebral discs, knee menisci, and certain tendon attachments.

Osteon

The basic structural unit of compact bone, consisting of concentric lamellae arranged around a central canal that houses blood vessels and nerves.

Canaliculi

Tiny channels in bone tissue that connect lacunae to each other and to the central canal.

Periosteum

A tough, fibrous sheath covering the external surface of bone tissue.

Erythrocytes

Red blood cells; formed elements in blood responsible for transporting $O_2$ and $CO_2$.

Leukocytes

White blood cells; formed elements in blood that defend against infections and other diseases.

Platelets

Cell fragments in blood involved in blood clotting and mechanisms that minimize blood loss.

Excitability

The ability of a cell to respond to an external stimulus by altering the electrical voltage across its plasma membrane; highly developed in muscle and nerve cells.

Neuroglia (Glial Cells)

Supporting cells of nervous tissue that protect, assist, and outnumber neurons.

Skeletal Muscle Tissue

Voluntary muscle tissue made of long, cylindrical, multinucleated striated cells; functions in moving the skeleton, facial expression, speech, and breathing.

Cardiac Muscle Tissue

Involuntary, striated muscle tissue composed of short, branched cells typically containing a single central nucleus; cells are joined end-to-end by intercalated discs; found exclusively in the heart.

Intercalated Discs

Specialized junctional complexes joining cardiac muscle cells end-to-end; contain gap junctions for electrical coupling and desmosomes for mechanical adhesion.

Smooth Muscle Tissue

Involuntary muscle tissue composed of short, fusiform (spindle-shaped) cells with a single central nucleus and no striations; located in the walls of viscera, blood vessels, iris, and piloerector muscles.

Tight Junction

An intercellular junction formed by linkage proteins that seal adjacent plasma membranes together, preventing molecules from passing between cells (e.g., in stomach and intestinal epithelia).

Desmosome

An intercellular patch that mechanically links adjacent cells using cadherin proteins anchored by keratin intermediate filaments; resists mechanical tension; prevalent in the epidermis and cardiac muscle.

Hemidesmosome

A half-desmosome junction that anchors the basal plasma membrane of epithelial cells to the underlying basement membrane.

Gap Junction

A junction formed by a ring of transmembrane proteins (connexons, made of connexins) that creates a pore allowing ions and small molecules to pass between cells; electrically couples cardiac and smooth muscle cells.

Exocrine Gland

A gland that maintains contact with an epithelial surface via a duct, through which its secretions are released.

Endocrine Gland

A ductless gland that releases its chemical secretions (hormones) directly into the bloodstream.

Eccrine (Merocrine) Secretion

A mode of glandular secretion where products are released via exocytosis without any loss of cellular cytoplasm (e.g., tear glands, pancreas, most sweat glands).

Holocrine Secretion

A mode of secretion in which cells synthesize and accumulate product and then disintegrate entirely, releasing cell fragments as the secretion (e.g., sebaceous glands).

Apocrine Secretion

A mode of secretion where lipid droplets bud from the apical cell surface enveloped by a small amount of cytoplasm and membrane (e.g., milk fat secretion in mammary glands).

Mucous Membrane (Mucosa)

A membrane lining passages opening directly to the exterior (digestive, respiratory, urinary, reproductive tracts); composed of epithelium, an areolar lamina propria, and sometimes muscularis mucosae.

Lamina Propria

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

Serous Membrane (Serosa)

A membrane consisting of simple squamous mesothelium over areolar connective tissue that lines closed internal body cavities, covers organs, and secretes lubricating serous fluid (e.g., pleura, pericardium, peritoneum).

Cutaneous Membrane

The skin; the body's largest membrane, consisting of stratified squamous epithelium (epidermis) overlying connective tissue (dermis); relatively dry.

Synovial Membrane

A connective tissue membrane lining joint cavities that secretes lubricating synovial fluid; contains no epithelium.

Hyperplasia

Tissue growth accomplished by an increase in cell numbers through cell multiplication.

Hypertrophy

Tissue enlargement resulting from an increase in the size of pre-existing cells rather than an increase in cell number (e.g., skeletal muscle enlarging with exercise).

Neoplasia

The development of a tumor (neoplasm), consisting of the abnormal growth of nonfunctional tissue.

Metaplasia

The transformation of one mature tissue type into another mature tissue type (e.g., ciliated pseudostratified columnar epithelium in smokers' bronchi changing into stratified squamous epithelium).

Totipotent Stem Cells

Stem cells (such as the zygote and earliest embryonic cells) capable of developing into any cell type, including the placenta and extraembryonic membranes.

Pluripotent Stem Cells

Stem cells derived from the blastocyst's inner cell mass that can develop into any embryonic cell type, but cannot develop into an entire organism or placental tissue.

Regeneration

The replacement of dead or damaged cells with the same functional cell type as before, fully restoring normal tissue function.

Fibrosis

The replacement of damaged tissue with scar tissue (primarily collagen) produced by fibroblasts; maintains organ integrity but does not restore normal physiological function.

Granulation Tissue

A soft, highly vascular connective tissue that forms during wound healing, characterized by proliferating capillaries, fibroblasts, and collagen deposition.

Atrophy

The shrinkage of a tissue resulting from a reduction in cell size or cell number; occurs as senile atrophy from aging or disuse atrophy from immobility.

Necrosis

Premature, pathological tissue death resulting from trauma, toxins, or infection (e.g., infarction, gangrene, decubitus ulcers).

Apoptosis

Programmed cell death; a normal physiological process in which completed cells shrink, fragment, and are phagocytized without eliciting inflammation.

Infarction

Sudden tissue necrosis that occurs when blood supply to a region is completely interrupted (e.g., myocardial infarction).

Gangrene

Tissue necrosis caused by an insufficient blood supply, frequently accompanied by infection.