6. Histology: Tissue Types, Structures, and Functions

Vocabulary practice flashcards covering tissue classifications, epithelial membranes, glandular functions, connective tissue types, muscle and nervous tissues, and cellular adaptations.

Tissue

A group of cells that are similar in structure and perform a common or related function.

Epithelial tissue (Epithelium)

A sheet of cells that covers body surfaces, lines body cavities, and forms glands; functions primarily in covering and protection.

Connective tissue

The most abundant and widely distributed tissue type; provides support, protection, insulation, binding, storage, and transportation.

Muscle tissue

A highly vascularized tissue composed of specialized contractile cells that produce movement via interactions between cytoskeletal elements.

Nervous tissue

Tissue responsible for body control, consisting of neurons that transmit impulses and supporting glial cells.

Intercellular junctions (Epithelium)

Specialized connections joining epithelial cells into continuous sheets, including tight junctions, desmosomes, hemidesmosomes, and gap junctions.

Avascular but innervated

Characteristic of epithelial tissue where it lacks a direct blood supply but contains nerve fibers.

Apical surface

The exposed or free side of an epithelial tissue sheet, which often possesses microvilli or cilia.

Basal surface

The attached side of an epithelial tissue sheet that faces the underlying connective tissue.

Basal lamina

A layer of glycoproteins secreted by epithelial cells that acts as a scaffold and selectively filters molecules diffusing from underlying connective tissue.

Reticular lamina

A layer of extracellular material belonging to connective tissue that is primarily composed of collagen fibers.

Basement membrane

A supporting structure reinforcing the epithelial sheet, composed of both the basal lamina and the reticular lamina.

Simple epithelium

An epithelium composed of a single layer of cells, typically found in locations specialized for absorption, filtration, and permeability.

Stratified epithelium

An epithelium composed of two or more layers of cells stacked on top of each other, primarily providing protection.

Epithelial cell shapes

The three morphological classifications of epithelial cells: squamous (flattened, scalelike), cuboidal (boxlike), and columnar (tall, column-shaped).

Endothelium

Specially named simple squamous epithelium that lines the lumen of blood vessels, lymph vessels, and the hollow organs of the cardiovascular system.

Mesothelium

Specially named simple squamous epithelium found within serous membranes lining body cavities.

Simple squamous epithelium

A single layer of thin, flat cells with centrally bulging nuclei; provides the thinnest barrier for rapid diffusion, filtration, and secretion.

Simple cuboidal epithelium

A single layer of cells about as tall as they are wide with spherical, centrally located nuclei; functions in absorption and secretion.

Nonciliated simple columnar epithelium

A single layer of tall cells with oval basal nuclei and microvilli; lines most of the digestive tract and functions in absorption and secretion.

Ciliated simple columnar epithelium

A single layer of tall ciliated cells with oval basal nuclei; lines larger lung bronchioles and uterine tubes to move mucus and oocytes.

Pseudostratified columnar epithelium

A single layer of cells with varying heights where all cells attach to the basement membrane, but not all reach the apical surface.

Keratinized stratified squamous epithelium

Multiple cell layers where apical cells are dead, flat, lack nuclei, and are packed with the protein keratin; forms the epidermis of the skin.

Nonkeratinized stratified squamous epithelium

Multiple cell layers where apical cells remain flat, living, nucleated, and lack keratin; lines moist areas like the oral cavity, esophagus, and vagina.

Stratified cuboidal epithelium

A relatively rare epithelium consisting of two or more layers of cells, with cuboidal cells at the apical surface; located in ducts of exocrine glands.

Stratified columnar epithelium

A rare epithelium consisting of two or more cell layers with columnar cells at the apical surface; found in parts of the male urethra and large glandular ducts.

Transitional epithelium

A stratified epithelium lining urinary organs whose apical cells appear polyhedral/rounded when relaxed and flattened when distended; contains binucleated cells.

Endocrine gland vs. Exocrine gland

Comparison showing endocrine glands (ductless glands that secrete hormones into extracellular fluid/blood) versus exocrine glands (duct-containing glands that release secretions onto body surfaces or cavities).

Goblet cell

A unicellular exocrine gland scattered within epithelial sheets that produces and secretes mucin by exocytosis.

Acinus

The secretory portion of a multicellular exocrine gland.

Duct

The conducting portion of a multicellular exocrine gland.

Simple gland

A multicellular exocrine gland characterized by having a single, unbranched duct.

Compound gland

A multicellular exocrine gland characterized by having branched ducts.

Merocrine gland

A gland that packages its unaltered secretion into vesicles and releases it via exocytosis, leaving the gland cell fully intact.

Apocrine gland

A gland where the apical portion of the secretory cell membrane and cytoplasm pinches off to form the secretion, after which the cell repairs itself.

Holocrine gland

A gland where secretory cells accumulate product and then completely rupture, with the disintegrated cell material becoming the secretion.

Mechanisms of exocrine gland secretion

The three modes of exocrine secretion: merocrine (exocytosis), apocrine (apical pinch-off), and holocrine (entire cell disintegration).

Connective tissue classification

The three primary classifications derived from mesenchyme: connective tissue proper (loose and dense), supporting connective tissue (cartilage and bone), and fluid connective tissue (blood and lymph).

Extracellular matrix (ECM)

The collective non-living material surrounding connective tissue cells, consisting of ground substance and protein fibers.

Ground substance

Non-living material produced by connective tissue cells containing water, proteoglycans, and cell adhesion proteins; varies from liquid to solid.

Proteoglycan

A large ground-substance macromolecule consisting of a protein core attached to multiple negatively charged glycosaminoglycans (GAGs).

Fibroblast

The primary resident cell type of connective tissue proper responsible for producing, maintaining, and repairing the extracellular matrix.

Mesenchymal cell

An adult stem cell type residing within connective tissue that can divide and differentiate to replace damaged cells.

Collagen fibers

Long, unbranched, tough, and flexible protein fibers resistant to stretching; account for approximately $25\%$ of all body protein.

Reticular fibers

Thin, glycoprotein-coated collagenous fibers that form branching, interwoven networks to support the stroma of organs.

Elastic fibers

Branching connective tissue fibers composed of elastin that allow tissues to stretch and recoil easily.

Loose connective tissue

A category of connective tissue proper containing relatively few, irregularly arranged fibers and abundant ground substance; includes areolar, adipose, and reticular tissues.

Dense connective tissue

A category of connective tissue proper dominated by densely packed protein fibers (primarily collagen) with comparatively little ground substance.

Areolar connective tissue

A loose connective tissue containing scattered fibroblasts, few elastic and collagen fibers, abundant ground substance, and many blood vessels; underlies epithelia.

Adipose connective tissue

A loose connective tissue dominated by closely packed adipocytes with peripherally pushed nuclei; stores energy, insulates, and cushions organs.

Reticular connective tissue

A loose connective tissue featuring a meshwork of reticular fibers, leukocytes, and fibroblasts that forms the stroma of lymphatic organs.

Dense regular connective tissue

A dense connective tissue containing parallel arrays of densely packed collagen fibers and scarce ground substance; forms tendons and ligaments.

Dense irregular connective tissue

A dense connective tissue featuring randomly arranged and clumped collagen fibers that withstands tensile stresses applied in multiple directions.

Elastic connective tissue

A dense connective tissue dominated by densely packed elastic fibers that allows stretching and recoil in structures like elastic arteries.

Cartilage

An avascular supporting connective tissue with a firm, semisolid extracellular matrix containing chondrocytes trapped in lacunae.

Bone (Osseous tissue)

A rigid supporting connective tissue characterized by an inorganic calcified matrix containing collagen fibers, osteocytes, and blood vessels.

Osteon

The structural unit of compact bone consisting of concentric lamellae arranged around a central canal containing blood vessels and nerves.

Blood

A fluid connective tissue consisting of formed elements (erythrocytes, leukocytes, platelets) suspended in a liquid ground substance called plasma.

Lymph

A fluid connective tissue derived from blood filtrate that lacks blood cells and major proteins, which is eventually returned to plasma.

Muscle tissue types

The three forms of muscle tissue: skeletal (multinucleated, striated, voluntary), cardiac (branching, intercalated discs, involuntary), and smooth (fusiform, unstriated, involuntary).

Neuron

A specialized cell of nervous tissue consisting of a cell body, dendrites, and an axon, capable of receiving, processing, and transmitting impulses.

Glial cells

Non-conducting cells of nervous tissue responsible for supporting, protecting, insulating, and nourishing neurons.

Organ

A structure composed of two or more tissue types that work together to accomplish a specific, complex biological function.

Body membranes

Epithelial and connective tissue sheets that cover surfaces and line cavities: mucous, serous, cutaneous, and synovial membranes.

Mucous membrane (Mucosa)

A membrane lining compartments and passageways that open to the external environment; consists of epithelium over connective tissue and produces mucus.

Serous membrane

A membrane lining closed internal body cavities, consisting of a parietal layer, a visceral layer, and a thin watery serous fluid to reduce friction.

Cutaneous membrane

The keratinized dry membrane covering the external surface of the body, commonly referred to as skin.

Synovial membrane

A membrane surrounding some joints consisting of areolar connective tissue and superficial squamous cells without a basement membrane, producing lubricating synovial fluid.

Stem cell division

The mitotic processes by which stem cells replicate: asymmetrical division (yielding one stem cell and one committed daughter cell) and symmetrical division (yielding two uncommitted daughter cells).

Atrophy

A decrease in the size of an organ or tissue resulting from a lack of normal stimulation.

Hypertrophy

An increase in the size of an organ or tissue resulting from overstimulation, characterized by enlarged individual cells.

Hyperplasia

An increase in the size of a tissue or organ caused by an accelerated, localized rate of cell division, leading to an increased number of cells.

Hypertrophy vs. Hyperplasia

Adaptive cellular changes where hypertrophy involves cells increasing in physical size, whereas hyperplasia involves an increase in the total number of cells.

Necrosis

Tissue death resulting from irreversible damage or disease.