Histology

Epithelial Tissues

  • General characteristics

    • Highest regeneration rate: constant replacement of lost cells

    • Avascular: lacks direct blood vessel connections

    • Polarity: apical (top, unattached) and basal (bottom, attached) surfaces; basement membrane attachment

    • Innervation: has nerve supply

    • Tight cell connections: tightly bound cells via junctions, forming continuous sheets

  • Classification by appearance

    • One layer: Simple epithelia

    • More than one layer: Stratified epithelia

  • Classification by shape

    • Squamous (flattened)

    • Cuboidal (cube-shaped)

    • Columnar (tall, rectangular)

  • Relation of structure to function (overview)

    • Simple: diffusion/filtration (squamous); secretion/absorption (cuboidal, columnar)

    • Stratified: protection; transitional types in urinary systems for stretching

  • Form and Function (selected examples)

    • Simple squamous:

    • Appearance: single layer of flattened cells

    • Function: diffusion/filtration

    • Locations: kidney glomeruli (filtration membrane), alveoli, endothelium/mesothelium, lining of capillaries

    • Simple cuboidal:

    • Appearance: single layer of cube-shaped cells

    • Function: secretion and absorption

    • Locations: kidney tubules, gland ducts, ovary surface

    • Simple columnar:

    • Appearance: single layer of tall, rectangular cells

    • Function: absorption and secretion; often has microvilli for increased surface area in the small intestine

    • Locations: line most of digestive tract (e.g., small intestine), some cilia in respiratory tract; may have goblet cells

    • Pseudostratified ciliated columnar:

    • Appearance: single layer of cells of differing heights, nuclei at different levels; appears stratified but isn't. Often has cilia and goblet cells.

    • Function: secretion (mucus) and propulsion of mucus by ciliary action

    • Locations: trachea and upper respiratory tract

    • Stratified squamous:

    • Appearance: multiple layers of flattened cells at the apical surface; basal cells are cuboidal or columnar

    • Function: protection against abrasion

    • Locations: keratinized (skin) vs nonkeratinized (mouth, esophagus, vagina)

    • Stratified cuboidal:

    • Appearance: generally two layers of cube-shaped cells

    • Function: protection and secretion

    • Locations: ducts of sweat, mammary, salivary glands

    • Transitional epithelium:

    • Appearance: basal cells are cuboidal or columnar; surface cells dome-shaped when relaxed, flattened when stretched

    • Function: stretches readily, permits distension of urinary organ by contained urine

    • Locations: lines urinary bladder, ureters, part of urethra (urinary system)

Connective Tissues

  • General characteristics

    • Cells: fibroblasts (fibrous), chondroblasts (cartilage), osteoblasts (bone), adipocytes (fat), leukocytes (WBCs)

    • Fibers: collagen (strength), elastic (stretch/recoil), reticular (delicate network)

    • Ground substance (matrix): fills space; varies by tissue type, making up the unique matrix characteristics

  • Connective tissue proper

    • Loose connective tissue (areolar, adipose, reticular)

    • Dense connective tissue (regular, irregular, elastic)

  • Supporting connective tissues

    • Cartilage (hyaline, elastic, fibrocartilage)

    • Bone (osseous tissue)

  • Fluid connective tissues

    • Blood and lymph

  • Areolar (loose)

    • Unique matrix characteristics: Gel-like matrix with all three fiber types loosely arranged

    • Cell types: Fibroblasts, macrophages, mast cells, and some white blood cells

    • Fiber types: Collagen, elastic, reticular

    • Function: wraps, cushions organs; immune cells fight pathogens; holds tissue fluid

    • Location: under epithelia (e.g., papillary layer of dermis), surrounds organs; around capillaries

  • Adipose (loose)

    • Unique matrix characteristics: Sparse matrix; closely packed adipocytes (fat cells)

    • Cell types: Adipocytes primarily

    • Fiber types: Few fibers (collagen, reticular)

    • Function: energy storage; cushions organs; insulates against heat loss; protection

    • Location: subcutaneous tissue (hypodermis); around kidneys/eyes; within abdomen; in breasts

  • Reticular (loose)

    • Unique matrix characteristics: Network of reticular fibers in a loose ground substance

    • Cell types: Reticular cells primarily; supports white blood cells

    • Fiber types: Reticular fibers

    • Function: forms a soft internal skeleton (stroma) that supports other cell types, including white blood cells, mast cells, and macrophages

    • Location: lymphoid organs (lymph nodes, spleen, bone marrow)

  • Dense regular

    • Unique matrix characteristics: Primarily parallel collagen fibers; few elastic fibers; major cell type is fibroblast

    • Cell types: Fibroblasts

    • Fiber types: Collagen fibers (predominantly parallel)

    • Function: tensile strength in one direction; attaches muscles to bones (tendons) and bones to bones (ligaments); withstands great tensile stress when pulling force is applied in one direction

    • Location: tendons, ligaments, aponeuroses

  • Dense irregular

    • Unique matrix characteristics: Primarily irregularly arranged collagen fibers; some elastic fibers; major cell type is fibroblast

    • Cell types: Fibroblasts

    • Fiber types: Collagen fibers (predominantly irregular)

    • Function: tensile strength in many directions; withstands tension exerted in many directions

    • Location: dermis, fibrous capsules of organs and joints

  • Dense elastic

    • Unique matrix characteristics: High proportion of elastic fibers

    • Cell types: Fibroblasts

    • Fiber types: Elastic fibers (predominantly)

    • Function: allows tissue recoil after stretching; maintains pulsatile flow in arteries; recoil of lungs

    • Location: walls of large arteries (e.g., aorta), certain ligaments (e.g., ligamentum nuchae), bronchial tubes

  • Cartilage (supporting connective tissue)

    • Unique matrix characteristics: Avascular; firm, flexible matrix (chondroitin sulfate & hyaluronic acid) with collagen and/or elastic fibers; cells embedded in lacunae

    • Cell types: Chondroblasts (produce matrix), Chondrocytes (mature cells in lacunae)

    • Hyaline cartilage:

    • Appearance: Amorphous but firm matrix; imperceptible collagen fibers

    • Function: resists compression; supports and reinforces; cushions; acts as a template for embryonic skeleton

    • Location: ends of long bones, costal cartilages, nose, trachea, larynx cartilages

    • Elastic cartilage:

    • Appearance: Similar to hyaline but with more elastic fibers in matrix

    • Function: maintains shape with flexibility

    • Location: external ear (pinna), epiglottis

    • Fibrocartilage:

    • Appearance: Matrix similar to hyaline but less firm; thick collagen fibers predominate

    • Function: absorbs compression shock; provides tensile strength with the ability to absorb compressive shock

    • Location: intervertebral discs, knee joints (menisci), pubic symphysis

  • Bone (osseous tissue)

    • Unique matrix characteristics: Hard, calcified matrix containing many collagen fibers; inorganic calcium salts provide rigidity

    • Cell types: Osteoblasts (form matrix), Osteocytes (mature cells in lacunae), Osteoclasts (resorb bone)

    • Fiber types: Collagen fibers

    • Function: supports and protects (by enclosing); provides levers for muscles to act on; stores calcium and other minerals; blood cell formation in marrow

  • Blood (fluid connective tissue)

    • Unique matrix characteristics: Fluid matrix (plasma) with no fibers (only soluble proteins that precipitate during clotting)

    • Cell types: Red blood cells (erythrocytes), White blood cells (leukocytes), Platelets

    • Fiber types: None (soluble proteins for clotting)

    • Function: transports gases (oxygen, CO2_{2}), nutrients, wastes, hormones, and immune products; immunity (white blood cells)

    • Location: Contained within blood vessels