Tissues

Tissue

Groups of cells similar in structure that perform common or related functions

Histology

The study of tissues

Four basic tissue types

Epithelial, connective, muscle, and nervous tissue

Epithelial tissue (epithelium)

A sheet of cells that covers body surfaces or cavities; includes covering and lining epithelia and glandular epithelia

Main functions of epithelial tissue

Protection, absorption, filtration, excretion, secretion, and sensory reception

Polarity of epithelial cells

Cells have a top (apical surface) and bottom (basal surface) with different structures and functions

Apical surface

The free upper side of an epithelial cell exposed to surface or cavity, often with microvilli or cilia

Basal surface

The lower attached side of an epithelial cell facing inward, attached to basal lamina

Basal lamina

Adhesive sheet that holds basal surface of epithelial cells to underlying connective tissues

Specialized contacts in epithelia

Include tight junctions and desmosomes that bind adjacent cells tightly together

Basement membrane

Layer made of basal and reticular lamina that reinforces epithelial sheet and defines its boundary

Avascular but innervated epithelia

Epithelial tissue lacks blood vessels but is supplied by nerve fibers

Regeneration in epithelia

High regenerative capacity stimulated by loss of polarity and broken contacts, needed due to exposure to friction and substances

Classification of epithelia

First name: number of cell layers (simple or stratified); Second name: shape (squamous, cuboidal, columnar)

Simple squamous epithelium

Single layer of flattened cells; facilitates rapid diffusion; found in kidney, lungs, and lining of blood vessels (endothelium) and serous membranes (mesothelium)

Simple cuboidal epithelium

Single layer of cube-like cells; involved in secretion and absorption; found in small gland ducts and kidney tubules

Simple columnar epithelium

Single layer of tall cells, sometimes with microvilli or cilia; involved in absorption and secretion; found in digestive tract and bronchi

Pseudostratified columnar epithelium

Single layer with varying cell heights, appears stratified; mostly ciliated; involved in mucus secretion and movement; found in respiratory tract

Stratified squamous epithelium

Multiple layers with flat surface cells; protects against abrasion; keratinized in skin, nonkeratinized in moist linings

Stratified cuboidal epithelium

Rare; usually two layers; found in sweat and mammary glands

Stratified columnar epithelium

Rare; only apical layer is columnar; found in pharynx, male urethra, and some glandular ducts

Transitional epithelium

Stratified tissue with cells that can change shape when stretched; found in urinary organs like bladder and ureters

Gland

One or more cells that make and secrete aqueous secretions

Endocrine glands

Ductless glands that secrete hormones into interstitial fluid and blood

Exocrine glands

Secrete products onto body surfaces or into body cavities through ducts; include sweat, oil, and salivary glands

Unicellular exocrine glands

Mainly goblet and mucous cells producing mucin to form mucus; located in respiratory and intestinal tracts

Multicellular exocrine glands

Have ducts and secretory units; supported by connective tissue capsule and blood supply; classified by structure (simple or compound) and secretion mode

Modes of secretion

Merocrine (exocytosis), Holocrine (rupture of whole cell), Apocrine (apical portion ruptures, controversial in humans)

Connective tissue

Most abundant and widely distributed primary tissue; functions in binding, support, protection, insulation, fuel storage, and transport

Four main connective tissue classes

Connective tissue proper, cartilage, bone, blood

Common connective tissue features

All arise from mesenchyme, varying vascularity, cells in extracellular matrix (ECM)

Extracellular matrix components

Ground substance and fibers

Ground substance

Gel-like material between cells facilitating diffusion; contains interstitial fluid, cell adhesion proteins, proteoglycans like chondroitin sulfate and hyaluronic acid

Connective tissue fibers

Collagen (strong, tensile), elastic (stretch and recoil), reticular (fine, branched networks)

Connective tissue “blast” cells

Immature, actively secrete ECM and ground substance; fibroblasts in connective tissue proper, chondroblasts in cartilage, osteoblasts in bone

Connective tissue “cyte” cells

Mature cells maintaining matrix health

Other connective tissue cells

Fat cells (adipocytes), white blood cells, mast cells (inflammatory response), macrophages (phagocytosis)

Loose connective tissues

Areolar (supports and binds tissues), adipose (nutrient storage, insulation), reticular (fibers form support mesh in lymphoid organs)

Dense connective tissues

Dense regular (parallel collagen fibers, high tensile strength, tendons, ligaments), dense irregular (thick collagen in various directions, dermis, joint capsules), elastic (elastic fibers, found in vertebral ligaments and large arteries)

Cartilage

Produced by chondroblasts and maintained by chondrocytes in lacunae; 80% water with collagen and proteoglycans; tough, flexible, avascular

Types of cartilage

Hyaline (most abundant), elastic (more elastic fibers), fibrocartilage (strong, between hyaline and dense regular)

Bone (osseous tissue)

Supports and protects, stores fat, makes blood cells; extracellular matrix rich in collagen and inorganic calcium salts; highly vascularized; osteoblasts produce matrix, osteocytes maintain it

Blood

Atypical connective tissue with fluid matrix (plasma); contains red and white blood cells, platelets; functions in transport and nutrients/waste exchange

Muscle tissue

Highly vascularized tissue responsible for movement; contains myofilaments actin and myosin

Three muscle tissue types

Skeletal (voluntary, striated, multinucleate), cardiac (involuntary, striated, branched, intercalated discs), smooth (involuntary, nonstriated, spindle-shaped)

Nervous tissue

Main tissue of nervous system; contains neurons (nerve impulse generators) and supporting cells (insulation and protection)

Covering and lining membranes

Composed of epithelium plus underlying connective tissue; types include cutaneous, mucous, and serous membranes

Cutaneous membrane

The skin; keratinized stratified squamous epithelium over connective tissue; dry membrane

Mucous membranes

Line body cavities open to exterior; moist membranes with epithelium over loose connective tissue (lamina propria); may secrete mucus

Serous membranes

Line closed ventral body cavities; simple squamous epithelium (mesothelium) over areolar connective tissue; moist membranes producing serous fluid; includes pleura, pericardium, peritoneum

Tissue repair

Occurs by inflammation followed by regeneration (replacement of destroyed tissue with same tissue) or fibrosis (replacement with connective tissue with loss of original function)

Steps in tissue repair

1) Inflammation: blood vessel dilation, clotting; 2) Organization: blood clot replaced by granulation tissue, epithelial regeneration, fibroblast collagen production; 3) Regeneration and fibrosis complete repair

Tissues with high regenerative capacity

Epithelial tissue, bone, areolar connective tissue, dense irregular connective tissue, blood-forming tissue

Tissues with moderate regenerative capacity

Smooth muscle, dense regular connective tissue

Tissues with poor regenerative capacity

Cardiac muscle and nervous tissue of brain and spinal cord

Embryonic germ layers

Ectoderm (forms nerve tissue), mesoderm (forms muscle and connective tissue), endoderm (forms some epithelial tissues)

Age-related tissue changes

Epithelia thin, tissue repair less efficient, atrophy of bone, muscle, nervous tissue; increased cancer risk due to DNA mutations