Memory tricks for the four tissue types

Epithelial tissue: covers and lines surfaces, forms glands. Avascular (no blood vessels) — gets nutrients by diffusion. Tightly packed cells, rapid regeneration. Connective tissue: most abundant — supports, protects, connects. Has cells scattered in extracellular matrix (ground substance + fibers). Muscle tissue: contraction. Skeletal (voluntary), cardiac (involuntary, striated), smooth (involuntary, non-striated). Nervous tissue: generates and transmits electrical signals. Neurons plus supporting neuroglia. Every organ contains at least 2-3 tissue types — the stomach contains all four.

Step 1 — Layers: Simple (one layer) or Stratified (multiple layers). Step 2 — Cell shape at apical surface: Squamous (flat, scale-like), Cuboidal (cube-shaped, nucleus in center), Columnar (tall, column-like, nucleus at base). Simple squamous: air sacs (alveoli), blood vessel lining (endothelium) — gas exchange and filtration. Simple cuboidal: kidney tubules, thyroid follicles — secretion and absorption. Simple columnar: GI tract lining — absorption and secretion. Stratified squamous: skin (keratinized), mouth/esophagus (non-keratinized) — protection from abrasion. Pseudostratified columnar: respiratory tract — appears layered but all cells touch basement membrane.

Glands are derived from epithelium and specialized for secretion. Exocrine glands: retain their duct — secretion delivered to a surface or lumen. Unicellular (goblet cells — mucus) or multicellular (sweat, salivary, pancreatic exocrine, liver). Types by secretion method: merocrine (secreted by exocytosis — most common, e.g. sweat, salivary), apocrine (cell apex pinches off with secretion — e.g. mammary glands), holocrine (whole cell ruptures — e.g. sebaceous/oil glands). Endocrine glands: no duct — hormones secreted directly into bloodstream. Pituitary, thyroid, adrenal glands. The pancreas is both — exocrine (digestive enzymes via duct) and endocrine (insulin/glucagon into blood).

All connective tissues share the same basic structure: cells scattered within an extracellular matrix. Matrix = ground substance (fluid to solid) + protein fibers. Collagen fibers: strongest, resist tension, white. Elastin fibers: elastic, return to shape after stretch, yellow. Reticular fibers: thin collagen, form delicate support networks. Three types of CT cells: fibroblasts (most common — make matrix), macrophages (phagocytosis — immune defense), mast cells (release histamine during inflammation). The ratio of cells to matrix, and the type of matrix, determines the CT type — from watery blood to rigid bone.

Loose CT: areolar (most widely distributed — wraps organs, under skin), adipose (fat — energy storage, insulation, cushion), reticular (lymph nodes, liver, spleen framework). Dense CT: dense regular (tendons and ligaments — parallel collagen for strength in one direction), dense irregular (dermis, joint capsules — collagen in multiple directions). Cartilage: hyaline (articular surfaces, costal cartilage, fetal skeleton — smooth, no blood vessels), fibrocartilage (intervertebral discs, menisci — compression resistance), elastic (ear, epiglottis — flexibility). Bone: rigid matrix mineralized with calcium phosphate. Blood: liquid matrix (plasma) with blood cells.

Skeletal muscle: long cylindrical fibers, multinucleated (nuclei at periphery), striated (alternating dark/light bands from sarcomere arrangement), voluntary control via somatic nervous system. Cardiac muscle: branching cells (shorter than skeletal), ONE or TWO central nuclei, striated, connected by intercalated discs (gap junctions for rapid signal spread), involuntary — SA node pacemaker. Smooth muscle: spindle-shaped cells (tapered at ends), single central nucleus, NO striations (actin and myosin arranged differently — no sarcomeres), involuntary — autonomic control. Found in walls of hollow organs (GI tract, blood vessels, bladder, uterus).

Nervous tissue has two cell types. Neurons: generate and conduct electrical signals. Cell body (soma) contains nucleus. Dendrites receive signals. Axon conducts signals away (one per neuron). Myelin sheath (from Schwann cells in PNS, oligodendrocytes in CNS) insulates axon for faster conduction. Neuroglia (support cells, 10:1 ratio): Astrocytes (BBB, nutrient support — most numerous CNS glia), Oligodendrocytes (myelin in CNS), Microglia (immune cells of CNS — phagocytosis), Ependymal cells (line ventricles, make CSF), Schwann cells (myelin in PNS), Satellite cells (support PNS ganglia cell bodies).

Hemostasis (minutes): blood vessel spasm, platelet plug, fibrin clot forms — stops bleeding. Inflammation (hours to days): vasodilation, increased permeability, WBC migration, phagocytosis of debris — redness, heat, swelling, pain. Regeneration: stem cells proliferate and replace damaged cells with same tissue type — best outcome. Fibrosis: scar tissue (collagen) replaces damaged area when regeneration fails — permanent structural change. Which tissues regenerate well: epithelium (excellent), liver (good), bone (good with proper alignment). Poor regeneration: cardiac muscle, nervous tissue (neurons cannot regenerate — why strokes cause permanent deficits).

Cutaneous membrane (skin): dry membrane, stratified squamous epithelium on dense irregular CT — covers external body surface. Serous membranes: simple squamous (mesothelium) on areolar CT — lines closed body cavities and covers organs. Parietal layer lines cavity, visceral layer covers organ. Secrete serous fluid to reduce friction. Pleura (lungs), pericardium (heart), peritoneum (abdomen). Mucous membranes: line body openings that connect to exterior (respiratory, GI, urinary, reproductive tracts) — secrete mucus. Synovial membranes: line joint cavities — NOT epithelium (only CT) — secrete synovial fluid for joint lubrication. Inflamed in rheumatoid arthritis.