Tissue: The Living Fabric
Epithelial, connective, muscle and nervous tissue — and how tissues repair
By the end of this unit you can…
- ✓List the four primary tissue types and their general functions
- ✓Classify epithelia by layers and cell shape and give a location for each
- ✓Compare endocrine and exocrine glands and the modes of secretion
- ✓Describe the matrix, fibers and cells of connective tissue and its main types
- ✓Compare the three muscle tissues and describe nervous tissue
- ✓Describe the four membranes and the steps of tissue repair
Key terms
Practice →1Four tissue types
A tissue is a group of cells similar in structure that perform a common function. Histology is the study of tissues. Every organ is built from just four primary types:
| Tissue | Job in one phrase |
|---|---|
| Epithelial | Covering — lines and covers surfaces; forms glands |
| Connective | Support — binds, protects, stores, transports |
| Muscle | Movement — contracts |
| Nervous | Control — sends electrical signals |
2Epithelial tissue: features
Epithelium is a sheet of cells that covers a body surface or lines a cavity. It forms boundaries, so its jobs are protection, absorption, filtration, excretion, secretion and sensory reception.
- Polarity — an apical surface (exposed, often with microvilli or cilia) and a basal surface (attached).
- Specialized contacts — cells packed tightly, joined by tight junctions and desmosomes.
- Supported by connective tissue — the basement membrane = basal lamina (made by epithelial cells) + reticular lamina (made by underlying connective tissue).
- Avascular but innervated — no blood vessels; nutrients diffuse from underlying connective tissue. Has nerve fibers.
- Regeneration — high rate of cell division replaces cells lost to friction.
Epithelia are named by two words: number of layers (simple = one layer; stratified = two or more) + shape of the cells (squamous = flat, cuboidal = cube, columnar = tall). Stratified epithelia are named for the shape of the cells in the apical layer.
3Types of epithelium
| Epithelium | Function | Where |
|---|---|---|
| Simple squamous | Diffusion & filtration (thin!) | Lung alveoli, kidney glomeruli, lining of vessels (endothelium) and serosae (mesothelium) |
| Simple cuboidal | Secretion & absorption | Kidney tubules, ducts of small glands, ovary surface |
| Simple columnar | Absorption; secretion of mucus (goblet cells) | Most of the digestive tract (stomach to rectum); ciliated version in uterine tubes |
| Pseudostratified columnar | Secretes and propels mucus (ciliated) | Trachea and most upper airways |
| Stratified squamous | Protects underlying tissue from abrasion | Keratinized: epidermis. Nonkeratinized: mouth, esophagus, vagina |
| Stratified cuboidal & columnar | Protection (rare) | Ducts of large glands; part of male urethra |
| Transitional | Stretches to allow distension | Urinary bladder, ureters, part of urethra |
4Glandular epithelia
A gland is one or more cells that make and secrete a product (secretion). Glands are classified two ways:
- Endocrine glands
- Ductless; secrete hormones into the extracellular space, which then enter the blood (thyroid, adrenal, pituitary).
- Exocrine glands
- Secrete products onto body surfaces or into body cavities through ducts (sweat, oil, salivary glands, liver, pancreas).
- Unicellular exocrine
- Goblet cells (and mucous cells) make mucin, which dissolves in water to form mucus.
- Multicellular exocrine
- A duct + a secretory unit; simple (unbranched duct) or compound (branched); secretory units tubular, alveolar (acinar) or tubuloalveolar.
| Mode | How | Example |
|---|---|---|
| Merocrine | Products released by exocytosis — cell stays intact | Most glands: pancreas, sweat (eccrine), salivary |
| Holocrine | Products accumulate until the whole cell ruptures; replaced by division | Sebaceous (oil) glands |
| Apocrine | Apex of the cell pinches off with the product (debated in humans) | Lipid secretion in mammary glands |
5Connective tissue: features
Connective tissue is the most abundant and widely distributed tissue. All types arise from embryonic mesenchyme and have lots of extracellular matrix — the matrix, not the cells, does most of the work (e.g. bears weight).
Main jobs: binding and support, protection, insulation, storing reserve fuel, and transporting substances (blood).
- Ground substance
- Unstructured material filling the space between cells: interstitial fluid + cell adhesion proteins + proteoglycans (with glycosaminoglycans such as hyaluronic acid). Holds water.
- Collagen fibers
- The strongest; high tensile strength (resist pulling). White fibers.
- Elastic fibers
- Made of elastin; stretch and recoil (skin, lungs, arteries).
- Reticular fibers
- Fine, branched collagen fibers forming delicate networks (supporting lymph nodes, spleen, bone marrow).
Cells: immature “-blasts” actively secrete matrix; mature “-cytes” maintain it. Fibroblasts (CT proper), chondroblasts → chondrocytes (cartilage), osteoblasts → osteocytes (bone), and hematopoietic stem cells (blood). Also: fat cells, white blood cells, mast cells (release histamine for inflammation and heparin, an anticoagulant) and macrophages (phagocytes).
6Types of connective tissue
| Type | Key features | Location |
|---|---|---|
| Areolar (loose) | Loose array of all three fibers; lots of ground substance; universal packing | Under epithelia (lamina propria), around organs and capillaries |
| Adipose (loose) | Packed fat cells; stores energy, insulates, cushions. Brown fat makes heat (infants) | Hypodermis, around kidneys and eyeballs, breasts, abdomen |
| Reticular (loose) | Network of reticular fibers; supports free blood cells | Lymph nodes, spleen, bone marrow |
| Dense regular | Parallel collagen fibers; great tensile strength in one direction; poorly vascularized | Tendons (muscle→bone), most ligaments (bone→bone) |
| Dense irregular | Irregular collagen bundles; resists tension in many directions | Dermis of skin, joint capsules, organ capsules |
| Elastic | Dense regular CT with many elastic fibers; recoils | Walls of large arteries, ligaments between vertebrae |
| Hyaline cartilage | Glassy matrix of fine collagen; chondrocytes in lacunae; most abundant cartilage | Ends of long bones (articular), costal cartilage, nose, trachea, embryonic skeleton |
| Elastic cartilage | Like hyaline but many elastic fibers — keeps shape, flexible | External ear (auricle), epiglottis |
| Fibrocartilage | Thick collagen fibers; tensile strength + absorbs compression | Intervertebral discs, pubic symphysis, knee menisci |
| Bone (osseous) | Hard calcified matrix; well vascularized; osteons | Skeleton |
| Blood | Fluid matrix (plasma); transports | Within blood vessels |
7Muscle & nervous tissue
| Skeletal | Cardiac | Smooth | |
|---|---|---|---|
| Striations | Yes | Yes | No |
| Nuclei | Many, peripheral | Usually one, central | One, central |
| Cell shape | Long cylinders (fibers) | Branching, joined by intercalated discs | Spindle-shaped |
| Control | Voluntary | Involuntary | Involuntary |
| Location | Attached to bones / skin | Heart wall | Walls of hollow organs |
Nervous tissue forms the brain, spinal cord and nerves. It contains neurons — which generate and conduct electrical impulses — and supporting neuroglia (glial cells) that insulate, support and protect them.
8Covering & lining membranes
Membranes are simple organs made of an epithelium bound to a connective tissue layer.
| Membrane | Made of | Location |
|---|---|---|
| Cutaneous (skin) | Keratinized stratified squamous epithelium + dense irregular CT (dermis) | Body surface — the only dry membrane |
| Mucous (mucosa) | Epithelium on loose CT (lamina propria) | Lines cavities open to the exterior: digestive, respiratory, urinary, reproductive tracts |
| Serous (serosa) | Simple squamous mesothelium on areolar CT | Lines closed ventral cavities and covers their organs (pleura, pericardium, peritoneum) |
| Synovial | Loose connective tissue only (no epithelium) | Lines joint cavities |
9Tissue repair
- 1Inflammation — injured cells release chemicals (histamine) → capillaries dilate and become permeable; clotting proteins wall off the area.
- 2Organization — the clot is replaced by granulation tissue (new capillaries + fibroblasts making collagen); macrophages clean up.
- 3Regeneration and fibrosis — surface epithelium regenerates over the area; underlying tissue matures into a fibrous scar.
| Regenerative capacity | Tissues |
|---|---|
| Excellent | Epithelia, bone, areolar CT, dense irregular CT, blood-forming tissue |
| Moderate | Smooth muscle, dense regular CT (tendons, ligaments) |
| Weak | Skeletal muscle, cartilage |
| Essentially none (scar tissue) | Cardiac muscle, nervous tissue of brain and spinal cord |