4.2 Epithelial Tissue

Epithelial tissue covers body surfaces, lines hollow organs and cavities, and forms glands. It's tightly packed, regenerates quickly, and performs functions like protection, absorption, and secretion. Understanding its features is key to grasping how the body maintains barriers and regulates exchanges between compartments.

Epithelial Tissue

Features of Epithelial Tissue

Every epithelial tissue shares a set of defining characteristics that distinguish it from other tissue types.

• Cellularity and packing: Cells are tightly packed with very little extracellular material between them. This dense arrangement is what makes epithelium effective as a barrier.
• Polarity: Every epithelial sheet has an apical surface (the free side exposed to the outside environment or the lumen of an organ) and a basal surface (the side attached to underlying connective tissue via the basement membrane). This polarity matters because the two surfaces often have different structures and functions.
• Avascularity: Epithelial tissue lacks its own blood vessels. It receives nutrients by diffusion from capillaries in the underlying connective tissue. However, it is innervated (has nerve supply) in some regions.
• Regeneration: Epithelial cells divide rapidly (high mitotic rate), which allows damaged tissue to replace itself quickly. This is why cuts to your skin or the lining of your mouth heal relatively fast.

Functions of epithelial tissue:

• Protection from abrasion, dehydration, and pathogen invasion
• Absorption of substances like nutrients and gases (intestinal lining, lung alveoli)
• Secretion of substances like mucus, enzymes, and hormones
• Filtration to regulate exchange between compartments, such as blood and interstitial fluid (kidney glomerulus)
• Sensation in specialized locations (taste buds, olfactory epithelium)

Simple vs. Stratified Epithelia

Epithelial tissues are classified by two criteria: the number of cell layers and the shape of cells at the apical surface. This naming system tells you both the structure and gives you strong clues about the tissue's function.

Simple epithelia consist of a single layer of cells. Because they're thin, they're suited for diffusion, filtration, absorption, and secretion. You won't find simple epithelia in areas subject to heavy mechanical stress.

• Simple squamous epithelium has thin, flat cells (think fried eggs viewed from above). Found where rapid diffusion or filtration occurs: capillary walls, air sacs of the lungs (alveoli), and serous membranes like the pericardium and pleura.
• Simple cuboidal epithelium has cube-shaped cells with a central, round nucleus. Involved in secretion and absorption. Found in kidney tubules and small gland ducts.
• Simple columnar epithelium has tall, rectangular cells with oval nuclei near the base. Lines much of the digestive tract (stomach and intestines). Some cells have microvilli (a brush border) that increase surface area for absorption. Goblet cells, which secrete mucus, are scattered among columnar cells in the respiratory and digestive tracts.
• Pseudostratified columnar epithelium looks like it has multiple layers, but every cell actually contacts the basement membrane. The illusion comes from nuclei sitting at different heights. Found primarily in the respiratory tract, where ciliated cells sweep mucus and trapped particles upward.

Stratified epithelia consist of multiple cell layers, making them much tougher. Their primary role is protection, and they're found in areas subject to abrasion or mechanical stress. The tissue is named for the shape of cells at the apical (top) layer, not the basal layer.

• Stratified squamous epithelium is the most common stratified type. The top layers are flattened squamous cells.
  • Keratinized: Surface cells are dead and filled with the protein keratin, which waterproofs the tissue. This is the epidermis of your skin.
  • Non-keratinized: Surface cells are living and kept moist by secretions. Found lining the esophagus, mouth, and vagina.
• Stratified cuboidal and stratified columnar epithelia are rare. They appear in some large gland ducts (e.g., salivary glands) and parts of the male urethra.
• Transitional epithelium (urothelium) is a special stratified type found in the urinary bladder, ureters, and part of the urethra. Its cells change shape: when the bladder is empty, the apical cells look rounded and dome-shaped; when it stretches to fill, they flatten out. This allows the organ to expand without tearing its lining.

Junctions in Epithelial Tissue

Because epithelial cells are packed so tightly, they need specialized junctions to hold together, seal gaps, and communicate. There are three main categories.

Tight junctions (zonula occludens) form a seal between adjacent cells near the apical surface. Transmembrane proteins (mainly claudins and occludins) fuse the outer membranes of neighboring cells together. This prevents molecules from leaking between cells (paracellular transport) and helps maintain cell polarity by keeping apical and basolateral membrane proteins in their respective domains. They're especially important in the intestinal lining and blood-brain barrier.

Anchoring junctions mechanically link cells to each other or to the basement membrane. They resist pulling and stretching forces.

• Desmosomes (macula adherens): Spot-like junctions that anchor to intermediate filaments (like keratin) inside the cell. They act like rivets, providing tensile strength. Abundant in skin and cardiac muscle.
• Adherens junctions (zonula adherens): Belt-like junctions that encircle the cell and anchor to actin filaments. They help coordinate cell movement and maintain tissue shape.
• Hemidesmosomes: Structurally similar to half a desmosome, these anchor the basal surface of epithelial cells to the basement membrane. They use integrins rather than cadherins.

Gap junctions (nexus) are communication channels. They're formed by connexin proteins that assemble into ring-shaped structures called connexons. When connexons from two adjacent cells align, they create a tunnel that allows ions, small nutrients, and signaling molecules to pass directly between cells. This coordinates cellular activities like contraction in smooth muscle and maintains homeostasis across the tissue.

Epithelial Tissue Organization and Interactions

The basement membrane is a thin, non-cellular layer that separates epithelial tissue from the underlying connective tissue. It has two components: the basal lamina (produced by epithelial cells) and the reticular lamina (produced by connective tissue). The basement membrane provides structural support, anchors the epithelium in place, and acts as a selective filter that regulates which molecules can pass between the epithelium and connective tissue.

Cell adhesion molecules (CAMs) are proteins that maintain epithelial integrity:

• Cadherins are calcium-dependent proteins that mediate cell-to-cell adhesion. They're the key adhesion proteins in desmosomes and adherens junctions.
• Integrins connect cells to extracellular matrix components in the basement membrane. They're the main adhesion proteins in hemidesmosomes.

Epithelial-mesenchymal transition (EMT) is a process where epithelial cells lose their polarity and cell-cell adhesion and gain migratory, mesenchymal properties. EMT is normal and essential during embryonic development (e.g., neural crest cell migration) and wound healing. However, it also plays a role in cancer metastasis, where tumor cells use this process to break free from the primary tumor and spread to other tissues.

Endocrine vs. Exocrine Glands

Glands are classified by how they release their secretions.

Endocrine glands are ductless. Their secretory cells release hormones directly into the surrounding interstitial fluid, from which hormones diffuse into blood capillaries and travel to distant target cells. Secretory cells are typically arranged in clusters or cords to maximize contact with capillaries. Examples include the pancreatic islets (insulin, glucagon) and the adrenal medulla (epinephrine). Secretion usually occurs via exocytosis of secretory vesicles.

Exocrine glands retain a connection to the epithelial surface through ducts. Their secretions (sweat, saliva, mucus, digestive enzymes, etc.) are delivered onto an epithelial surface rather than into the blood.

Exocrine glands are classified by structure:

1. Simple glands have a single, unbranched duct (can be tubular or acinar/rounded)
2. Compound glands have branched ducts (can be tubular, acinar, or tubuloalveolar)

Exocrine glands are also classified by secretion method, which describes how the product leaves the cell:

1. Merocrine (eccrine) secretion: The product is released by exocytosis. The cell remains completely intact. Most exocrine glands use this method (e.g., sweat glands, salivary glands, pancreas).
2. Apocrine secretion: The secretory product accumulates at the apical end of the cell, and that portion pinches off along with a small amount of cytoplasm. The cell repairs itself and repeats the cycle. The mammary glands use this method for milk fat secretion.
3. Holocrine secretion: The entire cell accumulates product, then ruptures and dies to release it. The cell itself becomes the secretion. New cells from the basal layer replace the lost ones. Sebaceous (oil) glands of the skin are the classic example.