8.3 Tissues Formed From Eukaryotic Cells: Epithelium & Connective Tissue
Key Takeaways
- Epithelium is avascular and polarized, resting on a basement membrane of laminin and type IV collagen, and receives nutrients by diffusion from the underlying connective tissue.
- Epithelial naming combines layering (simple, stratified, pseudostratified, transitional) with apical cell shape (squamous, cuboidal, columnar).
- Connective tissue is defined by sparse cells embedded in abundant extracellular matrix of ground substance plus protein fibers, and nearly all of it derives from mesoderm.
- Type I collagen dominates skin, bone and tendon; type II is the hyaline cartilage collagen; type III is the reticular fiber collagen of lymphoid organs and blood vessels.
- Blood is classified as a connective tissue because its formed elements are suspended in an abundant extracellular matrix, plasma, despite having no fibers until clotting occurs.
From Cells to Tissues
A tissue is a group of structurally similar cells and their extracellular products that perform a shared function. Animals build every organ from four primary tissue types: epithelial, connective, muscle and nervous. The AAMC content outline names epithelial and connective tissue specifically under Content Category 2A, because these two illustrate how the junctional complexes and cytoskeletal elements of Sections 8.1 and 8.2 scale up into functional architecture.
Epithelial Tissue
Epithelium covers body surfaces, lines cavities and forms glands. Four defining properties recur in passages:
- Cellularity and continuity. Cells are tightly packed with minimal extracellular matrix, joined by the tight junctions, desmosomes and gap junctions of Section 8.2.
- Polarity. Every epithelial sheet has an apical surface facing the lumen or external environment (often bearing microvilli or cilia) and a basolateral surface facing the interior. Tight junctions act as a fence preventing membrane proteins from diffusing between the two domains, which is what makes vectorial transport possible.
- Attachment to a basement membrane. The basal lamina is secreted by the epithelium and is built from laminin and type IV collagen; beneath it, a reticular lamina of type III collagen is contributed by the connective tissue. Hemidesmosomes anchor cells to it through integrins.
- Avascularity with high regenerative capacity. Epithelium contains no blood vessels; oxygen and nutrients diffuse from capillaries in the underlying connective tissue. It is richly innervated and mitotically active, replacing itself constantly.
Classification: Layering x Shape
| Layering | Description | Representative location |
|---|---|---|
| Simple | One cell layer; all cells touch basement membrane | Alveoli, capillary endothelium, nephron tubules |
| Stratified | Two or more layers; named for the apical layer | Epidermis, esophagus, vagina |
| Pseudostratified | One layer, but staggered nuclei create a layered illusion; all cells touch basement membrane | Respiratory tract (ciliated, with goblet cells) |
| Transitional (urothelium) | Cells change shape from cuboidal to squamous as the organ stretches | Bladder, ureter, renal pelvis |
| Shape | Description | Functional bias |
|---|---|---|
| Squamous | Flat, plate-like | Rapid diffusion and filtration |
| Cuboidal | Roughly as tall as wide | Secretion and absorption |
| Columnar | Taller than wide | Absorption and secretion; often with microvilli |
Combining the two axes gives names such as simple squamous (alveolar gas exchange), simple columnar (small intestine absorption), stratified squamous keratinized (epidermis) and pseudostratified ciliated columnar (trachea).
Glandular Epithelium
Epithelium that invaginates and specializes for secretion becomes glandular.
- Exocrine glands retain a duct and release product onto an epithelial surface — sweat, salivary, pancreatic acinar and gastric glands.
- Endocrine glands lose the duct and secrete hormones directly into interstitial fluid and blood — thyroid, adrenal, pancreatic islets (see Sections 9.3 and 9.4).
Clinical connection often used in passages: more than 85% of human cancers are carcinomas, tumors of epithelial origin, which is unsurprising given the high mitotic index of epithelium. Sarcomas arise from connective tissue and muscle, and are far rarer.
Connective Tissue
Connective tissue is the structural opposite of epithelium: few cells scattered within abundant extracellular matrix (ECM). Nearly all connective tissue derives from mesoderm (mesenchyme). Its ECM has two components:
1. Ground Substance
An amorphous hydrated gel of glycosaminoglycans (GAGs) such as hyaluronic acid and chondroitin sulfate, most of which are bound to core proteins to form proteoglycans, plus adhesive glycoproteins such as fibronectin. Their dense negative charge traps water, resisting compression and permitting solute diffusion.
2. Protein Fibers
| Fiber | Protein | Property | Where it dominates |
|---|---|---|---|
| Collagen | Type I | High tensile strength, low stretch | Skin dermis, bone, tendon, ligament |
| Type II | Resists compression | Hyaline and elastic cartilage | |
| Type III (reticular) | Fine branching mesh forming stroma | Lymph nodes, spleen, bone marrow, vessels | |
| Type IV | Sheet-like network, non-fibrillar | Basement membrane | |
| Elastic | Elastin + fibrillin | Recoils after stretch | Aorta, lung, elastic cartilage, dermis |
Resident Cell Types
- Fibroblasts — the workhorse; synthesize collagen, elastin and ground substance.
- Adipocytes — store triacylglycerol (see 5.4).
- Macrophages, mast cells and plasma cells — immune surveillance within the matrix (see 10.5).
- Chondroblasts/chondrocytes and osteoblasts/osteocytes — specialized matrix-secreting cells of cartilage and bone (see 11.4).
Categories of Connective Tissue
| Category | Subtype | Key feature |
|---|---|---|
| Connective tissue proper | Loose (areolar) | Loosely woven fibers, abundant ground substance; the packing tissue beneath epithelium |
| Adipose | Adipocytes dominate; insulation, cushioning, energy storage | |
| Dense regular | Parallel collagen bundles — tendons (muscle to bone) and ligaments (bone to bone) | |
| Dense irregular | Interwoven collagen resisting multidirectional stress — dermis, organ capsules | |
| Specialized | Cartilage | Chondrocytes in lacunae within a firm type II collagen and proteoglycan matrix; avascular |
| Bone (osseous) | Hydroxyapatite-mineralized type I collagen; highly vascular | |
| Blood | Formed elements suspended in plasma, a fluid matrix; fibers appear only on clotting |
Why blood counts as connective tissue. The defining criterion is cells dispersed in abundant extracellular matrix of mesodermal origin — not solidity. Plasma is that matrix, and soluble fibrinogen becomes the fiber component when it polymerizes into fibrin during coagulation (see 10.2).
Integrating Tissues Into Organs
An organ is two or more tissue types working together. The small intestine illustrates all four: simple columnar epithelium with microvilli absorbs nutrients; loose connective tissue in the lamina propria carries capillaries and lacteals; smooth muscle in the muscularis externa drives peristalsis; and the enteric nervous plexuses coordinate both. Recognizing which tissue does what lets you predict, in a passage, which layer a drug, toxin or mutation will affect.
A biopsy of the trachea shows a single layer of cells in which every cell contacts the basement membrane, but the nuclei sit at several different heights and many apical surfaces bear cilia. This tissue is best described as:
A mutation abolishes production of type II collagen. Which tissue would be most directly compromised?
Which feature explains why epithelial sheets can perform directional transport, such as moving glucose from the intestinal lumen into the bloodstream?