Histology: Epithelia, glands and connective tissues

🔬 Epithelia, glands and connective tissues. Histology is the study of normal tissues under the microscope. The body is built from four basic tissue types - epithelium, connective tissue, muscle and nervous tissue - and every organ is a combination of them. Recognising normal structure is the foundation for recognising disease: metaplasia, dysplasia, invasion through a basement membrane, fibrosis and inflammation all make sense only against the normal picture. This chapter covers epithelia, glands, the basement membrane and the connective tissues including cartilage and bone.
Your four questions when looking at a tissue: Which of the four basic tissues are present? For an epithelium - how many layers and what shape are the surface cells? What is the specialisation (cilia, microvilli, keratin, goblet cells)? Is the normal architecture preserved?

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Principles and the Four Basic Tissues

• Epithelium - sheets of closely packed cells covering surfaces and lining cavities and ducts; avascular (nourished by diffusion from underlying connective tissue); sits on a basement membrane; polarity (apical, lateral, basal surfaces); high turnover (labile).
• Connective tissue - cells scattered in an abundant extracellular matrix (fibres plus ground substance); supports, binds, transports, defends and stores; derived from mesoderm (mesenchyme).
• Muscle - contractile cells (skeletal, cardiac, smooth).
• Nervous tissue - neurons and supporting glial cells.
• Microscopy basics - H&E: nuclei blue/purple (basophilic), cytoplasm and collagen pink (eosinophilic); fat and glycogen are dissolved during processing (appear as clear spaces); size reference - a red cell is about 7–8 µm.
• Germ layer origins of epithelia - ectoderm (epidermis, cornea, lens), endoderm (lining of gut and respiratory tract, liver, pancreas, thyroid), mesoderm (endothelium, mesothelium, renal tubules, gonads).

Classification of Epithelia

• Classified by number of layers (simple = one; stratified = more than one) and shape of the surface cells (squamous - flat; cuboidal; columnar - taller than wide).

Type	Structure	Locations	Function
Simple squamous	Single layer of flat cells	Endothelium (vessels), mesothelium (pleura, pericardium, peritoneum), alveoli (type I pneumocytes), Bowman's capsule, loop of Henle (thin limb)	Diffusion, filtration, smooth lining
Simple cuboidal	Single layer of cube-shaped cells	Thyroid follicles, kidney tubules, small ducts, ovarian surface, choroid plexus	Secretion, absorption
Simple columnar	Single layer of tall cells, often with microvilli or cilia	Stomach (mucus-secreting, no goblet cells), small and large intestine (enterocytes with microvilli and goblet cells), gallbladder, uterine tube (ciliated), endometrium, endocervix	Absorption, secretion
Pseudostratified columnar	One layer - all cells touch the basement membrane but nuclei at different heights	Respiratory epithelium (ciliated with goblet cells - nose, trachea, bronchi), epididymis and vas deferens (stereocilia)	Mucociliary clearance
Stratified squamous non-keratinised	Many layers, flat surface cells, nuclei retained	Oral cavity, oesophagus, vagina, ectocervix, anal canal (below the pectinate line), cornea	Protection against abrasion (wet surfaces)
Stratified squamous keratinised	Surface layer of dead anucleate keratin	Epidermis (skin)	Waterproofing, protection
Stratified cuboidal/columnar	Two or more layers	Large ducts of sweat and salivary glands, conjunctiva, male urethra (part)	Protection
Transitional (urothelium)	Stratified with large dome-shaped "umbrella" surface cells; changes shape when stretched	Renal pelvis, ureters, bladder, proximal urethra	Distensible, impermeable to urine

• Junctions between epithelia are clinically important - the gastro-oesophageal junction (Z-line - Barrett's metaplasia when columnar epithelium extends upwards), the cervical transformation zone (squamocolumnar junction - where most cervical neoplasia arises), the anorectal junction (pectinate line - adenocarcinoma above, squamous cell carcinoma below; different lymphatic drainage and nerve supply).

Surface Specialisations and Junctions

• Microvilli - finger-like projections with an actin core; increase absorptive surface area (brush border of small intestine - enzymes such as lactase; proximal convoluted tubule). Lost in coeliac disease (villous atrophy) and microvillus inclusion disease.
• Cilia - motile, with a 9+2 microtubule axoneme and dynein arms; beat in coordinated waves (respiratory tract, uterine tubes, ependyma). Impaired by smoking (squamous metaplasia), viral infection, primary ciliary dyskinesia and cystic fibrosis (thick mucus).
• Stereocilia - long, non-motile microvilli (epididymis; inner ear hair cells - mechanotransduction).
• Keratin - intermediate filament protein; keratinisation in epidermis; keratin "pearls" in squamous cell carcinoma.
• Junctional complex - tight junctions (zonula occludens - seal), adherens junctions (zonula adherens - cadherins, actin), desmosomes (macula adherens - keratin filaments - pemphigus), gap junctions (connexins), hemidesmosomes (basal - pemphigoid).
• Goblet cells - unicellular glands secreting mucin; normal in intestine and respiratory tract; their presence in the oesophagus or stomach indicates intestinal metaplasia.

Glands and Secretion

• Exocrine glands secrete via ducts onto a surface; endocrine glands are ductless and secrete hormones into the blood (thyroid, adrenal, pituitary, islets of Langerhans). Some organs are both (pancreas, liver, gonads).
• Structure of exocrine glands: unicellular (goblet cells) or multicellular - simple (unbranched duct) or compound (branched duct); secretory units tubular, acinar (alveolar) or tubuloacinar.

Gland	Example
Simple tubular	Intestinal crypts (of Lieberkühn)
Simple coiled tubular	Eccrine sweat glands
Simple branched tubular	Gastric glands
Simple branched acinar	Sebaceous glands
Compound tubuloacinar	Salivary glands, pancreas, breast, prostate

• Secretion types: merocrine (exocytosis, cell intact - most glands, eccrine sweat, salivary, pancreas), apocrine (apical part of the cell pinched off - mammary gland lipid, apocrine sweat glands of axilla and groin - hidradenitis suppurativa), holocrine (whole cell disintegrates - sebaceous glands - acne).
• Serous acini (watery, enzyme-rich - parotid, pancreas; basophilic granular cytoplasm) versus mucous acini (viscous mucin - pale, foamy; sublingual) and mixed (submandibular - serous demilunes).
• Myoepithelial cells - contractile cells around acini and ducts (breast, salivary, sweat glands) under oxytocin in breast; preserved in benign breast lesions and DCIS but lost in invasive carcinoma (p63, SMA immunostains help diagnosis).

The Basement Membrane

• Basement membrane - thin extracellular layer between epithelium (or endothelium) and connective tissue: basal lamina (lamina lucida and lamina densa - type IV collagen, laminin, nidogen, perlecan) plus lamina reticularis (type III collagen - reticulin). PAS-positive.
• Functions - anchorage, barrier and filtration (glomerular basement membrane - charge and size selective), cell polarity and differentiation, scaffold for regeneration.
• Clinical relevance:
  • Invasion - carcinoma is invasive once it breaches the basement membrane; carcinoma in situ has not.
  • Glomerular disease - anti-GBM disease (antibodies to type IV collagen α3 chain - linear IgG), Alport syndrome (type IV collagen gene defect - haematuria, deafness, lens abnormalities), thin basement membrane nephropathy, diabetic nephropathy (thickened GBM).
  • Skin - epidermolysis bullosa (inherited defects of basement membrane zone proteins - blistering), bullous pemphigoid (BP180/BP230 hemidesmosomal antigens), epidermolysis bullosa acquisita (type VII collagen).

Connective Tissue Proper

• Resident cells: fibroblasts (make collagen, elastin and ground substance - spindle-shaped; active in repair), adipocytes, mast cells (histamine, heparin - IgE receptors; metachromatic granules), macrophages (histiocytes - tissue-specific names: Kupffer cells in liver, microglia in CNS, alveolar macrophages, osteoclasts in bone, Langerhans cells in skin), mesenchymal stem cells.
• Transient (wandering) cells: lymphocytes, plasma cells (eccentric "clock-face" nucleus, perinuclear halo - Golgi), neutrophils, eosinophils.
• Types:
  • Loose (areolar) - beneath epithelia (lamina propria, submucosa), around vessels; cells and fibres loosely arranged; site of inflammatory responses and oedema.
  • Dense irregular - fibres in many directions - dermis, organ capsules, periosteum.
  • Dense regular - parallel collagen bundles - tendons, ligaments, aponeuroses (poor blood supply - slow healing).
  • Reticular - type III collagen meshwork supporting lymph nodes, spleen, bone marrow and liver (silver stain).
  • Elastic - elastic laminae of large arteries (aorta), ligamenta flava.
  • Embryonic - mesenchyme and Wharton's jelly (umbilical cord).

Fibres and Ground Substance

Collagen type	Location	Associated disease
Type I (90% of body collagen)	Bone, skin (dermis), tendons, ligaments, dentine, mature scar, cornea	Osteogenesis imperfecta (fractures, blue sclerae, hearing loss, dentinogenesis imperfecta)
Type II	Cartilage (hyaline, elastic), vitreous humour, nucleus pulposus	Stickler syndrome, some skeletal dysplasias
Type III (reticulin)	Skin (fetal), blood vessels, uterus, granulation tissue, lymphoid organs	Vascular Ehlers–Danlos syndrome (arterial, bowel and uterine rupture)
Type IV	Basement membranes	Alport syndrome, anti-GBM disease
Type VII	Anchoring fibrils (epidermis to dermis)	Dystrophic epidermolysis bullosa

• Collagen synthesis - inside fibroblasts: procollagen chains, hydroxylation of proline and lysine (needs vitamin C - scurvy), glycosylation, triple helix formation; outside: cleavage to tropocollagen, cross-linking by lysyl oxidase (copper-dependent - Menkes disease).
• Elastic fibres - elastin core with fibrillin microfibrils; allow recoil (aorta, lung, skin). Marfan syndrome (fibrillin-1) - aortic root dilatation, lens dislocation; solar elastosis (sun-damaged skin); emphysema (elastin destruction by neutrophil elastase - α1-antitrypsin deficiency).
• Ground substance - glycosaminoglycans (hyaluronic acid, chondroitin, heparan, keratan and dermatan sulfate), proteoglycans (aggrecan in cartilage) and adhesive glycoproteins (fibronectin, laminin) - hydrated gel resisting compression. Accumulates in mucopolysaccharidoses and pretibial myxoedema.

Adipose Tissue

• White adipose tissue - large single lipid droplet per cell (unilocular), nucleus pushed to the edge ("signet ring" appearance); energy storage, insulation, cushioning; endocrine organ - leptin (satiety), adiponectin (insulin-sensitising - reduced in obesity), resistin, inflammatory cytokines (TNF-α, IL-6), aromatase (oestrogen production - breast and endometrial cancer risk after menopause).
• Brown adipose tissue - multiple small droplets (multilocular), many mitochondria with UCP-1 (thermogenin) for non-shivering thermogenesis; prominent in neonates (interscapular, around kidneys and great vessels); hibernoma is a benign tumour of brown fat.
• Visceral fat (omental, mesenteric) is metabolically more harmful than subcutaneous fat.
• Lipoma - benign tumour of mature fat (commonest soft tissue tumour); liposarcoma - malignant (retroperitoneum, thigh); deep, large (>5 cm), growing or painful soft tissue lumps need urgent imaging (sarcoma pathway).

Cartilage

• Avascular - nourished by diffusion from the perichondrium (or synovial fluid at joints) → limited capacity for repair (osteoarthritis, meniscal tears in the inner zone). Cells - chondroblasts and chondrocytes in lacunae; matrix rich in type II collagen and aggrecan.

Type	Features	Locations
Hyaline	Glassy matrix, type II collagen; perichondrium (except articular)	Articular surfaces, costal cartilage, nasal septum, larynx (thyroid, cricoid), trachea and bronchi (C-shaped rings), epiphyseal growth plates, fetal skeleton
Elastic	Hyaline plus elastic fibres - flexible	Pinna (auricle), external auditory canal, epiglottis, Eustachian tube
Fibrocartilage	Dense type I collagen with rows of chondrocytes; no perichondrium	Intervertebral discs (annulus fibrosus), menisci, pubic symphysis, TMJ disc, labrum, tendon insertions

• Clinical: relapsing polychondritis (autoimmune inflammation of cartilage - ear, nose, airway), osteoarthritis (articular cartilage loss, fibrillation, osteophytes, subchondral sclerosis and cysts), achondroplasia (FGFR3 mutation - growth plate cartilage proliferation failure), chondrocalcinosis (pseudogout - CPPD crystals), perichondritis of the pinna (Pseudomonas - piercing).

Bone

• Composition: organic osteoid (type I collagen) about 35% and inorganic hydroxyapatite (calcium phosphate) about 65%.
• Cells:
  • Osteoblasts - form osteoid and mineralise it; express RANKL; secrete ALP (raised in bone formation - growth, healing fractures, Paget's, osteoblastic metastases).
  • Osteocytes - osteoblasts trapped in lacunae; connected by canaliculi; sense mechanical load; secrete sclerostin (inhibits formation - romosozumab target) and FGF23 (phosphate regulation).
  • Osteoclasts - large multinucleated cells derived from monocytes/macrophages; resorb bone in Howship's lacunae using acid and cathepsin K; activated by RANKL binding RANK (blocked by denosumab); inhibited by osteoprotegerin, calcitonin and bisphosphonates (induce osteoclast apoptosis).
• Architecture:
  • Cortical (compact) bone - 80% of skeleton; osteons (Haversian systems) - concentric lamellae around a central Haversian canal with vessels; Volkmann's canals connect them.
  • Trabecular (cancellous, spongy) bone - lattice of trabeculae in vertebral bodies, ends of long bones and pelvis; high surface area and turnover - lost first in osteoporosis (vertebral and wrist fractures) and site of red marrow.
  • Woven bone - disorganised collagen; in fetal bone, fracture callus, Paget's disease, fibrous dysplasia, osteosarcoma - always abnormal in adults outside repair. Lamellar bone - mature, organised.
• Ossification: intramembranous (direct from mesenchyme - flat bones of the skull, clavicle, mandible) and endochondral (via a cartilage model - long bones, vertebrae, pelvis; growth plate zones - resting, proliferative, hypertrophic, calcification, ossification).
• Bone remodelling - coupled resorption and formation (basic multicellular units) throughout life; about 10% of the skeleton renewed yearly. Imbalance - osteoporosis (normal mineralisation, reduced mass), osteomalacia/rickets (defective mineralisation - wide osteoid seams), Paget's disease (chaotic remodelling - mosaic pattern of cement lines), osteopetrosis (osteoclast failure - dense but brittle bone, marrow failure).

At a Glance

Tissue	Location to remember
Simple squamous	Endothelium, mesothelium, alveoli
Simple cuboidal	Thyroid follicles, kidney tubules
Simple columnar	Stomach, intestine, gallbladder
Pseudostratified ciliated	Trachea and bronchi
Stratified squamous non-keratinised	Mouth, oesophagus, vagina, ectocervix
Stratified squamous keratinised	Epidermis
Transitional	Bladder, ureters, renal pelvis
Hyaline cartilage	Joints, trachea, growth plate
Elastic cartilage	Ear, epiglottis
Fibrocartilage	Discs, menisci, symphysis
Type I / II / III / IV collagen	Bone / cartilage / vessels and reticulin / basement membrane

Spot the Tissue

• Biopsy from the lower oesophagus showing columnar epithelium with goblet cells. → Barrett's oesophagus (intestinal metaplasia).
• Bronchial biopsy from a heavy smoker showing stratified squamous epithelium instead of ciliated columnar. → Squamous metaplasia.
• Epithelium with dome-shaped surface cells lining a hollow organ that stores fluid. → Urothelium (bladder).
• Bone with a mosaic of irregular cement lines and huge osteoclasts. → Paget's disease.
• Breast lesion with ducts lacking a myoepithelial layer on p63 staining. → Invasive carcinoma.

Exam-style Scenarios

• Scenario. Why does damaged articular cartilage heal poorly, while bone fractures usually heal well?
  Answer: Articular cartilage is avascular, aneural, lacks perichondrium, and its chondrocytes are trapped in lacunae with little capacity to divide or migrate - defects fill with weaker fibrocartilage at best. Bone is highly vascular with osteoprogenitor cells in periosteum and marrow, allowing callus formation and remodelling.
• Scenario. A child has recurrent fractures with minimal trauma, blue sclerae and hearing loss. Which tissue component is abnormal, and why is the diagnosis important?
  Answer: Type I collagen (osteogenesis imperfecta - COL1A1/COL1A2). It is important to distinguish it from non-accidental injury; management involves bisphosphonates, physiotherapy, orthopaedic and audiology follow-up, and genetic counselling.
• Scenario. Why do squamous cell carcinomas arise at the anorectal junction and the cervical transformation zone?
  Answer: Junctions between different epithelia (columnar and squamous) are sites of active metaplasia and cell turnover, and are particularly vulnerable to persistent high-risk HPV infection - the transformation zone and anal transition zone are where most HPV-related neoplasia arises.

Key Points

• Classify epithelia by layers and surface cell shape.
• Junctions between epithelia are common sites of metaplasia and cancer.
• The basement membrane separates in situ from invasive carcinoma.
• Know the collagen types and their diseases (I - OI; II - cartilage; III - vascular EDS; IV - Alport).
• Cartilage is avascular - poor healing; bone remodels throughout life.

Further Reading

• Young B et al. Wheater's Functional Histology
• Ross MH, Pawlina W. Histology: A Text and Atlas
• Stevens and Lowe's Human Histology