Study sheet: Epithelial Tissues Histology

📋 Course Outline

1. Glandular Architecture and Cell Types
2. Glandular Secretion Modes
3. Epithelial Surface Specializations
4. Epithelial Types and Functions
5. Cell Adhesion and Junctions
6. Basal Lamina Functions
7. Histological Sample Preparation
8. Epithelial Organization and Renewal

📖 1. Glandular Architecture and Cell Types

🔑 Key Concepts & Definitions

• Tubular gland : A tubular gland has a secretory portion organized as a tube, which may be simple straight, branched straight, or simple convoluted; examples include colonic glands, gastric fundic glands, and sweat glands.
• Acinar gland : An acinar gland has spherical functional units with narrow lumina, and its acini are assembled in grape-like clusters.
• Serous cell : A serous cell produces a fluid, protein-rich secretion with an enzymatic role, synthesized as proenzymes, and has abundant rough endoplasmic reticulum and small apical secretory vesicles.

★ Must-know

📌 Exocrine glands release their products onto an epithelial surface or through an excretory duct, whereas endocrine glands release their products directly into the blood.

• The main endocrine gland organizations and examples are:
  • trabecular: adrenal gland and anterior pituitary
  • diffuse: Leydig cells in the testis
  • follicular: thyroid

Further detail

• The three types of acini are:
  • serous acini
  • mucous acini
  • mixed acini

💡 Memory Hook

Tubular glands form tubes, whereas acini form grape-like spherical units.

📖 2. Glandular Secretion Modes

🔑 Key Concepts & Definitions

• Mucous cell : A mucous cell produces a viscous mucus rich in glycoproteins and proteoglycans called mucins, with a flattened basal nucleus, abundant supranuclear Golgi apparatus, and large PAS-positive secretory vacuoles.

★ Must-know

📌 Merocrine secretion releases the product by exocytosis without loss of cellular material, apocrine secretion releases an apical cytoplasmic fragment, and holocrine secretion releases whole glandular cells after degeneration and desquamation.

📌 Amphicrine glands perform both exocrine and endocrine functions; the liver secretes bile and releases glycogen-related products into the blood, while the pancreas secretes pancreatic juice and the hormones insulin and glucagon.

Further detail

• The characteristic examples are:
  • merocrine: salivary gland or exocrine pancreas
  • holocrine: sebaceous gland
  • apocrine: lactating mammary gland

💡 Memory Hook

Merocrine releases, apocrine decapitates, holocrine disintegrates.

📖 3. Epithelial Surface Specializations

🔑 Key Concepts & Definitions

• Microvillus : A microvillus is a finger-like apical projection specialized for absorption, approximately 0.1 µm in diameter and 1 µm long, that increases exchange surface and transport capacity.
• Stereocilium : A stereocilium is an immobile, thin, branched apical expansion related to microvilli that can reach 80 µm and contains an actin filament cytoskeleton.
• Cilium : A cilium is a motile apical projection that moves fluid or particles and measures 6–10 µm in length and 0.2 µm in diameter, with up to 200 cilia per epithelial cell.

📝 Essential Points

• The axoneme contains:

  • nine microtubule doublets
  • a central pair
  • nexine bridges
  • inner and outer dynein arms
  • radial spokes

• Basolateral folds are deep membrane invaginations rich in mitochondria that support active transport of water and electrolytes, as in the proximal convoluted renal tubule.

• Urothelial membrane plaques are rigid 12-nm-thick apical regions that unfold when the bladder fills and thereby increase the luminal surface.

💡 Memory Hook

Picture an epithelial cell with a brush, moving cilia, basal folds, and umbrella plaques.

📖 4. Epithelial Types and Functions

🔑 Key Concepts & Definitions

• Simple squamous epithelium : Simple squamous epithelium consists of one layer of flattened cells specialized for trans-epithelial exchange and is found in vascular endothelium and serous mesothelium.
• Pseudostratified epithelium : Pseudostratified epithelium has nuclei at different heights, but every cell rests on the basal lamina and some basal cells do not reach the surface.
• Transitional epithelium : Transitional epithelium is a stratified epithelium with basal cuboidal cells, intermediate racket-shaped cells, and superficial umbrella cells that allow the urinary tract to stretch during filling.

📝 Essential Points

• The principal epithelial functions are:

  • protection
  • absorption
  • metabolic exchange
  • substance movement
  • sensory reception
  • mechanical functions

• Endothelium lines closed blood-vessel cavities, mesothelium lines coelomic cavities such as the pleura, pericardium, and peritoneum, and epithelium lines body cavities communicating with the exterior.

💡 Memory Hook

Simple epithelia have one layer, whereas pseudostratified epithelia have nuclei at different heights but all cells contact the basal lamina.

📖 5. Cell Adhesion and Junctions

📝 Essential Points

• The three major junction classes are:
  • tight junctions
  • anchoring junctions
  • gap junctions

📌 Epithelial junctions maintain cell cohesion, control intercellular permeability, and prevent membrane proteins from diffusing between specialized membrane domains.

💡 Memory Hook

Occlude, anchor, communicate: tight junctions, anchoring junctions, gap junctions.

📖 6. Basal Lamina Functions

🔑 Key Concepts & Definitions

• Basal lamina : The basal lamina is a constant structure at the interface between epithelium and connective tissue, composed mainly of type IV collagen, laminin, entactin, fibronectin, and heparan sulfate glycosaminoglycan.

★ Must-know

📌 The basal lamina acts as a molecular sieve whose glycosaminoglycan arrangement determines pore size; in the kidney it permits water, ions, and small organic molecules but excludes large proteins.

Further detail

• The basal lamina supports epithelial adhesion through hemidesmosomes and focal contacts and is anchored to the underlying connective-tissue matrix.

• In bullous pemphigoid, circulating autoantibodies target basal-lamina components including hemidesmosomes and BPAg1 and BPAg2, causing skin detachment and blister formation.

💡 Memory Hook

Basal-lamina damage causes epithelial detachment and can permit tumor invasion.

📖 7. Histological Sample Preparation

★ Must-know

• 🔄 Paraffin preparation proceeds through: fixation, dehydration in increasing ethanol concentrations, clearing with xylene, infiltration with liquid paraffin at 56°C, formation of an oriented inclusion block

• Microtomy produces sections 2–10 µm thick for light microscopy, whereas ultramicrotomy produces 50–70 nm sections for electron microscopy.

Further detail

• After sectioning, the slide is stained to increase contrast and mounted by placing resin between the slide and coverslip.

• Common stains and their targets are:

  • hematoxylin: nucleic acids
  • eosin or erythrosine: cytoplasm
  • safran or light green: collagen fibers

💡 Memory Hook

Fix, dehydrate, clear, embed, cut, stain, mount.

📖 8. Epithelial Organization and Renewal

🔑 Key Concepts & Definitions

• Epithelial tissue : Epithelial tissue consists of cohesive, polarized cells arranged in one or more layers on a basal lamina; it is avascular and performs absorption, secretion, protection, and body-surface or cavity lining.

★ Must-know

• Glandular epithelia synthesize products using organelles adapted to proteins, mucins, steroids, or ions and then excrete those products either through a duct, onto an epithelial surface, or directly into blood.

• Epithelial cells are continuously renewed because their life span is a few hours to a few days and they are eliminated by desquamation or apoptosis.

Further detail

📌 Epithelial renewal is controlled by growth factors, and epithelial appearance can change under hormonal influences, cellular activity, or pathological metaplasia.

• Smoking can cause squamous metaplasia of the respiratory epithelium, while the uterine and vaginal epithelia change under the female hormonal cycle.

💡 Memory Hook

Rapid cell loss requires continuous germinal-cell renewal, while hormones and disease alter epithelial appearance.

📊 Synthesis Tables

Epithelial Surface Types

Type	Main feature	Example or location
Simple squamous	One flat cell layer; exchange	Endothelium and mesothelium
Simple cuboidal	Central nuclei; secretion and absorption	Renal tubules
Simple columnar	Basal nuclei; strong secretion and absorption	Digestive tract
Pseudostratified	All cells contact basal lamina; nuclei at different heights	Respiratory epithelium and epididymis
Transitional	Umbrella cells; stretchable	Ureter and bladder

Exocrine Secretion Modes

Mode	Cellular mechanism	Example
Merocrine	Exocytosis without cell loss	Salivary gland
Apocrine	Apical cytoplasmic fragment released	Lactating mammary gland
Holocrine	Whole cell degenerates and is released	Sebaceous gland