Epithelial and Connective Tissues: Structure, Function, and Classification

Epithelial Tissue

Overview of Epithelial Tissue

Epithelial tissue is a fundamental tissue type in the human body, serving as a protective barrier and involved in absorption, secretion, and sensation. It covers body surfaces, lines internal cavities, and forms glands.

• Key Characteristics: Closely packed cells, minimal extracellular matrix, avascular, high regenerative capacity.

• Main Functions: Protection, absorption, filtration, excretion, secretion, and sensory reception.

Classification of Epithelia

Epithelia are classified based on the number of cell layers and the shape of the cells at the surface.

• Simple epithelium: Single layer of cells.

• Stratified epithelium: Multiple layers of cells.

• Cell shapes: Squamous (flat), cuboidal (cube-like), columnar (tall).

Simple Epithelia

Simple Cuboidal Epithelium

Composed of a single layer of cube-shaped cells, often with central nuclei.

• Location: Kidney tubules, ducts of small glands, ovary surface.

• Function: Secretion and absorption.

• Example: Kidney tubule lining.

Simple Columnar Epithelium

Single layer of tall, column-shaped cells. Specialized for absorption and secretion.

• Location: Lines digestive tract from stomach to intestines.

• Features:

  • Microvilli increase surface area for absorption.

  • Goblet cells secrete mucus.

  • Some cells possess cilia for movement of substances.

Pseudostratified Columnar Epithelium

Appears layered due to varying cell heights, but all cells touch the basement membrane.

• Location: Most of the respiratory tract (trachea, bronchi).

• Features:

  • Ciliated cells move mucus.

  • Goblet cells secrete mucus.

Stratified Epithelia

Stratified Squamous Epithelium

Multiple layers of flat cells; thick and protective.

• Location: Epidermis of skin, lining of esophagus, rectum, anus, vagina.

• Keratinized: In skin, contains keratin for waterproofing and protection.

Stratified Cuboidal and Columnar Epithelium

Rare types, usually found in ducts of glands and small areas of the pharynx and urethra.

• Stratified Cuboidal: Usually two layers, found in ducts of some glands (e.g., sweat glands).

• Stratified Columnar: Found in small areas of the pharynx and urethra.

Transitional Epithelium

Specialized for stretching; cells change shape from cuboidal to squamous as the organ expands.

• Location: Lines hollow urinary organs (e.g., bladder, ureters).

• Function: Allows stretching and recoiling.

Glandular Epithelium

Overview

Glandular epithelium consists of cells specialized to produce and secrete substances. Glands can be classified by their mode of secretion and structure.

• Types: Endocrine (ductless, secrete hormones into blood/lymph) and Exocrine (have ducts, secrete onto surfaces or into cavities).

• Cellularity: Unicellular (e.g., goblet cells) or multicellular (formed by invagination of epithelium).

Endocrine Glands

• Structure: Ductless.

• Function: Secrete hormones by exocytosis into extracellular space, then into blood or lymph.

Exocrine Glands

• Structure: Have ducts.

• Function: Secrete products such as mucus, sweat, oil, and saliva.

• Examples: Liver, pancreas, sweat glands, salivary glands.

Unicellular Exocrine Glands

• Example: Goblet cell.

• Product: Mucin (a glycoprotein); mucin + water = mucus.

Multicellular Exocrine Glands

• Structure: Consist of a duct and an acinus (secretory portion).

Classification of Exocrine Glands by Duct Structure

Type	Description
Simple	Unbranched duct
Compound	Branched duct

Classification by Secretory Portion Shape

Shape	Description
Tubular	Secretory cells form tubes
Alveolar/Acinar	Secretory cells form sacs
Tubuloalveolar	Both tubular and alveolar regions

Modes of Secretion

• Merocrine: Secrete products by exocytosis (e.g., most sweat glands, salivary glands, pancreas).

• Holocrine: Entire cell ruptures to release product (e.g., sebaceous/oil glands of skin).

• Apocrine: Product accumulates just beneath the apex of the cell, then the apex pinches off.

Connective Tissue

Overview of Connective Tissue

Connective tissue is the most abundant and widely distributed tissue type in the body. It supports, binds, protects, insulates, and transports substances.

• Main Types: Connective tissue proper, cartilage, bone tissue, blood.

• Functions: Support, binding, protection, insulation, transportation.

Common Features of Connective Tissue

• Common Origin: All connective tissues arise from mesenchyme (embryonic connective tissue).

• Vascularity: Varies from avascular (cartilage) to highly vascular (bone).

• Extracellular Matrix: Composed of ground substance and fibers; provides structural and functional support.

Additional info: The extracellular matrix allows connective tissue to bear weight, withstand tension, and endure physical trauma.