Lecture 17 1&2 Epithelial cells and tissues

Epithelial Tissue Overview

Course Details

Course Code: BMS1025
Instructor: Dr. Patrizia Camelliti (Senior Lecturer in Cardiovascular Biology, University of Surrey)
Contact: p.camelliti@surrey.ac.uk

Outline of Epithelial Tissue

Definition of Epithelial Tissue

Epithelial tissue is a type of tissue that forms a continuous layer of closely packed cells that cover surfaces and line cavities throughout the body.

Location in the Body

Epithelial tissue can be found covering all internal and external body surfaces. Key locations include:

Skin: Acts as a barrier to protect underlying tissues.
Mouth: Involved in the first stages of digestion and protects underlying structures.
Blood Vessels: Endothelium (a specialized type of epithelium) regulates the passage of substances into and out of blood.
Stomach and Intestines: Lining involved in absorption and secretion processes.
Lungs: Alveoli provide a site for gas exchange.
Liver: Hepatic epithelium involved in the secretion of bile and metabolic processes.

Functions and Characteristics

Epithelial tissue serves various critical functions including:

Physical Protection: Protects underlying structures from mechanical damage, microbial invasion, dehydration, and chemical harm.
- Example: Skin's keratinized stratified squamous epithelium provides a tough protective barrier.
Control Permeability: Regulates what enters and exits the environment, crucial in areas such as capillaries and alveoli.
- Example: Alveolar epithelium controls gas exchange between air and blood.
Sensation: Houses specialized sensory cells that detect environmental stimuli.
- Example: Epithelial cells in the nose are involved in the sense of smell.
Absorption: Facilitates the uptake of nutrients, particularly in the digestive tract.
- Example: Microvilli in the intestines increase the surface area for absorption.
Secretion: Produces and releases substances like hormones, enzymes, and mucus through glandular epithelium.
- Example: Goblet cells secret mucus in the respiratory tract to trap foreign particles and pathogens.
Transport: Ciliated epithelia help in the movement of substances across epithelial surfaces, such as moving mucus up the respiratory tract to expel debris.

Classification (structure, location, function)

Epithelial tissue can be classified based on different criteria:

Number of Cell Layers:
- Simple: One cell layer (e.g., simple squamous, simple cuboidal, simple columnar).
- Stratified: More than one layer (e.g., stratified squamous, stratified cuboidal).
Cell Shape:
- Squamous: Flat and scale-like.
- Cuboidal: Cube-shaped with approximately equal height and width.
- Columnar: Tall and column-like, often with specialized structures such as cilia or microvilli.

Glands (Secretory Epithelium)

Glandular epithelial tissue specializes in secretion. Glands can be classified into:

Endocrine Glands: Secrete hormones directly into the bloodstream (e.g., thyroid, adrenal glands).
Exocrine Glands: Secrete substances via ducts to surfaces (e.g., sweat glands, salivary glands).
- Modes of secretion include:
  - Merocrine: Secretion through exocytosis, preserving the cell (e.g., pancreatic acinar cells).
  - Apocrine: Secretion involves budding off a portion of the cell (e.g., mammary glands).
  - Holocrine: Cells disintegrate to release their contents (e.g., sebaceous glands).

Levels of Structural Organisation

Epithelial tissue exists within a hierarchy of biological organization:

Chemical Level: Atoms combine to form molecules.
Cellular Level: Molecules form together to create cells.
Tissue Level: Groups of similar cells come together to perform a specific function (such as epithelial tissue).
Organ Level: Different tissue types combine to form organs (e.g., stomach composed of epithelial, muscular, and connective tissues).
Organ System Level: Organs cooperate to perform complex functions (e.g., the digestive system).
Organism Level: All bodily systems work together to maintain life.

Understanding Tissues

Definition: Tissues are groups of similar cells that work together for a common purpose. Epithelial tissue is one of four primary tissue types, along with connective, muscle, and nervous tissue.

Connective Tissue: Provides structural support and binds other tissues together.
Muscle Tissue: Facilitates movement, whether voluntary or involuntary.
Nervous Tissue: Transmits signals and communicates information throughout the body.

Additional Characteristics of Epithelial Tissue

Polarity: Epithelial cells exhibit polarity with distinct apical and basal surfaces, each serving unique functions.
Specialized Contacts: Tight junctions create impermeable barriers, while desmosomes provide mechanical support.
Attachment: The basal surface attaches to a basement membrane that anchors epithelial tissue to underlying structures.
Avascularity: Epithelial tissues lack blood vessels; they obtain nutrients through diffusion from underlying tissues.
Regeneration: Epithelial tissues have a remarkable ability to regenerate, with processes of cell division and repair occurring frequently.

Stem Cells in Epithelial Tissue

Definition: Stem cells are undifferentiated cells capable of self-renewal and differentiation into specialized cells.These cells are abundant in epithelial tissue, providing a high capacity for tissue repair and regeneration, whereas such potential is more limited in cardiac and nervous tissues.

Learning Outcomes

Understand the various functions and characteristics of epithelial tissue.
Recognize the classifications of epithelial tissues based on structure and function.
Describe different types of glands and their secretion mechanisms.
Grasp essential knowledge crucial for practical applications in later modules.