Epithelial Tissue - Lecture 10

Stratified Squamous Epithelium

Definition: A type of epithelium composed of multiple layers of flattened cells that are specialized for protection against mechanical stress and abrasion. This structure is particularly crucial in areas subject to friction and wear, highlighting its important role in maintaining tissue integrity.

Locations:

  • Esophagus: Protects against abrasion from food particles during swallowing, serving as a critical barrier for underlying tissues exposed to potentially damaging substances.

  • Oral Cavity: Protects against mechanical damage incurred during activities such as eating and chewing, and plays an essential role in the oral immune defense by serving as a frontline barrier against pathogens.

  • Vagina: Shields against abrasion during intercourse, accommodating physical changes while providing resistance to infection, thus contributing to reproductive health.

Characteristics:

  • The classification of stratified squamous epithelium is primarily determined by the shape of its outermost cell layer, where cell nuclei are noticeably elongated and flattened, resembling a squamous shape.

  • This epithelium is characterized by a multilayered structure that provides durability and strength, essential for protection against shear forces.

  • Constant exposure to abrasion leads to damage, necessitating that apical cells are regularly shed and replaced to maintain functional integrity.

Cell Replacement:

  • Basal Layer: This layer serves as the site of active cell division, where mitosis occurs to replace sloughed-off cells. New cells generated in this layer migrate upward to replenish the apical surface effectively.

  • Cell division is concentrated in the basal region, where cells have better access to nutrients supplied by underlying blood vessels due to the avascular nature of epithelial tissues. This allows for a steady and dynamic renewal process essential for sustaining tissue function and structure.

BRDU Assay

Overview: The BRDU assay is a vital technique used to measure cell division by incorporating 5-bromo-2'-deoxyuridine (BRDU), a synthetic nucleoside, into DNA during the S-phase of the cell cycle, thus enabling the assessment of cellular proliferation rates.

Procedure:

  1. Incorporation: Living tissue samples are exposed to a BRDU solution, which allows diving cells to incorporate this nucleoside into their DNA.

  2. Treatment: Following an appropriate incubation period, tissue samples undergo treatment with acid or heat to denature the DNA, making it accessible for antibody binding.

  3. Antibody Binding: An anti-BRDU antibody is applied to specifically bind to the incorporated BRDU, followed by the addition of a secondary antibody that is conjugated with horseradish peroxidase for signal amplification.

  4. Visualization: The horseradish peroxidase catalyzes a reaction that converts a colorless compound (DAD) and hydrogen peroxide into a brown-colored polymer, which can be visualized under a microscope, marking areas of active cell division.

Findings: The assay has revealed that active cell division in the basal layer of stratified squamous epithelium is crucial for tissue repair processes, highlighting its importance in maintaining the health and functional capacity of epithelial tissues.

Stratified Cuboidal Epithelium

Location: This type of epithelium is primarily found in sweat glands, salivary glands, and mammary glands, where it forms a protective layer around glandular structures.

Function:

  • Its main purpose is to protect underlying tissues from mechanical damage and to prevent leakage of glandular products, thus maintaining the integrity and functionality of glandular tissues.

  • It also provides structural support and can assist in secretion regulation, resisting pressure during secretion processes and facilitating the regulated transport of substances.

Stratified Columnar Epithelium

Occurrences: This type of epithelium is most commonly found in the male urethra and in certain gland ducts, playing a role in both secretion and protection.

Function: It provides a leak-resistant layer that lines areas where urine and semen pass, which is essential for the containment of various bodily fluids and aligns with the body’s needs in functions like urination and reproductive health.

Transitional Epithelium

Characteristics: This specialized type of epithelium is unique in its ability to undergo significant changes in shape. It lines the urinary bladder, ureters, and parts of the urethra.

Function: Transitional epithelium minimizes surface area to reduce the risk of bacterial attachment while allowing the bladder to expand and contract as needed, thus facilitating efficient urinary storage and excretion while providing a barrier to contaminants.

Keratinization

Definition: Keratinization is the biological process by which epithelial cells progressively become coated with keratin, a protective protein that plays a critical role in forming barriers against mechanical injury, pathogens, and dehydration.

Stratum Corneum: This outermost layer of the skin consists of many layers of dead, keratinized cells, creating a robust barrier protecting the underlying tissues from environmental insults and aiding in moisture retention.

Variability: The thickness of the stratum corneum varies significantly across different body parts; it is thicker in protective areas such as the heels and soles of the feet, while areas with higher touch sensitivity, such as the face and genitals, possess a thinner stratum corneum, balancing protection with sensory function.

Touch Sensitivity and Epithelial Thickness

Touch Receptors: Found primarily in the dermis and influenced by the thickness of the epidermis, thicker epithelial layers provide enhanced protection but tend to reduce sensitivity. This interplay between thickness and sensory function is key in various skin locations.

Microvilli and Stereocilia

Microvilli: Small apical extensions of the plasma membrane found in intestinal epithelial cells, microvilli significantly increase the surface area available for absorption, collectively forming the brush border. They are supported by an actin filament core, providing structural strength and enhancing functionality in nutrient absorption.

Stereocilia: These are longer extensions compared to microvilli and are found in the epididymis where they facilitate fluid reabsorption and contribute to sperm maturation, and also in inner ear hair cells, where they are vital for sound detection and auditory processing.

Recent Discoveries: Recent advancements have uncovered the role of nanobrushes, which regulate the spacing between microvilli, potentially impacting the efficiency of nutrient absorption in the intestines and offering exciting avenues for further research in epithelial biology.

In Situ Hybridization

Purpose: In situ hybridization is an important technique used to detect specific RNA or DNA sequences within tissue sections, providing valuable insights into gene expression patterns and the presence of particular genetic information in the genome.

Application: This method is widely applied in research to study keratin gene expression in various developing structures, aiding in the understanding of developmental biology and tissue-specific gene regulation.