Epithelial Tissue: Structure and Function

• The presentation is about epithelial tissue

Introduction

• Epithelial tissue consists of cells closely joined by union complexes with minimal extracellular matrix
• It originates from all three blastodermic layers and is primarily derived from the ectoderm
• Epithelial tissue is located on external surfaces such as skin, lining hollow organs (mucous membranes), lining closed cavities (serous membranes, mesothelium), in vessels (endothelium), and in glands (exocrine and endocrine)
• Epithelia are classified as either lining epithelia or glandular epithelia

Importance of Epithelial Tissue

• Epithelial tissue is a key part of organ structure, forming the parenchyma, while connective tissue makes up the stroma
• Epithelia are responsible for various bodily functions, including protection, nutrition, exchange, absorption, and secretion
• In medicine, histological study of epithelial tissue is essential for diagnosis of tumors and inflammatory processes, understanding absorption and secretion alterations, immune reactions, tissue regeneration, and new regenerative therapies

General Characteristics of Epithelial Tissue

• Intercellular cohesion exists
• Very little extracellular matrix is present
• It sits on top of connective tissue
• It is avascular
• It has a short average lifespan
• Exhibits a high renewal and regeneration rate
• Cells have a defined shape
• It has polarity
• It has contents
• It serves functions

Intercellular Cohesion

• Cells tightly joined by complexes
• Includes adherents
• Includes occludents
• Includes the nexus type unions (gap or communicating junctions)

Extracellular Matrix

• There is a minimal space between cells, and this space is filled with extracellular fluid

Seating on Connective Tissue

• Epithelia rest on connective tissue and are separated by a basement membrane
• The connective tissue is often called the lamina propria
• Epithelial cells are connected via hemidesmosomes to the basement membrane
• Pemphigoid is not the same as epidermolysis bullosa

Avascularity

• Epithelial tissues are generally avascular, lacking blood vessels
• The vascular stria of the Corti organ is an exception
• Nutrition is supplied by diffusion from underlying connective tissue
• Avascularity contributes to a short lifespan

Lifespan

• Epithelial cells are classified by their cycle as labile (continuous renewal), stable (slow renewal), or permanent (no regeneration)
• Epithelial cells are labile with a short cycle
• Aged cells undergo apoptosis
• Epidermis renews in 47 days under normal conditions
• In psoriasis, skin renewal is much faster (8-10 days), with increased epidermal thickness and psoriatic plaques

Renovation and Regeneration

• Renovation replaces cells under physiological conditions using germ cells
• Repair occurs after injury through regeneration, where damaged cells proliferate from non-injured areas, or scar formation, a connective tissue response is when injuries are significant
• Epithelial tissue has high renewal and regeneration capabilities

DHAM: Growth and Differentiation Adaptations

• Hypertrophy involves an increase in cell size
• Hyperplasia involves an increase in cell number in response to a stimulus
• Atrophy involves a size decrease due to disuse or poor nutrition
• Metaplasia is a reversible change where one differentiated cell type is replaced by another

Dysplasia, Anaplasia, and Neoplasia

• Dysplasia shows abnormal changes like pleomorphism, hyperchromatic nuclei, and tissue disorganization; considered premalignant
• Anaplasia is when cells lose differentiation and do not resemble healthy ones, signifying malignancy
• Neoplasia or tumors are abnormal tissue masses having excessive, uncoordinated growth; tumors can be benign or malignant

Cell shape

• The shapes of the cells can be categorized as squamous, cuboidal, or columnar
• Squamous cells have greater width than height
• Cuboidal cells have consistent width, height, and depth
• Columnar or prismatic cells are taller

Polarity

• Polarized cells have three domains; apical, basal, and lateral

Kartagener Syndrome or Ciliary Dyskinesia

• Kartagener syndrome or ciliary dyskinesia involves axoneme alterations, that include missing dinein arms
• This affects ciliated epithelia and sperm, causing respiratory infections, infertility, and situs inversus

Lateral Domain

• Connections with neighboring cells
• Occluding junctions (tight)
• Adhering junctions
• Desmosomes provide intercellular cohesion
• Communicating junctions (gap)

Cellular Content

• Epithelial cells, especially secretory, exhibit organelle polarity
• They show an ordered distribution of organelles following the cell’s function, generally basal-apical
• Includes intermediate filaments (keratofilaments or tonofilaments) for durability

Functions

• Epithelial tissue offers protection to the skin and mucous membranes by working as a mechanical barrier
• It acts as chemical - and innate immune barrier
• It serves metabolic functions such as exchanging substances, nutrients, and gases
• It serves absorptive purposes in the intestine
• It assists in secretion in the glands
• It assists Serous membranes and is specialized cells for sensory perception

Classification of Epithelia

• These types of epithelia include skin and epidermis
• Include organs within the mucosa along with the epithelia
• Include vessels as well as endothelia
• Includes Serosaas, and Mesothelia
• Epitelios Glandulares contain cells speclized in secretion and formations like glands

Mucosa

• It was previously thought of as a membrane covered in mucus
• Comprises an epithelium for lining, a connective tissue lamina propria, and sometimes a smooth muscle layer
• Mucous membranes line cavities that open to the body's exterior, such as the digestive and respiratory tracts
• Not all mucous membranes are mucus-lined

Serosa

• Serosal membranes make up the visceral and parietal layers of the peritoneum, pericardium, and pleura
• Structurally, serosa includes a mesothelium and a layer of connective tissue
• Serosal membranes are moistened with a fluid, enabling slip