Histology of Epithelium, Overview and Types
Epithelial cells form from ectoderm, mesoderm, and endoderm, which explains why epithelial line body cavities and cover most body and organ surfaces. There are many arrangements of epithelial cells, such as squamous, cuboidal, and columnar, that organize as simple, stratified, pseudostratified, and transitional.
Nivedita Dar20 Nov, 2024
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Histology of Epithelium: unmasking microscopic features of the cells that lines our body surfaces
What is Epithelium?
Epithelial tissue is one of the four major types of tissue found in the body. It covers internal and external surfaces and its structure varies based on its location in the body.Structure of Epithelial Tissue
Epithelial cells are the basic building blocks of epithelial tissue. These cells are unique in having different structures in their cell membrane known as domains. The domains are apical, lateral, and basal, each exhibiting different functions such as housing receptors or channels. Some variations of epithelium possess microvilli, cilia, and stereocilia, which help increase the surface area for absorption, remove foreign particles from the surface, and transport foreign particles from the extracellular matrix into the cell.Basement Membrane: How It Forms
Epithelial cells are organized into one or more layers, separated by thin sheets of extracellular matrix. The lowest layer (basal region) generates a specialized extracellular matrix known as the basement membrane, which separates the epithelial tissue from the underlying connective tissue.Junctions of Epithelial Cells
Junctions are proteinaceous complex structures on the cell membrane. They help the cell maintain polarity, interact with adjacent cells, and anchor the tissue. There are five types of junctions: tight junctions, adherens junctions (also known as desmosomes), gap junctions, and anchoring junctions (also called hemidesmosomes).Types of Epithelium
Epithelial tissue has been classified into different types based on the shape of cells and the layers of cells in the tissue.Classification Based on Cell Shape
1. Squamous Epithelium: These cells are flat and have a sheet-like appearance. They have an oval-shaped nucleus and are thinner compared to their height. 2. Columnar Epithelium: These cells resemble columns, being taller than they are wide. They have an elongated oval nucleus and may have microvilli or cilia on their surface. 3. Cuboidal Epithelium: These cells resemble cubes, having almost equal height, width, and depth. They have a large, round, and centrally located nucleus.Classification Based on Layers of Cells
1. Simple Epithelium: This type has a single layer of cells. Depending on the shape of the cells, it can be classified as simple squamous, simple columnar, or simple cuboidal epithelium.- Simple Squamous Epithelium: Found in the capillaries, endothelium of blood vessels, and alveoli of the lungs.
- Simple Columnar Epithelium: Found in the stomach, intestines, and gallbladder, it helps in protection, secretion, absorption, and excretion.
- Simple Cuboidal Epithelium: Found in areas involved in protection due to its thickness.
- Stratified Squamous Epithelium: Found in the esophagus, larynx, and vagina.
- Stratified Cuboidal Epithelium: Found in the excretory ducts of sweat glands, large ducts of excretory glands, the anorectal junction, and surrounding ovarian follicles.
- Stratified Columnar Epithelium: Found in the retina of the eye, organ of Corti, and taste buds on the tongue. A variant, keratinized epithelium, is characterized by dead cells in the superficial layer containing the protein keratin.
Glandular Epithelium
Glandular cells are modified epithelial cells specialized to pick up substances from the blood, modify them, and release them as products. They can be unicellular glands (e.g., goblet cells in the intestinal epithelium) or form glandular organs (e.g., thyroid gland). Glands are classified as exocrine (releasing secretions to external body surfaces or internal organs) or endocrine (releasing products into the blood).Histology of Epithelium FAQs
What is pseudostratified epithelium and where is it located in the body?
Pseudostratified epithelium is a variation of simple columnar epithelium that appears as multiple layers but is a single layer where all cells connect to the basement membrane. It is found in the upper respiratory tract.
What are the different types of junctions in epithelial cells, and what are their functions?
Epithelial cells have five types of junctions: tight junctions, adhering junctions, gap junctions, and anchoring junctions (hemidesmosomes). These junctions maintain cell polarity, facilitate interactions between adjacent cells, and anchor the tissue.
What are glandular cells, and how are they classified based on their secretion?
Glandular cells are modified epithelial cells specialized in picking up substances from the blood and modifying them into products released from the cell. They can be exocrine glands (releasing secretions to external body surfaces or internal organs) or endocrine glands (releasing products into the blood).
What is keratinized epithelium?
Keratinized epithelium is a specialized stratified epithelium characterized by dead cells in the most superficial layer containing the protein keratin.
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