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Epithelium - Coggle Diagram
Epithelium
Classification according to function
Surface Epithelium
Defintion
Specialized layer of tissue formed by closely aggregated cells that line the outer surface of organs, blood vessels, the skin, and the inner surface of body cavities.
Divided into
Columnar
Cuboidal
Squamous
Have a slightly irregular shape, and a small, centrally located nucleus. These cells can be stratified into layers, as in (Right) this human cervix specimen.
Function
Protects against mechanical/chemical effects and harmful
radiation.
Resorption: e.g., nutrients in the gastrointestinal tract.
Classification
Surface epithelia are classified
according to the number of layers and cell shape.
Criteria for Classification
Layers
Simple Epithelium
A single layer of epithelial cells with all cells attached to the basement membrane.
Pseudostratified Epithelium
A single layer of cells with the appearance of multiple layers as a result of the nuclei positioned at different levels.
Stratified Epithelium
Two or more cell layers with the basal layer attached to the basement membrane.
Cell Shape
Cell shape of the upper cell layer of a type
of epithelium determines the name for the whole epithelium.
Flat (Squamous Epithelium)
Appearance: Upper cells have a squamous (flat) shape (width >
height).
Examples: skin , esophagus , cervix , serous
membrane (e.g., peritoneu).
Cube-shaped (Cuboidal Epithelium)
Appearance: Upper cells have a cube-like shape (width = height).
Examples: bile ducts , renal tubules
Column-shaped (Columnar Epithelium)
Appearance: Upper cells have a rectangular or cylindrical shape (width < height).
Examples: intestine , respiratory tract (respiratory epithelium).
Epithelial Specialization
Keratinization (horny layer)
Differentiated into the horny layer (keratinization) and cell
projections (microvilli, stereocilia, kinocilia).
Only found on the skin (stratum
corneum). The epidermis is a keratinized stratified
squamous epithelium
Cell Projections
Microvilli
Microvilli are tiny finger-like projections on cells, especially in the small intestine, aiding in nutrient absorption by increasing surface area.
They form structures like the brush border, found on both absorptive and secretory surfaces, and vary in length and composition. In the immune system, they are common on lymphocytes.
Stereocilia (Stereovilli)
Stereocilia are non-motile cell modifications, similar to microvilli but longer and thicker.
They contain actin, lack an axoneme, and are found in the vas deferens, epididymis, and inner ear sensory cells.
Unlike cilia, stereocilia lack a basal body, may or may not have a glycocalyx coating, and have no fixed arrangement.
Cilia
Cilia are small hair-like structures on mammalian cell surfaces, involved in both locomotion and mechanoreception.
They can be single or multiple, and organisms with cilia are called ciliates, using them for feeding and movement.
Types of Cilia
Motile Cilia
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Non-motile Cilia
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Glandular Epithelium
Glandular epithelium, whether single cells or groups, produces and secretes specific products like mucin.
It typically invaginates into connective tissue but is separated by the basal lamina.
Glandular epithelium can be either endocrine (secreting into the bloodstream) or exocrine (secreting onto a surface).
Classification
The type of secretion associated with it
Serous Glands
Low viscosity, protein, and enzyme-rich secretion, characterized by acinar glands, basal-rich rough endoplasmic reticulum, and apical granular secretions. Examples include the parotid gland and pancreas.
Mucous Glands
Viscous, mucin-rich secretion characterized by tubular glands with pale, bright cytoplasm. Examples include sublingual gland, esophageal glands, Brunner's glands, and all intraepithelial glands.
Seromucous Glands
Low viscosity secretion containing both mucin and enzymes. Terminal end is mainly tubuloacinar with a serous demilune. Gland cells exhibit characteristics of both serous and mucous cells. Example: Submandibular gland.
Secretory Mechanism
Exocrine glands are intraepithelial glands or extraepithelial
glands that release their secretions on the inner (e.g.,
intestinal lumen) or outer (e.g., skin) surfaces of the body.
Eccrine Secretion
Release of ions and small molecules facilitated by transmembrane proteins through membrane protein transport. Found in almost all exocrine gland cells.
Merocrine Secretion
Secretion release rich in proteins occurs through cytosolic vesicle fusion with the cell membrane, utilizing the mechanism of exocytosis. Commonly found in exocrine glandular cells, such as salivary and sweat glands.
Apocrine Secretion
Lipid-rich secretions are released as a portion of the apical cytoplasm buds off, involving the cell membrane. This process, known as apocrine secretion, occurs exclusively in apocrine sweat glands and lactating mammary glands.
Holocrine Secretion
Entire cytoplasmic content is released through gland cell destruction via apoptosis. This mechanism, called holocrine secretion, is exclusive to sebaceous glands.
Location & Shape
Intraepithelial Glands
Glandular cells that are located within the surface
epithelium.
Shape
Goblet Cells
Goblet cells are single intraepithelial glandular cells found in the small and large intestines, nasal mucosa, and respiratory tract.
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They have a characteristic vacuole filled with mucus and function to secrete mucins.
Secretory Epithelium
Columnar epithelial cell conglomerates with bright cytoplasm (mucus-filled), lacking a vacuole.
Found in locations like the surface epithelium of the stomach and cervical canal, where they form entirely secretory epithelium.
Extraepithelial Glands
Extraepithelial glands migrate during embryogenesis from surface epithelium into underlying connective tissue, resulting in an elaborate structure.
Terminal Ends
Closed gland connected to excretory ducts for secretion production, with various terminal ends.
Functions include the production and release of secretions into the lumen, draining into the excretory duct system.
Shapes of the terminal ends of exocrine glands
Acinar Glands
Form: berry-like , narrow lumen
Examples: parotid gland, pancreas
Alveolar Glands
Shape: sac-like , wide lumen
Examples: apocrine sweat glands
Tubular Glands
Shape: tubular; wide, elongated lumen Examples: gastric glands (foveolas in the stomach),
colonic crypts
Subtypes
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Mixed Forms
Tubuloacinar, tubuloalveolar
Excretory Ducts
Epithelial-lined ducts connect glandular cells to the gland opening, facilitating secretion drainage and modifying its nature.
Duct shapes vary due to subdivision into lobules by connective tissue septa in extraepithelial glands.
Intralobular Ducts
Within lobuli, drain secretions to larger interlobular ducts
Interlobular Ducts
In broad connective tissue septa, lead to excretory ducts, the thickest segment
Excretory Ducts
Collect secretions from terminal ends and divert them to the gland opening.
Examples include bilayered columnar epithelium in terminal excretory ducts of various glands, such as apocrine and eccrine sweat glands and esophageal glands.
Distribution
Covers all inner and outer surfaces of the body (skin and mucous membranes).
Exception
Synovial Membrane (synovium)
Composed of synoviocytes (not epithelial cells) which line the inner surfaces of joint capsules, synovial bursa, and tendon
sheaths.
Definition
Is present in almost all organs of the human
body as lining and glandular tissue.
Formation
Different types of epithelia are formed
from the three germ layers.
Ectoderm
Epithelium of skin and skin appendages.
Mesoderm
Epithelium of serous membranes (mesothelium): Peritoneum,
pleural, and pericardial cavities
Cells lining the inner surface of blood and lymphatic vessels
(endothelium).
Endoderm
Epithelium of the respiratory and
gastrointestinal tract.
