Epithelial cells 상피세포에 대하여

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상피세포

신경세포

근육세포

결합조직세포

Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. 

Functions of epithelial cells include secretion, selective absorption, protection, transcellular transport and detection of sensation. In Greek "ἐπί" ("epi") means "on" or "upon", and "θηλή" ("thēlē") means "nipple".^[1] Epithelial layers are avascular, so they must receive nourishment via diffusion of substances from the underlying connective tissue, through the basement membrane.^[2][3] Epithelia can also be organized into clusters of cells that function as exocrine and endocrine glands.

상피세포의 구조

Cells in epithelium are very densely packed together like bricks in a wall, leaving very little intercellular space. The cells can form continuous sheets which are attached to each other at many locations by adherens junctions, tight junctions and desmosomes.^[4]

[edit]Basement membrane

All epithelial cells rest on a basement membrane, which acts as a scaffolding on which epithelium can grow and regenerate after injuries.^[5] Epithelial tissue is innervated, but avascular. This epithelial tissue must be nourished by substances diffusing from the blood vessels in the underlying tissue, but they don't have their own blood supply. The basement membrane acts as a selectively permeable membrane that determines which substances will be able to enter the epithelium.^[2][3]

[edit]Cell junctions

Cell junctions are especially abundant in epithelial tissues. They consist of protein complexes and provide contact between neighbouring cells, between a cell and the extracellular matrix, or they build up the paracellular barrier of epithelia and control the paracellular transport.^{[citation needed]}

Cell junctions are the contact points between plasma membrane and tissue cells. There are mainly 5 different types of cell junctions. They are tight junctions, adherens junctions, desmosomes, hemidesmosomes, and gap junctions. Tight junctions are a pair of trans-membranar protein fused on outer plasma membrane. Adherens junctions are a plaque (protein layer on the inside plasma membrane) which attaches both cells' microfilaments. Desmosomes attach to the microfilaments of cytoskeleton made up of keratin protein. Hemidesmosomes resemble desmosomes on a section. They are made up of the integrin (a transmembraner protein) instead of cadherin. They attach the epithelial cell to the basement membrane. Gap junctions connect the cytoplasm of two cells and are made up of proteins called connexins (six of which come together to make a connexon).

상피세포의 분류

Tissues are generally classified by the morphology of their cells, and the number of layers they are composed of.^[2][4][6] Epithelial tissue that is only one cell thick is known as simple epithelium.^[7] If it is two or more cells thick, it is known as stratified epithelium.^[8] However, when taller simple epithelial cells (see columnar, below) are viewed in cross section with several nuclei appearing at different heights, they can be confused with stratified epithelia. This kind of epithelium is therefore described as "pseudostratified"epithelium.^[9]

There are three principal morphologies associated with epithelial cells. Squamous epithelium has cells which are wider than they are tall (flat and scale-like). Cuboidal epithelium has cells whose height and width are approximately the same (cube shaped). Columnar epithelium has cells taller than they are wide (column shaped). In addition, the morphology of the cells in transitional epithelium may vary from squamous to cuboidal, depending on the amount of tension on the epithelium.^[10]

[edit]Simple epithelium

Simple epithelium is one cell thick, that is, every cell is in direct contact with the underlying basement membrane. It is generally found where absorption and filtration occur. The thinness of the epithelial barrier facilitates these processes.^[4]

Simple epithelial tissues are generally classified by the shape of their cells. The four major classes of simple epithelium are: (1) simple squamous; (2) simple cuboidal; (3) simple columnar; (4) pseudostratified.^[4]

Simple squamous epithelium is found lining areas where passive diffusion of gases occur, including the walls of capillaries, the linings of the alveoli of the lungs, and the linings of the pericardial, pleural, and peritoneal cavities.

상피세포의 기능

The primary functions of epithelial tissues are: (1) to protect the tissues that lie beneath it from radiation, desiccation, toxins, invasion by pathogens, and physicaltrauma; (2) the regulation and exchange of chemicals between the underlying tissues and a body cavity; (3) the secretion of hormones into the blood vascular system, and/or the secretion of sweat, mucus, enzymes, and other products that are delivered by ducts glandular epithelium;^[11] (4) to provide sensation.

[edit]Secretory epithelia

As stated above, secretion is one major function of epithelial cells. Glands are formed from the invagination / infolding of epithelial cells and subsequent growth in the underlying connective tissue. There are two major classifications of glands: endocrine glands and exocrine glands. Endocrine glands secrete their product into the extracellular space where it is rapidly taken up by the blood vascular system. The exocrine glands secrete their products into a duct that then delivers the product to the lumen of an organ or onto the free surface of the epithelium.

In arthropods, the integument, or external "skin", consists of a single layer of epithelial ectoderm from which arises the cuticle,^[12] an outer covering of chitin the rigidity of which varies as per its chemical composition.

[edit]Sensing the extracellular environment

"Some epithelial cells are ciliated, and they commonly exist as a sheet of polarised cells forming a tube or tubule with cilia projecting into the lumen." Primary cilia on epithelial cells provide chemosensation, thermosensation and mechanosensation of the extracellular environment by playing "a sensory role mediating specific signalling cues, including soluble factors in the external cell environment, a secretory role in which a soluble protein is released to have an effect downstream of the fluid flow, and mediation of fluid flow if the cilia are motile."^[13]

발생학적 성장

In general, there are epithelial tissues deriving from all of the embryological germ layers^{[citation needed]}:

• from ectoderm (e.g., the epidermis);
• from endoderm (e.g., the lining of the gastrointestinal tract);
• from mesoderm (e.g., the inner linings of body cavities).

However, it is important to note that pathologists do not consider endothelium and mesothelium (both derived from mesoderm) to be true epithelium. This is because such tissues present very different pathology. For that reason, pathologists label cancers in endothelium and mesothelium sarcomas, whereas true epithelial cancers are called carcinomas. Also, the filaments that support these mesoderm-derived tissues are very distinct. Outside of the field of pathology, it is, in general, accepted that the epithelium arises from all three germ layers.^{[citation needed]}

^{When growing epithelium in culture, one can determine whether or not a particular cell is epithelial by examining its morphological characteristics. Epithelial cells tend to cluster together, and have a "characteristic tight pavementlike appearance". But this is not always the case, such as when the cells are derived from a tumor. In these cases, it is often necessary to use certain biochemical markers to make a positive identification. The intermediate filament proteins in the cytokeratin group are almost exclusively found in epithelial cells, and so are often used for this purpose.[14]}

^{상피세포의 위치}

Epithelium lines both the outside (skin) and the inside cavities and lumen of bodies. The outermost layer of our skin is composed of dead stratified squamous,keratinized epithelial cells.^{[citation needed]}

Tissues that line the inside of the mouth, the esophagus and part of the rectum are composed of nonkeratinized stratified squamous epithelium. Other surfaces that separate body cavities from the outside environment are lined by simple squamous, columnar, or pseudostratified epithelial cells. Other epithelial cells line the insides of the lungs, the gastrointestinal tract, the reproductive and urinary tracts, and make up the exocrine and endocrine glands. The outer surface of the corneais covered with fast-growing, easily regenerated epithelial cells. Endothelium (the inner lining of blood vessels, the heart, and lymphatic vessels) is a specialized form of epithelium. Another type, mesothelium, forms the walls of the pericardium, pleurae, and peritoneum.^{[citation needed]}

System	Tissue	Epithelium	Subtype
circulatory	blood vessels	Simple squamous	endothelium
digestive	ducts of submandibular glands	Stratified columnar	-
digestive	attached gingiva	Stratified squamous, keratinized	-
digestive	dorsum of tongue	Stratified squamous, keratinized	-
digestive	hard palate	Stratified squamous, keratinized	-
digestive	oesophagus	Stratified squamous, non-keratinized	-
digestive	stomach	Simple columnar, non-ciliated	gastric epithelium
digestive	small intestine	Simple columnar, non-ciliated	intestinal epithelium
digestive	large intestine	Simple columnar, non-ciliated	intestinal epithelium
digestive	rectum	Simple columnar, non-ciliated	-
digestive	anus	Stratified squamous, non-keratinized superior to Hilton's white line Stratified squamous, keratinized inferior to Hilton's white line	-
digestive	gallbladder	Simple columnar, non-ciliated	-
endocrine	thyroid follicles	Simple cuboidal	-
nervous	ependyma	Simple cuboidal	-
lymphatic	lymph vessel	Simple squamous	endothelium
integumentary	skin - dead superficial layer	Stratified squamous, keratinized	-
integumentary	sweat gland ducts	Stratified cuboidal	-
integumentary	mesothelium of body cavities	Simple squamous	mesothelium
reproductive - female	ovaries	Simple cuboidal	germinal epithelium (female)
reproductive - female	Fallopian tubes	Simple columnar, ciliated	-
reproductive - female	endometrium (uterus)	Simple columnar, ciliated	-
reproductive - female	cervix (endocervix)	Simple columnar	-
reproductive - female	cervix (ectocervix)	Stratified squamous, non-keratinized	-
reproductive - female	vagina	Stratified squamous, non-keratinized	-
reproductive - female	labia majora	Stratified squamous, keratinized	-
reproductive - male	tubuli recti	Simple cuboidal	germinal epithelium (male)
reproductive - male	rete testis	Simple cuboidal	-
reproductive - male	ductuli efferentes	Pseudostratified columnar	-
reproductive - male	epididymis	Pseudostratified columnar, with stereocilia	-
reproductive - male	vas deferens	Pseudostratified columnar	-
reproductive - male	ejaculatory duct	Simple columnar	-
reproductive - male (gland)	bulbourethral glands	Simple columnar	-
reproductive - male (gland)	seminal vesicle	Pseudostratified columnar	-
respiratory	oropharynx	Stratified squamous, non-keratinized	-
respiratory	larynx	Pseudostratified columnar, ciliated	respiratory epithelium
respiratory	larynx - True vocal cords	Stratified squamous, non-keratinized	-
respiratory	trachea	Pseudostratified columnar, ciliated	respiratory epithelium
respiratory	respiratory bronchioles	Simple cuboidal	-
sensory	cornea	Stratified squamous, non-keratinized	corneal epithelium
sensory	nose	Pseudostratified columnar	olfactory epithelium
urinary	kidney - proximal convoluted tubule	Simple cuboidal, with microvilli	-
urinary	kidney - ascending thin limb	Simple squamous	-
urinary	kidney - distal convoluted tubule	Simple cuboidal, without microvilli	-
urinary	kidney - collecting duct	Simple cuboidal	-
urinary	renal pelvis	Transitional	urothelium
urinary	ureter	Transitional	urothelium
urinary	urinary bladder	Transitional	urothelium
urinary	prostatic urethra	Transitional	urothelium
urinary	membranous urethra	Pseudostratified columnar, non-ciliated	-
urinary	penile urethra	Pseudostratified columnar, non-ciliated	-
urinary	external urethral orifice	Stratified squamous	-