Week 2- Epithelial Tissue Flashcards
Epithelial Tissue aka Epithelium
-Composed of one or more layers of closely packed cells
-Contains little to no extracellular matrix between these cells
-covers body surfaces, lines the body and organ cavities, and forms glands
Common Characteristics
1) Cellularity
2) Polarity
3) Attachment to basement membrane
4) Avascularity
5) Extensive innervation
6) High regeneration capacity
Cellularity
-Epithelial tissue is composed almost entirely of tightly packed cells
-There is a minimal amount of extracellular matrix between cells
Polarity
Epithelium has an apical and basal surface
Apical surface
-Exposed either to the external environment or to some internal body space
-Can have either microvilli or cilia
-Lateral surfaces may contain membrane (intercellular) junctions
Microvilli
-Small membranous projections on apical surface of the cell that increase its surface area for secretion and absorption
-Individual microvilli can’t be distinguished with light microscopy, rather they appear collectively as a brush border
Cilia
Numerous, slightly longer, membranous projections that move fluid, mucus, and materials past the cell surface
Basal surface
-a fixed or deep surface
-Epithelium is attached to a basement membrane with underlying connective tissue
Basement membrane
-Binds basal surface of epithelial layer
-Consists of 3 molecular levels that can be viewed under electron microscope (using a light microscope, it appears as a single noncellular/molecular layer)
-These molecular layers are formed by secretions of both the epithelium and the underlying connective tissue
-Composed of collagen, glycoproteins, and proteoglycans
-Molecular structures of membrane act like glue to strengthen attachment and form a selective molecular barrier between the epithelium and underlying connective tissue
Avascularity
-All epithelial tissues lack blood vessels
-Get nutrients either directly across apical surface or by diffusion across the basal surface from blood vessels within underlying connective tissue
Extensive innervation
Richly innervated (supplied with nerves) to detect changes in the environment at that body or organ region
High regeneration capacity
-Undergo cell division frequently
-Allows tissue to regenerate at high rate, a necessary condition for a tissue that is often exposed to the environment and lost by abrasion and damage
-Continual replacement occurs through cell division of the deepest epithelial cells (stem cells), which are adjacent to basement membrane
Functions of Epithelial Tissue
1) Physical protection
2) Selective permeability
3) Secretions
4) Sensations
Physical protection
Protects both external and internal surfaces from dehydration, abrasion, and destruction by physical, chemical, or biological agents
Selective permeability
-Exhibits range of permeabilty
-May be relatively non-permeable to some substances, while promoting/assisting passage of other ions/molecules
-All substances entering and leaving the body must pass through epithelium
-Acts as body’s gatekeepers
Secretions
-Some are specialized to produce and release secretion
-These cells form glands
-Glands may be individual cells scattered among other cell types in an epithelium (ex: goblet cells) or arranged in small, organized clusters within a multicellular gland
Sensations
-Innervated by sensory nerve endings to detect or respond to a stimulus
-These nerve endings and those in underlying connective tissue continuously relay sensory input to CNS concerning touch, pressure, temp, and pain
What are the two things used to classify epithelial tissue types?
1) Number of cell layers
2) Shape of cells at apical surface
Simple epithelium
-One layer of cells
-All cells are in direct contact with basement membrane
-Found in areas where stress in minimal and secretion, absorption, or filtrations is primary function
Stratified epithelium
-Contains 2 or more layers of cells
-Only cells in basal (deepest) layer are in contact with basement membrane
-Provides either more structural support or better protection for underlying tissue
-Cells in basal layer are continuously regenerated as cells in the apical layer are lost due to abrasion or stress
Pseudostratifed epithelium
-Appears layered because cells’ nuclei are distributed at different levels between apical and basal surfaces
-All cells are attached to basement membrane, but some of them do not reach its apical surface
Squamous Cells
-Flat, wide, and somewhat irregular in shape
-Cells are arranged like floor tiles, and nucleus is somewhat flattened
Cuboidal Cells
-About as tall as they are wide
-Don’t resemble perfect cubes because edges are somewhat rounded
-Cells nucleus is spherical and located in center of cell
Columnar cells
-Slender and taller than they are wide
-Nucleus is oval and usually oriented lengthwise and in the basal region
Transitional cells
-Can readily change shape from polyhedral to more flattened, depending upon degree to which epithelium is stretched
-Shape change occurs when epithelium cycles between distended and relaxed states (such as lining of bladder, which fills with urine and is later emptied)
Simple squamous epithelium
-Consists of single layer of flattened cells
-Extremely delicate and represents thinnest possible barrier to allow rapid movement of molecules and ions across epithelium by membrane transport process
-Forms lining of air sacs (alveoli) of lung
-well suited for exchange of O and CO2 between blood and inhaled air
Endothelium
Name of the simple squamous epithelium that lines both blood vessels and lymph vessels
Mesothelium
-Name given to simple squamous epithelium that forms serous membrane of body cavities
-Gets its name from the embryonic primary germ layer called mesoderm, from which it’s derived
Simple cuboidal epithelium
-Contains one layer of uniformly shaped cells that are about as tall as they are wide with a centrally located, spherical nucleus
-Allows for both absorption and secretion
-Uniform shape makes them ideal to form structural components of glands
-Location: lining of kidney tubules, thyroid gland follicles, surface of ovary, and secretory regions and ducts of most exocrine glands
Simple columnar epithelium
-Single layer of cells that are taller than they are wide
-Nucleus is oval, oriented lengthwise, and located in basal region
-Ideal for both absorptive and secretory functions
-2 types: ciliated and nonciliated
Nonciliated simple columnar epithelium
-Often contains microvilli and a scattering of unicellular glands called goblet cells
-Goblet cells secrete mucin (a glycoprotein that when hydrated forms mucus)
-Lines most of GI tract from stomach to upper portion of anal canal
Ciliated simple columnar epithelium
-Has cilia that project from apical surfaces of cells
-Mucus covers apical surface and moves by the beating of the cilia
-Goblet cells are interspersed throughout this epithelium
-Lines larger bronchioles in lung and uterine tubes (helps move oocyte from ovary to uterus)
Pseudostratified columnar epithelium
-Not really stratified because all its cells are in direct contact with basement membrane, but nuclei are scattered at different distances from basal surface
-Not all cells reach apical surface
-Columnar cells always reach apical surface and shorted basal cells are stem cells that give rise to the columnar cells
-2 types: ciliated and nonciliated
Pseudostratified ciliated columnar epithelium
-Contains cilia on apical surface
-Houses goblet cells that secrete mucin (hydrates to mucus which traps foreign particles and is moved by beating cilia)
-Lines larger air passageways of respiratory system (nasal cavity, part of pharynx, part of larynx, trachea, and bronchi)
Pseudostratified nonciliated columnar epithelium
-Lacks goblet cells and cilia
-Rare and found primarily in membranous part of male urethra and epididymis
Stratifed squamous epithelium
-Has multiple cell layers and only deepest layer is in direct contact with basement membrane
-Cells in basal layers have cuboidal/polyhedral shape vs apical cells have flattened, squamous shape
-Protects underlying tissues from damage caused by abrasion and friction
-Stem cells in basal layer continuously divide to produce a new stem cell and a committed cell that is gradually displaced toward surface to replace those cells that have been lost
-Exists in 2 form: keratinized and nonkeratinized
Keratinized Stratified Squamous Epithelium
-Superficial layers are composed of cells that are dead
-Cells lack nuclei and all organelles
-Filled with protein keratin (tough, protective protein that strengthens tissue)
-Ex: Epidermis (outer layer) of skin
Nonkeratinized Stratified Squamous Epithelium
-Lack keratin and remain alive
-Since cells are alive, nuclei is visible throughout tissue
-Kept moist with secretions like saliva or mucus
-Form surface of mucous membranes that line oral cavity, part of pharynx, part of larynx, esophagus, vagina, and anus
Stratified Cuboidal Epithelium
-Contains 2 or more layers of cells and superficial cells tend to be cuboidal in shape
-Forms tubes and ducts
-Protection and secretion
-Forms walls of ducts of most exocrine glands (such as ducts of sweat glands in skin and periphery of ovarian follicles
Stratified columnar epithelium
-Relatively rare in body
-2 or more layers of cells, but only cells at apical surface are columnar
-Protects and secretes
-Found in large salivary glands, conjunctiva covering the eye, and segment of male urethra
Transitional epithelium
-Limited to urinary tract
-Varies in appearance depending upon whether it is in a relaxed state or distended state
-In relaxed state basal cells appear cuboidal or polyhedral and apical cells are large and rounded
-When it stretches, it thins and apical cells flatten and become almost squamous
-Some binucleated cells are present
-By being able to stretch, ensures urine does not seep into underlying tissues of these organs
Glands
-Individual cells or multicellular organs composed mostly of epithelial tissue
-Secrete substances either for use elsewhere in body or for elimination from body
-Glandular secretions include mucin, ions, hormones, enzymes, urea, ect
Endocrine glands
-Lack ducts and secrete their products (hormones) into the blood to be transported throughout body
-Ex: Thyroid and adrenal glands
Hormones
Act as chemical messengers to influence cell communication
Exocrine glands
-Originate from an invagination of epithelium that burrows into underlying connective tissue
-Maintain connection with epithelial surface through a duct
-Ex: sweat glands, mammary glands, and salivary glands
Duct
Epithelium lined tube through which gland secretions are discharged onto epithelial surface
Unicellular exocrine glands
-Do not contain duct
-Located close to surface of epithelium in which they reside
-Most common type is goblet cell (found in simple columnar and pseudostratified ciliated columnar)
Multicellular exocrine glands
-Contain numerous cells that work together to produce a secretion
-Consists of acini and one or more smaller ducts (merge to form a larger duct that transports secretion to epithelial surface)
-Typically surrounded by fibrous capsule and extensions of capsule called septa partition gland into lobes
Acini
Clusters of cells that produce a secretion
Septa
Extensions of fibrous capsule that partition gland into lobes
How are exocrine glands classified?
Anatomic form or method of secretion
Simple glands
Have single unbranched duct
Compound glands
Have branched ducts
Tubular
Secretory portion and duct have same diameter
Acinar
Secretory portion forms an expanded sac
Tubuloacinar
Has both tubules and acini
Different Classifications of Exocrine Glands
1) Simple tubular
2) Simple branched tubular
3) Simple coiled tubular
4) Simple acinar
5) Simple branched acinar
6) Compound tubular
7) Compound acinar
8) Compound tubuloacinar
Merocrine glands
-Package secretions into secretory vesicles and release secretions by exocytosis
-Glandular cells remain intact and are not damaged by producing secretion
-Ex: lacrimal (tear) glands, salivary glands, sweat glands, exocrine glands of pancreas, and gastric glands of stomach
Apocrine glands
-Produce secretory material where cell’s apical portion pinches off, releasing cytoplasmic content
-Afterwards, cell repairs itself in order to repeat secretory activity
-Ex: mammary glands and ceruminous glands of ear
Holocrine glands
-Formed from cells that accumulate a product; entire cell then disintegrates
-Holocrine secretion: viscous mixture of cell fragments and product of cell produced prior to its disintegration
-Ruptured dead cells are continuously replaced by other epithelial cells undergoing cellular division
-Ex: Oil-producing (sebaceous) glands in skin
