Epithelial Transport Flashcards
What drives the process of epithelial renewal?
epithelial stem cells
How do epithelial tissue get metabolite, gases, and secreted ligands?
- avascular
- they must diffuse through connective tissue and basal laminae to reach epithelia or blood
What are some of the functions served by epithelial?
- barrier
- selective absorption and transport of various molecules from the environment
- selective secretion
- movement of particles, solutions, and cell elements through passageways
- large scale biochemical modification/conversion/metabolism
- communication to and from other tissues and organs
- reception of sensory stimuli
What are endothelia?
line blood and lymphatic vessels (made of endothelial cells)
What is mesothelium?
sheets of cells that line enclosed internal spaces of body cavities
Embryonic epithelia often disassemble and move into the _______________, where they may migrate to other locations to form new __________ or may transform into distinct _____________ cell lineages that give rise to other tissues.
Embryonic epithelia often disassemble and move into the mesenchymal (connective) tissues, where they may migrate to other locations to form new epithelia or may transform into distinct non-epithelial cell lineages that give rise to other tissues.
What are the two layers of mucosae?
- outer epithelium
- the connective tissue directly underneath called the lamina propria
What does the lamina propria contain?
(connective tissue directly underneath the epithelial layer of mucosae)
- immune cells, small blood vessels
What is submucosa?
- deeper connective tissue layer under the lamina propria
- contains bigger vessels and muscles, nerve axon bundles, etc.
What are the general layers of epithelial sheets?
Space (lumen) - Epithelia - epithelial basal lamina - connective tissue - other connective embedded tissues (blood vessels, muscles, nerves - with their own basal laminae that connect them to the CT)
What is simple epithelia?
all cells arranged in a single layer or sheet
What is stratified epithelia?
more than one layer of cells in which the cells of the outer layers do not directly contact the basal lamina
What is pseudostratified epithelia?
some cells do not reach the free surface but all directly rest on the basal lamina
What is transitional epithelia?
(found in the bladder)
- epithelia are stratified but when stretched change shape from cuboidal to squamous
What are tight junctions?
- highly selective barrier that limits or prevents diffusion of substances between epithelial cells
What are the key core proteins of tight junctions?
occluding and claudins
What are adherence junctions?
promote attachment, morphological organization, and stem cell behavior within the epithelial sheet
What proteins do adherence junctions contain?
specific cadherins that link to actin filaments and other adapter/signaling proteins in the cytoplasm
What are desmosomes?
promote mechanical strength and resist shearing forces and promote structural organization of the epithelial sheet
What proteins do desomosomes contain?
different class of cadherins that link to intermediate filaments and other adapter proteins
What are gap junctions?
promote rapid communication between epithelial cells through diffusion of ions and small molecules
Why is the membrane surrounding the lateral and basal side of an epithelial cell called the basolateral membrane?
because the protein and lipid content is similar on both the lateral and basal sides
What are the two key aspects of epithelial cell polarity?
- the plasma membrane composition is locally segregated into domains (apical and basal)
- the organelles and cytoplasm is polarized ( cytoskeleton, especially microtubules, is asymmetric or polar in orientation)
What is transcytosis?
the endocytosis of substances from one membrane region, followed by the trans-cellular transport of the vesicles and their exocytosis from another membrane region
Epithelial cell polarity is crucial to allow _____________ to or from one side of the epithelium.
unidirectional secretion and/or absorption of molecules
What are two apical surface modifications?
microvilli and cilia
What are microvilli?
cell surface extensions that contain actin bundles connected to cytoskeletal elements in the cell interior
What is the primary function of microvilli?
to increase surface area, which greatly increases the rate/efficiency of membrane transport and secretion
What is a basolateral surface modification?
infolds and outfolds of the basolateral membrane (lack structural organization of the apical microvilli and cilia)
What is the basal lamina?
- thin sheet of extracellular material that underlies the basal surface of each epithelial tissue, blood vessels, muscle, and nerve tissue
- made of collagen (esp. collagen IV) and extracellular glycoproteins (lamins and entactin)
What are the functions of basal laminae?
- mediate attachment of epithelia to underlying connective tissue
- contribute to selective filtration of substances diffusing to or from the epithelia
- necessary for establishment and maintenance of epithelial polarity
- serve as specific highways for migration of cells through connective tissue
- barrier to movement of invading microbes or cancerous cell into other tissues
- control the gene expression of cells to affect their proliferation or development
- control development, organization, repair from injury or disease (tissue scaffolding)
How do epithelial cells directly connect to the basal laminae?
by attachment of integrins (hemidesmosomes and focal adhesions) on the basal surface of the epithelial cell to components of the basal lamina
What do focal adhesions connect to inside the epithelial cell?
actin filaments
What are the three key properties of stem cells?
- they are competent for cell division
- stem cells must self renew (at least one mother stem cell is regenerated with each division)
- stem cells produce anywhere from one to many distinct, differentiated cell types specific to each tissue
What are transit amplifying cells?
- daughters of stem cells that proliferate at faster rates
- middle step - transitional intermediate
- transit amplifying cells then produce differentiated cells
What regulates cell division and differentiation in stem cells?
- in the stem cell niche, neighboring cells secrete regulatory factors that tightly control stem cell division and differentiation programs
- signals from distant sources (hormones and endocrine glands)
What is the typical cell signaling pathway for stem cells?
- extracellular ligand secreted by signaling cells
- receptor in receiving cells binds and is activated/inactivated by the ligand
- downstream effector proteins in the cytoplasm and nucleus
- numerous modulator proteins that promote or suppress pathway components
What are two key principles of stem cell signaling pathways?
- each pathway controls multiple, different stem cell systems (some are unique, but many are shared)
- a single signaling pathway will have different development outcomes in different stem cell lineages
What are exocrine glands?
secrete material onto epithelia lined surfaces or the outside world
How do glands secrete their substances?
- exocytosis
- total cell disintegration (e.g. oily sebum onto hair and skin)
What are the main components of multicellular exocrine glands?
- secretory units (clumps of secretory epithelial cells)
- ducts (tubular structures that emanate from the secretory units)
What are the three general types of exocrine glands products?
- mucous (viscous glycoprotein-rich fluids)
- serous (low viscosity watery fluids)
- mised
Do endocrine glands have ducts?
no
How do endocrine glands secrete hormones when they’re surrounded by basal lamina?
hormone molecules must cross the basal surface and basal lamina of the epithelium, so most endocrine cells secrete these molecules from the basolateral membrane
What regulates secretions from the exocrine and endocrine glands?
autonomic nervous system and/or hormones from the blood
What are adenocarcinomas derived from?
glandular epithelium
