Rests on some other tissue, usually connective, to which it is attached
Supported by a thin, tough sheet of ECMatrix, called the basal lamina, composed of a specialized type of collagen (Type IV) and other macromolecules, one includes laminin, which provides adhesive sites for integrin molecules in the PM of epithelial cells, and thus serves a linking role like that of fibronectin in connective tissues.
Absorptive cells, which take up nutrients, and goblet cells, which secrete mucus, in the intestines
Tight Junctions Make an Epithelium Leak-Proof and Separate Its Apical and Basal Surfaces
Cell junctions – Specialized region of connection between two cells or between a cell and the ECM
Each cadherin molecule is tethered inside its cell, via several linker proteins, to actin filaments.
Form a continuous adhesion belt, located near the apical end of the cell just below the TJs, around each of the interacting epithelial cells. Actin is therefore connected form cell to cell across the epithelium.
By shrinking the apical surface along one axis, the sheet can roll itself up into a tube.
Or it can make a cup-shaped concavity and eventually create a vesicle that may pinch off from the rest of the epithelium
Gap junction – communicating cell-cell junction that allows ions and small molecules to pass from the cytoplasm of one cell to the cytoplasm of the next.
Connexons form the channels across two PMs and allow inorganic ions and small water-soluble molecules to move directly from cytosol to cytosol.
Creates an electrical and metabolic coupling between the cells.
GJs in heart muscle cells proved electrical coupling that allows electrical waves of excitation to spread through the tissue.
These waves trigger the coordinated contraction of the cells, producing a regular heart beat.
Can be opened or closed as needed in response of extracellular signals.
Dopamine, a neurotransmitter, reduces GJ communication within a class of neurons in the retina in response to an increase in light intensity.
This reduction in GJ permeability changes the pattern of electrical signaling and help the retina switch from using rod photoreceptors, which are good detectors of low light, to cone photoreceptors, which detect color and fine detail in bright light.
Plasmodesmata in plant cells are the functional couterpart of GJs
Cytoplasmic channels lined with PM, and the cytoplasm becomes continuous form one cell to the next.
Divides to another stem cell and then to a TD cell
RBCs derive from a hemopoietic stem cell found in the bone marrow
Specific Signals Maintain the Stem-Cell Populations
Signals are from the stem cells themselves, progeny, and surrounding tissues
Wnt proteins, a class of signaling molecules, serve to keep the stem cells and precursor cells at the base of each intestinal crypt in a proliferative state: the cells in these regions both secrete Wnt proteins and express the receptors for these proteins; and through positive feedback, stimulate themselves to continue dividing.
Stem Cells Can Be Used to Repair Damaged Tissues
Mutagens, agents that cause changes in the nucleotide sequence of DNA
Most human cancer cells not only contain many mutations but also are genetically unstable
This genetic instability results from mutations that interfere with the accurate replication and maintenance of the genome and thereby increase the mutation rate itself.
Cancer Cells Evolve Properties that Give Them a Competitive Advantage
Natural selection favors cells carrying mutations that enhance cell proliferation and cell survival regardless of the effects on neighbors
A general list of key behaviors of cancer cells distinguish them from normal cells:
Reduced dependence on signals from other cells for growth, survival, and division
Mutations in the Ras gene can cause intracellular signals for proliferation to be produced even in the absence of extracellular signals
Cancer cells are less prone than normal cells to kill themselves by apoptosis
About 50% of all human cancers have lost or suffered a mutation in the p53
Cancer cells can often proliferate indefinitely
Telomerase is not produced so telomeres become too short.
Most cancer cells are genetically unstable, with an increased mutation rate
Cancer cells are abnormally invasive, mostly due to the lack of cell-adhesion molecules, such as cadherins that hold normal cells in place
Cancer cells can often survive and proliferate in foreign tissues to form secondary tumors (metastases), whereas most normal cells die when misplaced.
Many Diverse Types of Genes Are Critical for Cancer
Oncogene – any abnormally activated gene that can make a cell cancerous. Typically a mutant for of a normal gene (proto-oncogene) involved in the control of cell growth or division
Proto-oncogene – the corresponding normal form of the gene
For cells, the danger lies in mutations that destroy gene function
Tumor suppressor gene – a gene that in a normal tissue cell inhibits progress through the cell cycle. Loss or inactivation of both copies of such a gene from a diploid cell can cause it to divide as a cancer cell.
Some of these genes code for growth factors, for receptors, like Ras
Others code for DNA repair proteins or mediators like p53
Colorectal Cancer Illustrates How Loss of a Gene Can Lead to Growth of a Tumor
The abnormlity can be traced to deletion or inactivation of a gene called the Adenomatous Polyposos Coli (APC) gene.
Affected individuals inherit one mutant copy of the gene and one normal copy.
People with a functioning APC gene have been found to have developed two independent somatic mutations which causes them to develop colon cancer.
APC encodes an inhibitory protein that normally restricts the activation of the Wnt signaling pathway, which is involved in stimulating cell proliferation in the crypts of the gut lining as described earlier.
An Understanding of Cancer Cell Biology Opens the Way to New Treatments
Blocking blood vessels that normally invade a growing tumor.