Cell Bio Chapter 20
Compare, in terms of structure and composition, the cell walls found in wood and those found in a leaf.
plant cells themselves synthesize secrete and control the composition of this extracellular matrix: a cell wall can be thick and hard as in wood, or thin and flexible as in a leaf.
Explain how collagen fibrils become oriented in developing tissues
when fibroblast are mixed with a meshwork of randomly oriented collagen fibrils that form a gel in a culture dish, the fibroblast tug on the meshwork, drawing in collagen from their surrounding and compacing it. if two small pieces of embryonic tissue containing fibroblast are placed far apart on a collagen gel, the intervening collagen becomes organized into dense band of aligned fibers that connect the two explants.
Provide examples of stem cells that give rise to several types of differentiated cells.
a single type of stem cell gives rise to several types of differentiated progeny; the stem cells of the the intestine for example produce absorptive cells, goblet cells and several other secretory cell types. the process of blood cell formation or hemopoiesis provides and extreme example of this phenomenon
Explain how a lack of normal cell-cycle control mechanisms or DNA damage response may help make cancer cells particularly vulnerable to therapeutic intervention
Most of our anti-tumor therapeutics (chemo, radiation) induce DNA damage in the tumor cells. We know these cells lack mechanisms that halt the cell cycle to allow for DNA damage repair--therefore, they would try to continue to survive in the event of radiation damaging their DNA. They wouldn't be successful though because they can't repair their DNA, making them more vulnerable to killing
Outline how cellulose is produced by plant cells and how the orientation of the microfibrils is regulated.
cellulose is produced in a radically different way from most other extracellular macromolecules. instead of being made inside the cell and then exported by exocytosis, it is synthesized on the outer surface of the cell by enzymes complexes embedded in the plasma membrane. these complexes transport glucose monomers from the cytosol across the plasma membrane and incorporate them into a set of growing cellulose chains at their points of membrane attachment.
Describe the structure of proteoglycans and outline their functions in the extracellular matrix
chains of GAGs are usually covalently linked to a core protein to form proteoglycans which are extremely diverse in size, shape, and chemistry. are a major component of the animal extracellular matrix, the "filler" substance existing between cells in an organism. Here they form large complexes, both to other proteoglycans, to hyaluronan, and to fibrous matrix proteins, such as collagen.
Outline how collagen molecules are synthesized, secreted, and assembled into fibrils
collagen fibrils must be correctly aligned, they are woven in a wickerwork pattern or in alternating layers with different orientations so as to resist tensile stress in multiple directions. the connective-tissue cells that produce collagen control this orientation first by depositing the collagen in an oriented fashion and then by rearranging it. during development of the tissue, fibroblast work on the collagen they have secreted, crawling over it and pulling on it--helping to compact it into sheets and draw it out into cables. this mechanical role of fibroblasts in shaping collagen matrices has been demonstrated dramatically in cell culture.
Compare the extracellular matrix of connective tissues such as bone or tendon to that of muscle or epidermis
connective tissue, extracellular matrix is abundant and carries the mechanical load. in other tissues such as epithelia, extracellular matrix is sparse and the cells are directly joined to one another and carry the mechanical load themselves. animal connective tissue are enormously varied, they can be tough and flexible like tendons or dermis of the skin.
Indicate the composition of the basal lamina
consists of a thin, tough sheet of extracellular matrix, composed of mainly of specialized type of collagen and protein called laminin. laminin provides adhesive sites for integrin molecules in the basal plasma membrane of epithelial cells and it thus serves a linking role like that of fibronectin in other connective tissue.
Summarize how stem cells divide to produce a continuous supply of terminally differentiated cells
differentiated cells: specialized cell that need continual replacement are themselves unable to divide. the cells that replace the terminally differentiated cells that are lost are generated from a stock of proliferating precursor cells, which themselves usually derive from a much smaller number of self-renewing stem cells.
Outline the main functions common to epithelia and list some functions of specialized epithelia
epithelia cover the external surface of the body and line all its internal cavities, and they must have been an early feature in the evolution of animals. cells join together into an epithelial sheet create a barrier which has the same significance for the multicellular organism that the plasma membrane has for a single cell. it keeps molecules in and others out; it takes up nutrients and exports wastes it contains receptors for environmental signals and it protects the interior of the organism from invading microorganisms.
Present potential obstacles to the use of human embryonic stem cells for the replacement of damaged tissues in disorders such as Parkinson's disease or diabetes.
experiments in mice suggest that it should be possible to use cells derived from human ES cells to replace the skeletal muscle fibers that degenerate in victims of muscular dystrophy, the nerve cells that die in patients with parkinson's disease. -one major problem concerns immune rejection: if the transplanted cells are genetically different from the cells of the patient into whom they are grafted they are likely to be rejected and destroyed by the immune system. beyond the practical scientific difficulties there have been ethical concerns about the use of human embryos to produce human ES cells.
Contrast the extracellular matrix and cell wall, and recall what roles these materials have in the supportive tissues of animals and plants
extracellular matrix: internal framework of cytoskeletal filaments. the material that cells secrete around themselves; it is this matrix that gives supportive tissues such as bone or wood their strength. cell wall: the supportive matrix is called cell wall like a boxlike structure that encloses, protects, immobilized and shapes each cell.
Distinguish the locations and functions of fibroblasts, osteoblasts, and osteoclasts.
fibroblast: connective-tissue cells that manufacture and inhabit the extracellular matrix go by various names according to their tissue type: in skin, tendon, and many other connective tissues, they are called fibroblasts. in bone, they are called osteoblasts. these cells make both collagen and the other macromolecules of the matrix. Osteoclasts are the cells that degrade bone to initiate normal bone remodeling and mediate bone loss in pathologic conditions by increasing their resorptive activity.
Explain how the orientation of cellulose microfibrils in a plant cell wall influences the shape of a plant tissue.
for a plant cell to grow or change its shape, the cell wall has to stretch or deform. because the cellulose microfibrils resist stretching, their orientation governs the direction in which the growing cell enlarges; if for example they are arranged circumferentially as a corset the cell will grow more readily in length than in girth. by controlling the way it lays down its cell wall.
Review how induced pluripotent stem cells are produced and compare their properties to those of embryonic stem cells
it is now possible to produce pluripotent stem cells without the use of embryos. differentiated cells can be taken from an adult mouse or human tissue grown in culture and reprogrammed into an ES-like state by artificially driving the expression of a set of three or four transcription regulators. this treatment is sufficient to permanently convert fibroblasts into cells with practically all the properties of ES cells, including the ability to proliferate indefinitely, differentiated in diverse ways and in the case of the mouse cells, contribute to the formation of any tissue. these ES-like cells are called induced pluripotent stems cells.
Define pluripotency and summarize how embryonic stem cells can be induced to differentiate into a variety of cell types.
it is possible through cell culture, to derive from early mouse embryos an extraordinary class of stem cells called embryonic stem cells or ES cells. Under appropriate conditions these cells can be kept proliferating indefinitely in culture and yet retain unrestricted developmental potential and are thus said to be pluripotent; if cells from the culture dish are put back into an early embryo, they can give rise to all the tissues and cell types in the body, including the reproductive germ line cells.
Review the composition of a plant cell wall and identify the role of cellulose microfibrils in this matrix.
like all extracellular matrices, plant cell walls derive their tensile strength from long fibers oriented along the lines of stress. in higher plants the long fibers are generally made from the polysaccharides cellulose the most abundant organic macromolecules. these cellulose microfibrils are interwoven with other polysaccharides and some structural proteins, all bonded together to form a complex structure that resists both compression and tension.
Review how genetic instability can be produced and its role in cancer progression
many human cancer cells are able to speed up this acquisition of mutations because they are also genetically unstable. this genetic instability results from mutations that interfere with the accurate replication and maintenance of the genome and thereby increase rate at which mutations accumulate.
Distinguish passenger mutations from cancer-critical or driver mutations.
most mutations do not contribute to cancer--even though they may happen to be found in a cancer cell; these mutations are called passenger mutations. at the same time for those mutations is generally not sufficient. precisely how many of these cancer critical or driver mutations are required is still a matter of debate, but for most full blown cancers it could be at least 10-- and as we will see, they have to affect the right type of gene. these mutations do not all occur at once but sequentially, usually over a period of many years.
Differentiate between the primary and secondary cell walls
most newly formed cells in a plant initially make relatively thin primary cell walls, which can slowly expand to accomodate cell growth. once cell growth stops and the wall no longer needs to expand a more rigid secondary cell wall is often produced either by thickening of the primary wall or by deposition of new layers with a different composition underneath the old ones.
Chapter 20: Review the relationship among cells, tissues, and organs.
most of the cells in multicellular organisms are organized into cooperative assemblies called tissues, such as the nervous, muscular, epithelial and connective tissues found in vertebrates, tissue in turn are organized into organs such a heart lung, brain and kidney.
Define an epithelium and describe the four main types of epithelia.
multicellular sheets in which adjacent cells are joined tightly together. in some cases the sheet is many cells thick or stratified as in the epidermis ( the outer layer of the skin) in other cases it is a simple epithelium only one cell thick (lining of the gut). -mainly as a protective barrier (biochemical functions) -some secrete specialized product such as hormones -absorb the nutrients -detect signals
Outline how tumors evolve through repeated rounds of mutation, proliferation, and natural selection
mutations that lead to cancer do not cripple the mutant cells. on the contrary, they give these cells a competitive advantage over their neighbors. it is this advantage enjoyed by the mutant cells that lead to disaster for the organism as a whole. as an initial population of mutant cells grows, it slowly evolves; new chance mutations occur, some of which are favored by natural selection because they enhance cell proliferation, cell survival, or both. this process of random mutation followed by selection culminated in the genesis of cancer cells that run riot within the population of cells that form the body upsetting its regular structure.
Contrast cell turnover rates in different tissues, such as skin, bone, blood, and intestinal epithelium
one extreme is the intestinal epithelium, in which cells are replaced every three to six days, at the other extreme is a nervous tissue in which most of the nerve cells last a lifetime without replacement. between these extremes there is a spectrum of different styles tissue renewal. bone has a turnover time of about ten years and it involves renewal of the matrix as well as of cells.
List the mixture of cell types that form tissues such as skin and describe the function of each cell type
skin can be viewed as a large organ composed of two main tissues; epithelial tissue (the epidermis) on the outside, and connective tissue on the inside. the outermost layer of the epidermis consists of flat dead cells, whose intracellular organelles have disappeared. the underlying connective tissue consists of the tough dermis (from which leather is made) and the deeper fatty hypodermis. the dermis and hypodermis are richly supplied with blood vessels and nerves; some of the nerves extend into the epidermis.
Outline how different signaling mechanisms contribute to maintaining the complex organization of a stem-cell system such as that of the intestine
stem cell systems requires control mechanisms to ensure that new differentiated cells are generated in the appropriate places and in the right numbers. the controls depend on extracellular signals exchanged between stem cells, their progeny and other cell types. -almost all these signaling mechanisms contribute to the task of maintaining the complex organization of a stem cell system such as that of the intestine. in this system a class of signals molecules known as the Wnt proteins serves to promote the proliferation of the stem cells and precursor cells at the base of each intestinal crypt.
Describe the general appearance, distribution, and abundance of stem cells in an adult mammal.
stem cells are not differentiated and can divide without limit (or at least for the lifetime of the animal). when stem cell divides though each daughter has a choice either it can remain a stem cell or it can embark on a course leading to terminal differentiation, usually via a series of precursor-cell divisions. job of the stem cells is not to carry out specialized function of differentiated cells but rather produce cells that will.
Identify the key regulatory pathways that are altered in almost all human cancers.
that many of the driver mutations in individual tumors affect genes that fall into small number of key regulator pathways; those that govern cell proliferation, cell growth, cell survival, and the cell's response to DNA damage and stress. only a single gene tends to be mutated in each pathway but not always the same gene; it is the under or overactivity of the pathway as whole that matters for cancer development, not the way which malfunction is achieved.
Review how epithelial sheets are polarized in structure and function
the apical and basal faces of an epithelium are different each contains a different set of molecules that reflect the polarized organization of the individual epithelial cells; each has a top and a bottom, with different properties and functions, this polarity is crucial for epithelial that lines the small intestine of a mammal.
Summarize the structure and function of tight junctions
the barrier function of epithelial sheets is made possible by tight junctions. these junctions seal neighboring cells together so that water-soluble molecules cannot easily leak between them. if a small tracer molecule is added to one side of an epithelial cell sheet, it will usually not pass beyond the tight junction. is formed from proteins called claudins and occludins, which are arranged in strands along the lines of the junction to create the seal
Compare the structures of collagen molecules, collagen fibrils, and collagen fibers
the collagens are a family of proteins that come in many varieties, are the chief proteins in bone, tendon, and skin. the characteristic feature of a typical collagen molecule is it's a long stiff, triple-stranded helical structure in which three collagen polypeptide chains are wound around one another in a ropelike superhelix. some types of collagen molecules, in turn, assemble into ordered polymers called collagen fibrils which are thin cables these can pack together into still thicker collagen fibers.
Describe the driving force for plant cell growth
the driving force for cell growth is the same as that keeping the lettuce crisp, a swelling pressure called turgor pressure that develops as the result of an osmotic imbalance between the interior of the plant cell and its surroundings
Contrast how cell replacement occurs in the skin epidermis with replacement in the intestinal epithelium
the pattern of cell replacement varies from one stem cell based tissue to another. in the lining of the small intestine for example the absorptive and secretory cells are arranged as single-layered, simple epithelium covering the surfaces of the finger like villi that project into the gut lumen. this epithelium is continuous with the epithelium lining the crypts which descend into the underlying connective tissue. in the epidermis, proliferating stem cells and precursors cells are confined to the basal layer, adhering to the basal lamina.
Identify the component that provides tensile strength in the connective tissues of animals, and outline what gives different connective tissues their distinctive characteristics
the tensile strength--whether great or small--is chiefly provided not by a polysaccharide as it is in the cell wall of plants but by fibrous proteins principally collagens. the various types of collagen that they contain to its quantity and most importantly to the other molecules that are interwoven with its varying proportions. these other molecules include the rubbery protein elastin which gives the walls of arteries their resilience as blood pulses through them as well as a host of specialized polysaccharide molecules.
Explain how loss of the tumor suppressor gene APC can give rise to colorectal cancer, both in families predisposed to the condition and in patients with no family history of the disease
adenomatous polyposis coli: affected individuals inherit one normal gene copy and one mutant copy. although one normal gene copy is enough for normal cell behavior, all the cells of these individuals are only one mutantational step away from total loss the gene's function as compared to two steps away from person who inherits two normal copies. the individual tumors arise from cells that have undergone a somatic mutation that inactivates the remaining good copy of APC.
Explain how induced pluripotent stem cells are currently being used to study human disease, cell fate, and the development of organs
can be used to generate large, homogeneous populations of differentiated human cells of specific types in culture; these can be used to test for potential toxic or beneficial effects of candidate drugs. it is possible to generate IPs cells from patients who suffer from a genetic disease and to use these IPS cells to produce affected, differentiated cell types, which can then be studied to learn more about the disease mechanism and to search for potential treatments. -IPS cells produced from such individuals have been coaxed to differentiate in culture into neurons and heart muscle cells which are now being used to study the physiological consequences of the Ca 2+ channel abnormality and to hunt drugs that can correct the defects.
Recall the two heritable properties that define cancer cells
cancer cells are defined by two heritable properties: they and their progeny (1) proliferate in defiance of the normal constraints and (2) invade and colonize territories normally reserved for other cells.
Explain why cancer is most often a disease of old age.
cancer is therefore, is most often a disease of old age, because it takes a long time for an individual clone of cells--those derived from a common founder-- to accumulate a sufficient number critical cancer mutations.
Review how integrins allow cells to attach to collagen in the extracellular matrix, and explain how these attachments are dynamically formed and broken
cells are able to interact with the collagen in the extracellular matrix thanks to a family of transmembrane receptor proteins called integrins. the extracellular domain of an integrin bind to components of the matrix, while it is intracellular domain interacts with the cell cytoskeleton. this mooring provides a strong and stable point of attachment; without it, integrins would be easily torn from the flimsy lipid bilayer and cells would be unable to anchor themselves to the matrix.
Summarize the characteristics that distinguish cancer cells from normal cells and how they provide cancer cells with a competitive advantage
1. cancer cells have a reduced dependence on signals from other cells for their survival, growth and division. often this is because contain mutations in components of the cell signaling pathway that normally respond to such a stimuli 2. cancer cells can survive levels of stress and internal derangement that would cause normal cells to kill themselves by apoptosis. this avoidance of cell suicide if often the result of mutations in genes that regulate the intracellular death program responsible for apoptosis 3. can often proliferate indefinitely, (cell senescence: occurs after which they permanently stop this cessation of proliferation) cancer cells typically break through this proliferation barrier by reactivating production of telomerase enabling them to maintain telomere length indefinitely 4. are genetically unstable, with greatly increased mutation rate and an abnormal number of chromosomes 5. are abnormally invasive at least partly because they often lack certain cell adhesion molecules such as cadherins that help hold normal cells in their proper place 6. abnormally avid for nutrients, which they use to generate much of their ATP by glycolysis in the cell cytosol 7. can survive and proliferate in abnormal locations whereas most normal cells die when misplaced. this colonization of unfamiliar territory may result from the ability of cancer cells to produce their own extracellular survival signals and to suppress their apoptosis program 8. they secrete signals that influence the behavior of cells in the surrounding connective tissue, thereby modifying the tumor's microenvironment. cells in the remodel microenvironment in return produce signals that support the survival and proliferation of cancer cells, which renders the microenvironment more hospitable for tumor growth.
Review the three main factors that contribute to tissue stability
1. cell communication: each type of specialized cell continually monitors its environment for signals from other cells and adjusts its behavior accordingly; the proliferation and even the survival of most vertebrate cells depend on social signalling. ensure that new cells are produced and survive only when and where they are required 2. selective cell adhesion: because different cell types have different cadherins and other cell adhesion molecules in their 3. cell memory: specialized patterns gene expression evoked by signals that acted during embryonic development are afterward stably maintained so that cells autonomously preserve their distinctive character and pass it on to their progeny. a fibroblast divides to produce more fibroblast an endothelial cell divides to produce more endothelial cells and so on.
Contrast benign tumors, malignant tumors, and metastases
Benign Tumor: cells that have the first property but not the second proliferate excessively but remain clustered together in a single mass forming a tumor. it can usually be easily removed cleanly and completely by surgery Malignant Tumor: a tumor is cancerous only if its cell has the ability to invade surrounding tissue, in which case the tumor is said to be. these cells with this invasive property often break loose from the primary tumor and enter the bloodstream or lymphatic vessels. Metastases: from where they form secondary tumors or metastases at other sites in the body
Distinguish adherens junctions, desmosomes, and hemidesmosomes in terms of structure, location, and function
adherens junctions: each cadherin molecule is tethered inside a cell, via several linker proteins to actin filaments. often the adherens junctions to form a continuous adhesion belt around each of the interacting epithelial cells located near the apical end of the cell. desmosome: a different set of cadherin molecules connects to keratin filaments-- the intermediate filaments found specifically in epithelial cells. bundles of ropelike keratin filaments criss-cross the cytoplasm and are "spot-welded" via desmosome junctions to the bundles of keratin filaments in adjacent cells. hemidesmosomes: these attachments of epithelial cells to the basal lamina beneath them are therefore. the extracellular domains of these integrins bind to laminin in the basal lamina; inside the cell the integrin.
Describe the distribution of cells in a connective tissue.
animal connective tissue are enormously varied, they can be tough and flexible like tendons or dermis of the skin; hard and dense like bone; resilient and shock absorbing like cartilage or soft and transparent like the jelly that fills the interior of the eye. in all these examples the bulk of the tissue is occupied by extracellular matrix and the cells that produce the matrix are scattered within it like raisins in a pudding; the tensile strength--whether great or small--is chiefly provided not by a polysaccharide as it is in the cell wall of plants but by fibrous proteins principally collagens.
Differentiate between oncogenes and tumor suppressor genes, and outline the types of genetic events that alter the activity of each
for many critical cancer genes the most dangerous mutations are the ones that render the encoded protein hyperactive. these gain of function mutations have a dominant effect; only one gene copy needs to be mutated to promote and development of cancer. the resulting mutant gene is called an oncogene and the corresponding normal form of the gene is called proto-oncogene. the danger lies in mutation that destroy their activity, these loss of function mutations are generally recessive both copies of the gene must be lost or inactivated to contribute to cancer development; the normal gene is called a tumor suppressor gene. in additions to genetic alteration tumor suppressor genes can also be silenced by epigenetic changes, which alter gene expression without changing the genes nucleotide sequence.
Compare gap junctions and plasmodesmata in terms of structure, location, and function
gap junction: appear as regions where the plasma membrane of two cells lie close together and exactly parallel, with a very narrow gap between them. the gap, however, is not entirely empty; it is spanned by the protruding end of many identical, transmembrane protein complexes that reside in the plasma membrane of two apposed cells. plasmodesmata: the cytoplasm of adjacent plant cells are connected via minute communicating through channels called plasmodesmata which span the intervening cell walls. are cytoplasmic channels lined with the plasma membrane.
Describe the structure of glycosaminoglycans and summarize how they allow connective tissue to resist compression.
glycosaminoglycan: (GAGs) negatively charged polysaccharides chains made of repeating disaccharide units. while collagen provides tensile strength to resist stretching, a completely different group of macromolecules in the extracellular matrix of animal tissue provides the complementary function, resisting compression
Describe how an accumulation of mutations can turn a polyp in the lining of the colon into an invasive or metastatic cancer.
identify APC as a tumor suppressor gene and knowing its sequence and mutant phenotype, one can begin to decipher how its loss helps to initiate the development of cancer. as explained the APC gene was found to encode 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. when APC function is lost the pathway is hyperactive and epithelial cells proliferate to excess, generating a polyp. within this growing mass of tissue, further driver mutations occur, sometimes resulting in invasive cancer. the effect of mutation in a variety of tumor suppressor genes and proto oncogene including those involved in colorectal cancer.
