unit 7 - cellular communities Flashcards
types of tissues
- epithelial tissue
- connective tissue
- nervous tissue
- muscle tissue
connective tissue general characteristics
most abundant tissue in the body, supports and binds other tissues. consists of cells (fibroblasts) scattered throughout an extracellular matrix
extracellular matrix in connective tissue
protein (usually collagen) fibers embedded in ground substance (carbohydrates).
collagen structure
- triple helices, 3 polypeptide chains. 1.5 nm diameter
*unique amino acid (hydroxyproline) - repeats of Gly-X-Y, X is typically proline and Y is typically hydroxyproline
- form collagen fibrils that are 10-300 nm in diameter
why does lack of vitamin C cause scurvy?
enzyme prolyl hydroxylase that converts proline to hydroxyproline requires vitamin C. without hydroxyproline, the collagen structure is disrupted. scurvy develops as a result of weakened connective tissue. results in skin lesions and blood vessel hemorrhages
synthesis of collagen and mechanisms of production
endogenous collagen: synthesized by the body
exogenous collagen: comes from outside sources
- pro-collagen fibers are produced with protective caps on each end to prevent them from assembling into fibrils. cell would become choked with its own products if they assembled inside
- pro-collagen is secreted
- extracellular procollagen proteinase enzymes cut off terminal domains and allow assembly
- mature collagen forms into fibrils outside the cell
- collegen is found in specific orientations - fibroblasts deposit them in a certain orientation and help reposition them by crawling over them and pulling on them, compacting them into sheets or drawing them out into cables
bone marrow transplant for repair
bone marrow transplant to transfuse hematopoietic stem cells, which can proliferate and differentiate into other blood cells to replace blood. used to treat leukemia
1. body is dosed with radiation to kill diseased/cancerous bone marrow stem cells.
2. replaced with donor stem cells
3. donor stem cells repopulate the bone marrow to make new, cancer free cells
umbilical cord cells
- umbilical cord blood contains hematopoietic and mesenchymal stem cells
- if collected just after umbilical cord is cut, can be used to treat many different cancers, immune deficiencies, and genetic disorders
- downside: most diseases treatable with these cells are hereditary, so the genetic defect would also be in the stem cells and not help
embryonic stem cells for repair
- obtained from the inner cell mass of the blastocyst, which forms 3-7 days after an egg cell is fertilized by sperm
- if grown in vitro, they can retain the pluripotent properties of stem cells to differentiate into any cell in the body
- could be an inexhaustible supply of cells to study, test treatments, or replace and repair damaged mature tissues
- could be used to replace skeletal muscle fibers that degenerate with muscular dystrophy, insulin-secreting cells lost in type 1 diabeties, nerve cells that die in those with Parkinson’s, cardiac muscle cells that die during a heart attack
- usually derived from blastocysts created by IVF that are no longer needed
- downsides: ethical concerns over using human embryos, immune rejection
reproductive vs. theraputic cloning
reproductive cloning: uses nuclear transplantation to clone the entire multicellular animal
1. nucleus of an unfertilized egg is removed and replaced with the nucleus from an adult somatic cell (diploid, full genome to be cloned) - zygote is created
2. grown in culture for a few days - blastocyst develops
3. blastocyst is implanted in the uterus of a surrogate where it develops normally and is born
theraputic cloning: creates personalized embryonic stem cells. similar procedure without transferring to a surrogate
1. nucleus of an unfertilized egg is removed and replaced with the nucleus from an adult somatic cell (diploid, full genome to be cloned) - zygote is formed
2. grown in culture for a few days to reach blastocyst stage
3. embryonic stem cells can be harvested from the inner cell mass to treat someone without fear of immune rejection
induced pluripotent stem cells
- alternative to embryonic stem cells, with less ethical concerns and bans because they originate from adult cells
1. specific genes are introduced to adult differentiated cells (Oct3/4, Sox2, and Klf4)
2. genes reprogram the cells to act as pluripotent embryonic stem cells - expanded the field of regenerative medicine, can be used to make a bunch of cells to test new drugs
examples of genetic defects in connective tissue
- ehlers-danlos syndrome: mutations to the procollagen proteinases causes hyper extensible skin with a lower tensile strength
fibronectin structure and function
- protein ‘bridges’ that extend out of cells to connect them to collagen fibers in the extracellular matrix
*dimer of polypeptide chains, joined by disulfide bonds near the c-terminus. sites to bind integrins, collagen, and proteoglycans
integrin structure and function
- transmembrane receptor proteins that attach to actin filaments in the cell cortex inside the cell and anchor fibronectin to the cell membrane outside the cell.
- since it connects collagen to the strong cytoskeleton rather than the lipid bilayer, it prevents cell tearing when there is tension between the cell and the extracellular matrix
- transmembrane heterodimers of an alpha and beta subunit
stem cell classifications and examples in tissues
- totipotent: embryonic stem cells, only occur for a limited time during the morula stage. can give rise to any cell type
-
pluripotent: embryonic stem cells, inner mass cells in the blastocyst. can give rise to many cell types in their germ layer
* ectoderm - exoskeleton
* mesenchymal stem cells - mesoderm-derived cells like fat, bone, cartilage
* endoderm - inner lining of organs -
multipotent: somatic adult stem cells, can give rise to different cells in a certain group
* stem cells in bone marrow (hematopoietic blood stem cells) can differentiate into different types of blood cells
* oval cells in the liver can differentiate into hepatocytes and other liver cells to replaced damage tissue. a rat with 1/3 of a liver can regenerate the rest in a few days -
unipotent: somatic/adult stem cells, can only differentiate into one type of cell
* in skeletal muscle tissue - satellite cells
* in basal layer of skin - epidermal stem cells
* in intestinal epithelium - stem cells at the bottom of intestinal crypts divide several times, moving upwards, differentiate near the surface, surface epithelium are continuously shed
role of proteoglycans and GAGs
- ground substance of connective tissue
- proteoglycans: core protein attached to a long chain of glycosaminoglycans (complex, negatively charged sugars)
- form large aggregates
collegen makes up what percentage of the total protein mass in mammals?
25%
where is collagen found?
- tendons that attach muscles to bone. parallel formation so they are flexible but inelastic
- demis layer of the skin, woven into a basket-like pattern in multiple layers in different orientations. this helps it resist tensile stress in multiple directions
examples of tissue renewal
- nerve cells and muscle cells last a lifetime and are never replaced, while cells lining the intestines are replaced every 4-5 days
- bone is renewed every ten years by osteogenic cells, a type of stem cell on the surface of bones
- blood cells are generated in the bone marrow by hematopoietic stem cells. depending on what part of the blood, it is replaced anywhere from every 5 days to every few years
- outer layers of the epidermis of the skin are continuously flaking off and replaced from below via stem cells in the basal layer of the skin
stem cell division for tissue renewal
stem cell divides into 2 daughter cells, one that remains a stem cell to replace the mother cell and one transit-amplifying cell (precursor to the tissue cell), which goes through several more divisions to proliferate and then differentiates (terminally differentiated cell that cannot divide)
growth responses to growth factors
VEGF: vascular endothelial growth factor
* secreted by cells deprived of oxygen, which stimulates the proliferation of endothelial cells to grow extra capillaries into the tissue to increase blood/oxygen supply
EGF: epidermal growth factors
* secreted by platelets when skin is injured, cause fibroblasts in connective tissue to proliferate because they secrete collagen (patch the hole)
