specialized tissues, stem cells and tissue renewal

unspecialized cells capable of renewing themselves can be induced to differentiate into tissue or organ specific cells with specialized functions

gut, bone marrow, and skin

pancreas, brain, and heart

not terminally differentiated can divide without limit able to renew themselves (preventing depletion of stem cells) undergo slow division

ability of a cell to give rise to all cells of an organism, including embryonic and extra-embryonic tissues (placenta) e.g. zygote

ability of a cell to give rise to all cells of the embryo and subsequently adult tissues e.g. embryonic stem cells

ability of a cell to give rise to different cell types of a given lineage e.g. adult stem cells

asymmetric division independent choice

creates 2 cells, one with stem cell characteristics and another with the ability to differentiate (due to presence of determinate in cytosol in original stem cell) first round of division: 50/50 split

division makes 2 identical cells but the outcome is random and/or influenced by environment (not 50/50 split but depends on the needs of the cell) first round of division: 50/50 split; or both stem cells; or both differentiated cells e. g. if everything in cell/tissue is good –> independent choice allows for both daughter cells to become stem cells e. g. if tissue is injured –> independent choice allows for both daughter cells to become differentiated

how existing stem cells rapidly increase their numbers e.g. doesn’t explain injury situation

gene expression

microenvironment • chemicals secreted by neighboring cells • physical contact with neighboring cells • stem cell - stem cell interaction • molecules in the microenvironment: extracellular matrix; growth factors; oxygen tension; ionic strength; pH; metabolites like ATP

each organ/tissue has fixed number of founder stem cell populations programmed to have fixed number of divisions founder stem cells divide and give rise to one daughter cell that remains a stem cell and a set of cells that have a set number of transit amplifying divisions define the size of large final structures

transit from a cell with stem cell characteristics to a differentiated cell programmed to divide for a limited number of times strategy for growth control

are tissue specific multipotent serve as an internal repair system (divide without limit to replenish damaged cells) need a specialized microenvironment where they can stay as a stem cell

outer covering of skin creates a water barrier made of epithelial cells continuously repaired and renewed every 30 days

second layer of skin rich in collagen provides toughness

deepest layer of skin fatty subcutaneous layer

loose connective tissue dense connective tissue

secrete extracellular matrix providing mechanical support

endothelial cells blood vessels function to supply nutrients and oxygen and remove waste products

macrophages and dendritic cells provide defense against microbes and pathogens

adaptive immune response