Cellular Development: Stem Cells

Cellular Development Stem Cells: Stem cells originate in the zygote as the cell begin dividing via mitosis and must posses two main qualities: 1. Self renewal where at least one of the daughter cells remains a stem cell (Obligate asymmetric replication when one goes on to be differentiated) 2. Can differentiate into different cell types (potency shows how many different cell types that stem cell can differentiate into) In the blastocyst there are embryonic cells (inner cell mass) which go on to become the embryo and are also known as stem cells (which can then go on to differentiate into any of the more than 200 cell types and are specifically known as pluripotent) Somatic stem cells are used as a repair mechanism for the body in order to replenish tissues that need to be replaced For example, skin cells are constantly needing to be replaced and therefore this is the role of epidermal stem cells Mature cells are already differentiated and are not the same as stem cells (unspecialized cells) Hematopoietic stem cells are what differentiate into red blood cells and white blood cells and is located in the bone marrow Mesenchymal stem cells is located in the bone marrow and are what differentiate into skeletal tissue (cartilage and bone) and the fat found in bone marrow Stochastic differentiation is when a stem cell divides into two daughter cells (the other stem cells will divide into two mother cells in order to make up for this loss of a stem cell) Induced pluripotent stem cells (iPS) are when you introduce a gene into an already differentiated cell and it will “forget” that it is differentiated and reprogram back to a pluripotent stem cell Cellular Communication: Direct cell-cell communication is when there is direct cell to cell interaction, meaning that they physically touch (such as a macrophage and a helper t cell) Neural communication is when there is a message passed through a small distance, such as a neurotransmitter diffusing across the synaptic cleft from one neuron to another Paracrine signaling is when there is a message passed through a small distance, such as with mast cells when they detect an allergen in the vicinity Endocrine signaling help communicate to cells all over the body, such as through the pituitary gland Mitochondria, Apoptosis, & Oxidative Stress: Apoptosis is known as programmed cell death (controlled) → occurs in development in order to form the fingers and is caspase regulated Necrosis is also known as programmed cell death (but not through a controlled process) and is not caspase regulated DNA damage, infection (especially by viruses), environmental stress (such as lower oxygen levels), and reactive oxygen species (unstable oxygen species such as O2- or H2O2) can induce cell death In fact, about 4% of the oxygen is improperly reduced into a reactive oxygen species Cytochrome C found within the mitochondria (a membrane of the ETC) are released from the mitochondria during apoptosis in order to activate proteins known as caspases (which is an enzyme that breaks down proteins specifically with the aspartate residue using its cysteine residue → “C - ASP - ASE”) Cells that die can be reused by surrounding cells Cellular Specialization (differentiation): With different proteins and genes active, the cells are differentiated into different types of cells Basically a stem cell turns on certain genes in order to change how the cell looks One differentiated, cells can de-differentiate without intervention from humans Stem cells differentiate based on cues: 1. Internal cues occurs when transcription factors are only located on one side of the cell so that when it differentiates only a couple of the cells are divided with the transcription factors (AKA asymmetric segregation of cellular determinants) 2. External cues, also known as induction, occurs where one group of cells induces differentiation in other stem cells through paracrine signaling, gap junctions (which are connections between cells), or even direct contact (“peer pressure”) Remember, the whole point of differentiation is to turn on/off certain genes since ALL cells have the SAME genetic information Telomeres & Cell Senescence: Mitotic cells are cells that are actively able to divide (such as epithelial cells, endothelial cells, fibroblasts, and even stem cells) in order to regenerate tissues Post Mitotic cells do not undergo mitosis and are therefore incapable of proliferation/repair of tissue (neurons, heart muscle cells, etc.) and is basically a very slow process of repair, if it ever even does repair Telomeres are located at the ends of the DNA in order to protect the rest of the DNA and do not code for anything (you lose some telomere every time DNA is copied) Once telomeres get too short, the cell gives off a DNA damage response and no longer is able to divide in order to prevent further shortening (becomes a senescent cell which is a non-dividing cell) Hayflick limit is the number of times a cell can divide before reaching senescence (about 60 or so times) DNA mutation can also lead to senescence Telomerase is found in stem cells in order to maintain their telomere lengths even through consistent replication and therefore don’t really ever reach a hayflick limit Cancer cells can develop a mutation in order to express telomerase, allowing them to continuously divide without really ever reaching the hayflick limit Cell Movement: Flagella in sperm is made up microtubules that are connected by dynein proteins allowing for the sperm’s “tail” to whip around and cause the sperm cell to move (made different in bacteria and archaea) Neutrophils literally get a signal and move out of the blood vessel and into the surrounding tissue Cytoskeletal Model of Movement: the leading edge acts to move the cell by combining actin to form actin filaments while the rear is used to steer or as an anchor (microtubules in the back) Membrane Flow Model of Movement: bites of the plasma membrane endocytosis into the cell and move to the front of the membrane in order to add membrane to the front Some membrane is just plasma membrane and others are ones with little foot proteins known as integrins