Stem Cells

• Specialized, or differentiated, cells have distinct functions
• As differentiation of a cell increases, potency decreases.
• Potency describes the development potential of a cell
• Differentiation = a process that occurs through many cellular divisions, where a cell eventually gives rise to one or many different specialized cell types
• zygote = a fertilized egg, a one-cell embryo
• progenitor cell = gives rise to specialized cells
• Cells become specialized from cues from their environment, including signals from within or outside of the cell
• germ layers = embryonic structures that give rise to specific, pre-defined differentiated cell types
• the germ layers are...

  • ectoderm = outer layer
  • mesoderm = middle layer
  • endoderm = internal layer
• gene expression changes during the process of differentiation, where stem cell genes are switched off and cell specific genes are switched on
• stem cell = a cell that has the ability of self-renewal (to create a copy of itself) and a differentiated cell
• self-renewal = the process during cell division whereby a stem cell produces an identical copy of itself as one of the daughter cells
• totipotent = has the ability to give rise to an entire organism (for most mammals, this includes the placenta!)
• pluripotent = in mammals, has the ability to give rise to all the cells of the body, but not the extraembryonic cells (ex. for the placenta or umbilical cord)
• multipotent = the ability of a tissue-specific stem cell to give rise to the cell types in that particular tissue
• unipotent = has the ability to generate only one type of cell
• embryonic stem (ES) cells = undifferentiated cells found in embryos, derived from the inner cell mass (for mammals), almost always pluripotent
• inner cell mass = the inner layer of the blastocyst that will become the embryo, used for ES cells
• ES cells are usually pluripotent, while adult stem cells are usually multipotent or unipotent
• fate mapping = a technique used to determine the fate of a particular cell in an early blastocyst; it involves injecting a vital or fluorescent dye into the embryo and mapping its location in later developmental stages and as an adult organism
• Why can fate mapping be misleading?

  Just because a cell does not give rise to a particular tissue in the embryo, this does not mean that it cannot give rise to that tissue in vitro or under other conditions
• morula = the embryo at about 8 or 16 cells; consists of a small group of internal cells surrounded by a larger group of external cells
• blastula or blastocyst = embryo after the morula stage; at this point there is a fluid-filled blastocoel cavity
• cleavage = the series of rapid cell divisions that occur after fertilization; occurs throughout the morula and blastula stages, but usually slows down during the blastula stage
• compaction = occurs around 8 cell stage, where the cells of the embryo begin to stick together very tightly and form a single sphere-like structure
• cavitation = the process of the embryo making the blastocoel
• blastocoel = the fluid-filled space in the blastula or blastocyst
• implantation = the process by which the embryo invades the uterine lining, so that it can establish a connection with the parental blood supply
• trophoblasts = the cells that make up the outer portion of a blastocyst and will eventually become part of the placenta but not the embryo proper (unlike some cells of the ICM)
• epiblasts = the cells in the ICM that go on to make the cells of the embryo, including all three germ layers
• hypoblast = the cells of the ICM that go on to form parts of the yolk sac and separates the epiblasts from the blastocyst cavity
• gastrulation = a series of events characterized by coordinated movement of groups of cells within the embryo, also including differentiation and reorganization of the germ layers
• euchromatin = loosely packed chromatin

  heterochromatin = densely packed chromatin
• histone acetylation = addition of acetyl groups to the lysines of histone tails, which loosens the chromatin and promotes gene expression

  histone methylation = addition of methyl groups to condense the chromatin, usually causing gene repression
• molecular modification for gene expression regulation can occur at the level of both the histones and the DNA
• chromatin modifications can be passed down, making up a sort of epigenetic memory of gene expression
• DNA methylation occurs at CpG cytosines
• MeCP2 binds to methylated CpG's and recruits either histone deacetylase or histone methyltransferase, both of which contribute to gene repression
• enhancers can bind transcription factors to increase transcription, while promoters also are where RNA polymerase II binds to initiate transcription
• cytoplasmic determinants = substances in the cell that influence early development; they get distributed unevenly between cells as the embryo undergoes cell division, contributing to differences in gene expression