Differentiation, Life and Death: Stem Cells and Apoptosis

Flashcards covering stem cell properties, potency, telomere maintenance, and mechanisms of programmed vs. accidental cell death.

Stem Cells

Special cells with two important properties: they are able to renew themselves (make more cells like themselves) and differentiate (become other cells that perform different functions).

Totipotent

Stem cells capable of giving rise to an entire organism and all supporting tissues; includes the zygote and early blastomeres up to the 4-to-8-cell stage.

Pluripotent

Stem cells that can differentiate into almost any cell type but cannot develop into a whole organism on their own; examples include embryonic stem cells from the inner cell mass of a blastocyst.

Multipotent

Stem cells that can only differentiate into a closely related family of cells, such as adult stem cells that only become blood cells.

Unipotent

Stem cells that can only produce one specific cell type.

Induced Pluripotent Stem Cells (iPSCs)

Adult cells that have been genetically reprogrammed to behave like embryonic stem cells.

Morula

A compressed ball of cells formed as the embryo transitions from the 8-cell stage before becoming a blastocyst.

Blastocyst

An embryo at day 5 to 6 consisting of approximately 150 cells, where the inner cell mass is pluripotent and the outer layer forms the placenta.

Telomeres

Protective, non-coding repeating units of $(TTAGGG)_n$ at chromosome ends; they typically have 1,500 to 3,000 repeats ($18\,kb$) in new chromosomes.

Hayflick Limit

The point at which telomeres become critically short, triggering cellular senescence via a DNA-damage response.

Telomerase

An enzyme that acts as a reverse transcriptase, synthesizing fresh $TTAGGG$ repeats back onto chromosome ends after division to maintain immortality.

Telomere Sister-Chromatid Exchange (T-SCE)

A recombination trick used by early embryo cleavage cells to rapidly lengthen chromosomes without using telomerase.

Asymmetric Division Shielding

A mechanism where an adult stem cell creates one new stem cell (retaining pristine template DNA) and one progenitor cell that undergoes rapid division and telomere erosion.

TRF2

A protein that arranges DNA at the chromosome end into a "telomere-loop" structure; its removal is lethal in adult cells but not in pluripotent stem cells.

Paternal Age at Conception (PAC) effect

The phenomenon where aging males produce sperm with increasingly longer telomeres due to robust telomerase expression in the testes.

Alternative Lengthening of Telomeres (ALT)

A recombination-based mechanism used by early embryos (2-cell to blastocyst stage) to equalize telomere lengths by using longer telomeres as templates.

Immortal Strand Hypothesis

The theory that adult stem cells selectively retain older DNA strands during division to minimize the accumulation of mutations.

Adipogenesis Induction Cocktail

A chemical mixture containing Insulin, IBMX, and Indomethacin (which activates PPAR) used to differentiate mesenchymal stem cells into fat.

Apoptosis

Programmed cell death (Greek for "falling off") characterized by DNA breakup, organelle fragmentation, cell shrinkage, blebbing, and phagocytosis; takes 6 to 24 hours.

Ced-9

The "Master regulator" protein found in the outer mitochondrial membrane that regulates apoptosis in C. elegans by controlling Ced-3 and Ced-4.

Caspases

Proteases produced as inactive pro-versions; they are activated by removing the prodomain and cleaving at Asp (D) residues to form a heterotetramer.

Extrinsic Pathway

An apoptotic signaling pathway initiated from outside the cell via death receptors and FADD (fas associated death domain) to activate DED-containing caspases.

Intrinsic Pathway

An apoptotic pathway where DNA damage activates BH3-proteins, antagonizing Bcl-2 and activating Bax and Bak to form mitochondrial channels for Cytochrome C release.

Phosphatidylserine

A phospholipid that flips from the inner to the outer leaflet during early apoptosis, acting as a marker that interacts with Annexin V.

Karyorrhexis

The process occurring during apoptosis where the nucleus of the cell breaks up.

DNA Laddering

A pattern detected in electrophoresis resulting from endogenous DNases cutting internucleosomal regions into $180-200\,bp$ fragments during apoptosis.

Necrosis

Accidental, passive cell death not regulated by homeostatic mechanisms; typically caused by hypoxia, toxins, or extreme temperature and involves calcium or ROS mediation.

Autophagy

"Self eating" cell process for removing damaged proteins and organelles; includes macroautophagy (lysosome fusion), microphagy, and chaperone-mediated types.

Turner Syndrome

A viable monosomy ($45; XO$) in humans that can result in webbing on the neck due to alterations in developmental apoptosis.