Mol Precept March 23

Stem cells possess the ability to differentiate into various cell types but have limitations on what they can become based on their characteristics.

Totipotency vs. Pluripotency
- Totipotency
- Definition: Refers to the ability of a single cell to divide and develop into a full organism.
- Example: A zygote can develop into an entire organism as it has the complete potential.
- Pluripotency
- Definition: Refers to the capability of a stem cell to differentiate into multiple cell types within an organism but not to form a whole organism.
- Example: When put in a dish, pluripotent cells will usually differentiate into specific types of cells without forming a complete organ.

After the zygote stage, cells go through stages (2-cell stage to 32-cell stage) where they become specific cell types such as ectoderm, mesoderm, and endoderm.

Autonomous Specification
- Definition: Cells determine their fate based on intrinsic factors without external signals.
- Mechanism: Cells may divide asymmetrically, leading daughter cells to develop differently based on internal characteristics (e.g., size or location).
- Example: Some proteins (e.g., a specific protein expressed on one side of the cell) lead to differential differentiation during asymmetric cell division.
Inductive (Non-Autonomous) Specification
- Definition: Cells are influenced by signals from neighboring cells to determine their fate.
- Mechanism: Includes cell-to-cell signaling (e.g., paracrine signaling).
- Example: A stem cell's environment or niche sends signals triggering differentiation, such as direct cell adhesion or chemically transmitting signals through free space.

Germ cells consist of sperm and ova (oocytes). They hold significant importance in reproductive biology.

Primordial Germ Cells (PGCs)
- Definition: Precursors to germ cells that are present from early embryonic development.
- Specification: Primordial germ cells are induced to form by surrounding cells in specific regions of the embryo.
- Migration: PGCs migrate to form gonads (testes or ovaries) during embryonic development rather than being produced by the gonads.

Gonads are the reproductive organs (testes or ovaries).
PGCs must migrate to the gonads to form eggs or sperm during later developmental stages.

Inductive Signals: They arise from adjacent cells guiding PGCs to become germ cells when gonads develop.
Hormonally Influenced Sex Determination: Gonadal differentiation occurs when hormones from the developing organism (like the presence of an SRY gene for testes development) guide differentiation into either testes or ovaries.

In Drosophila, germ cells are directed by a structure called germplasm deposited at the posterior of the embryo.
As with humans, there’s an embryonic folding process that allows for the migration of germ cells to gonads, albeit with maternal deposit in flies, not in humans.

Every somatic cell carries the same genome (identical DNA), but distinct cell types arise from differential gene expression.

Transcription Factors: The variation in gene expression results from different combinations of transcription factors present in each cell type.
Enhancers:
- Definition: DNA sequences located near genes that, when bound by transcription factors, enhance the expression of that gene.
- Function: They play a crucial role in tissue-specific gene expression by modulating the rate of gene transcription.

Serpentine Mouse: The study of this mouse showed that a specific enhancer's deletion led to the absence of limbs, demonstrating the enhancers' role in limb development.
Techniques for Gene Expression Assessment:
- Western Blot: Examines protein levels.
- RNA Sequencing: Measures mRNA levels for differential expression of genes.
- In Situ Hybridization: Detects mRNA localization through fluorescent tagging.
- CRISPR and Reporter Genes: These methods allow real-time visualization of gene expression by integrating reporter genes into the organism's DNA.