Untitled Flashcard Set

1. Sex Organ Development (The Bipotential Gonad)

Early on, embryos have the plumbing for both sexes (Wolffian ducts for male, Müllerian ducts for female). The choice comes down to two pathways fighting to the death.

  • The Male (XY) Pathway: The SRY gene activates Sox9. Sox9 creates a positive feedback loop with Fgf9 to keep itself turned on. Sox9 directly destroys the female pathway (blocks -catenin) and turns on AMH, which degrades the female Müllerian ducts.

  • The Female (XX) Pathway: Wnt4 and Rspo1 stabilize -catenin. -catenin turns on ovary genes (like Follistatin) and directly destroys the male pathway (blocks Sox9). Without AMH around, the Müllerian ducts become the uterus and oviducts.

2. The Big Four Signaling Pathways

You just need to know the trigger, the main player, and the result.

  • FGF / RTK: FGF binds a receptor tyrosine kinase (RTK). This triggers a domino effect of kinases (Ras Raf MEK ERK). ERK enters the nucleus to turn on transcription factors.

  • Hedgehog (Hh): Without Hh, a receptor called Patched keeps a protein called Smoothened trapped. When Hh binds Patched, Smoothened is freed. Smoothened stops Gli from being chopped into a repressor, allowing full-length Gli to activate target genes.

  • Wnt / -catenin: Without Wnt, a "destruction complex" constantly shreds -catenin. Wnt binds the Frizzled receptor, which uses Disheveled to shut down the destruction complex. -catenin survives, enters the nucleus, and binds TCF to turn on genes.

  • TGF- / BMP: Ligands bind kinase receptors, which phosphorylate specific Smad proteins (Smad 1/5/8 for BMP; Smad 2/3 for TGF-/Nodal). These bind Smad4 and go to the nucleus to change gene expression.

3. Asymmetric Cell Division in C. elegans

The unfertilized egg has no front or back.

  • The Trigger: The sperm enters, and its centrosome physically defines the posterior (back) end of the embryo.

  • The PAR Protein Turf War: The sperm centrosome physically pushes anterior PARs (PAR-3, PAR-6, PKC-3) to the front. This allows posterior PARs (PAR-1, PAR-2) to lock onto the back. They mutually repel each other to split the cell into two distinct zones.

  • The Result: The first cell division is uneven, creating a large front cell (AB) and a small back cell (P1).

4. Gastrulation Cell Movements (The Physical Mechanics)

Embryos change shape using five specific movements.

  • Invagination: Poking the balloon. Cells undergo apical constriction (pulling their tops tight) to fold a sheet of cells inward, creating a dent like the blastopore lip.

  • Involution: Rolling inward. A sheet of cells marches over an edge (the blastopore lip) and crawls along the inside roof of the embryo. (Requires fibronectin to crawl on).

  • Epiboly: Stretching. An outer layer of cells thins out and stretches to completely cover the outside of the embryo.

  • Convergent Extension: Squeezing the tube. Cells move toward the middle and squeeze between each other (intercalation), making the tissue narrower and much longer.

  • Ingression: Dropping in. Individual cells break off from a sheet and drop freely into the hollow center of the embryo.

5. The Spemann-Mangold Organizer (Xenopus)

This is the master control center for the frog embryo.

  • Where it forms: Exactly 180 degrees opposite of where the sperm entered.

  • How it forms: Sperm entry causes the outer edge of the egg to rotate (cortical rotation). This rotation physically drags Disheveled to the dorsal side. Disheveled saves -catenin from destruction. -catenin teams up with Smad2 to turn on Organizer genes (like Siamois, Twin, and Goosecoid).

  • What it does: It turns into the notochord, tells the tissue above it to become the neural tube, and coordinates all gastrulation movements.