Week 9 - MOLECULAR DEVELOPMENT Part 4: Mammalian development and body axis formation Flashcards
(22 cards)
What is shown in early human embryo development?
Stages from fertilization to early organ formation.
Highlights stages like cleavage, blastocyst formation, and gastrulation.
What happens during early stages before implantation?
Zygotic genome starts being expressed (early gene activity begins).
Polar bodies are formed — extra chromosomal material discarded after cell divisions.
What is “compaction” and why is it important?
At 8-cell stage, blastomeres stick tightly together using adhesion molecules (like E-cadherin).
Cells become tightly packed and communicate by exchanging small molecules.
Key step before blastocyst formation.
What happens to cells before compaction?
Cells become radially polarized — different proteins are expressed on inner vs outer sides.
Adhesion molecules like E-cadherin begin localizing to cell junctions.
What are the two major cell types in the blastocyst?
Inner Cell Mass (ICM): Forms the embryo.
Trophoblast: Forms part of the placenta.
Each blastomere initially has the potential to become either ICM or trophoblast.
Why does the blastocyst need to hatch?
The Zona Pellucida (ZP) is a protective shell around the embryo.
Blastocyst must hatch out of ZP to implant into the uterus wall.
If it hatches too early → ectopic (tubal) pregnancy risk.
What needs to happen before gastrulation?
Gastrulation is when the embryo goes from a simple ball of cells to a layered structure that will become all parts of the body.
A-P axis (head-to-tail) needs to be established first.
Gastrulation then reshapes the embryo, forming three germ layers.
Occurs through formation of the primitive streak.
How is the Anterior-Posterior axis formed?
Posterior side: High levels of morphogens from primitive streak.
Anterior side: Protected by “antagonist” proteins from the Node.
Antagonists block posterior signals to maintain head identity.
How do mechanical forces help embryo patterning?
Uterus physically shapes the embryo — squeezes and narrows it.
Distal cells migrate toward one side and become Anterior Visceral Endoderm (AVE).
AVE blocks posterior morphogens and starts head gene expression.
What happens if anterior genes like Lim-1 are missing?
Mice without Lim-1 gene fail to form head structures (“headless mice”).
Only ears (most anterior structures) are present.
What do migrating cells form during gastrulation?
Cells moving anteriorly form:
Endoderm (gut lining)
Head mesoderm (muscle/bone of head)
Notochord (primitive spinal cord)
Cells moving posteriorly form:
Mesoderm (muscle, bone, blood)
What is EMT (Epithelial to Mesenchymal Transition)?
Organized, stuck-together epithelial cells lose attachment and become migratory.
Critical for cells to move through primitive streak and form body layers.
What is the “HOX Code”?
Each body region expresses a unique combination of HOX genes.
4 clusters of HOX genes (Hoxa, Hoxb, Hoxc, Hoxd) are activated sequentially.
Their position on the chromosome matches the order of body parts they control (colinearity).
How are HOX genes turned on?
Primitive streak morphogens activate HOX genes indirectly.
Cdx (caudal-type) transcription factors are key middle steps.
How does chromatin structure affect HOX gene activity?
Chromatin (DNA + proteins) opens up over time, letting HOX genes be activated at the right place and time.
How do HOX patterns vary across animals?
Chickens and mice have same number of vertebrae.
BUT their vertebrae types differ — because HOX gene boundaries differ.
What happens when HOX genes are knocked out?
Deleting Hox10: Lumbar vertebrae become thoracic (extra ribs).
Deleting Hox11: Sacral vertebrae become lumbar.
How is Left-Right symmetry broken in mammals?
Cilia near the node move fluid from right to left.
Flow triggers Nodal expression on the left side.
Nodal activates Pitx2, setting up left-side organs.
How do cilia drive Left-Right axis formation?
Motile cilia: Beat fluid leftward.
Immotile crown cells: Sense flow and trigger calcium signals.
Left-side calcium signal increases Nodal expression.
What do motile and primary cilia do in embryos?
Motile cilia: Move fluid directionally (right to left).
Primary (immotile) cilia: Sense and transmit environmental signals.
How do Nodal, Lefty, and Pitx2 interact?
Nodal: Induces itself (positive feedback) and triggers Lefty.
Lefty: Stops Nodal signaling to limit it.
Pitx2: Turns on left-specific body genes.
What is Primary Ciliary Dyskinesia (PCD)?
A recessive genetic disease where cilia can’t beat properly.
Symptoms:
Chronic sinus and ear infections
Male infertility
50% chance of situs inversus (organs reversed).
