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The left-right (L/R) axis is fundamental in the anatomical orientation of vertebrate organisms.
Abnormalities in L/R axis can lead to significant congenital defects, affecting organ placement and functioning.

Source: The Annals of Thoracic Surgery
Description: An otherwise healthy adolescent presents to the emergency department (ED) with bruised ribs after a car accident. Concern about internal bleeding leads to radiological imaging.
Result of Imaging: Disbelief in the findings of the images.

CT Scan Findings:
- Coronal View: Reveals normal chest view.
- Normal Abdomen View: Highlights the typical placement of organs in a healthy patient.

Diagram Components:
- Normal Heart Structure: Illustrated with: Aorta, Right Atrium (RA), Left Atrium (LA), Left Ventricle (LV), Right Ventricle (RV), and Patent Ductus Arteriosus.
- Transposed Great Arteries: Changes in the orientation of blood vessels lead to anomalies.
Source: Journal of the American Society of Echocardiography.

Situs Solitus: Correct orientation of organs (normal).
- Organ placements include: Right Lung (RL), Left Lung (LL), Left Atrium (LA), Right Atrium (RA), Liver, Stomach.
Situs Inversus: Reversal of organ symmetry.
Heterotaxia: Abnormal placement of internal organs.
Connection to Transposition: Organ placement is crucial in the context of anomalies such as transposition of the great arteries which affects blood circulation and oxygenation.

Historical Studies:
- Levin et al, 1995: Asymmetric gene expression identified before organ migration in chick embryos.
- Additional studies in Xenopus, zebrafish, and mouse (Lustig et al, 1996; Schottenfeld et al, 2007).

Components of the Pathway:
- Key Proteins: Nodal, EGF-CFC, FoxH1, Pitx2, Lefty.
- Interaction: Lefty inhibits Nodal signaling.
- Significant mutation in zebrafish: Gene squint assists in L/R patterning.

Outcomes of Defects:
- Situs Solitus, Situs Inversus, and Heterotaxia can occur due to disruptions in Nodal signaling pathways.
Organ Effects: Heart, Liver, Pancreas can have altered placements leading to severe physiological consequences.

Various studies (Shiratori and Hamada, 2006; Shweikert et al., 2010; Lenhart et al., 2011) have shown that Nodal expression begins bilaterally and highlights the importance of asymmetry in embryonic development.

Key question: What breaks symmetry in the embryo?
Research Impact: Insights from studies on mutant mouse models (Supp et al., 1997 and 1999) highlight the "Left-Right Dynein" mutations associated with L/R defects.

LRD Expression: LRD mRNA is found in cilia at the mouse node, critical for L/R axis determination.
Structure of Cilia:
- Components:
- Basal body, Flagellum or cilium, Central bridge, Dynein arms
- Ciliary Functionality: Essential for normal fluid movement across the embryonic node, influencing asymmetry in invasion and placement of organs.

Structural comparison: Sperm flagella share features with cilia.
Importance of microtubule arrangements: Doublets, central pair, and specificity of dynein and nexin links.

Kartagener’s Syndrome: Phenotypes related to cilia dysfunction.
- Key symptoms include: Bronchial cilia involvement, mucus buildup, sterility, Situs Inversus leading to difficulty in breathing.

Definition: Left-Right Organizers (LROs) in embryos equipped with components like Cilia, contribute to establishing L/R patterning.
Embryonic Structures: Mouse node, Zebrafish Kupffer's vesicle, and Xenopus gastrocoel roof plate identified as centers for L/R determination.

Cilia at the node generate a leftward fluid flow, fundamentally linked to L/R development.
Loss of Left-Right Dynein (LRD) leads to immotile cilia, disrupting asymmetric flow critical for establishing the L/R axis.

Question of causality: While flow correlates with asymmetry, research (Nonaka et al, 2002) indicates leftward flow activates left Nodal expression critical for organ placement.
Two Proposed Models for the mechanism of action:
- A) Determinant Molecule.
- B) Mechanical Stress - artificial flow experiments suggest model A may be more relevant.

Pkd2 Mutant Phenotype: A mouse mutant for polycystic kidney disease 2 (pkd2) displays left-right patterning defects along with an absence of asymmetric Nodal expression.
Functionality: Pkd2 is identified as a stretch-activated calcium channel present in all cilia at the node, crucial for maintaining leftward fluid flow.

Studies (McGrath et al., 2003) show that Ca²⁺ levels are increased on the left node side but are absent when there's a disruption in fluid flow or when Pkd2 is not functional.

Hypothesis Overview: Left-sided Nodal signaling leads to asymmetric organ placement and development.
Crucial Mechanism: Calcium gradient manipulated by fluid flow supports asymmetric signaling and gene expression maintaining L/R organ placement through phases in embryogenesis.

Maintaining asymmetry after Nodal signaling induction involves: differentiation of molecular midline barriers, including genes such as spaw and lefty1 relevant to L/R development.

Summary: Asymmetric Gene Expression connects to early and late stages of organogenesis, and symmetry-breaking events are vital for establishing the left-right axis in vertebrates, showcasing complex interactions and mechanisms involved in embryonic development.