left/right axis specification

Definition

Target type: biologicalprocess

The establishment, maintenance and elaboration of the left/right axis. The left/right axis is defined by a line that runs orthogonal to both the anterior/posterior and dorsal/ventral axes. Each side is defined from the viewpoint of the organism rather of the observer (as per anatomical axes). [GOC:dph, GOC:gvg, GOC:mah]

The establishment of the left-right (L/R) axis is a fundamental process in animal development, defining the body plan and ensuring the correct placement of organs. This complex process involves a series of molecular and cellular events, beginning with the initial asymmetry observed in the early embryo.

**1. Early Asymmetry:**

The first sign of L/R asymmetry often appears in the form of cilia, tiny hair-like structures on the surface of cells. In many animals, a group of specialized cilia, known as nodal cilia, are located in the node (a region of the developing embryo) and beat in a leftward direction. This coordinated movement generates a leftward flow of fluid, creating a localized asymmetry in the concentration of signaling molecules.

**2. Nodal Signaling and Lefty:**

The leftward flow of fluid activates downstream signaling pathways, primarily involving the Nodal signaling pathway. Nodal is a secreted signaling molecule that activates downstream transcription factors, including Pitx2, which plays a crucial role in specifying left-sided identity. Importantly, the right side of the node expresses a molecule called Lefty, which acts as an antagonist of Nodal, preventing its diffusion to the right side.

**3. Establishing Left and Right Identities:**

The asymmetric distribution of Nodal and Lefty leads to differential gene expression on the left and right sides of the embryo. This results in the establishment of distinct left and right identities, with specific genes and proteins being expressed on each side. For instance, the expression of Pitx2 on the left side is essential for the development of left-sided organs, such as the heart and stomach.

**4. Organ Asymmetry and Function:**

The establishment of the L/R axis is crucial for proper organ development and function. Many organs, including the heart, lungs, liver, and gut, exhibit distinct left and right asymmetry. For example, the heart is positioned on the left side of the body, while the liver is located on the right side. This asymmetry is essential for proper organ function, as each side of the body performs specific tasks.

**5. Conservation and Evolution:**

The L/R axis specification pathway is highly conserved across different animal species, suggesting its fundamental importance in development. However, variations in the mechanisms and genes involved in this process have been observed, reflecting the evolutionary adaptations of different animal groups.

**6. Defects in Axis Specification:**

Disruptions in the L/R axis specification pathway can lead to serious developmental defects, such as situs inversus (a complete reversal of organ positions), heterotaxia (a mixture of left and right organ positions), and congenital heart defects. These conditions highlight the importance of proper L/R axis establishment for normal development and function.'
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Proteins (2)

Compounds (16)