determination of bilateral symmetry

Definition

Target type: biologicalprocess

The establishment of an organism's body plan or part of an organism with respect to a single longitudinal plane. The pattern can either be symmetric, such that the halves are mirror images, or asymmetric where the pattern deviates from this symmetry. [GOC:go_curators]

Bilateral symmetry, a fundamental body plan in animals, is established through a complex and intricate process involving multiple signaling pathways, gene expression, and cellular interactions. This process begins during early embryonic development and is influenced by a combination of intrinsic and extrinsic factors.

**1. Establishment of the Dorsal-Ventral Axis:**
The first step in bilateral symmetry determination is the establishment of the dorsal-ventral axis, which defines the back and belly of the organism. This process is often initiated by maternal factors deposited within the egg, such as the maternally derived mRNA of the bicoid gene in Drosophila. The bicoid protein acts as a morphogen, creating a concentration gradient along the anterior-posterior axis, with higher concentrations at the anterior end. This gradient triggers the expression of other genes, including dorsal, which establishes the dorsal-ventral axis.

**2. Specification of the Left-Right Axis:**
Once the dorsal-ventral axis is established, the left-right axis is specified. This process is often influenced by the Nodal signaling pathway, which is activated in a left-sided fashion. Nodal signaling is crucial for the development of left-right asymmetry in various organs, including the heart, lungs, and gut. In vertebrates, the left-right axis is also influenced by cilia, which are hair-like structures that generate fluid flow, contributing to the asymmetric activation of Nodal signaling.

**3. Segmentation and Pattern Formation:**
After the establishment of the body axes, the embryo undergoes segmentation, dividing the body into repeating units called segments. The segmentation process is regulated by a series of genes, including the homeobox genes, which control the identity and development of individual segments. These genes act in a hierarchical manner, with master control genes like the Hox genes specifying the identity of different segments along the anterior-posterior axis.

**4. Organogenesis and Morphogenesis:**
Once the body plan is established, specific organs and tissues develop within each segment. This process involves cell differentiation, migration, and growth, regulated by various signaling pathways and gene expression patterns. Interactions between different cell types and tissues ensure the proper formation and organization of organs and body structures, ultimately leading to the establishment of bilateral symmetry.

**5. Fine-Tuning and Maintenance:**
Bilateral symmetry is not a static state but rather a dynamic process that is constantly refined throughout development. Fine-tuning of body symmetry occurs through ongoing interactions between cells and tissues, ensuring the accurate positioning and development of organs and limbs. Moreover, specific mechanisms exist to maintain bilateral symmetry in adult organisms, ensuring the appropriate function of organs and tissues over time.

In summary, bilateral symmetry determination is a complex and multifaceted process involving a delicate interplay of genetic and developmental factors. This process is crucial for the proper development and function of organisms, ensuring the formation of a body plan that allows for efficient movement, sensory perception, and overall survival.'
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