Chapter 20, Problem 1DQ

Summary Introduction

To review:

The molecular and cellular events responsible for the gut looping, as the gut tube rotates in specific directions, locate the stomach near the heart and the appendix on the right hand side.

Introduction:

The mammalian gut development starts at two sites that undergo migration towards each other and ultimately fuse in the center. Initially, the gut cells forms a flat sheet beneath the embryo, followed by the formation of the gut tube. The cells from the lateral parts of anterior endoderm moves ventrally in the forgut, and leads to the formation of tube of anterior intestinal portal (AIP) whereas the caudal intestinal portal (CIP) is derived from the posterior endoderm. The initiation of counterclockwise coiling of the intestine is carried by leftward tilt of the primitive gut tube, which is a result of left-right asymmetries in the architecture of the dorsal mesentery. The gut coiling in specific directions is associated with regulated molecular and cellular events.

Explanation of Solution

In a number of vertebrates, organs adopt asymmetric positions with respect to the midline, not much is known about the movement of the tissue and cellular changes which regulates the gene expression to produce this left-right asymmetry. The evidences based on the looping of the zebrafish gut suggests that this process results from the asymmetric migration of the neighboring lateral plate mesoderm (LPM). LPM autonomously provides a motive force for the gut displacement. The small domain of the gastrulating embryo in mammals appears to mediate the action of cilia present on the cells at the Node. This process triggers a cascade of molecular events which results in unilateral expression of the TGF-β-related ligand Nodal. In silico analysis suggests that left-right asymmetries is derived by synergistic changes in its epithelium and mesenchyme. Cells are more densely packed on the left side compare to right, in the mesenchymal compartment. Extracellular matrix (ECM) and cell: cell adhesion properties also vary, dorsal mesentery ECM follows left-right asymmetric condition when adhesion molecule N-cadherin is expressed exclusively on the left side. The asymmetric expression of two transcription factors Pitx2 and Isl1 is also crucial for these asymmetries.

Conclusion

Thus it is concluded that a number of molecular and cellular events are responsible for the gut looping, like lateral plate mesoderm (LPM), Extracellular matrix (ECM) and cell: cell adhesion properties and gene expression of transcription factors Pitx2 and Isl1, along with many other factors.