Chapter 17, Problem 1DQ

Summary Introduction

To review:

The factor which triggers the early epigenetic modification in the chromatin of 3′ Hox genes which allows 3′ to 5′ loosening in chromatin states of Hox clusters over the course of paraxial mesoderm development. Whether additional regulators required to propel this transition to the 5′ end of the cluster. Also explain the mechanism that stablize the inheritance of epigenetic modification of Hox genes in a given segment.

Introduction:

The Hox genes are the important homeobox containing gene family which confer the positional information to the axial and paraxial tissues over the course of mesoderm development. These genes follow the sequential activation in time and space, and reflects their organization in clusters in the genome. The co-linearity of expression of Hox genes has been conserved during evolution, but not fully understood at molecular level. The somites resemble each other, but leads to the formation of different dtructures along the rostral to caudal axis.

Explanation of Solution

The anterior-posterior patterning of the somites is determines by the regulated expression of Hox genes. In more anterior regions of the paraxial mesoderm, the Hox genes present in the 3′ region of the genome cluster are likely to express. Whereas the genes present in the 5′ region likely to express in the posterior region. Any alteration in this Hox gene expression can leads to a change in specification of the mesoderm. The initiation of Hox gene expression is proposed by a progress zone model which explains the progressive activation of 3′ to 5′ Hox genes. This initial Hox transcription and the early rostral expansion of Hox expression domains depends upon the events which are related to the emergence and extension of the primitive streak. Wnt signaling may involve in the regulation of Hox gene expression during its anteriorward spreading. Fgf signalling also modulate the morphogenetic movement of the mesoderm, so also play an important role in the Hox gene expression. Retinoic acid (RA) also play a role in initiating Hox gene expression as RA is detected endogenously in the posterior part of early post-implantation embryos. Thus a number of signalling molecules progressively modulate Hox gene expression and also stabilize the epigenetic modification of Hox genes.

Conclusion

Thus it is concluded that the anterior-posterior patterning of the somites is determines by the regulated expression of Hox genes. Wnt signaling may involve in the regulation of Hox gene expression during its anteriorward spreading. Fgf signalling also modulate the morphogenetic movement of the mesoderm, Retinoic acid (RA) also play an important role in the Hox gene expression.