Question

4. Somites. What is the relationship between the determination front and cycling levels of Notch activity in the formation of somites?This is the question that was given.

PLEASE EXPLAIN IN LEAST TWO PARAGRAPHS

Answer 1

The formation of a segmented body plan is a conserved feature of embryogenesis for all vertebrate species. This process leads to the formation of transient embryonic segments, called somites. Somites are precursors of vertebrae and ribs, associated skeletal muscles, and some dermis .

The Notch signalling pathway plays a role in atleast two aspects of somitogenesis.

1. The oscillating pattern of gene expression known as the segmental clock, and

2. The establishment of anterior-posterior polarity of somites.

Somites develop sequentially beginning in the anterior of the embryo and progress towards the extending tail bud. Some genes that help form the boundaries between developing somites also control the development of the somites themselves . These genes turn on and off in a wave-like pattern, with each wave corresponding to the formation of a new somite. The timing of these oscillations is critical for proper somite formation and segmentation of the embryo, and is called the segmental clock.

The second role of Notch signalling is to establish anterior posterior polarity of somites through interactions with proteins that belong to the fibroblast growth factor (FGF) signaling pathway. In contrast to Notch, FGF proteins secreted from cells form a concentration gradient along the embryo with the highest levels in the extending tail bud. The Notch signal starts in the tail bud and moves through the presomitic mesoderm until it reaches a region where there is little FGF protein. At this position in the embryo, the Notch signal ceases to oscillate, which causes some cells to form a somite. The position where the Notch signal terminates marks the front, or anterior half of the new somite, and initiates the expression of genes that form the boundary between somites. When the anterior boundary of the somite forms, different genes begin to make their products that will define the anterior or posterior halves of the somite. Thus, not only do somites distinguish themselves from neighboring somites in what tissues they are the precursors of, but the poles within individual somites also form distinct tissues.