Uncoupling segmentation and somitogenesis in the chick presomitic mesoderm

Area of Science:

• Developmental Biology
• Molecular Biology
• Embryology

Background:

• The molecular mechanisms driving somite subdivision into rostrocaudal compartments are largely unknown.
• While rostrocaudal identity is acquired at the presomitic mesoderm (PSM) level, it's unclear if this is fully determined in the PSM or requires environmental cues.
• Avian homologues of mouse Delta-like1 (C-Delta-1) and Notch1 (C-Notch-1) are implicated in somite rostrocaudal polarity specification.

Purpose of the Study:

• To investigate whether somite rostrocaudal patterning is determined autonomously within the PSM or relies on extrinsic maintenance cues.
• To examine the expression profiles and regulation of C-Delta-1 and C-Notch-1 in the developing chick embryo.
• To determine if segmentation is intrinsically linked to somite epithelialization.

Main Methods:

• In vitro culture of PSM explants from chick embryos.
• Analysis of gene expression patterns for C-Delta-1 and C-Notch-1.
• Comparison of gene expression in PSM explants with and without overlying ectoderm.

Main Results:

• C-Delta-1 and C-Notch-1 are expressed in distinct, overlapping domains within the PSM and caudal somites.
• PSM explants cultured without ectoderm maintained C-Delta-1 expression stripes, indicating autonomous patterning.
• Epithelialization was blocked in explants lacking ectoderm, but rostrocaudal patterning persisted.

Conclusions:

• Somite rostrocaudal patterning is an autonomous process intrinsic to the PSM.
• Segmentation can occur independently of complete somite formation and epithelialization.
• The PSM possesses intrinsic mechanisms for establishing rostrocaudal polarity.