The posterior limit of the area pellucida (pPL) as a reliable proxy for the end of the primitive streak in chick elongation studies

Abstract

Early embryo elongation involves coordinated cellular and tissue behaviors that are readily observable in the chick embryo vertebrate model system. Easily identifiable morphological landmarks are crucial to obtain reliable morphometric data, particularly when assessing tissue elongation over time. The posterior end of the primitive streak marks the caudal end of the chick embryonic tissue. However, the identification of its precise location is ambiguous, especially to the untrained eye. Herein, we assessed if the posterior limit of the area pellucida (pPL), which is readily recognizable due to the optical contrast with the area opaca, is a valid proxy for the caudal limit of the primitive streak. Measurements of total embryo length were performed in multiple images of chick embryos over time using both caudal landmarks. We found that the pPL offered greater precision and a higher degree of inter-user reproducibility, when compared to the end of the primitive streak. Importantly, our work uncovers a quantitative proportionality between embryo length measurements using the end of the primitive streak and the pPL as caudal landmarks. We have thus validated the pPL as a reliable morphological proxy for the end of the primitive streak in chick embryo elongation studies.