A morphometric characterization of early CHICK embryo elongation

The chicken embryo has long been a pivotal model system to understand the cellular and molecular mechanisms driving amniote embryo development. Its easy access for in vivo experimentation, together with the development of ex ovo culture techniques, has made it a choice model system for elaborate experimental manipulations. Temporal progression of chick embryo development is classically categorized using the Hamburger and Hamilton staging system (Hamburger, V., & Hamilton, 1951). However, this offers limited temporal resolution when comparing embryos within the same developmental stage and may further be hindered by experimental conditions that directly impact the morphological structures used for stage identification. Here, we performed timelapse imaging of early chick embryonic stages HH4 to HH10 and obtained quantitative elongation data of multiple embryonic portions, yielding two valuable and freely accessible data resources for the chick research community. We identified length measurements capable of describing developmental time, thus enabling the alignment of independent embryos with temporal resolution. Notably, the head-fold (C-HF) showed a strong time correlation, even though it elongates above the primary embryonic axis. A morphometric characterization of HH stages further showed that C-HF length can discriminate HH stages of development, albeit with limited resolution. Finally, we present ChEEQ: Chicken Embryo Elongation Quantification (https://colab.research.google.co m/github/EmbryoClock/ChickElong/blob/main/ChEEQ/ChEEQ.ipynb), a new morphometric tool describing HH4-HH10 embryo elongation, that allows the comparison of user-input data with our reference dataset and is capable of inferring quantitative alterations to embryo developmental time using length measurements alone. Together, these resources open new avenues for investigating vertebrate embryo elongation and quantitatively assessing the effects of experimental interventions on development.