Abstract
The early stages of development of the chick embryo, leading to primitive streak formation (the start of gastrulation), have received renewed attention recently, especially for studies of the mechanisms of large-scale cell movements and those that position the primitive streak in the radial blastodisc. Over the long history of chick embryology, the terminology used to define different regions has been changing, making it difficult to relate studies to each other. To resolve this objectively requires precise definitions of the regions based on anatomical and functional criteria, along with a systematic molecular map that can be compared directly to the functional anatomy. Here, we undertake these tasks. We describe the characteristic cell morphologies (using scanning electron microscopy and immunocytochemistry for cell polarity markers) in different regions and at successive stages. RNAseq was performed for 12 regions of the blastodisc, from which a set of putative regional markers was selected. These were studied in detail by in situ hybridization. Together this provides a comprehensive resource allowing the community to define the regions unambiguously and objectively. In addition to helping with future experimental design and interpretation, this resource will also be useful for evolutionary comparisons between different vertebrate species.
Highlights
Like most amniotes, the avian egg is fertilized internally within the mother’s reproductive system
The polarity of the blastodisc remains plastic until the appearance of the streak, which is most dramatically demonstrated by cutting the embryo into fragments: as long as each fragment contains a portion of the inner area pellucida (AP) and of the marginal zone (MZ), they can all give rise to a primitive streak and thereafter to a complete embryo [12,13,14]
The aspect ratio (AR) between the major and the minor axis of each epiblast cell was investigated by manual segmentation and the ratio represented as a pseudo-colour heat map
Summary
The avian egg is fertilized internally within the mother’s reproductive system. The early zygote spends about 20 h inside the mother’s reproductive system while initial meroblastic cell divisions take place, before being laid as a flat blastodisc, estimated to contain about 20 000 cells [1,2]. This blastodisc comprises a central area pellucida (AP) and a peripheral, extraembryonic area opaca (AO), separated by a thin ring, the marginal zone (MZ). The polarity of the blastodisc remains plastic until the appearance of the streak, which is most dramatically demonstrated by cutting the embryo into fragments: as long as each fragment contains a portion of the inner AP and of the MZ, they can all give rise to a primitive streak and thereafter to a complete embryo [12,13,14]
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