In Ovo Gain- and Loss-of-Function Approaches to Study Gut Morphogenesis

The polarity of cellular components is essential for cellular shape changes, oriented cell migration, and modulating intra- and intercellular mechanical forces. However, many aspects of polarized cell behavior-especially dynamic cell shape changes during the process of morphogenesis-are almost impossible to study in cells cultured in plastic dishes. Avian embryos have always been a treasured model system to study vertebrate morphogenesis for developmental biologists. Avian embryos recapitulate human biology particularly well in the early stages due to their flat disc gastruloids. Since avian embryos can be manipulated in ovo they present paramount opportunities for highly localized targeting of genetic mechanisms during cellular and developmental processes. Here, we review the application of these methods for both gain of function and loss of function of a gene of interest at a specific developmental stage during left-right (LR) asymmetric gut morphogenesis. These tools present a powerful premise to investigate various polarized cellular activities and molecular processes in vivo in a reproducible manner.

Avian Embryos as a Model to Study Vascular Development

The use of live imaging is indispensable for advancing our understanding of vascular morphogenesis. Imaging fixed embryos at a series of distinct developmental time points, although valuable, does not reveal the dynamic behavior of cells, as well as their interactions with the underlying ECM. Due to the easy access of chicken embryos to manipulation and high-resolution imaging, this model has been at the origin of key discoveries. In parallel, known through its extensive use in quail-chick chimera studies, the quail embryo is equally poised to genetic manipulations and paramount to direct imaging of transgenic reporter quails. Here we describe ex ovo time-lapse confocal microscopy of transgenic quail embryo slices to image vascular development during gut morphogenesis. This technique is powerful as it allows direct observation of the dynamic endothelial cell behaviors along the left-right (LR) axis of the dorsal mesentery (DM), the major conduit for blood and lymphatic vessels that serve the gut. In combination with in ovo plasmid electroporation and quail-chick transplantation, these methods have allowed us to study the molecular mechanisms underlying blood vessel assembly during the formation of the intestine. Below we describe our protocols for the generation of embryo slices, ex ovo time-lapse imaging of fluorescently labeled cells, and quail-chick chimeras to study the early stages of gut vascular development.

"A narrow bridge home": The dorsal mesentery in primordial germ cell migration

Specification of primordial germ cells (PGCs) in all vertebrates takes place in extragonadal sites. This requires migration of PGCs through embryonic tissues towards the genital ridges by both passive and active types of migration. Commonly, colonization in the genital ridges follows migration of the PGCs along the thin tissue of the dorsal mesentery. Here we review the anatomy of the dorsal mesentery, the role it plays in migration of PGCs, and the interactions of PGCs with different cell types, extracellular matrix and signaling pathways that are all essential for attraction and orientation of PGCs along the dorsal mesentery towards the gonad anlage.