Watching the assembly of an organ a single cell at a time using confocal multi-position photoactivation and multi-time acquisition

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.

Molecular profiling of the developing avian telencephalon: regional timing and brain subdivision continuities

In our companion study (Jarvis et al. [2013] J Comp Neurol. doi: 10.1002/cne.23404) we used quantitative brain molecular profiling to discover that distinct subdivisions in the avian pallium above and below the ventricle and the associated mesopallium lamina have similar molecular profiles, leading to a hypothesis that they may form as continuous subdivisions around the lateral ventricle. To explore this hypothesis, here we profiled the expression of 16 genes at eight developmental stages. The genes included those that define brain subdivisions in the adult and some that are also involved in brain development. We found that phyletic hierarchical cluster and linear regression network analyses of gene expression profiles implicated single and mixed ancestry of these brain regions at early embryonic stages. Most gene expression-defined pallial subdivisions began as one ventral or dorsal domain that later formed specific folds around the lateral ventricle. Subsequently a clear ventricle boundary formed, partitioning them into dorsal and ventral pallial subdivisions surrounding the mesopallium lamina. These subdivisions each included two parts of the mesopallium, the nidopallium and hyperpallium, and the arcopallium and hippocampus, respectively. Each subdivision expression profile had a different temporal order of appearance, similar in timing to the order of analogous cell types of the mammalian cortex. Furthermore, like the mammalian pallium, expression in the ventral pallial subdivisions became distinct during prehatch development, whereas the dorsal portions did so during posthatch development. These findings support the continuum hypothesis of avian brain subdivision development around the ventricle and influence hypotheses on homologies of the avian pallium with other vertebrates.

Developmental imaging: the avian embryo hatches to the challenge

The avian embryo provides a multifaceted model to study developmental mechanisms because of its accessibility to microsurgery, fluorescence cell labeling, in vivo imaging, and molecular manipulation. Early two-dimensional planar growth of the avian embryo mimics human development and provides unique access to complex cell migration patterns using light microscopy. Later developmental events continue to permit access to both light and other imaging modalities, making the avian embryo an excellent model for developmental imaging. For example, significant insights into cell and tissue behaviors within the primitive streak, craniofacial region, and cardiovascular and peripheral nervous systems have come from avian embryo studies. In this review, we provide an update to recent advances in embryo and tissue slice culture and imaging, fluorescence cell labeling, and gene profiling. We focus on how technical advances in the chick and quail provide a clearer understanding of how embryonic cell dynamics are beautifully choreographed in space and time to sculpt cells into functioning structures. We summarize how these technical advances help us to better understand basic developmental mechanisms that may lead to clinical research into human birth defects and tissue repair.

From seeing to believing: labelling strategies for in vivo cell-tracking experiments

Intravital microscopy has become increasingly popular over the past few decades because it provides high-resolution and real-time information about complex biological processes. Technological advances that allow deeper penetration in live tissues, such as the development of confocal and two-photon microscopy, together with the generation of ever-new fluorophores that facilitate bright labelling of cells and tissue components have made imaging of vertebrate model organisms efficient and highly informative. Genetic manipulation leading to expression of fluorescent proteins is undoubtedly the labelling method of choice and has been used to visualize several cell types in vivo. This approach, however, can be technically challenging and time consuming. Over the years, several dyes have been developed to allow rapid, effective and bright ex vivo labelling of cells for subsequent transplantation and imaging. Here, we review and discuss the advantages and limitations of a number of strategies commonly used to label and track cells at high resolution in vivo in mouse and zebrafish, using fluorescence microscopy. While the quest for the perfect label is far from achieved, current reagents are valuable tools enabling the progress of biological discovery, so long as they are selected and used appropriately.

Transgenesis and imaging in birds, and available transgenic reporter lines

Avian embryos are important model organism to study higher vertebrate development. Easy accessibility to developing avian embryos enables a variety of experimental applications to understand specific functions of molecules, tissue-tissue interactions, and cell lineages. The whole-mount ex ovo culture technique for avian embryos permits time-lapse imaging analysis for a better understanding of cell behaviors underlying tissue morphogenesis in physiological conditions. To study mechanisms of blood vessel formation and remodeling in developing embryos by using a time-lapse imaging approach, a transgenic quail model, Tg(tie1:H2B-eYFP), was generated. From a cell behavior perspective, Tg(tie1:H2B-eYFP) quail embryos are a suitable model to shed light on how the structure and pattern of blood vessels are established in higher vertebrates. In this manuscript, we give an overview on the biological and technological background of the transgenic quail model and describe procedures for the ex ovo culture of quail embryos and time-lapse imaging analysis.

Evidence for dynamic rearrangements but lack of fate or position restrictions in premigratory avian trunk neural crest

Neural crest (NC) cells emerge from the dorsal trunk neural tube (NT) and migrate ventrally to colonize neuronal derivatives, as well as dorsolaterally to form melanocytes. Here, we test whether different dorsoventral levels in the NT have similar or differential ability to contribute to NC cells and their derivatives. To this end, we precisely labeled NT precursors at specific dorsoventral levels of the chick NT using fluorescent dyes and a photoconvertible fluorescent protein. NT and NC cell dynamics were then examined in vivo and in slice culture using two-photon and confocal time-lapse imaging. The results show that NC precursors undergo dynamic rearrangements within the neuroepithelium, yielding an overall ventral to dorsal movement toward the midline of the NT, where they exit in a stochastic manner to populate multiple derivatives. No differences were noted in the ability of precursors from different dorsoventral levels of the NT to contribute to NC derivatives, with the exception of sympathetic ganglia, which appeared to be 'filled' by the first population to emigrate. Rather than restricted developmental potential, however, this is probably due to a matter of timing.

Photomodulatable fluorescent proteins for imaging cell dynamics and cell fate

An organism arises from the coordinate generation of different cell types and the stereotypical organization of these cells into tissues and organs. Even so, the dynamic behaviors, as well as the ultimate fates, of cells driving the morphogenesis of an organism, or even an individual organ, remain largely unknown. Continued innovations in optical imaging modalities, along with the discovery and evolution of improved genetically-encoded fluorescent protein reporters in combination with model organism, stem cell and tissue engineering paradigms are providing the means to investigate these unresolved questions. The emergence of fluorescent proteins whose spectral properties can be photomodulated is one of the most significant new developments in the field of cell biology where they are primarily used for studying protein dynamics in cells. Likewise, the use of photomodulatable fluorescent proteins holds great promise for use in developmental biology. Photomodulatable fluorescent proteins also represent attractive and emergent tools for studying cell dynamics in complex populations by facilitating the labeling and tracking of individual or defined groups of cells. Here, we review the currently available photomodulatable fluorescent proteins and their application in model organisms. We also discuss prospects for their use in mice, and by extension in embryonic stem cell and tissue engineering paradigms.

Targeted deletion of Hand2 in enteric neural precursor cells affects its functions in neurogenesis, neurotransmitter specification and gangliogenesis, causing functional aganglionosis

Targeted deletion of the bHLH DNA-binding protein Hand2 in the neural crest, impacts development of the enteric nervous system (ENS), possibly by regulating the transition from neural precursor cell to neuron. We tested this hypothesis by targeting Hand2 deletion in nestin-expressing neural precursor (NEP) cells. The mutant mice showed abnormal ENS development, resulting in lethal neurogenic pseudo-obstruction. Neurogenesis of neurons derived from NEP cells identified a second nestin non-expressing neural precursor (NNEP) cell in the ENS. There was substantial compensation for the loss of neurons derived from the NEP pool by the NNEP pool but this was insufficient to abrogate the negative impact of Hand2 deletion. Hand2-mediated regulation of proliferation affected both neural precursor and neuron numbers. Differentiation of glial cells derived from the NEP cells was significantly decreased with no compensation from the NNEP pool of cells. Our data indicate differential developmental potential of NEPs and NNEPs; NNEPs preferentially differentiate as neurons, whereas NEPs give rise to both neurons and glial cells. Deletion of Hand2 also resulted in complete loss of NOS and VIP and a significant decrease in expression of choline acetyltransferase and calretinin, demonstrating a role for Hand2 in neurotransmitter specification and/or expression. Loss of Hand2 resulted in a marked disruption of the developing neural network, exemplified by lack of a myenteric plexus and extensive overgrowth of fibers. Thus, Hand2 is essential for neurogenesis, neurotransmitter specification and neural network patterning in the developing ENS.

Optical highlighter molecules in neurobiology

The development of advanced optical methods has played a key role in propelling progress in neurobiology. Genetically-encoded fluorescent molecules found in nature have enabled labeling of individual neurons to study their physiology and anatomy. Here we discuss the recent use of both native and synthetic optical highlighter proteins to address key problems in neurobiology, including questions relevant to synaptic function, neuroanatomy, and the organization of neural circuits.

Dynamic lineage analysis of embryonic morphogenesis using transgenic quail and 4D multispectral imaging

We describe the development of transgenic quail that express various fluorescent proteins in targeted manners and their use as a model system that integrates advanced imaging approaches with conventional and emerging molecular genetics technologies. We also review the progression and complications of past fate mapping techniques that led us to generate transgenic quail, which permit dynamic imaging of amniote embryogenesis with unprecedented subcellular resolution.

Quantifying subcellular distribution of fluorescent fusion proteins in cells migrating within tissues

The movement of proteins within cells can provide dynamic indications of cell signaling and cell polarity, but methods are needed to track and quantify subcellular protein movement within tissue environments. Here we present a semiautomated approach to quantify subcellular protein location for hundreds of migrating cells within intact living tissue using retrovirally expressed fluorescent fusion proteins and time-lapse two-photon microscopy of intact thymic lobes. We have validated the method using GFP-PKCζ, a marker for cell polarity, and LAT-GFP, a marker for T-cell receptor signaling, and have related the asymmetric distribution of these proteins to the direction and speed of cell migration. These approaches could be readily adapted to other fluorescent fusion proteins, tissues and biological questions.