The ectoderm is the outermost of the three germ layers of the early embryo that arise during gastrulation. Once the germ layers are established, the complex interplay of cellular proliferation, differentiation, and migration results in organogenesis. The ectoderm is the progenitor of both the surface ectoderm and the neural ectoderm. Notably, the surface ectoderm develops into the epidermis and its associated appendages, nails, external exocrine glands, olfactory epithelium, and the anterior pituitary. Specification, development, and homeostasis of these organs demand a tightly orchestrated gene expression program that is often dictated by epigenetic regulation. In this review, we discuss the recent discoveries that have highlighted the importance of chromatin regulatory mechanisms mediated by transcription factors, histone and DNA modifications that aid in the development of surface ectodermal organs and maintain their homeostasis post-development.

The neural tube, the embryonic precursor to the brain and spinal cord, begins as a flat sheet of epithelial cells, divided into non-neural and neural ectoderm. Proper neural tube closure requires that the edges of the neural ectoderm, the neural folds, to elevate upwards and fuse along the dorsal midline of the embryo. We have previously shown that members of the claudin protein family are required for the early phases of chick neural tube closure. Claudins are transmembrane proteins, localized in apical tight junctions within epithelial cells where they are essential for regulation of paracellular permeability, strongly involved in apical-basal polarity, cell-cell adhesion, and bridging the tight junction to cytoplasmic proteins. Here we explored the role of Claudin-3 (Cldn3), which is specifically expressed in the non-neural ectoderm. We discovered that depletion of Cldn3 causes folic acid-insensitive primarily spinal neural tube defects due to a failure in neural fold fusion. Apical cell surface morphology of Cldn3-depleted non-neural ectodermal cells exhibited increased membrane blebbing and smaller apical surfaces. Although apical-basal polarity was retained, we observed altered Par3 and Pals1 protein localization patterns within the apical domain of the non-neural ectodermal cells in Cldn3-depleted embryos. Furthermore, F-actin signal was reduced at apical junctions. Our data presents a model of spina bifida, and the role that Cldn3 is playing in regulating essential apical cell processes in the non-neural ectoderm required for neural fold fusion.

The single cell layer of surface ectoderm (SE) which overlies the closing neural tube (NT) plays a crucial biomechanical role during mammalian NT closure (NTC), challenging previous assumptions that it is only passive to the force-generating neuroepithelium (NE). Failure of NTC leads to congenital malformations known as NT defects (NTDs), including spina bifida (SB) and anencephaly in the spine and brain respectively. In several mouse NTD models, SB is caused by misexpression of SE-specific genes and is associated with disrupted SE mechanics, including loss of rostrocaudal cell elongation believed to be important for successful closure. In this study, we asked how SE mechanics affect NT morphology, and whether the characteristic rostrocaudal cell elongation at the progressing closure site is a response to tension anisotropy in the SE. We show that blocking SE-specific E-cadherin in ex utero mouse embryo culture influences NT morphology, as well as the F-actin cable. Cell border ablation shows that cell shape is not due to tension anisotropy, but that there are regional differences in SE tension. We also find that YAP nuclear translocation reflects regional tension heterogeneity, and that its expression is sensitive to pharmacological reduction of tension. In conclusion, our results confirm that the SE is a biomechanically important tissue for spinal NT morphogenesis and suggest a possible role of spatial regulation of cellular tension which could regulate downstream gene expression via mechanically-sensitive YAP activity.

During embryonic development, complex changes in cell behavior generate the final form of the tissues. Extension of cell protrusions have been described as an important component in this process. Cellular protrusions have been associated with generation of traction, intercellular communication or establishment of signaling gradients. Here, we describe and compare in detail from live imaging data the dynamics of protrusions in the surface ectoderm of chick and mouse embryos. In particular, we explore the differences between cells surrounding the lens placode and other regions of the head.
Our results showed that protrusions from the eye region in mouse embryos are longer than those in chick embryos. In addition, protrusions from regions where there are no significant changes in tissue shape are longer and more stable than protrusions that surround the invaginating lens placode. We did not find a clear directionality to the protrusions in any region. Finally, we observed intercellular trafficking of membrane puncta in the protrusions of both embryos in all the regions analyzed.
In summary, the results presented here suggest that the dynamics of these protrusions adapt to their surroundings and possibly contribute to intercellular communication in embryonic cephalic epithelia.

OBJECTIVE: To investigate how signals from lens regulate retinal vascular development and neovascularization.
METHODS: Le-Cre transgenic mouse line was employed to inactivate Smad4 in the surface ectoderm selectively. Standard histological and whole-mount retina staining were employed to reveal morphological changes of retinal vasculature in Smad4 defective eye. cDNA microarray and subsequent analyses were conducted to investigate the molecular mechanism underlying the vascular phenotype. Quantitative polymerase chain reaction (qPCR) was carried out to verify the microarrays results.
RESULTS: We found that inactivation of Smad4 specifically on surface ectoderm leads to a variety of retinal vasculature anomalies. Microarray analyses and qPCR revealed that Sema3c, Sema3e, Nrp1, Tie1, Sox7, Sox17, and Sox18 are significantly affected in the knockout retinas at different developmental stages, suggesting that ocular surface ectoderm-derived Smad4 can signal to the retina and regulates various angiogenic signaling in the retina.
CONCLUSIONS: Our data suggest that the cross-talk between ocular surface ectoderm and retina is important for retinal vasculature development, and Smad4 regulates various signaling associated with sprouting angiogenesis, vascular remodeling and maturation in the retina of mice.

During primary neurulation, the separation of a single-layered ectodermal sheet into the surface ectoderm (SE) and neural tube specifies SE and neural ectoderm (NE) cell fates. The mechanisms underlying fate specification in conjunction with neural tube closure are poorly understood. Here, by comparing expression profiles between SE and NE lineages, we observed that uncommitted progenitor cells, expressing stem cell markers, are present in the neural plate border/neural fold prior to neural tube closure. Our results also demonstrated that canonical Wnt and its antagonists, DKK1/KREMEN1, progressively specify these progenitors into SE or NE fates in accord with the progress of neural tube closure. Additionally, SE specification of the neural plate border via canonical Wnt signaling is directed by the grainyhead-like 3 (Grhl3) transcription factor. Thus, we propose that the fate specification of uncommitted progenitors in the neural plate border by canonical Wnt signaling and its downstream effector Grhl3 is crucial for neural tube closure. This study implicates that failure in critical genetic factors controlling fate specification of progenitor cells in the neural plate border/neural fold coordinated with neural tube closure may be potential causes of human neural tube defects.