Stem cells are crucial players during development, homeostasis and tissue regeneration and their interactions with the surrounding microenvironment are key to regulate stem cell fate. The skull's stem cell niches reside in the fibrous joints that connect flat bones of the skull. In the embryo, bone and sutures develop in concert to form a complex, multi-facted structure that requires interaction with multiple differentiating cell types to maintain balance between growth and differentiation. Disruption of this balance drives changes in size and shape of skull bones and can severely impact quality of life. Cranial sutures, often seen as simple extracellular matrix-rich structures bridging the rigid plates of the skull, are major actors in craniofacial morphogenesis of as they harmonize bone growth with expansion of the developing brain and participate in providing osteoblasts during repair. The complexity of the extracellular environment and the important role for sutures in skeletal development makes these niches a compelling structure to investigate how interactions with the surrounding microenvironment can modulate stem cells fate. The key role of sutures in development is highlighted by the numerous severe dysmorphisms arising from failure to maintain suture patency. The ability of the suture to respond to brain growth or trauma and the dysmophisms presented by patients with defective sutures is mediated by both biochemical and mechanical cues but the cell biology of these niches remains elusive, especially during their development. In particular, few studies have shed light on the underlying cellular behaviors behind microenvironmental regulation of cranial suture stem cell fate and what role mechanical inputs play in the establishment of this niche. In my thesis, I addressed gaps in our understanding of suture biology by characterizing the suture stem cell niche microenvironment and exploring how cell-ECM interactions serve as regulators of suture stem cell fate. Making use of various microscopy and analytical techniques I first characterized the composition of the microenvironment in a developing suture niche, such as organization of ECM, cytoskeleton and nuclear morphologies. My work builds on an incomplete transcriptional understanding of suture cell development, such that specific genetic markers are rarely useful for identifying distinct suture cell populations during its morphogenesis. By applying shape description tools to parse suture cells and test whether shape correlates to cell identity, we concluded that suture nuclei are distinct and less spherical than those of other cranial tissues. Using 'global' markers such as nuclear stains, I have also identified physical distinctions between suture nuclei and neighboring tissues, indicating that cell shape is an integral part of midline suture identity and can be used to explore coordination of fate choice and morphogenesis in this enigmatic structure. In addition, I present evidence that supports that maturation of extracellular matrix begins during early stages of suture development. In particular, embryonic midline sutures express high levels of fibrillary collagen, which contributes to the formation of a complex extracellular environment that provides the suture with physical properties distinct from those of developing bones. My work shows the presence of cell-ECM and cell-cell adhesions in the developing midline sutures, as well as a complex actin cytoskeleton that is, in part, mediated by physical stresses resultant from underlying brain expansion. Secondly, I aimed to address how perturbations in ECM composition can affect cell specification. To investigate the importance of ECM maturation in regulating suture cell fate I inhibited the function of lysyl oxidase, a collagen crosslinker, during embryonic development. Disruption of collagen crosslinking altered expression of collagen and ECM receptor encoding genes. In addition, this inhibition induced changes in the shape and size of collagen fibers in the embryonic midline suture and decreased tissue bulk stiffness relative to WT. These abnormal properties of the ECM impact tissue delineation in the cranial mesenchyme through nuclear shape analyses. This might be explained by observed changes in the composition of the nuclear envelop of suture cells as we find altered lamin concentration and localization upon lysyl oxidase inhibition. The work developed during myPhD steps away from the traditional genetic approaches used to study the embryonic suture and provides the first in-depth analysis of the physical properties of the developing midline suture at stages preceding known establishment of the niche. The various methods and analyses applied reveal a complex organization of embryonic suture ECM and its tight relationship with shape and fate in this tissue. This work serves as a foundation for future studies that can explore the mechanisms through which ECM regulates fate and development of the suture niche, and potentially skeletal development more generally
