The mechano-chemical circuit drives skin organoid self-organization.

Stem cells in organoids self-organize into tissue patterns with unknown mechanisms. Here, we use skin organoids to analyze this process. Cell behavior videos show that the morphological transformation from multiple spheroidal units with morphogenesis competence (CMU) to planar skin is characterized by two abrupt cell motility-increasing events before calming down. The self-organizing processes are controlled by a morphogenetic module composed of molecular sensors, modulators, and executers. Increasing dermal stiffness provides the initial driving force (driver) which activates Yap1 (sensor) in epidermal cysts. Notch signaling (modulator 1) in epidermal cyst tunes the threshold of Yap1 activation. Activated Yap1 induces Wnts and MMPs (epidermal executers) in basal cells to facilitate cellular flows, allowing epidermal cells to protrude out from the CMU. Dermal cell-expressed Rock (dermal executer) generates a stiff force bridge between two CMU and accelerates tissue mixing via activating Laminin and ß1-integrin. Thus, this self-organizing coalescence process is controlled by a mechano-chemical circuit. Beyond skin, self-organization in organoids may use similar mechano-chemical circuit structures.

Instructive role of melanocytes during pigment pattern formation of the avian skin.

Animal skin pigment patterns are excellent models to study the mechanism of biological self-organization. Theoretical approaches developed mathematical models of pigment patterning and molecular genetics have brought progress; however, the responsible cellular mechanism is not fully understood. One long unsolved controversy is whether the patterning information is autonomously determined by melanocytes or nonautonomously determined from the environment. Here, we transplanted purified melanocytes and demonstrated that melanocytes could form periodic pigment patterns cell autonomously. Results of heterospecific transplantation among quail strains are consistent with this finding. Further, we observe that developing melanocytes directly connect with each other via filopodia to form a network in culture and in vivo. This melanocyte network is reminiscent of zebrafish pigment cell networks, where connexin is implicated in stripe formation via genetic studies. Indeed, we found connexin40 (cx40) present on developing melanocytes in birds. Stripe patterns can form in quail skin explant cultures. Several calcium channel modulators can enhance or suppress pigmentation globally, but a gap junction inhibitor can change stripe patterning. Most interestingly, in ovo, misexpression of dominant negative cx40 expands the black region, while overexpression of cx40 expands the yellow region. Subsequently, melanocytes instruct adjacent dermal cells to express agouti signaling protein (ASIP), the regulatory factor for pigment switching, which promotes pheomelanin production. Thus, we demonstrate Japanese quail melanocytes have an autonomous periodic patterning role during body pigment stripe formation. We also show dermal agouti stripes and how the coupling of melanocytes with dermal cells may confer stable and distinct pigment stripe patterns.

Self-organization process in newborn skin organoid formation inspires strategy to restore hair regeneration of adult cells.

Organoids made from dissociated progenitor cells undergo tissue-like organization. This in vitro self-organization process is not identical to embryonic organ formation, but it achieves a similar phenotype in vivo. This implies genetic codes do not specify morphology directly; instead, complex tissue architectures may be achieved through several intermediate layers of cross talk between genetic information and biophysical processes. Here we use newborn and adult skin organoids for analyses. Dissociated cells from newborn mouse skin form hair primordia-bearing organoids that grow hairs robustly in vivo after transplantation to nude mice. Detailed time-lapse imaging of 3D cultures revealed unexpected morphological transitions between six distinct phases: dissociated cells, cell aggregates, polarized cysts, cyst coalescence, planar skin, and hair-bearing skin. Transcriptome profiling reveals the sequential expression of adhesion molecules, growth factors, Wnts, and matrix metalloproteinases (MMPs). Functional perturbations at different times discern their roles in regulating the switch from one phase to another. In contrast, adult cells form small aggregates, but then development stalls in vitro. Comparative transcriptome analyses suggest suppressing epidermal differentiation in adult cells is critical. These results inspire a strategy that can restore morphological transitions and rescue the hair-forming ability of adult organoids: (i) continuous PKC inhibition and (ii) timely supply of growth factors (IGF, VEGF), Wnts, and MMPs. This comprehensive study demonstrates that alternating molecular events and physical processes are in action during organoid morphogenesis and that the self-organizing processes can be restored via environmental reprogramming. This tissue-level phase transition could drive self-organization behavior in organoid morphogenies beyond the skin.

Mapping stem cell activities in the feather follicle.

It is important to know how different organs 'manage' their stem cells. Both hair and feather follicles show robust regenerative powers that episodically renew the epithelial organ. However, the evolution of feathers (from reptiles to birds) and hairs (from reptiles to mammals) are independent events and their follicular structures result from convergent evolution. Because feathers do not have the anatomical equivalent of a hair follicle bulge, we are interested in determining where their stem cells are localized. By applying long-term label retention, transplantation and DiI tracing to map stem cell activities, here we show that feather follicles contain slow-cycling long-term label-retaining cells (LRCs), transient amplifying cells and differentiating keratinocytes. Each population, located in anatomically distinct regions, undergoes dynamic homeostasis during the feather cycle. In the growing follicle, LRCs are enriched in a 'collar bulge' niche. In the moulting follicle, LRCs shift to populate a papillar ectoderm niche near the dermal papilla. On transplantation, LRCs show multipotentiality. In a three-dimensional view, LRCs are configured as a ring that is horizontally placed in radially symmetric feathers but tilted in bilaterally symmetric feathers. The changing topology of stem cell activities may contribute to the construction of complex feather forms.

Global feather orientations changed by electric current.

During chicken skin development, each feather bud exhibits its own polarity, but a population of buds organizes with a collective global orientation. We used embryonic dorsal skin, with buds aligned parallel to the rostral-caudal body axis, to explore whether exogenous electric fields affect feather polarity. Interestingly, brief exogenous current exposure prior to visible bud formation later altered bud orientations. Applying electric pulses perpendicular to the body rostral-caudal axis realigned bud growth in a collective swirl, resembling an electric field pointing toward the anode. Perturbed buds show normal molecular expression and morphogenesis except for their altered orientation. Epithelial-mesenchymal recombination demonstrates the effects of exogenous electric fields are mediated through the epithelium. Small-molecule channel inhibitor screens show Ca2+ channels and PI3 Kinase are involved in controlling feather bud polarity. This work reveals the importance of bioelectricity in organ development and regeneration and provides an explant culture platform for experimentation.

The river of stem cells.

In this issue of Cell Stem Cell, Greco et al. (2009) characterize the hair germ as a novel stop between bulge stem cell and transient amplifying cells during hair regeneration. The work implies stem cell states can be regulated to form different numbers of intermediate stops, depending on physiological requirements.

Wnt3a gradient converts radial to bilateral feather symmetry via topological arrangement of epithelia.

The evolution of bilaterally symmetric feathers is a fundamental process leading toward flight. One major unsolved mystery is how the feathers of a single bird can form radially symmetric downy feathers and bilaterally symmetric flight feathers. In developing downy feather follicles, barb ridges are organized parallel to the long axis of the feather follicle. In developing flight-feather follicles, the barb ridges are organized helically toward the anterior region, leading to the fusion and creation of a rachis. Here we discover an anterior-posterior molecular gradient of wingless int (Wnt3)a in flight but not downy feathers. Global inhibition of the Wnt gradient transforms bilaterally symmetric feathers into radially symmetric feathers. Production of an ectopic local Wnt3a gradient reoriented barb ridges toward the source and created an ectopic rachis. We further show that the orientation of the Wnt3a gradient is dictated by the dermal papilla (DP). Swapping DPs between wing covert and breast downy feathers demonstrates that both feather symmetry and molecular gradients are in accord with the origin of the DP. Thus the fates of feather epidermal cells are not predetermined through some molecular codes but can be modulated. Together, our data suggest feathers are shaped by a DP--> Wnt gradient-->helical barb ridge organization-->creation of rachis-->bilateral symmetry sequence. We speculate diverse feather forms can be achieved by adjusting the orientation and slope of molecular gradients, which then shape the topological arrangements of feather epithelia, thus linking molecular activities to organ forms and novel functions.

Morphoregulation of avian beaks: comparative mapping of growth zone activities and morphological evolution.

Avian beak diversity is a classic example of morphological evolution. Recently, we showed that localized cell proliferation mediated by bone morphogenetic protein 4 (BMP4) can explain the different shapes of chicken and duck beaks (Wu et al. [2004] Science 305:1465). Here, we compare further growth activities among chicken (conical and slightly curved), duck (straight and long), and cockatiel (highly curved) developing beak primordia. We found differential growth activities among different facial prominences and within one prominence. The duck has a wider frontal nasal mass (FNM), and more sustained fibroblast growth factor 8 activity. The cockatiel has a thicker FNM that grows more vertically and a relatively reduced mandibular prominence. In each prominence the number, size, and position of localized growth zones can vary: it is positioned more rostrally in the duck and more posteriorly in the cockatiel FNM, correlating with beak curvature. BMP4 is enriched in these localized growth zones. When BMP activity is experimentally altered in all prominences, beak size was enlarged or reduced proportionally. When only specific prominences were altered, the prototypic conical shaped chicken beaks were converted into an array of beak shapes mimicking those in nature. These results suggest that the size of beaks can be modulated by the overall activity of the BMP pathway, which mediates the growth. The shape of the beaks can be fine-tuned by localized BMP activity, which mediates the range, level, and duration of locally enhanced growth. Implications of topobiology vs. molecular blueprint concepts in the Evo-Devo of avian beak forms are discussed.

Molecular shaping of the beak.

Beak shape is a classic example of evolutionary diversification. Beak development in chicken and duck was used to examine morphological variations among avian species. There is only one proliferative zone in the frontonasal mass of chickens, but two in ducks. These growth zones are associated with bone morphogenetic protein 4 (BMP4) activity. By "tinkering" with BMP4 in beak prominences, the shapes of the chicken beak can be modulated.