Ting-Xin Jiang

‘Cyclic alopecia’ in Msx2 mutants: defects in hair cycling and hair shaft differentiation

Msx2-deficient mice exhibit progressive hair loss, starting at P14 and followed by successive cycles of wavelike regrowth and loss. During the hair cycle, Msx2 deficiency shortens anagen phase, but prolongs catagen and telogen. Msx2-deficient hair shafts are structurally abnormal. Molecular analyses suggest a Bmp4/Bmp2/Msx2/Foxn1 acidic hair keratin pathway is involved. These structurally abnormal hairs are easily dislodged in catagen implying a precocious exogen. Deficiency in Msx2 helps to reveal the distinctive skin domains on the same mouse. Each domain cycles asynchronously — although hairs within each skin domain cycle in synchronized waves. Thus, the combinatorial defects in hair cycling and differentiation, together with concealed skin domains, account for the cyclic alopecia phenotype.

Evo-Devo of feathers and scales: building complex epithelial appendages

The vertebrate body is covered by either scales, feathers or fur to provide warmth and protection. Comparing and contrasting the formation of these different integument appendages may provide insights into their common embryonic origin as well as evolutionary divergence. The reptile integument is mainly made of scales [1]. In birds, there are two major integument appendages: scales on the foot and feathers on most of the rest of the body [2••]. Scales provide protection and prevent water loss. The major innovation of the avian integument was the evolution of feathers, which provide novel functions such as insulation, display (communication), and flight. Chickens have three major types of scales, which are morphologically similar to reptile scales (Figure 1a,b [1,3]). Reticulate scales are found on the foot pad: they are radially symmetric and express α-keratin only. Scutate scales are large and rectangular and are the major type found on the anterior meta-tarsal shank and dorsal part of the toes. Scutella scales are distributed lateral to the scutate scales and are smaller in size but are also rectangular. Both scutate and scutella scales have anterior–posterior polarity, with an outer surface composed of β-keratin and an inner surface and a hinge region composed of α-keratin. Cell proliferation is distributed diffusely in scales [4•] without a localized growth zone (e.g. hair matrix or feather collar), dermal papillae, or follicular structures. Figure 1 Morphology of scales and feathers: (a) reptile scales; (b) avian foot scales; and (c) avian feathers. Avian reticulate scales are similar in shape to reptile tuberculate scales. Avian scutate and scutella scales are similar in shape to reptile overlapping ... Feathers are arranged in specific tracts over the body which are divided by apteric zones (regions without feathers [2••]). The base of each feather follicle contains protected tissues, permitting the epithelial–mesenchymal interactions (epidermal collar and dermal papillae) that provide a source for continuous feather elongation and molting. Epithelial and dermal sheaths lie along the exterior part of the feather, whereas pulp is found within the epithelial cylinder during development. A typical feather is composed of a rachis (primary shaft), barbs (secondary branches), and barbules (tertiary branches; Figure 1c). The variation in feather size, shape and texture is complex. With regard to size, feathers of the same bird are of different length and diameter, and often distributed in a gradient. For shape, types range from down feathers that are mainly radially symmetric (the rachis is either absent or very short) and contour feathers the symmetry of which is mainly bilateral. Flight feathers are bilaterally asymmetric (Figure 1c). For texture, feathers can either be fluffy or form a firm vane. The barbules can be bilaterally symmetric to each other and therefore fluffy (plumulaceous), or the distal barbule can form a hooklet enabling it to interweave with the proximal barbule of the next barb in a ‘velcro-like’ mechanism (pennaceous). The calamus is the region of a shaft without barbs. A feather can have different ratios of these structures, thus providing an enormous number of permutations of structural and functional variations [2••,5]. The feather is the most complex vertebrate integument appendage ever evolved. How is a flat piece of epidermis transformed into a three level branched structure? Here we present ten complexity levels of integument appendages that correspond to developmental stages of chicken skin and feather precursors recently identified in dinosaur/primitive bird fossils. Cellular and molecular events that convert one complexity level to the next are discussed, including those converting avian foot scales to feathers.

Morpho-Regulation of Ectodermal Organs : Integument Pathology and Phenotypic Variations in K14-Noggin Engineered Mice through Modulation of Bone Morphogenic Protein Pathway

Ectodermal organs are composed of keratinocytes organized in different ways during induction, morphogenesis, differentiation, and regenerative stages. We hypothesize that an imbalance of fundamental signaling pathways should affect multiple ectodermal organs in a spatio-temporal-dependent manner. We produced a K14-Noggin transgenic mouse to modulate bone morphogenic protein (BMP) activity and test the extent of this hypothesis. We observed thickened skin epidermis, increased hair density, altered hair types, faster anagen re-entry, and formation of compound vibrissa follicles. The eyelid opening was smaller and ectopic cilia formed at the expense of Meibomian glands. In the distal limb, there were agenesis and hyperpigmentation of claws, interdigital webbing, reduced footpads, and trans-differentiation of sweat glands into hairs. The size of external genitalia increased in both sexes, but they remained fertile. We conclude that modulation of BMP activity can affect the number of ectodermal organs by acting during induction stages, influence the size and shape by acting during morphogenesis stages, change phenotypes by acting during differentiation stages, and facilitate new growth by acting during regeneration stages. Therefore during organogenesis, BMP antagonists can produce a spectrum of phenotypes in a stage-dependent manner by adjusting the level of BMP activity. The distinction between phenotypic variations and pathological changes is discussed.

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.

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. Developmental Dynamics 235:1400–1412, 2006.

Successive formative stages of precartilaginous mesenchymal condensations in vitro: modulation of cell adhesion by Wnt-7A and BMP-2.

High‐density chick limb bud cell culture is a useful model to study mesenchymal condensatifons and chondrogenesis. Most previous studies have focused on the effects of soluble reagents on terminal chondrogenic differentiation and have not defined the early cellular processes and signaling events. In this study, we defined five successive stages in the differentiation process: 1) dissociated cells, 2) small aggregates, 3) formation of cell clusters, 4) precartilaginous condensations, and 5) cartilage nodule. We used RCAS retrovirus‐mediated Wnt‐7a gene transduction to test the effect of Wnt‐7a on the differentiation process. We found that Wnt‐7a suppressed chondrogenic differentiation. Wnt‐7a did not inhibit the initiation of condensation formation but blocked the progression of precartilaginous condensations to cartilage nodules. The Wnt‐7a‐transduced cultures showed characteristics of a less mature culture with persistent expression of NCAM, N‐cadherin, wider distribution of integrin β1 and fibronectin, and suppression of tenascin‐C. BMP‐2 is known to enhance chondrogenic differentiation in these cultures by promoting cell clusters to form continuous sheet‐like precartilaginous condensations. However, cultures exposed to both BMP‐2 and Wnt‐7a showed inhibition of chondrogenic differentiation. Different signaling molecules such as Wnt‐7a and BMP‐2 may have antagonistic effects on cartilage differentiation and the gradient of the two molecules may be involved in defining the boundaries of the initial precartilaginous condensation. We propose that the shape of the precartilaginous condensations may be modulated by local concentrations of signaling molecules, such as Wnt‐7a and BMP‐2, which act to alter cell‐substrate and cell‐cell adhesions. J. Cell. Physiol. 180:314–324, 1999.

Molecular histology in skin appendage morphogenesis

Classical histological studies have demonstrated the cellular organization of skin appendages and helped us appreciate the intricate structures and function of skin appendages. At this juncture, questions can be directed to determine how these cellular organizations are achieved. How do cells rearrange themselves to form the complex cyto‐architecture of skin appendages? What are the molecular bases of the morphogenesis and histogenesis of skin appendages? Recently, many new molecules expressed in a spatial and temporal specific manner during the formation of skin appendages were identified by molecular biological approaches. In this review, novel molecular techniques that are useful in skin appendage research are discussed. The distribution of exemplary molecules from different categories including growth factors, intracellular signaling molecules, homeobox genes, adhesion molecules, and extracellular matrix molecules are summarized in a diagram using feather and hair as models. We hope that these results will serve as the ground work for completing the molecular mapping of skin appendages which will refine and re‐define our understanding of the developmental process beyond relying on morphological criteria. We also hope that the listed protocols will help those who are interested in this venture. This new molecular histology of skin appendages is the foundation for forming new hypotheses on how molecules are mechanistically involved in skin appendage development and for designing experiments to test them. This may also lead to the modulation of healing and regeneration processes in future treatment modalities. Microsc. Res. Tech. 38:00–00, 1997.

Specialized stem cell niche enables repetitive renewal of alligator teeth

Reptiles and fish have robust regenerative powers for tooth renewal. However, extant mammals can either renew their teeth one time (diphyodont dentition) or not at all (monophyodont dentition). Humans replace their milk teeth with permanent teeth and then lose their ability for tooth renewal. Here, we study tooth renewal in a crocodilian model, the American alligator, which has well-organized teeth similar to mammals but can still undergo life-long renewal. Each alligator tooth is a complex family unit composed of the functional tooth, successional tooth, and dental lamina. Using multiple mitotic labeling, we map putative stem cells to the distal enlarged bulge of the dental lamina that contains quiescent odontogenic progenitors that can be activated during physiological exfoliation or artificial extraction. Tooth cycle initiation correlates with β-catenin activation and soluble frizzled-related protein 1 disappearance in the bulge. The dermal niche adjacent to the dermal lamina dynamically expresses neural cell adhesion molecule, tenascin-C, and other molecules. Furthermore, in development, asymmetric β-catenin localization leads to the formation of a heterochronous and complex tooth family unit configuration. Understanding how these signaling molecules interact in tooth development in this model may help us to learn how to stimulate growth of adult teeth in mammals.

Topology of Feather Melanocyte Progenitor Niche Allows Complex Pigment Patterns to Emerge

Feather Features Diversification of feather pigment patterns is essential for avian speciation and adaptation. Yet the identity of feather pigment progenitor cells and the cellular and molecular basis for feather pigment pattern formation is poorly understood. Lin et al. (p. 1442, published online 25 April) report that, compared to the localized pigment cell niche in hair, a more dispersed circular topology of the feather pigment progenitor niche loosens spatial constraints and allows for more freedom in patterning possibilities. Multiple cellular mechanisms are co-opted and choreographed in this multidimensional space to create patterns. The patterns of colors in feathers are produced via temporal and spatial regulation of melanocyte stem cells. Color patterns of bird plumage affect animal behavior and speciation. Diverse patterns are present in different species and within the individual. Here, we study the cellular and molecular basis of feather pigment pattern formation. Melanocyte progenitors are distributed as a horizontal ring in the proximal follicle, sending melanocytes vertically up into the epithelial cylinder, which gradually emerges as feathers grow. Different pigment patterns form by modulating the presence, arrangement, or differentiation of melanocytes. A layer of peripheral pulp further regulates pigmentation via patterned agouti expression. Lifetime feather cyclic regeneration resets pigment patterns for physiological needs. Thus, the evolution of stem cell niche topology allows complex pigment patterning through combinatorial co-option of simple regulatory mechanisms.

Physiological Regeneration of Skin Appendages and Implications for Regenerative Medicine

The concept of regenerative medicine is relatively new, but animals are well known to remake their hair and feathers regularly by normal regenerative physiological processes. Here, we focus on 1) how extrafollicular environments can regulate hair and feather stem cell activities and 2) how different configurations of stem cells can shape organ forms in different body regions to fulfill changing physiological needs.