Skip to Main Content

Making a Tissue: Epidermal Development

At gastrulation (embryonic day 7.5, E7.5), the ectoderm moves from inside the embryo to the surface. There, it forms the neuroectoderm that will invaginate to generate the neural crest and nervous system, and the epidermis, which begins as a monolayer of unspecified progenitors. These progenitors will progress not only to form the mature, stratified epidermis, but also its appendages, including hair follicles, sebaceous glands, sweat glands, and even mammary glands. The ectoderm also develops the oral epithelium of the head and neck region.

As the embryo grows, epidermal divisions are initially parallel relative to the plane of the epithelium. WNT-signaling within some of these cells prompts them to coalesce to form an epithelial placode, whose cells divide perpendicularly to the epidermal plane to form a bud-like structure. At its earliest stage, could be a hair bud, tooth bud, mammary bud or sweat gland bud. BMP signaling plays a critical role in bud fate decision. To make a hair follicle, BMP signaling must be blocked within the bud; to make a sweat gland, BMP signaling must be activated within the bud (Lu et al., 2016). In mice and most mammals, eccrine sweat glands only form on the paws, whereas hair follicles occur elsewhere on body skin. By contrast, humans evolved to have both sweat glands and hair follicles on body skin, enabling them to better control body temperature and withstand temperature extremes. Probing mechanism, we discovered that in humans, body skin begins in a BMP-inhibitory state, causing buds to become hair follicles. Then, a spike of BMP occurs, causing the last bud waves to become glands. Moreover downstream, BMP-inhibition promotes expression of sonic hedgehog (SHH), while BMPs antagonize SHH production, and this antagonism is essential for proper fate specification. Our findings not only solve a fascinating evolutionary conundrum but also have important implications for future therapeutics of patients suffering from sweat gland loss in genetic disorders and/or bad burns.

integrins As epidermal stratification begins, spindle orientations become oblique, giving rise to one basal and one suprabasal daughter (Williams et al., 2014). Basal cells make and secrete extracellular matrix (ECM) composed of laminin 5, collagen IV and fibronectin, which act as receptors for transmembrane integrins composed of α and β subunits. In epidermis, two major integrin heterodimers, α3β1 and α6β4, utilize laminin 5 as their primary ECM ligand. However, α3β1 engages focal adhesion kinase (FAK) and mobilizes actin dynamics to control basement membrane assembly (Raghavan et al., 2000; 2003; Schober et al., 2007), while α6β4 forms the transmembrane core of hemidesmosomes, ultrastructurally elaborate structures essential for strong adherence to the underlying basement membrane (Guo et al., 1995; Dowling et al., 1996). α6β4 engages BPAG2 (collagen XVII) a transmembrane collagen that reinforces this extracellular anchorage, and BPAG1, which binds to the keratin cytoskeleton whose filaments are composed of keratins 14 and 5 (Dowling et al., 1996; Guo et al., 1995). The K14-K5 keratin network provides an essential mechanical framework to these developing basal epidermal cells (Vassar et al., 1989; Coulombe et al., 1991).

closeup thumbDuring embryonic development, the monolayer of epidermal progenitors begins to polarize its integrins towards the basement membrane. This polarity is accompanied by the elaboration of hemidesmosomes. Polarity is also accompanied by increased cell-cell “adherens” junctions composed of the core transmembrane component, E-cadherin, and its associated proteins α-catenin and β-catenin. β-Catenin links E-cadherin to α-catenin, while α-catenin associates with filamentous actin and with actin-associated proteins. These junctions coordinate movements within the basal epidermal sheet and participate in the stratification process (Vasioukhin et al., 2000; 2001a; Vaezi et al., 2003; Kobielak et al., 2004; Livshitz et al., 2012). Suprabasally, the cytoskeletal network and its adhesions strengthen through formation of desmosomes, whose core desmosomal cadherins bind plakoglobin (related to β-catenin), which binds desmoplakin (related to BPAG1) to interconnect with the keratin filament network (Kouklis et al., 1993; Gallicano et al., 1998; Vasioukhin et al., 2001b).Desmosome

The maturation of epithelial junctions and enhanced epidermal polarity is typified by the localization of polarity proteins, Par3, Par6 and atypical protein kinase C (aPKC), at the apical actin cortex (Luxenburg et al., 2011; 2014). These maturation steps are also associated with a change in spindle orientation. By E15.5, 70% of mitotic spindles become polarized perpendicularly to the underlying basement membrane (Lechler et al., 2005; Williams et al., 2014). This shift in alignment from parallel/oblique to perpendicular enhances epidermal stratification and the formation of spinous cells, typified by expression of keratins 1 and 10 (Fuchs and Green, 1980; Cheng J et al., 1992). Genetic mutations in β1, α-catenin and proteins involved in actin dynamics show that cell substratum adhesion, cell-cell adhesion, and polarization of the cortical actin cytoskeleton are all essential for this perpendicular orientation of spindle microtubules (Lechler et al., 2005; Luxenburg et al., 2011; 2014).

Probing mechanism, we discovered that these asymmetric cell divisions utilize an ancient mechanism that has been previously been described to characterize the differential specification of daughter cell fates that occurs in the early C. elegans embryo and Drosophila neuroblast. These divisions asymmetrically partition fate determinants such that one daughter cell retains progenitor status, while the other differentiates (reviewed by Williams and Fuchs, 2014).

stem popup 7th image blanpain The core components of this pathway are LGN and mINSC, which become stabilized soon after the asymmetrically dividing cell enters mitosis (Lechler and Fuchs, 2005). mINSC binds to Par3, placing it at the apical inner surface of the cortical actin. LGN binds to Gαi, a G-coupled protein receptor which is also apically localized. Additionally, LGN can bind mINSC to link the Gαi and Par3 complexes. LGN can also bind to NuMA, a protein that can bind astral microtubules and link them to the apical cortical complex (Lechler and Fuchs, 2005; Williams et al., 2011). Although LGN cannot bind to mINSC and NuMA simultaneously, Par3 and the Gαi both participate in spindle orientation, and with LGN provide multiple ways in which the complex can be stabilized (Williams et al., 2011; 2014).

LGN/mINSC are also integrally linked to Notch signaling, as they are in Drosophila neuroblasts (Williams et al., 2011; 2014). Importantly, Notch signaling is essential for epidermal differentiation and governs the transition between basal and spinous cell fates (Blanpain et al., 2006). Notably, genetic defects in LGN alone or combined loss of mInsc and Gαi lead to mostly planar cell divisions, and accompanying defects in epidermal differentiation (Williams et al., 2011; 2014).

As the epidermis stratifies and matures, and basal cell proliferation slows down, the percentage of perpendicular divisions begin to wane in the epidermis. At this time, the epidermis enters a state of equilibrium. To maintain the surface barrier that keeps microbes out and retains body fluids, an inner layer of living, multiplying cells, which include stem cells. When a basal cell ceases to divide and begins its journey to the skin surface, it embarks on a program of terminal differentiation. As spinous cells form, they protect themselves by producing a mechanically durable, dense intracellular framework of filaments composed of keratin. Through abundant intercellular junctions, differentiating cells form continuous, interconnected cellular sheets. Later, as the spinous cells generate the granular layers, the cells assemble an indestructible proteinaceous envelope that serves as a scaffold on which lipids are extruded and organized to form the epidermal barrier. The molecular details of the process, which involve complete loss of all cellular organelles, including the nucleus, remain poorly understood.

Recently, we addressed how the epidermis maintains its fitness during development. If a mistake is made and a less fit cell is generated at a time when the embryonic skin is rapidly expanding, the tissue risks developing a clonal patch of poorly performing cells. Interestingly, healthy neighbors within the epidermis prevent this from happening, by recognizing the loser cell engulfing and digesting it (Ellis et al., 2019). With the onset of stratification and differentiation, the epidermis switches from this mechanism to one where the loser cell is instead eliminated through asymmetric cell divisions and terminal differentiation. These fascinating mechanisms of cellular competition are critical to optimizing tissue fitness, and it is likely that they are operative in other tissues.

An endless sea of questions remain. What controls the flux of cells from the innermost proliferative layer to the skin surface, where they are shed from the skin surface? What happens during the later stages of terminal differentiation when cells suddenly lose their organelles, including the nucleus? How is this death program regulated? How does this change in aging? How does temperature affect skin biology? Your internal tissues are 98.6 degrees Fahrenheit, but the skin epidermis is subject to a wide range in temperature. Similarly, the epidermis must confront changes in humidity and pH—how does it do so? And the epidermis is one of the few tissues that are not vascularized—does it have to fight to get ample nutrients or does its terminally differentiating cells benefit by moving further away from a food source? Indeed, we’ve just begun to scratch the surface of skin’s many mysteries.

 

At gastrulation (embryonic day 7.5, E7.5), the ectoderm moves from inside the embryo to the surface. There, it forms the neuroectoderm that will invaginate to generate the neural crest and nervous sys