The p63 gene generates transactivating and N-terminally truncated transcripts (⌬Np63) initiated by different promoters. Alternative splicing gives rise to three different C termini, designated ␣, ␤, and ␥. In the ocular epithelium, the corneal stem cells, which are segregated in the basal layer of the limbus, contain the ␣ isoform but not ␤ or ␥. Holoclones derived from the limbus are rich in ␣, meroclones contain little, and paraclones contain none. In normal resting corneal epithelium, p63 of all isoforms is absent. Upon corneal wounding, cells originating from the limbus and containing ␣ migrate progressively through the epithelium of the peripheral and central cornea. In the absence of an attached limbus, no ␣ isoform appears in the corneal epithelium. When migrating cells containing the ␣ isoform appear in the wounded corneal epithelium, they are confined to the basal layer, but the suprabasal cells, not only of the cornea but of the limbus as well, contain mRNA encoding ␤ and ␥. These data support the concept that the ␣ isoform of p63 is necessary for the maintenance of the proliferative potential of limbal stem cells and their ability to migrate over the cornea. The ␤ and ␥ isoforms, being suprabasal and virtually absent from the resting limbus, are not stem cell markers but are likely to play a role in epithelial differentiation specifically during the process of corneal regeneration. The corneal epithelium provides an ideal experimental system in which to distinguish keratinocyte stem and TA cells (8). Corneal stem cells are segregated in the basal layer of the limbus, which is the zone encircling the cornea and separating it from the bulbar conjunctiva. TA cells that migrate from the limbus form the corneal epithelium (8). That the limbus is the site of stem cell precursors of the corneal epithelium is clear for several reasons: (i) the basal layer of the limbus lacks keratin 3 (a marker for corneal differentiation), whereas limbal suprabasal layers and all layers of the corneal epithelium express keratin 3 (9); (ii) the limbus contains slow-cycling cells and holoclone-forming cells, but the corneal epithelium does not (7, 10); (iii) the corneal epithelial cells are not self-sustaining; they divide only a few times during their migration from the limbus to the central cornea (11); (iv) restoration of destroyed limbal͞corneal epithelium requires limbal transplantation (12) or grafts of autologous limbal cultures (13-15).The gene with the most striking effects on the development of stratified epithelia is p63 (16-19). Ablation of the p63 gene in mice results in the absence of these epithelia (17,18). In humans, mutations of the p63 gene cause disorders of the epithelia and of nonepithelial structures whose development depends on the epithelial functions (20). The p63 gene generates six isoforms (21). The transactivating isoforms are generated by the activity of an upstream promoter; the ⌬N isoforms are produced from a downstream intronic promoter and lack the transactivation domain. For both transcripts,...

Retinitis pigmentosa (RP) is a group of inherited disorders affecting 1 in 3000-7000 people and characterized by abnormalities of the photoreceptors (rods and cones) or the retinal pigment epithelium of the retina which lead to progressive visual loss. RP can be inherited in an autosomal dominant, autosomal recessive or X-linked manner. While usually limited to the eye, RP may also occur as part of a syndrome as in the Usher syndrome and Bardet-Biedl syndrome. Over 40 genes have been associated with RP so far, with the majority of them expressed in either the photoreceptors or the retinal pigment epithelium. The tremendous heterogeneity of the disease makes the genetics of RP complicated, thus rendering genotype-phenotype correlations not fully applicable yet. In addition to the multiplicity of mutations, in fact, different mutations in the same gene may cause different diseases. We will here review which genes are involved in the genesis of RP and how mutations can lead to retinal degeneration. In the future, a more thorough analysis of genetic and clinical data together with a better understanding of the genotype-phenotype correlation might allow to reveal important information with respect to the likelihood of disease development and choices of therapy.

Human limbal stem cells produce transit amplifying progenitors that migrate centripetally to regenerate the corneal epithelium. Coexpression of CCAAT enhancer binding protein δ (C/EBPδ), Bmi1, and ΔNp63α identifies mitotically quiescent limbal stem cells, which generate holoclones in culture. Upon corneal injury, a fraction of these cells switches off C/EBPδ and Bmi1, proliferates, and differentiates into mature corneal cells. Forced expression of C/EBPδ inhibits the growth of limbal colonies and increases the cell cycle length of primary limbal cells through the activity of p27Kip1 and p57Kip2. These effects are reversible; do not alter the limbal cell proliferative capacity; and are not due to apoptosis, senescence, or differentiation. C/EBPδ, but not ΔNp63α, indefinitely promotes holoclone self-renewal and prevents clonal evolution, suggesting that self-renewal and proliferation are distinct, albeit related, processes in limbal stem cells. C/EBPδ is recruited to the chromatin of positively (p27Kip1 and p57Kip2) and negatively (p16INK4A and involucrin) regulated gene loci, suggesting a direct role of this transcription factor in determining limbal stem cell identity.

Blood trauma caused by medical devices is a major concern. Complications following the implantation/application of devices such as prosthetic heart valves, cannulae, blood pumps, tubing, and throttles lead to sublethal and lethal damage to platelets and erythrocytes. This damage is provided by the alterations in fluid dynamics, providing a mechanical load on the blood corpuscle's membrane by means of the shear stress. An appropriate quantification of the shear-induced hemolysis of artificial organs is thought to be useful in the design and development of such devices in order to minimize device-induced blood trauma. To date, a power-law mathematical relationship using the time of exposure of a blood corpuscle to a certain mechanical load and the shear stress itself (derived under the peculiar condition of uniform shear stress) has served as a basic model for the estimation of the damage to blood, investigated by means of numerical and/or experimental fluid dynamical techniques. The aim of the present article is to highlight the effect of a time-varying mechanical loading acting on blood cells based on the usual power-law model; furthermore, the effect of the loading history of a blood particle is discussed, showing how the past history of the shear acting on a blood corpuscle is not taken into account, as researchers have done until now. The need for a reassessment of the power-law model for potential blood trauma assessment is discussed by using a mathematical formulation based on the hypotheses of the existence of damage accumulation for blood with respect to time and with respect to shear stress, to be applied in complex flow fields such as the ones established in the presence of artificial organs.