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Pilomatricomas are relatively rare tumors of ectodermal origin from the outer root sheath cell of the hair follicle. They are usually asymptomatic, solitary, firm or hard, freely mobile, dermal or subcutaneous nodules. The purpose of this article is to present a case that illustrates the diagnostic difficulty encountered by oral surgeons and pathologists and to review the literature regarding pilomatricomas of the auricular region.
Embryogenesis in hydra includes a variable period of dormancy; and this period, as well as subsequent stages through hatching, takes place within a thick cuticle that hinders observation. Thus, although the early stages of development have been well-characterized qualitatively, the middle and later stages are only poorly understood. Here, we provide a detailed description of the stages of embryogenesis, including the time required to traverse each of the stages, and the changes that occur in the type and number of cells throughout the stages. The events of cleavage and gastrulation occur within the first 48 h. Cleavage is holoblastic and unipolar and leads to a single-layered coeloblastula. Gastrulation occurs by ingression and is followed by the deposition of the thick cuticle. Thereafter, during the variable period of dormancy ranging from 2-24 weeks, little occurs; the important events are the conversion of the outer layer into an ectoderm and the appearance of the interstitial cell lineage. During the last 2 days before hatching, the endoderm and gastric cavity form, while stem cells of the interstitial cell lineage proliferate and differentiate into neurons, nematocytes, and secretory cells. Finally, the cuticle cracks, and the hatchling enlarges and emerges from the cuticle as a functional animal. The formation of the gastric cavity and the hatching of the embryo are both explicable in terms of the osmotic behavior of the animal and the hydrostatic forces generated by this behavior. Characteristics of development that are common to hydra and triploblastic phyla are presented.
At E4.0 the inner cell mass of the mouse blastocyst consists of a core of embryonic ectoderm cells surrounded by an outer layer of primitive (extraembryonic) endoderm, which subsequently gives rise to both visceral endoderm and parietal endoderm. Shortly after blastocyst implantation, the solid mass of ectoderm cells is converted by a process known as cavitation into a pseudostratified columnar epithelium surrounding a central cavity. We have previously used two cell lines, which form embryoid bodies that do (PSA1) or do not (S2) cavitate, as an in vitro model system for studying the mechanism of cavitation in the early embryo. We provided evidence that cavitation is the result of both programmed cell death and selective cell survival, and that the process depends on signals from visceral endoderm (Coucouvanis, E. and Martin, G. R. (1995) Cell 83, 279-287). Here we show that Bmp2 and Bmp4 are expressed in PSA1 embryoid bodies and embryos at the stages when visceral endoderm differentiation and cavitation are occurring, and that blocking BMP signaling via expression of a transgene encoding a dominant negative mutant form of BMP receptor IB inhibits expression of the visceral endoderm marker, Hnf4, and prevents cavitation in PSA1 embryoid bodies. Furthermore, we show that addition of BMP protein to cultures of S2 embryoid bodies induces expression of Hnf4 and other visceral endoderm markers and also cavitation. Taken together, these data indicate that BMP signaling is both capable of promoting, and required for differentiation of, visceral endoderm and cavitation of embryoid bodies. Based on these and other data, we propose a model for the role of BMP signaling during peri-implantation stages of mouse embryo development.
When presumptive ectoderm is treated with high concentrations of activin A, it mainly differentiates into axial mesoderm (notochord, muscle) in Xenopus and into yolk-rich endodermal cells in newt (Cynops pyrrhogaster). Xenopus ectoderm consists of multiple layers, different from the single layer of Cynops ectoderm. This multilayer structure of Xenopus ectoderm may prevent complete treatment of activin A and subsequent whole differentiation into endoderm. In the present study, therefore, Xenopus ectoderm was separated into an outer layer and an inner layer, which were individually treated with a high concentration of activin A (100 ng/mL). Then the differentiation and inductive activity of these ectodermal cells were examined in explantation and transplantation experiments. In isolation culture, ectoderm treated with activin A formed endoderm. Ectodermal and mesodermal tissues were seldom found in these explants. The activin-treated ectoderm induced axial mesoderm and neural tissues, and differentiated into endoderm when it was sandwiched between two sheets of ectoderm or was transplanted into the ventral marginal zone of other blastulae. These findings suggest that Xenopus ectoderm treated with a high concentration of activin A forms endoderm and mimics the properties of the organizer as in Cynops.
After lentectomy through the pupillary hole, the outer cornea of larval Xenopus laevis can undergo transdifferentiation to regenerate a new lens. This process is elicited by inductive factor(s) produced by the neural retina and accumulated into the vitreous chamber. During embryogenesis, the outer cornea develops from the outer layer of the presumptive lens ectoderm (PLE) under the influence of the eye cup and the lens. In this study, we investigated whether the capacity of the outer cornea to regenerate a lens is the result of early inductive signals causing lens-forming bias and lens specification of the PLE, or late inductive signals causing cornea formation or both signals. Fragments of larval epidermis or cornea developed from ectoderm that had undergone only one kind of inductive signals, or both kinds of signals, or none of them, were implanted into the vitreous chamber of host larvae. The regeneration potential and the lens-forming transformations of the implants were tested using an antisense probe for pax6 as an earlier marker of lens formation and a monoclonal antibody anti-lens as a definitive indicator of lens cell differentiation. Results demonstrated that the capacity of the larval outer cornea to regenerate a lens is the result of both early and late inductive signals and that either early inductive signals alone or late inductive signals alone can elicit this capacity.
Previous studies have indicated that the outer cornea can undergo transdifferentiation to form a lens in the tadpole larva of Xenopus laevis following removal of the original lens. This transformation appears to require an interaction with the neural retina. In the present study, we carried out a series of experiments to determine if the matured tadpole eye can also elicit lens formation in embryonic ectoderm. Labeled embryonic ectoderm was removed from the presumptive lens-forming region, or from the belly region (ventral ectoderm), at various stages of development (stages 11-19, gastrula to neural tube stages) and implanted into the eye cavity (posterior chamber) of advanced stage 52-55 tadpoles. After 3 days, we examined the tadpoles and their implanted tissues for lens cell formation using lens-specific antibodies. Implanted presumptive lens ectoderm differentiated lens cells in a large number of cases. The percentage of cases forming lens cells and the extent of morphological differentiation increased with increasing age of the implanted tissue. Implanted ventral ectoderm also formed lens cells, although at a reduced frequency and with limited morphological differentiation. These results indicate that the environment of the matured tadpole eye cavity stimulates lens cell formation in both presumptive lens and nonlens ectoderm. The development of the implanted tissues was compared to that found in previous studies where these tissues were cultured as explants or transplanted to lens-forming regions during early development and subjected to various periods of embryonic lens induction. Together, these findings suggest that the process of embryonic lens formation is related to that involved in transdifferentiation of the tadpole cornea during "lens regeneration." However, the inductive effect of the matured tadpole eye is qualitatively different from that of the early period of embryonic lens induction and, while more intense, may be more closely related to that which takes place via the optic vesicle during the later phase of embryonic lens induction.
AIMS: To investigate the changes in PAX6 expression in the developing human eye. METHODS: Six developing human eyes from 6 to 22 weeks' gestation were evaluated. Frozen sections were immunohistochemically stained with monoclonal antibody to chick Pax6 (amino acids 1-223). To verify antibody specificity, western blot analysis was carried out using cell lysates from P19 cells transfected with the human PAX6 gene. RESULTS: Western blot analysis demonstrated that the antibody reacted to human PAX6 protein. Positive immunostainings for PAX6 were seen in the surface ectoderm, lens vesicle, inner and outer layers of the optic cup, and optic stalk at 6 weeks, and in the corneal epithelia and conjunctiva, lens, and non-pigmented ciliary epithelia from 8 to 22 weeks. In the retina, positive cells were seen in the entire retina from 8 to 10 weeks, and were restricted to the ganglion cell layer and the inner and outer portions of the inner nuclear layer after 21 weeks. CONCLUSIONS: PAX6 is expressed on the surface and neuroectoderms at an early stage, then in the differentiating cells in the cornea, lens, ciliary body, and retina through development. PAX6 may play a role in determining cell fate in the morphogenesis of various human ocular tissue.
We have identified and cloned a novel type of homeobox gene that is composed of two homeodomains and is expressed in the Drosophila endoderm. Mutant analysis reveals that its activity is required at the foregut/midgut boundary for the development of the proventriculus. This organ regulates food passage from the foregut into the midgut and forms by the infolding of ectoderm and endoderm-derived tissues. The endodermal outer wall structure of the proventriculus is collapsed in the mutants leading to a failure of the ectodermal part to invaginate and build a functional multilayered organ. Lack-of-function and gain-of-function experiments show that the expression of this homeobox gene in the proventriculus endoderm is induced in response to Wingless activity emanating from the ectoderm/endoderm boundary whereas its expression in the central midgut is controlled by Dpp and Wingless signalling emanating from the overlying visceral mesoderm.
In the present investigation we localized binding sites for the lectins WGA (wheat germ agglutinin), RCA I (Ricinus communis agglutinin), LFA (Limax flavus agglutinin) and SBA (soya bean agglutinin) in the 7-day-old mouse embryo at the ultrastructural level. Lectin binding sites were localized on formaldehyde fixed embryos, embedded in LR-Gold, using gold-labelled lectins. Binding sites for WGA and RCA I were observed at the surface of the embryonic ectoderm oriented towards the proamnion cavity and the outer surface of the extraembryonic and the embryonic endoderm. Staining for SBA and LFA binding sites was seen in the basement membrane of the ectoderm. Moreover, binding sites for LFA were observed in the nucleoli of cells of the ectodermal, the mesodermal and the endodermal layer and in free ribosomes located in the cytoplasm of these cells.
The skin of Xenopus embryos contains a population of specialized ciliated cells that are distributed in an evenly spaced pattern. Here we describe two successive steps that govern the differentiation and the generation of the spacing pattern of these ciliated cells. The first step occurs in the inner or sensorial layer of the non-neural ectoderm where a subset of cells are chosen to differentiate into ciliated-cell precursors. This choice is under the control of lateral inhibition mediated by a Suppressor of Hairless-dependent Notch signaling pathway, in which X-Delta-1 is the putative ligand driving the selection process, and a new Enhancer-of-Split-related gene is an epidermal target of Notch signaling. Because nascent ciliated-cell precursors prevent neighboring cells from taking on the same fate, a scattered pattern of these precursors is generated within the deep layer of the non-neural ectoderm. Ciliated-cell precursors then intercalate into the outer layer of cells in the epidermis. We show that the intercalation event acts as a second step to regulate the spacing of the mature ciliated cells. We propose that the differentiation of the ciliated cells is not only regulated by Notch-mediated lateral inhibition, but is also an example where differentiation is coupled to the movement of cells from one cell layer to another.
Most triploblastic animals including vertebrates have a coelomic cavity that separates the outer and inner components of the body. The coelom is lined by two different tissue components, somatopleure and splanchnopleure, which are derived from the lateral plate region. Thus, the coelom is constructed as a result of a binary decision during early specification of the lateral plate. In this report we studied the molecular mechanisms of this binary decision. We first demonstrate that the splitting of the lateral plate into the two cell sheets progresses in an anteroposterior order and this progression is not coordinated with that of the somitic segmentation. By a series of embryological manipulations we found that young splanchnic mesoderm is still competent to be respecified as somatic mesoderm, and the ectoderm overlying the lateral plate is sufficient for this redirection. The lateral ectoderm is also required for maintenance of the somatic character of the mesoderm. Thus, the ectoderm plays at least two roles in the early subdivision of the lateral plate: specification and maintenance of the somatic mesoderm. We also show that the latter interactions are mediated by BMP molecules that are localized in the lateral ectoderm. Evolutionary aspects of the coelom formation are also considered.
Receptors for three lectins with restricted specificities, namely fucose-binding protein of Lotus tetragonolobus (FBP), peanut agglutinin (PNA) and Dolichos biflorus agglutinin (DBA), were distinctively located in 6- and 7-day mouse embryos and in embryoid bodies of teratocarcinoma OTT6050 grown in vivo. Thus, FBP reacted mainly with the inner cells (embryonic ectoderm and teratocarcinoma stem cells), DBA reacted with the outer cells (endoderm) and PNA reacted with all the germ layers including mesoderm. Upon in vitro culture of the embryoid bodies, the exposed stem cells express DBA receptors. Since the receptors for the three lectins in teratocarcinomas are known to be carried by the large carbohydrate chains characteristic of early embryonic cells, the present result suggests that terminal structure of the large carbohydrates is altered according to the direction of the differentiation or to the position of the cells in embryos and in teratocarcinomas.
The segregation of cells into germ layers is one of the earliest events in the establishment of cell fate in the embryo. In the zebrafish, endoderm and mesoderm are derived from cells that involute into an internal layer, the hypoblast, whereas ectoderm is derived from cells that remain in the outer layer, the epiblast. In this study, we examine the origin of the zebrafish endoderm and its separation from the mesoderm. By labeling individual cells located at the margin of the blastula, we demonstrate that all structures that are endodermal in origin are derived predominantly from the more dorsal and lateral cells of the blastoderm margin. Frequently marginal cells give rise to both endodermal and mesodermal derivatives, demonstrating that these two lineages have not yet separated. Cells located farther than 4 cell diameters from the margin give rise exclusively to mesoderm, and not to endoderm. Following involution, we see a variety of cellular changes indicating the differentiation of the two germ layers. Endodermal cells gradually flatten and extend filopodial processes forming a noncontiguous inner layer of cells against the yolk. At this time, they also begin to express Forkhead-domain 2 protein. Mesodermal cells form a coherent layer of round cells separating the endoderm and ectoderm. In cyclops-mutant embryos that have reduced mesodermal anlage, we demonstrate that by late gastrulation not only mesodermal but also endodermal cells are fewer in number. This suggests that a common pathway initially specifies germ layers together before a progressive sequence of determinative events segregate endoderm and mesoderm into morphologically distinct germ layers.
The embryonic ectoderm of the pig differentiated and became part of the outer barrier of the blastocyst (earlier formed by the trophectoderm alone) before shedding of the overlying polar trophectoderm around Day 10, thus securing the integrity of the rapidly expanding blastocyst. Ferritin, added to the medium of the blastocyst, was taken up rapidly by trophectoderm cells, but did not reach the blastocoele, and consequently no tracer was found within hypoblast cells. Embryonic ectoderm cells did not absorb the macromolecule, before or after loss of the polar trophectoderm. When ferritin was injected into the blastocoele, trophectoderm, hypoblast and embryoblast cells all absorbed the tracer. At Day 11, blastocyst diameter and embryoblast cell number varied widely and were hardly correlated. We suggest that embryoblast development may be a more reliable indicator for the developmental stage of a blastocyst than its diameter, which may merely be an indication of the viability of the trophoblast.
BACKGROUND: The assembly of complex tissues during embryonic development is thought to depend on differential cell adhesion, mediated in part by the cadherin family of cell-adhesion molecules. The protocadherins are a new subfamily of cadherins; their extracellular domains comprise cadherin-like repeats but their intracellular domains differ significantly from those of classical cadherins. Little is known about the ability of protocadherins to mediate the adhesion of embryonic cells, or whether they play a role in the formation of embryonic tissues. RESULTS: We report the isolation and characterization of a novel protocadherin, termed NF-protocadherin (NFPC), that is expressed in Xenopus embryos. NFPC showed a striking pattern of expression in early embryos, displaying predominant expression within the deep, sensorial layer of the embryonic ectoderm and in a restricted group of cells in the neural folds, but was largely absent from the neural plate and surrounding placodal regions. Ectopic expression in embryos demonstrated that NFPC could mediate cell adhesion within the embryonic ectoderm. In addition, expression of a dominant-negative form of NFPC disrupted the integrity of embryonic ectoderm, causing cells in the deep layer to dissociate, though leaving the outer layer relatively intact. CONCLUSIONS: Our results indicate that NFPC is required as a cell-adhesion molecule during embryonic development, and its function is distinct from that of classical cadherins in governing the formation of a two-layer ectoderm. These results suggest that NFPC, and protocadherins in general, are involved in novel cell-cell adhesion mechanisms that play important roles in tissue histogenesis.
PURPOSE: To study the development of the rat lens vesicle in relation to apoptosis. MATERIALS AND METHODS: Fetuses of Wistar Kyoto rats were removed by laparotomy on day 10-15 of gestation. Some fetuses were fixed in 2% paraformaldehyde and embedded in paraffin for a TUNEL technique examination of DNA fragmentation. Macrophages were stained immunohistochemically with antibody. Some fetuses were fixed in 4% glutaraldehyde and 1% osmic acid and embedded in Luveak 812, then examined with a transmission electron microscope (TEM). RESULTS: On day 11 of gestation (E11) before the start of lens invagination, apoptotic changes were noted in the cells between the surface ectoderm and optic vesicle, with the appearance of phagocytic cells. Apoptotic cells were present at the junction of the surface ectoderm and the lens placode, in the ventral and dorsal thirds of the lens placode and in the outer layer of the optic vesicle in the same axes on E12. Apoptotic changes appeared in the lens stalk, surface ectoderm and the anterior lens epithelium on E12.5. The lens vesicle was detached completely from the surface ectoderm by E13 and some cells had the typical characteristics of macrophages in the extracellular space between the surface ectoderm and the anterior lens epithelium. Apoptotic changes were confirmed by the TUNEL method, and macrophages were stained immunohistochemically. CONCLUSIONS: Apoptosis may have a major role during the whole process of lens vesicle development. Apoptosis may eliminate the cells between the surface ectoderm and the optic vesicle, help trigger invagination and facilitate separation from the ectoderm. Apoptosis might aid in the bowing of the optic vesicle during lens invagination.
Nucleosome assembly proteins (NAPs) bind core histones, facilitate chromatin remodeling, and can act as transcriptional coactivators. We previously described the isolation of a Xenopus NAP1-like (xNAP1L) cDNA, which encodes a member of this protein family. Its zygotic expression is restricted to neural cells, the outer cells of the ventral blood island (VBIs), and the ectoderm overlying the blood precursors. Here, we report that depletion of zygotic xNAP1L in embryos produces no obvious morphologic phenotype, but ablates alpha-globin mRNA expression in the VBIs. Transcript levels of the hematopoietic precursor genes SCL and Xaml (Runx-1) are also reduced in the VBIs. SCL expression can be rescued by injection of xNAP1L mRNA into the ectoderm, showing that the effect of xNAP1L can be non-cell autonomous. Fli1 and Hex, genes expressed in hemangioblasts but subsequently endothelial markers, were unaffected, suggesting that xNAP1L is required for the hematopoietic lineage specifically. Our data are consistent with a requirement for xNAP1L upstream of SCL, and injection of SCL mRNA into xNAP1L-depleted embryos rescues alpha-globin expression. Thus, xNAP1L, which belongs to a family of proteins previously believed to have general roles, has a specific function in hematopoiesis.
Adult Hydra attenuata with vitally stained gastrodermal cells were dissociated into their component cells which were then randomly reaggregated into pellets by low-speed centrifugation. Representative examples of these preparations, which develop into normal adult hydra if left undisturbed, were examined fresh at low magnification and at higher magnification in fixed, stained, and sectioned specimens. The actual pellet stage lasts less than 1 hour because the adult ectodermal and gastrodermal cells rapidly sort themselves into an inner and outer layer and seem to secrete a new mesoglea immediately thereafter. The "embryo" becomes trilaminar and attains a central cavity by extruding a large amount of cellular debris at the end of the first day. At about this same time, new tentacles begin to differentiate from rapidly dividing and undifferentiated interstitial cells. Regulation of tentacle number and position occurs at the end of two days, and the body form is essentially reestablished within 60 hours by further differentiation of the hypostomes and body wall. Complete separation of the preparation into individual polyps does not occur until about 190 hours of development.