Midgut epithelium formation in Thermobia domestica (Packard) (Insecta, Zygentoma)

The origin of midgut epithelium may begin either from yolk cells (energids), tips of stomo- and proctodaeum (ectoderm), inner layer (endoderm) or from both kinds of the above mentioned cells. The origin of the midgut epithelium in wingless insects (Apterygota) has still not been determined. In Thermobia domestica the formation of midgut is much delayed, and it completes in the post-embryonic stage, while the stomo- and the proctodaeum are well-developed in the embryonic period. The energids, which remain inside the yolk, start to migrate to its periphery, where they arrange singly close to cell membrane. The yolk mass with the energids at the 14th day of embryogenesis are referred to as the primary midgut. During the first instar larval stage more and more energids migrate to the yolk periphery and the cell membrane starts to form numerous foldings surrounding the groups of energids, which in turn lead to formation of isolated regenerative cell groups. Eventually the cell membrane invaginations reach the center of the yolk mass. Large cells of the primary epithelium, surrounding the newly formed midgut lumen are formed. The cells of the primary epithelium are filled with yolk and are equipped with microvilli pointing to the midgut lumen. As the yolk is being digested, the process of the primary epithelium cells degeneration begins. The cells are getting shorter and start to degenerate. The definitive midgut epithelium is formed from proliferating regenerative cells. It consists of regularly spaced regenerative cell groups as well as the epithelial cells. The ultrastructure of both these cell groups has been described.     

The morphogenesis of the stomach and intestine in the salamander Ambystoma maculatum

The major events associated with the morphogenesis of the amphibian alimentary tract are described and illustrated with a series of photomicrographs that present a continuous account of the differentiation process from its onset at stage 38 until the initiation of feeding at stage 46. Histological evidence is presented for the normal disappearance of the midgut region of the archenteron and the de novo formation of the intestine through the yolk mass. The mechanics of intestinal lumen formation are discussed in terms of the dynamic autonomous actions and interactions of the endoderm and the splanchnic mesoderm. The opening of the intestinal lumen as a consequence of cytolysis or cellular digestion is discounted. The relation of the present observations to the previously described polar endoderm cells is considered.     

Expression and function of the homoeotic genes Antennapedia and Sex combs reduced in the embryonic midgut of Drosophila

Drosophila homoeotic genes control the formation of external morphological features of the embryo and adult, and in addition affect differentiation of the nervous system. Here we describe the morphogenetic events in the midgut that are controlled by the homoeotic genes Sex combs reduced (Scr) and Antennapedia (Antp). The midgut is composed of two cell layers, an inner endoderm and an outer visceral mesoderm that surround the yolk. Scr and Antp are expressed in the visceral mesoderm but not in the endoderm. The two genes are required for different aspects of the midgut morphogenesis. In Scr null mutant embryos the gastric caeca fail to form. Scr is expressed in the visceral mesoderm cells posterior to the primordia of the gastric caeca and appears to be indirectly required for the formation of the caeca. Antp is expressed in visceral mesoderm cells that overlie a part of the midgut where a constriction will form, and Antp null mutant embryos fail to form this constriction. An ultrastructural analysis of the midgut reveals that the visceral mesoderm imposes the constriction on the endoderm and the yolk. The mesodermal tissue contracts within the constriction and thereby penetrates the layer of the midgut endoderm. Microtubules participate in the morphological changes of the visceral mesoderm cells. The analysis of the expression of Scr in Antp mutant embryos revealed a case of tissue-specific regulation of Scr expression by Antp. In the epidermis, Antp has been shown to negatively regulate Scr, but it positively regulates Scr in the visceral mesoderm.     

Alimentary canal formation in the stonefly,Kamimuria tibialis (pictet) (Plecoptera : Perlidae)

The alimentary canal formation in the stonefly, Kamimuria tibialis (Plecoptera : Perlidae) is described. The stomodaeum is formed as in other insect embryos. The proctodaeum is derived from the ectodermal fold an the caudal end of the embryo without the contribution of the amnion. The 3 Malpighian tubules develop from the blind end of the proctodaeum. The rectal pad is formed by the thickening of the dorsal wall of the proctodaeum. The midgut epithelium rudiment arises only from the blind end of the proctodaeum, i.e. it is completed by unipolar formation instead of bipolar. The yolk cells do not contribute to the formation of the midgut epithelium. The alimentary canal is transformed during the 1st nymphal instar and becomes functional in the next instar. The stomodaeum is differentiated into 3 parts: pharynx, oesophagus, and proventriculus. The midgut becomes shortened and its epithelium is well developed. Gastric caeca with tapering processes are formed.     

Cell fate, morphogenetic movement and population kinetics of embryonic endoderm at the time of germ layer formation in the mouse

The fate of the embryonic endoderm (generally called visceral embryonic endoderm) of prestreak and early primitive streak stages of the mouse embryo was studied in vitro by microinjecting horseradish peroxidase into single axial endoderm cells of 6.7-day-old embryos and tracing the labelled descendants either through gastrulation (1 day of culture) or to early somite stages (2 days of culture). Descendants of endoderm cells from the anterior half of the axis were found at the extreme cranial end of the embryo after 1 day and in the visceral yolk sac endoderm after 2 days, i.e. they were displaced anteriorly and anterolaterally. Descendants of cells originating over and near the anterior end of the early primitive streak, i.e. posterior to the distal tip of the egg cylinder, were found after 1 day over the entire embryonic axis and after 2 days in the embryonic endoderm at the anterior intestinal portal, in the foregut, along the trunk and postnodally, as well as anteriorly and posteriorly in the visceral yolk sac. Endoderm covering the posterior half of the early primitive streak contributed to postnodal endoderm after 1 day (at the late streak stage) and mainly to posterior visceral yolk sac endoderm after 2 days. Clonal descendants of axial endoderm were located after 2 days either over the embryo or in the yolk sac; the few exceptions spanned the caudal end of the embryo and the posterior yolk sac. The clonal analysis also showed that the endoderm layer along the posterior half of the axis of prestreak- and early-streak-stage embryos is heterogeneous in its germ layer fate. Whereas the germ layer location of descendants from anterior sites did not differ after 1 day from that expected from the initial controls (approx. 90% exclusively in endoderm), only 62% of the successfully injected posterior sites resulted in labelled cells exclusively in endoderm; the remainder contributed partially or entirely to ectoderm and mesoderm. This loss from the endoderm layer was compensated by posterior-derived cells that remained in endoderm having more surviving descendants (8.4 h population doubling time) than did anterior-derived cells (10.5 h population doubling time). There was no indication of cell death at the prestreak and early streak stages; at least 93% of the cells were proliferating and more than half of the total axial population were in, or had completed, a third cell cycle after 22 h culture.(ABSTRACT TRUNCATED AT 400 WORDS)     

Fate mapping study of the endoderm of the 1.5-day-old chick embryo

Various portions of the endoderm between the levels of the first and the 10th somite of 1.5-day-old chick embryos were marked by local application of the vital dye Dil, and the fate of marked cells was analyzed after cultivation of the embryos for 2 days in vitro.The presumptive area of digestive tract ranging from the posterior pharynx to the jejunum was found to extend bilaterally from the midline of the 1.5-day embryo with a width two or three times as great as the distance between the midline and the lateral edge of the somite. Either side of this area contributed to the same side of the endodermal tube of digestive tract. The anterior and posterior portions generally contributed to the anterior and posterior regions of the digestive tract, respectively, and the cells originating from the portion farther from the midline took the more ventral and posterior position in the digestive tract endoderm. Most of the presumptive areas of the digestive organs in the endoderm of 1.5-day embryo were located in a more anterior position than those in the splanchnic mesoderm.     

Origin of the brushborder in the differentiating midgut of Melasoma saliceti (Chrysomelidae, Coleoptera) embryos

The embryonic development of Melasoma saliceti takes eight days at room temperature. At the beginning of the 5th day the endoderm cells have already formed a unilayered epithelium of the midgut primordium. The midgut epithelium is formed by flat cells that are not connected by specialized intercellular junctions. Large vesicles can be seen in dilated intercellular spaces of the epithelium. Cytoplasmic projections, similar to microvilli, appear in the vesicles. During the 5th day ofdevelopment, the vesicles grow and become enclosed by the intercellular junctions of a zonula adherens type. During the 6th day of development the cell junctions surrounding the vesicles become transformed into a septate type. On the 8th day of development the vesicles come close to the apical sides of the midgut cells and open towards the yolk. At the same time the microvilli spread over the apical surface of the midgut primordium to form the regular brushborder of the larval midgut. In the species studied the vesicles appear to "prefabricate" the apical surfaces of the future midgut epithelium.     

Fate mapping study of the endoderm in the posterior part of the 1.5-day-old chick embryo

Various endodermal sites posterior to the caudal-most somite were marked in ovo with the vital dye Dil, and the fate of marked endoderm was analyzed after 2 or 3 days' reincubation. The endoderm in this area became gut epithelium posterior to the caudal jejunum and yolk sac. The area occupied by the cells that were to contribute to the dorsal part of the digestive tube lay centrally around the area overlaid by axial and paraxial mesoderm, with the preventral digestive area lying outside with considerable overlapping, which was surrounded by the preyolk sac area. During the formation of the posterior digestive tube, the endoderm was elongated anteroposteriorly to a considerable degree. Cells that contributed to the cloaca and those that produced descendants in the large intestine occupied similar areas posterior to the center of the sinus rhomboidalis, which were included in the pre-ileal area extending more anteriorly. Prejejunal cells generally localized in a more anterior position than pre-ileal cells. Pre-allantoic cells were located in a rather small area around the posterior primitive streak.     

Cdx2 initiates histodifferentiation of the midgut endoderm

Null mutation or haploinsufficiency of Cdx2 results in the development of heterotopic lesions with a gastric phenotype in the midgut endoderm. Conversely transgenic expression of Cdx2 in the stomach causes the endoderm to differentiate into intestinal-type mucosa. We demonstrate that the mesoderm adjacent to intestinal heterotopic areas expresses stomach specific Barx1 while the surrounding mesoderm is Barx1 negative. We conclude that the initiation of gut histodifferentiation lies in the endodermal expression of Cdx2 and that endodermal/mesodermal cross-talk involving Barx1 with appropriate feedback loops results in the development of the postnatal gut phenotype.     

Development of chick axial mesoderm: specification of prechordal mesoderm by anterior endoderm-derived TGFbeta family signalling

Two populations of axial mesoderm cells can be recognised in the chick embryo, posterior notochord and anterior prechordal mesoderm. We have examined the cellular and molecular events that govern the specification of prechordal mesoderm. We report that notochord and prechordal mesoderm cells are intermingled and share expression of many markers as they initially extend out of Hensen's node. In vitro culture studies, together with in vivo grafting experiments, reveal that early extending axial mesoderm cells are labile and that their character may be defined subsequently through signals that derive from anterior endodermal tissues. Anterior endoderm elicits aspects of prechordal mesoderm identity in extending axial mesoderm by repressing notochord characteristics, briefly maintaining gsc expression and inducing BMP7 expression. Together these experiments suggest that, in vivo, signalling by anterior endoderm may determine the extent of prechordal mesoderm. The transforming growth factor (beta) (TGFbeta) superfamily members BMP2, BMP4, BMP7 and activin, all of which are transiently expressed in anterior endoderm mimic distinct aspects of its patterning actions. Together our results suggest that anterior endoderm-derived TGFbetas may specify prechordal mesoderm character in chick axial mesoderm.     

A Drosophila growth factor homolog, decapentaplegic, regulates homeotic gene expression within and across germ layers during midgut morphogenesis

The decapentaplegic (dpp) gene product, a member of the transforming growth factor-beta family, is required in Drosophila embryos for normal gastrulation and the establishment of dorsal-ventral polarity in the embryo. dpp is also expressed at specific positions in the visceral mesoderm along the developing midgut. We find that mutations that eliminate the visceral mesoderm expression of dpp lead to defects in midgut morphogenesis and alter the spatially localized expression of the homeotic genes Sex combs reduced (Scr), Ultrabithorax (Ubx), and Antennapedia (Antp) in the visceral mesoderm. The extracellular dpp protein migrates from the visceral mesoderm across the apposing endodermal cell layer in a region of the endoderm that expresses the homeotic gene labial (lab). Mesodermal expression of dpp is required for the expression of lab in these endodermal cells indicating that dpp mediates an inductive interaction between the two germ layers. We propose that extracellular dpp protein regulates gut morphogenesis, in part, by regulating homeotic gene expression in the visceral mesoderm and endoderm of the developing midgut.     

Regionalization of cell fates and cell movement in the endoderm of the mouse gastrula and the impact of loss of Lhx1(Lim1) function

Investigation of the developmental fates of cells in the endodermal layer of the early bud stage mouse embryo revealed a regionalized pattern of distribution of the progenitor cells of the yolk sac endoderm and the embryonic gut. By tracing the site of origin of cells that are allocated to specific regions of the embryonic gut, it was found that by late gastrulation, the respective endodermal progenitors are already spatially organized in anticipation of the prospective mediolateral and anterior-posterior destinations. The fate-mapping data further showed that the endoderm in the embryonic compartment of the early bud stage gastrula still contains cells that will colonize the anterior and lateral parts of the extraembryonic yolk sac. In the Lhx1(Lim1)-null mutant embryo, the progenitors of the embryonic gut are confined to the posterior part of the endoderm. In particular, the prospective anterior endoderm was sequestered to a much smaller distal domain, suggesting that there may be fewer progenitor cells for the anterior gut that is poorly formed in the mutant embryo. The deficiency of gut endoderm is not caused by any restriction in endodermal potency of the mutant epiblast cells but more likely the inadequate allocation of the definitive endoderm. The inefficient movement of the anterior endoderm, and the abnormal differentiation highlighted by the lack of Sox17 and Foxa2 expression, may underpin the malformation of the head of Lhx1 mutant embryos.     

Tissue-specific requirements for the proprotein convertase furin/SPC1 during embryonic turning and heart looping

Furin, the mammalian prototype of a family of serine proteases, is required for ventral closure and axial rotation, and formation of the yolk sac vasculature. Here we show additionally that left-sided expression of pitx2 and lefty-2 are also perturbed in Furin-deficient embryos. These tissue abnormalities are preceded by a marked delay in the expansion of the definitive endoderm during gastrulation. Using a chimera approach, we show that Furin activity is required in epiblast derivatives, including the primitive heart, gut and extraembryonic mesoderm, whereas it is nonessential in the visceral endoderm. Thus, chimeric embryos, derived by injecting wild-type embryonic stem (ES) cells into fur(-/-) blastocysts, develop normally until at least 9.5 d.p.c. In contrast, Furin-deficient chimeras developing in the context of wild-type visceral endoderm fail to undergo ventral closure, axial rotation and yolk sac vascularization. Fur(-/-) cells are recruited into all tissues examined, including the yolk sac vasculature and the midgut, even though these structures fail to form in fur mutants. The presence of wild-type cells in the gut strikingly correlates with the ability of chimeric embryos to undergo turning. Overall, we conclude that Furin activity is essential in both extraembryonic and precardiac mesoderm, and in definitive endoderm derivatives.     

Drosophila endoderm development requires a novel homeobox gene which is a target of Wingless and Dpp signalling.

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.     

Fate and plasticity of the endoderm in the early chick embryo

In vertebrates, the endoderm is established during gastrulation and gradually becomes regionalized into domains destined for different organs. Here, we present precise fate maps of the gastrulation stage chick endoderm, using a method designed to label cells specifically in the lower layer. We show that the first population of endodermal cells to enter the lower layer contributes only to the midgut and hindgut; the next cells to ingress contribute to the dorsal foregut and followed finally by the presumptive ventral foregut endoderm. Grafting experiments show that some migrating endodermal cells, including the presumptive ventral foregut, ingress from Hensen's node, not directly into the lower layer but rather after migrating some distance within the middle layer. Cell transplantation reveals that cells in the middle layer are already committed to mesoderm or endoderm, whereas cells in the primitive streak are plastic. Based on these results, we present a revised fate map of the locations and movements of prospective definitive endoderm cells during gastrulation.     

Signals from lateral plate mesoderm instruct endoderm toward a pancreatic fate

During embryonic development, organs arise along the gut tube as a series of buds in a stereotyped anterior-posterior (A-P) pattern. Using chick-quail chimeras and in vitro tissue recombination, we studied the interactions governing the induction and maintenance of endodermal organ identify focusing on the pancreas. Though several permissive signals in pancreatic development have been previously identified, here we provide evidence that lateral plate mesoderm sends instructive signals to the endoderm, signals that induce expression of the pancreatic genes Pdx1, p48, Nkx6.1, glucagon, and insulin. Moreover, this instructive signal directs cells to form ectopic insulin-positive islet-like clusters in endoderm that would otherwise form more rostral organs. Once generated, endocrine cells no longer require interaction with mesoderm, but nonendocrine cells continue to require permissive signals from the mesoderm. Stimulation of activin, BMP, or retinoic acid signaling is sufficient to induce Pdx1 expression in endoderm anterior to the pancreas. Lateral plate mesoderm appears to pattern the endoderm in a posterior-dominant fashion as first noted in the patterning of the neural tube at the same embryonic stage. These findings argue for a central role of the mesoderm in coordinating the A-P pattern of all three primary germ layers.     

Cell autonomous commitment to an endodermal fate and behaviour by activation of Nodal signalling

In vertebrates the endoderm germ layer gives rise to most tissues of the digestive tract and controls head and heart morphogenesis. The induction of endoderm development relies on extracellular signals related to Nodals and propagated intracellularly by TGFbeta type I receptors ALK4/Taram-A. It is unclear, however, whether Nodal/ALK4/Taram-A signalling is involved only in the specification of endodermal precursors or plays a more comprehensive role in the activation of the endodermal program leading to the irreversible commitment of cells to the endodermal fate. Using cell transplantation experiments in zebrafish, we show that marginal cells become committed to endoderm at the onset of gastrulation and that commitment to endoderm can be reached by intracellular activation of the Nodal pathway induced by expression of an activated form of the taram-A receptor, Tar*. In a manner similar to endoderm progenitors, Tar*-activated blastomeres translocate from their initial site of implantation in the blastoderm to reach the surface of their migration substratum, the yolk syncitial layer, where they join endogenous endodermal derivatives during gastrulation and differentiate according to their anteroposterior position. We demonstrate that Nodal/Tar*-induced commitment does not rely on a secondary signal released by Tar*-expressing cells or a signal released by endogenous endoderm since Tar*-expressing wild-type cells can restore endoderm derivatives when transplanted into the endoderm-deficient mutant casanova. Likewise, the YSL does not appear essential for the maintenance of endodermal identity during gastrulation once the Nodal pathway has been activated. Thus, our results demonstrate that the activation of Nodal signalling is sufficient to commit cells both to an endodermal fate and behaviour. Wild-type endoderm implantation into casanova embryos rescues, in a non-autonomous fashion, the defective fusion of the two heart primordia in the midline, highlighting the importance of endoderm for normal heart morphogenesis.     

Differentiation of the embryonic disc, amnion, and yolk sac in the rhesus monkey

The inner cell mass of the blastocyst has differentiated into epiblast and hypoblast (primitive endoderm) prior to implantation. Since endoderm cells extend beyond the epiblast, it can be considered that both parietal and visceral endoderm are present. At implantation, epiblast cells begin to show marked evidence of polarity. They form a spherical aggregate with their basal ends toward the basal lamina and apical ends toward the interior. The potential for an internal space is formed by this change in polarity of the cells. No cytological evidence of separation of those cells that will form amniotic epithelium from the rest of the epiblast is seen until a cavity begins to form. The amniotic epithelium is originally contiguous with overlying cytotrophoblast, and a diverticulum remains in this position during early development. Epiblast forms a pseudostratified columnar epithelium, but dividing cells are situated toward the amniotic cavity rather than basally. The first evidence of a trilaminar disc occurs when a strand of cells contiguous with epiblast is found extending toward visceral endoderm. These presumptive mesoderm cells are undifferentiated, whereas extraembryonic mesoderm cells are already a distinct population forming extracellular materials. After implantation, visceral endoderm cells proliferate forming an irregular layer one to three cells thick. Visceral endoderm cells have smooth apical surfaces, but very irregular basal surfaces, and no basal lamina. At the margins of the disc, visceral endoderm is continuous with parietal endoderm and reflects back over the apices of the marginal visceral endoderm cells. This sacculation by visceral endoderm cells precedes pinching off of the secondary yolk sac from the remaining primary yolk sac.