Regional specification during embryogenesis in the craniiform brachiopod Crania anomala

A fate map has been constructed for the embryo of Crania. The animal half of the egg forms the ectodermal epithelium of the larva's apical lobe. The vegetal half of the egg forms endoderm, mesoderm, and the ectoderm of the mantle lobe. The vegetal pole is the site of gastrulation; this site becomes the posterior ventral region of the mantle lobe of the larva. The plane of the first cleavage goes through the animal-vegetal axis of the egg; it bears no relationship to the future plane of bilateral symmetry of the larva. The timing of regional specification was examined by isolating animal, vegetal, or meridional halves from oocytes, eggs, or embryos from prior to germinal vesicle breakdown through gastrulation. Animal halves isolated from oocytes formed either the epithelium of the apical lobe or a larva with all three germ layers. Animal halves isolated from unfertilized eggs and eight-cell embryos formed only apical lobe epithelium. Beginning at the blastula stage, animal halves formed mantle in addition to apical lobe epithelium. In animal halves isolated after gastrulation, the mantle lobe was always truncated. Vegetal halves isolated at all stages prior to gastrulation gastrulated and formed apical and mantle lobes with endoderm and mesoderm; however, the relative size of the apical lobe that formed decreased substantially when vegetal halves were isolated at later developmental stages. When meridional halves were isolated from unfertilized eggs and two- to four-cell embryos, both halves frequently formed normally proportioned larvae. Beginning at the blastula stage, a number of pairs frequently had a member that lacked dorsal setae on its mantle lobe while the other member of the pair formed setae, indicating that the dorsoventral axis had been set up. The process of regional specification in Crania is compared to those of Discinisca and Glottidia in the brachiopod subphylum Linguliformea and Phoronis in the phylum Phoronida.     

Regional specification during embryogenesis in Rhynchonelliform brachiopods

Fate maps have been constructed for embryos of Hemithiris and Terebratulina, representatives of two orders in the class Rhynchonellata that have been separated since the middle Ordovician. These fate maps are identical. The animal region of the egg forms the ectodermal covering of the apical lobe and the vegetal region is the site of gastrulation; the vegetal region forms the ectoderm of the ventral and posterior regions of the larva, endoderm and mesoderm. The cells that make up the animal region shift 90 degrees with respect to the vegetal pole during gastrulation. The timing and mode of regional specification in these two species are also identical. In each case, the animal region of the unfertilized egg has the intrinsic ability to form apical lobe ectoderm, while the vegetal region has the ability to form a normal larva. During embryogenesis, the vegetal region interacts with the animal region to suppress apical tuft differentiation in the apical lobe and to promote mantle lobe ectodermal differentiation, while the ability of the vegetal half to regulate by forming apical lobe structures is lost. The plane of bilateral symmetry of the larva begins to be set up between the late blastula and early gastrula stage. The fate maps and the processes of regional specification are compared in the four subphyla that make up the Brachiopoda and used to test a developmental model that provides an explanation for the variety of different body plans generated during the Cambrian.     

Blood cells and coelomocytes of the inarticulate brachiopod Lingula anatina

Three cell types are described from the coelomic cavity of the pedicle of the brachiopod Lingula anatina . Erythrocytes are abundant in the blood vessels of the mantle and also occur, in reduced numbers, in the pedicle. Phagocytic amoebocytes, characterized by a variable number of electrondense, homogeneous granules are common in the coelomic fluid of the pedicle. The enigmatic spindle bodies described by earlier authors constitute the most common cell type encountered in the pedicle coelom of aquarium-maintained specimens. The origin of spindle bodies from muscle cells is suggested.     

Predatory borings in the inarticulate brachiopod Artiotreta from the Silurian of Oklahoma

A sample containing inarticulate acrotretid brachiopods, from the Silurian Clarita Formation of Oklahoma, was studied to determine the occurrence, nature and frequency of boring in the shells, and the possible identity of the boring organism. Specimens of Artiotreta parva Ireland, 1961 are abundant and specimens of Opsiconidion n. sp. and Acrotretella siluriana Ireland, 1961 are rare in the samples studied. All of the borings discovered are in A. parva except for one, which is in a specimen of O. n. sp. The boreholes are considered to be of predatory origin, and are similar in morphology to borings made by Recent gastropods. The small countersunk boreholes are equally frequent on pedicle and brachial valves, but preferentially located in particular regions of each valve. Boring frequency is high. The identity of the boring organism has not been determined, but is considered to be a soft bodied predator or an archaeo-gastropod.     

The nature of siliceous mosaics forming the first shell of the brachiopod Discinisca

The juvenile shell of the brachiopod Discinisca consists of a mosaic of micrometer-sized siliceous tablets embedded in a chitinous substrate. The first-formed tablets are secreted on glycocalyx by a newly differentiated collective of outer epithelial cells. They are mainly rhombic but may also be ellipsoidal, discoidal, or deformed and sporadically overlap one another. On the surrounding juvenile shell, secreted by an incipient outer mantle lobe, the tablets are nearly all perfect rhombic plates in rhombic arrays. Their constant size, arrangement, and centripetal crystallization suggest intracellular assembly. The tablets, which are normally bilamellar, consist of discrete aggregates of crystalline spherules of silica in rhombic arrays within an organic matrix of fibrous protein and, presumably, a soluble polysaccharide(s). Mosaic secretion ceases at about the time when juveniles settle on the sea bed, which more or less coincides with the secretion of a ring of lamellae around the mosaic, induced by rapid advances and retractions of the outer mantle lobe prior to deposition of the organophosphatic mature shell. Energy dispersion X-ray analyses of pelagic and newly settled juveniles show that phosphatic secretion, even in the site of the first-formed outer epithelial collective, does not begin until all siliceous secretion has ceased.     

Step-wise specification of retinal stem cells during normal embryogenesis

The specification of embryonic cells to produce the retina begins at early embryonic stages as a multi-step process that gradually restricts fate potentials. First, a subset of embryonic cells becomes competent to form retina by their lack of expression of endo-mesoderm-specifying genes. From these cells, a more restricted subset is biased to form retina by virtue of their close proximity to sources of bone morphogenetic protein antagonists during neural induction. During gastrulation, the definitive RSCs (retinal stem cells) are specified as the eye field by interactions with underlying mesoderm and the expression of a network of retina-specifying genes. As the eye field is transformed into the optic vesicle and optic cup, a heterogeneous population of RPCs (retinal progenitor cells) forms to give rise to the different domains of the retina: the optic stalk, retinal pigmented epithelium and neural retina. Further diversity of RPCs appears to occur under the influences of cell-cell interactions, cytokines and combinations of regulatory genes, leading to the differentiation of a multitude of different retinal cell types. This review examines what is known about each sequential step in retinal specification during normal vertebrate development, and how that knowledge will be important to understand how RSCs might be manipulated for regenerative therapies to treat retinal diseases.     

Expression of evolutionarily conserved eye specification genes during Drosophila embryogenesis

Eye specification in Drosophila is thought be controlled by a set of seven nuclear factors that includes the Pax6 homolog, Eyeless. This group of genes is conserved throughout evolution and has been repeatedly recruited for eye specification. Several of these genes are expressed within the developing eyes of vertebrates and mutations in several mouse and human orthologs are the underlying causes of retinal disease syndromes. Ectopic expression in Drosophila of any one of these genes is capable of inducing retinal development, while loss-of-function mutations delete the developing eye. These nuclear factors comprise a complex regulatory network and it is thought that their combined activities are required for the formation of the eye. We examined the expression patterns of four eye specification genes, eyeless (ey), sine oculis (so), eyes absent (eya), and dachshund (dac) throughout all time points of embryogenesis and show that only eyeless is expressed within the embryonic eye anlagen. This is consistent with a recently proposed model in which the eye primordium acquires its competence to become retinal tissue over several time points of development. We also compare the expression of Ey with that of a putative antennal specifying gene Distal-less (Dll). The expression patterns described here are quite intriguing and raise the possibility that these genes have even earlier and wide ranging roles in establishing the head and visual field.     

The role of micromere signaling in Notch activation and mesoderm specification during sea urchin embryogenesis.

In the sea urchin embryo, the micromeres act as a vegetal signaling center. These cells have been shown to induce endoderm; however, their role in mesoderm development has been less clear. We demonstrate that the micromeres play an important role in the induction of secondary mesenchyme cells (SMCs), possibly by activating the Notch signaling pathway. After removing the micromeres, we observed a significant delay in the formation of all mesodermal cell types examined. In addition, there was a marked reduction in the numbers of pigment cells, blastocoelar cells and cells expressing the SMC1 antigen, a marker for prospective SMCs. The development of skeletogenic cells and muscle cells, however, was not severely affected. Transplantation of micromeres to animal cells resulted in the induction of SMC1-positive cells, pigment cells, blastocoelar cells and muscle cells. The numbers of these cell types were less than those found in sham transplantation control embryos, suggesting that animal cells are less responsive to the micromere-derived signal than vegetal cells. Previous studies have demonstrated a role for Notch signaling in the development of SMCs. We show that the micromere-derived signal is necessary for the downregulation of the Notch protein, which is correlated with its activation, in prospective SMCs. We propose that the micromeres induce adjacent cells to form SMCs, possibly by presenting a ligand for the Notch receptor.     

Oocyte and egg organization in the patellogastropod Lottia and its bearing on axial specification during early embryogenesis

In the basal gastropod Lottia, the apical region of the oocyte is normally the site where the meiotic apparatus attaches and polar body formation occurs following fertilization. This site marks the animal-vegetal axis of the egg. A stereotypical cleavage pattern is organized, and the segregation of developmental potential occurs along this axis during early development. The segregation of developmental potential is a relatively late event and probably does not start until after cleavage begins. By compressing oocytes during the process of germinal vesicle breakdown, the position where the meiotic apparatus attaches to the cell membrane can be altered so that it no longer corresponds to the apical end of the oocyte. This new site of polar body formation sets up a new animal-vegetal axis that organizes cleavage and the segregation of developmental potential. The timing of animal-vegetal axis specification in Lottia is much later than it is in derived gastropods with a precocious specification of the D quadrant.     

Pattern formation: Regional specification in the early C. elegans embryo

Recent findings suggest that C. elegans, albeit displaying an invariant cell lineage for embryonic development, uses the same basic strategy for embryogenesis as other organisms. The early embryo is regionalised by cell-cell interactions.     

Regional specification of cutaneous appendages in mammals

Summary The problem of the regional specification of snout vibrissae and dorsal pelage hairs has been analysed in mouse embryos. Reconstituted homo-and heterotopic skin explants, consisting of epidermis and dermis from both regions, were cultured on the chorioallantoic membrane of the chick embryo.Recombinants of 12.5-day upper lip dermis and 12.5-day dorsal epidermis developed a small number of large vibrissal type follicles arranged in a recognizable rectangular vibrissal pattern. The reverse combinations of 12.5- or 14.5-day dorsal dermis and 11- to 12.5-day upper lip epidermis formed a single population of numerous and small follicles arranged in a typical pelage hair pattern (trio groups) or gave rise to a mixed population of follicles with both whiskers and pelage hairs.It is concluded that the dermis is responsible for the regional specification of the cutaneous appendages and their distribution pattern. However, at the time it was isolated, the upper lip epidermis already possesses the information for the morphogenesis of vibrissae, but remains malleable and responsive to the dermal influence.This work was supported in part by DGRST and CNRS     

Auxin triggers transient local signaling for cell specification in Arabidopsis embryogenesis

The Arabidopsis embryonic root meristem is initiated by the specification of a single cell, the hypophysis. This event critically requires the antagonistic auxin response regulators MONOPTEROS and BODENLOS, but their mechanism of action is unknown. We show that these proteins interact and transiently act in a small subdomain of the proembryo adjacent to the future hypophysis. Here they promote transport of auxin, which then elicits a second response in the hypophysis itself. Our results suggest that hypophysis specification is not the direct result of a primary auxin response but rather depends on cell-to-cell signaling triggered by auxin in adjacent cells.     

Regional specification of the endoderm in the early chick embryo

In the avian embryo, the endoderm, which forms a simple flat-sheet structure after gastrulation, is regionally specified in a gradual manner along the antero-posterior and dorso-ventral axes, and eventually differentiates into specific organs with defined morphologies and gene expression profiles. In our study, we carried out transplantation experiments using early chick embryos to elucidate the timing of fate establishment in the endoderm. We showed that at stage 5, posteriorly grafted presumptive foregut endoderm expressed CdxA , a posterior endoderm marker, but not cSox2 , an anterior endoderm marker. Conversely, anteriorly grafted presumptive mid-hindgut endoderm expressed cSox2 but not CdxA . At stage 8, posteriorly grafted presumptive foregut endoderm also expressed CdxA and not cSox2 , but anteriorly grafted presumptive mid-hindgut endoderm showed no changes in its posterior-specific gene expression pattern. At stage 10, both posteriorly grafted foregut endoderm and anteriorly grafted mid-hindgut endoderm maintain their original gene expression patterns. These results suggest that the regional specification of the endoderm occurs between stages 8 and 10 in the foregut, and between stages 5 and 8 in the mid-hindgut.     

Position-specific expression of the annulin protein during grasshopper embryogenesis.

Annulin, named for its annular expression in developing limb buds, is a approximately 100 kDa membrane-associated protein that is expressed in a complex and changing pattern during grasshopper embryogenesis. Its expression is dynamic along the developing midline and in the mesoderm, transient in neuroepithelial sheath cells around mitotic neuroblasts, and position-specific in circumferential stripes in each limb bud segment. Annulin expression begins along the midline of the embryo at the onset of gastrulation. Mesoderm cells express the protein as they migrate away from the midline as do new cells that come to lie at the midline. During neurogenesis, annulin expression disappears from many midline cells until only a specific subset of midline glial cells expresses high levels of the protein. Starting at the beginning of neurogenesis, sheath cells express annulin in correlation with the mitotic activity of the neuroblasts they surround.     

Tissue-specific expression of onco-fetal antigens during embryogenesis.

It has become evident from recent literature that especially in tumor virus systems, cell transformation leads to an arrest of differentiation or to a retrodifferentiation. This may be reflected by the expression of embryonic antigens and it is therefore particularly important to characterize such antigens according to their specificity as well as to their specificity during embryogenesis. We have demonstrated the expression of embryonic antigens which are cross-reactive in avian fibroblasts transformed either by Rous sarcoma virus or by methylcholanthrene. This paper is intended to demonstrate that these embryonic antigens are detected only at a certain period of embryogenesis and particularly in muscle cells. They are detected only occasionally or not at all in cells of other tissues such as brain, liver, lung, and the digestive organs. These antigens are absent from the target cells before transformation and are consequently induced by the transforming agent, either viral or chemical. Therefore, these results suggest that by transformation mechanism, cells become specifically reverted to an earlier stage of differentiation (retrodifferentiation).