Vasa expression and germ-cell specification in the spider mite Tetranychus urticae

The specification of germ cells is an important process during the development of all animals. Expression of an evolutionarily conserved gene such as vasa can be used as a marker for germ cell fate. We have isolated a vasa-related gene from the two-spotted spider mite (Tetranychus urticae) and used it to examine the segregation of germ cells in this animal. In spider mites, vasa expression first appears in a group of cells that do not join the initial blastoderm surface. Instead, these cells remain in the interior of the blastoderm and then migrate to posterior regions of the embryo, where they form a cluster that appears in regions of the embryo consistent with the gonads. The expression pattern of this spider mite vasa homologue implies a novel process acts to specify germ cells in this species and that the specification of germ cells is an evolutionarily labile process.

Induction of the avian coelom with associated vitelline blood circulation by Rauber's sickle derived junctional endoblast and its fundamental role in heart formation

In histological sections through chicken blastoderms of different ages we describe the temporospatial relationship between junctional endoblast, the formation of blood islands (appearing first from a peripherally migrating mesoblastic blastema), and the formation of coelomic vesicles developing later in/and from a more superficially extending mesoblastic blastema (coelomic mesoblast). After unilateral removal of the Rauber's sickle-derived junctional endoblast in early streak blastoderms (stage 2-4; Vakaet [1970] Arch Biol 81:387-426) and culture to stage 11 (Hamburger and Hamilton [1951] J Morphol 88:49-92), we observed that the early formation of the coelomic cavity was locally or totally disturbed in the operated area. Besides the simultaneous absence of blood islands, the coelomic vesicles did not form normally. Instead of regularly aligned coelomic vesicles, progressively forming the coelomic cavity by fusion, some voluminous irregular cavities appeared. Thus, the extent of the coelomic cavity was greatly reduced and the operated side was considerably smaller than the unoperated side. Furthermore, in the youngest operated blastoderms the cranial portion of the involved coelomic cavity (hemipericardial cavity) exhibited rudimentary development and usually did not reach the region of the foregut endoderm. This resulted in the absence of the myoepicardium and associated endocardium at this side. In another experiment, after removal of the junctional endoblast at one side of the chicken blastoderm, a fragment of quail junctional endoblast was placed isotopically. This resulted, after further in vitro culture, in the restoration of the formation of coelomic vesicles and accompanying subjacent blood islands in the immediate neighborhood of the apposed quail junctional endoblast. Also, the pericardium and primary heart tube developed normally. Similarly, by using the quail-chicken chimera technique, we demonstrated that the splanchnic mesoderm cells of the pericardium develop in intimate association with the most cranial part of the junctional endoblast (derived from the Rauber's sickle horns). Our experiments indicate that the coelom and, in particular, the pericardium and primary heart tube form progressively (in time and space) under the inductory influence of Rauber's sickle and junctional endoblast.

Expression of pair-rule gene homologues in a chelicerate: early patterning of the two-spotted spider mite Tetranychus urticae

Embryo segmentation has been studied extensively in the fruit fly, Drosophila. These studies have demonstrated that a mechanism acting with dual segment periodicity is required for correct patterning of the body plan in this insect, but the evolutionary origin of the mechanism, the pair-rule system, is unclear. We have examined the expression of the homologues of two Drosophila pair-rule genes, runt and paired (Pax Group III), in segmenting embryos of the two-spotted spider mite (Tetranychus urticae Koch). Spider mites are chelicerates, a group of arthropods that diverged from the lineage leading to Drosophila at least 520 million years ago. In T. urticae, the Pax Group III gene Tu-pax3/7 was expressed during patterning of the prosoma, but not the opisthosoma, in a series of stripes which appear first in even numbered segments, and then in odd numbered segments. The mite runt homologue (Tu-run) in contrast was expressed early in a circular domains that resolved into a segmental pattern. The expression patterns of both of these genes also indicated they are regulated very differently from their Drosophila homologues. The expression pattern of Tu-pax3/7 lends support to the possibility that a pair-rule patterning mechanism is active in the segmentation pathways of chelicerates.

How one becomes many: blastoderm cellularization in Drosophila melanogaster

Embryonic development in Drosophila melanogaster begins with a rapid series of mitotic nuclear divisions, unaccompanied by cytokinesis, to produce a multi-nucleated single cell embryo, the syncytial blastoderm. The syncytium then undergoes a process of cell formation, in which the individual nuclei become enclosed in individual cells. This process of cellularization involves integrating mechanisms of cell polarity, cell-cell adhesion and a specialized form of cytokinesis. The detailed molecular mechanism, however, is highly complex and, despite extensive analysis, remains poorly understood. Nevertheless, new insights are emerging from recent studies on aspects of membrane polarization and insertion, which show that membrane components from intracellular organelles are involved. In addition, actin and actin-associated proteins have been heavily implicated while new evidence shows that microtubule cytoskeletal elements are mechanistically involved in all aspects of cellularization. This review will draw on both the traditional models and the new data to provide a current perspective on the nature of cellular blastoderm formation in Drosophila melanogaster.

Slow as Molasses is required for polarized membrane growth and germ cell migration in Drosophila

Drosophila germ cell migration is directed by attractive and repulsive guidance cues. We have identified a novel gene, slow as molasses (slam), which is required for germ cell migration. In slam zygotic mutants, germ cells fail to transit off the midgut into the mesoderm. We show that slam is required at this stage in parallel to HMG Coenzyme A reductase, a previously identified germ cell migration gene. Removal of both zygotic and maternal slam results in an earlier defect: a failure to form a cellular blastoderm. Consistent with this phenotype, we found that slam is one of the earliest genes to be transcribed in the embryo, and Slam protein localizes to the growing basal-lateral membrane during blastoderm formation, but Slam is not detected during later stages of embryogenesis. Because slam RNA and protein are expressed earlier than the time when we observe defects in germ cell migration, we propose that Slam is required for the localization of a signal to the basal side of blastoderm cells that is needed later in the posterior midgut to guide germ cells.

In the absence of Rauber's sickle material, no blood islands are formed in the avian blastoderm

Using the quail-chick chimera technique, we followed the fate of Rauber's sickle cells in older whole blastoderms (cultured for approximately 2 days): after removal of the autochthonous Rauber's sickle from an unincubated chicken blastoderm, a quail Rauber's sickle was grafted isotopically and isochronically in its place. In transverse sections through these chimeras, the grafted quail Rauber's sickle cells were seen to have transformed into a broad row or ridge of quail junctional endoblast cells extending at the inner border of the area containing blood islands. After unilateral removal of the junctional endoblast from an intermediate streak chicken blastoderm (Stage 3; Hamburger and Hamilton [1951] J Morphol 88:49-92), we observed during further in vitro culture that at the operated side, in the area previously occupied by this junctional endoblast, blood islands no longer developed. If after such a unilateral removal of the chicken junctional endoblast quail junctional endoblast was apposed in its place, then blood islands reappeared in the operated area. The intimate contact between the apposed quail junctional endoblast and the recently formed blood islands, derived from peripherally migrating mesoderm, was very obvious on sections through such chimeras. We further demonstrate that Rauber's sickle vs. junctional endoblast is indispensable for the anlage of blood islands in avian blastoderms. Indeed, in the absence of Rauber's sickle material no blood islands develop (even when mesoderm is present after ingression of the upper layer via a primitive streak) in the isolated central region of the area centralis of unincubated chicken blastoderms after culture in vitro. Also, no junctional endoblast and no sickle canal appear in these explants. By contrast, if a Rauber's sickle fragment is placed on such an isolated central blastoderm region, then blood islands develop. These blood islands start to develop from peripherally migrating mesoderm in the neighborhood of the Rauber's sickle-derived junctional endoblast.

The germ cell lineage identified by vas-mRNA during the embryogenesis in goldfish

vas RNA has been identified in germ-line cells and its precursors in zebrafish, with the result that the germ-line lineage can be traced throughout embryogenesis. In the present study, we described vas localization and the migration of vas-positive cells in goldfish, using whole mount in situ hybridization. The signals of vas mRNA localization appeared at the marginal part of the first to third cleavage planes. The eight signals were detected during the period from the 8- cells to the 512-cell stage. At the late-blastula stage, additional numbers of vas-positive cells were observed, suggesting the proliferation of these cells. At the segmentation period, vas-positive cells showed a long extended distribution along the embryonic axis, but did not form any clusters. vas-positive cells were occasionally distributed at the head region, especially around the future otic vesicle. These signals were inherited to the primordial germ cells, suggesting that vas-positive cells were primordial germ cells (PGCs) in goldfish.

Post-nodal mesoblast caudalizes the host axis and inhibits cell population growth, and induces new primitive streaks in chick embryos

One to eight post-nodal fragments (PN) or Hensen's nodes (HN) from full primitive streak stage chick embryos were transplanted onto the area pellucida or area opaca of stage 4 embryos and cultured for 20 h. Thirteen morphological and numerical parameters were affected in the host embryos and analyzed by multiple logistic regression for parametric hierarchy. In the area pellucida, both PN and HN transplants inhibited cell population growth while only PN caudalized the host axis and induced supernumerary primitive streaks expressing the mesoderm-specific gene Brachyury. In the area opaca, neither grafts influenced host axis morphogenesis, but PN inhibited the cell population growth significantly. Tracking [(3)H]TdR labeled grafts showed that PN cells migrated towards the host axis and participated in the formation of supernumerary somites and hearts. When placed near the host axis, PN caudalized it and inhibited cell population growth.

Localization of the fertilized germinal disc in the chicken egg before incubation

At the time of oviposition, the embryo in the fertilized chicken egg has developed the germinal disc on top of the yolk and contains thousands of blastodermal cells. The germinal disc escapes exact localization because it is surrounded by albumen and shell membranes under a calcified shell. This study investigated whether or not ultrasonography or nuclear magnetic resonance imaging (MRI) are suited to localize the germinal disc and, if so, to estimate the accuracy of localization. Hatchability of treated eggs was also recorded. The ultrasound waves were reflected by the shell. The germinal disc could be seen only after removal of the eggshell and the outer shell membrane. In all, 39 intact eggs, MRI localized the germinal disc within 2 mm thick image slices. The mean position of the germinal disc deviated 3.0+/-2.1 mm from the maximum vertical plane of the egg with a mean distance to the inner surface of the shell of 2.7+/-1.1 mm. Incubation resulted in 84.2 and 77.5% hatched chicks for imaged eggs and controls, respectively. The localization results of MRI were verified manually in 38 open shell cultures. Correlation coefficients for the position of the germinal disc were r = 0.86 to 0.99 for the x-axis, r = 0.75 to 0.89 for the y-axis, and r = 0.63 to 0.76 for the z-axis. The study thus shows that MRI is a reliable tool to localize the germinal disc within the intact freshly laid egg.

Localization of cells of the prospective neural plate, heart and somites within the primitive streak and epiblast of avian embryos at intermediate primitive-streak stages

By constructing avian transplantation chimeras using fluorescently-labeled grafts and antibodies specific for grafted cells, we have generated a prospective fate map of the primitive streak and epiblast of the avian blastoderm at intermediate primitive-streak stages (stages 3a/3b). This high-resolution map confirms our previous study on the origin of the cardiovascular system from the primitive streak at these stages and provides new information on the epiblast origin of the neural plate, heart and somites. In addition, the origin of the rostral endoderm is now documented in more detail. The map shows that the prospective neural plate arises from the epiblast in close association with the rostral end of the primitive streak and lies within an area extending 250 microm rostral to the streak, 250 microm lateral to the streak and 125 microm caudal to the rostral border of the streak. The future floor plate of the neural tube arises within the midline just rostral to the streak, confirming our earlier study, but unlike at the late-primitive streak stages when both Hensen's node and the midline area rostral to Hensen's node contribute to the floor plate, only the area rostral to the primitive streak contributes to the floor plate at intermediate primitive-streak stages. Instead of contributing to the floor plate of the neural tube, the rostral end of the primitive streak at intermediate primitive-streak stages forms the notochord as well as the rostromedial endoderm, which lies beneath the prechordal plate mesoderm and extends caudolaterally on each side toward the cardiogenic areas. The epiblast lateral to the primitive streak and caudal to the neural plate contributes to the heart and it does so in rostrocaudal sequence (i.e., rostral grafts contribute to rostral levels of the straight heart tube, whereas progressively more caudal grafts contribute to progressively more caudal levels of the straight heart tube), and individual epiblast grafts contribute cells to both the myocardium and endocardium. The prospective somites (i.e., paraxial mesoderm) lie within the epiblast just lateral to the prospective heart mesoderm. Comparing this map with that constructed at late primitive-streak stages reveals that by the late primitive-streak stages, prospective heart mesoderm has moved from the epiblast through the primitive streak and into the mesodermal mantle, and that some of the prospective somitic mesoderm has entered the primitive streak and is undergoing ingression.

Selection of genetically modified chicken blastodermal cells by magnetic-activated cell sorting

The use of chicken blastodermal cells (CBC) in the production of transgenic chickens requires incorporation of the desired stable genetic modification into CBC. With greater proportions of stably transfected blastodermal cells in the embryo inoculum, the frequency of intermediate chimeric birds is greater. Magnetic-activated cell sorting (MACS) was evaluated as a method for enrichment of transfected CBC. This approach requires surface expression of a molecule that can be recognized by an antibody. Chicken blastodermal cells from fertilized Barred Plymouth Rock eggs were coelectroporated with pmiwZ and pMACS Kk and were sorted magnetically by expression of the mouse H-2Kk molecule on the surface of successfully transfected cells. The effectiveness of sorting was assessed using X-gal staining to detect lacZ expression from the pmiwZ plasmid. After 48 h of culture, lacZ-positive cells appeared to be enriched 1.4-fold in the MACS selected population. Cells from this enriched pool contributed to extra-embryonic and intra-embryonic tissues of 72-h White Leghorn recipient embryos with a marginal increase in levels of intra-embryonic contribution. Our demonstration that transfected, cultured, and magnetically sorted CBC maintain their ability to contribute to ectodermal and mesodermal lineages of intra-embryonic tissues illustrates the potential value of this technique for introducing genetic modifications into birds.

Morphogenesis of gut and distribution of the progenitors of gut endocrine cells at cranial somite levels of the chick embryo

The primary objective of this study was to establish the distribution of the progenitors of selected gut endocrine cell types at cranial somite levels. In addition, analysis of the material has provided new information about the location of the presumptive territories of certain gut regions and of the pancreas. Narrow transverse strips of full-thickness blastoderm two or three somites in length were excised at the levels of somites 1 to 5 of 8.5- to 18-somite chick embryos and cultured as chorioallantoic grafts to an age equivalent to 20 days of incubation. The grafts were analysed by immunocytochemistry, and their morphology was evaluated. Individual grafts exhibited up to five different types of gut morphology, including those of oesophagus, proventriculus, gizzard, pyloric region, small intestine, and pancreas. The morphologic survey yielded new information about the location, extent, or both, of the territories of the pyloric region, the small intestine, and the pancreas. In general, the progenitors of gut endocrine cell types identified were those expected for the different morphologic regions: in only a few instances were ectopic endocrine cell types detected. The available evidence points to the progenitors of bombesin/gastrin-releasing peptide cells being located cranial to somite 5 at the stages studied. Based on the morphology and the proportion of insulin cells, the development of pancreas in grafts appeared compromised compared with grafts of the intact dorsal pancreatic bud: this may relate to the likely exclusion of dorsal pancreatic bud mesoderm from the graft area. The results show that presumptive small intestinal endoderm in grafts can differentiate in the absence of homologous (i.e., small intestinal) mesoderm: this accords with the view that the primary source of positional information in the gut is in the endoderm.

Activation of epiblast gene expression by the hypoblast layer in the prestreak chick embryo

Axis formation is a highly regulated process in vertebrate embryos. In mammals, inductive interactions between an extra-embryonic layer, the visceral endoderm, and the embryonic layer before gastrulation are critical both for anterior neural patterning and normal primitive streak formation. The role(s) of the equivalent extra-embryonic endodermal layer in the chick, the hypoblast, is still less clear, and dramatic effects of hypoblast on embryonic gene expression have yet to be demonstrated. We present evidence that two genes later associated with the gastrula organizer (Gnot-1 and Gnot-2) are induced by hypoblast signals in prestreak embryos. The significance of this induction by hypoblast is discussed in terms of possible hypoblast functions and the regulation of axis formation in the early embryo. Several factors known to be expressed in hypoblast, and retinoic acid, synergistically induce Gnot-1 and Gnot-2 expression in blastoderm cell culture. The presence of retinoic acid in prestreak embryos has not yet been directly demonstrated, but exogenous retinoic acid appears to mimic the effects of hypoblast rotation on primitive streak extension, raising the possibility that retinoid signaling plays some role in the pregastrula embryo.

Restriction of proliferation of primordial germ cells by the irradiation of Japanese quail embryos with soft X-rays

Primordial germ cells (PGCs) are the progenitor cells for the gametes. Avian PGCs are located in the central region of the area pellucida at the blastoderm stage. Shortly after further incubation, they migrate to the extra-embryonic germinal crescent, and then as soon as the blood vessels form, they enter the circulation and finally settle in the gonadal primordium. We have developed a simple method using soft X-ray irradiation (18 kV power, 20 cm distance) to reduce the number of PGCs in Japanese quail embryos, which should be useful in preparing recipient embryos for PGC-transfer studies. When embryos were exposed to the soft X-rays for 40 s before incubation, the concentration of circulating PGCs was less than one-fifth that in controls after 2 days of incubation. Embryos at day 6 of incubation contained approximately half the number of PGCs compared to controls when they were exposed before or at day 2 of incubation. Irradiation for 40 s is recommended taking into consideration the restriction of proliferation of PGCs, and viability and hatchability.

Cell cycle progression and cell division are sensitive to hypoxia in Drosophila melanogaster embryos

We and others recently demonstrated that Drosophila melanogaster embryos arrest development and embryonic cells cease dividing when they are deprived of O2. To further characterize the behavior of these embryos in response to O2 deprivation and to define the O2-sensitive checkpoints in the cell cycle, embryos undergoing nuclear cycles 3-13 were subjected to O2 deprivation and examined by confocal microscopy under control, hypoxic, and reoxygenation conditions. In vivo, real-time analysis of embryos carrying green fluorescent protein-kinesin demonstrated that cells arrest at two major points of the cell cycle, either at the interphase (before DNA duplication) or at metaphase, depending on the cell cycle phase at which O2 deprivation was induced. Immunoblot analysis of embryos whose cell divisions are synchronized by inducible String (cdc25 homolog) demonstrated that cyclin B was degraded during low O2 conditions in interphase-arrested embryos but not in those arrested in metaphase. Embryos resumed cell cycle activity within ~20 min of reoxygenation, with very little apparent change in cell cycle kinetics. We conclude that there are specific points during the embryonic cell cycle that are sensitive to the O2 level in D. melanogaster. Given the fact that O2 deprivation also influences the growth and development of other species, we suggest that similar hypoxia-sensitive cell cycle checkpoints may also exist in mammalian cells.

Localization of primordial germ cells or their precursors in stage X blastoderm of chickens and their ability to differentiate into functional gametes in opposite-sex recipient gonads

This study was performed to determine the distribution of primordial germ cells and their precursors in stage X blastoderm of chickens. The blastoderm (Barred Plymouth Rock chickens) isolated from the yolk was separated into three portions: the central disc, the marginal zone and the area opaca. The dissociated blastodermal cells derived from the central disc, marginal zone and area opaca were transferred into a recipient blastoderm (White Leghorn chicken) from which a cell cluster was removed from the centre of the central disc. The manipulated embryos were cultured in host eggshells until hatching. The chicks were raised until sexual maturity and test mated with Barred Plymouth Rock chickens to assess the donor cell contribution to the recipient germline. Germline chimaeric chickens were produced efficiently (46.7%, 7/15) when the blastodermal cells derived from the central disc were transferred into recipient embryos of the same sex, whereas no germline chimaeric chickens were produced when the blastodermal cells derived from the marginal zone or area opaca were transferred into recipient embryos of the same sex (0/12). Germline chimaeric chickens were also produced by transfer of blastodermal cells derived from the central disc (6.7%, 1/15), marginal zone (10.0%, 1/10) or area opaca (11.1%, 1/9) into recipient embryos of the opposite sex. It is concluded that primordial germ cells are induced during or shortly after stage X and that the cells derived from the central disc have the highest potential to give rise to germ cells. Cells derived from the marginal zone and area opaca can also give rise to germ cells, although the frequency is low.

Molecular genetic control of axis patterning during early embryogenesis of vertebrates

Formation of the axis and its subsequent patterning to establish the tube-within-a-tube body plan characteristic of vertebrates are initiated during gastrulation. In higher vertebrates (i.e., birds and mammals), gastrulation involves six key events: establishment of the rostrocaudal/mediolateral axis; formation and progression of the primitive streak and organizer; epiboly of the epiblast, ingression of prospective mesodermal and endodermal cells through the primitive streak, and migration of cells away from the primitive streak; regression of the primitive streak; establishment of the right-left axis; and formation of the tail bud. Over 50 years of study of these processes have provided a morphological framework for understanding how these events occur, and recent advances in imaging, microsurgical intervention, and cell tracking are beginning to elucidate the underlying cell behaviors that drive morphogenetic movements. Moreover, homotopic transplantation and dye microinjection studies are being used to generate high-resolution fate maps, and heterotopic transplantation studies are revealing the cell-cell interactions that are sufficient as well as required for mesodermal and ectodermal commitment. Additionally, the roles of the organizer and secondary signaling centers in establishing the body plan are being defined. With the advent of the molecular/genetic age, the molecular basis for axis formation is beginning to become understood. Thus, it is becoming clear that secreted growth factors/signaling molecules produced by localized signaling centers induce and pattern the axis, presumably through downstream activation of signal-transduction proteins and cascades of transcription factors.

Expression of protein tyrosine kinases and stem cell factor in chicken blastodermal cells

Chicken blastodermal cells (CBC) from Stage X embryos, which were isolated from newly laid, fertile, unincubated eggs, are pluripotent cells and can produce somatic and germline chimeras when injected into recipient stage X embryos. The CBC retain their pluripotential ability for up to 7 d in vitro. The molecular mechanisms that control proliferation and differentiation of CBC are largely unknown, although protein tyrosine kinases (PTK) are known to play important roles in these processes in similar cells. To understand better the molecular mechanisms of proliferation and differentiation in CBC, expression profiles of PTK and stem cell factor (SCF) were analyzed by reverse transcription polymerase chain reaction (RT-PCR) using gene-specific and degenerate oligonucleotide primers. Seventeen distinct PTK, including 14 receptor-type and 3 nonreceptor-type PTK and SCF were identified by RT-PCR. Expression of all of the genes was confirmed by northern blot analysis. The northern blot analysis showed that all probes hybridized with one or more transcripts at various expression levels. The expression of the 17 PTK and SCF genes in CBC suggests that they might play a role in signal transduction pathways that control the proliferation or differentiation in CBC.

Centrosomes and the Scrambled protein coordinate microtubule-independent actin reorganization

In Drosophila syncytial blastoderm embryos, centrosomes specify the position of actin-based interphase caps and mitotic furrows. Mutations in the scrambled locus prevent assembly of mitotic furrows, but do not block actin cap formation. The scrambled gene encodes a protein that localizes to the mitotic furrows and centrosomes. Sced localization, actin reorganization from caps into mitotic furrows, and centrosome-coordinated assembly of actin caps are not blocked by microtubule disruption. Our results indicate that centrosomes may coordinate assembly of cortical actin caps through a microtubule-independent mechanism, and that Scrambled mediates a second microtubule-independent process that drives mitotic furrow assembly.

Improvement of hatchability of chicken eggs injected by blastoderm cells

In our first experiment, we studied the effect of injection method of blastoderm cells (BC) into the subgerminal cavity of White Leghorn embryos on hatchability of chicken chimeras. Freshly laid eggs were injected through a hole made in the equatorial plane of the eggshell (Method A). In Method B, eggs were stored pointed end down for 5 to 7 d prior to injection, and a hole was cut in the blunt end of the eggshell. An advantage of Method B was that the early embryonic mortality was reduced (P < or = 0.01) and resulted in higher hatchability (41.0%; 43/105) than Method A (9.8%; 14/143). In the second experiment, we studied chicken hatchabililty as influenced by windowing (no hole, Group 1; hole in the equatorial plane, Group 2; hole in the blunt end of egg, Groups 3 and 4) and egg turning (Groups 1 and 4) or not (Groups 2 and 3) during incubation. The hatchability percentages were as follows: 67.9 (Group 1) 0.0, (Group 2) 23.3, (Group 3), and 56.8 (Group 4). A statistically significant difference (P < or = 0.05) was noted between Group 1 or 4 and the other groups. We found no statistically significant differences in the weight changes (g) but did note certain differences in the egg weight loss (%) among different egg treatments. In the third experiment, we investigated the influence of origins of BC donors: Rhode Island Red (RIR), Barred Plymouth Rock (BPR), and Green-legged Partridgelike (GP) on hatchability of putative and somatic chimera chickens. The hatchability of chimeras was dependent on the adequate assortment of BC of the donor and ranged from 7.4% (RIR) to 56.1% (GP). In the case of BC injection of the GP breed, good hatchability was accompanied by very high percentage (86.9; 20/23) of somatic chimeras.

Production of chicken chimeras by fusing blastodermal cells with electroporation

AIM

To establish techniques for producing somatic and germline chimeric chicken by transferring blastodermal cells fused with electroporation.

METHODS

Stage-X blastodermal cells isolated from freshly laid fertile unincubated white Leghorn and Rhode Island red chicken eggs were fused with electroporation. The treated cell suspension was transferred to the recovery medium (DMEM containing 10% FBS) and was injected into the subgerminal cavity of recipient unincubated embryos (stage X).

RESULTS

Of 177 recipient embryos injected with the fusing blastodermal cells, 6 (3.4%) survived to hatching. Somatic chimerism was examined in the melanocyte of the feather. The presence of feathers originating from the donor cell was observed in 1 bird (16.7%) out of the 6 hatched birds. After 21 days of incubation two birds out of five embryos were subjected to polymerase chain reaction (PCR) analysis for W-chromosome-specific DNA for each tissue. One bird possessed W-chromosome-specific DNA in the stomach, and the other exhibited the same DNA in the left and right gonads and other tissues, but not the stomach.

CONCLUSION

Recipient embryo having electrofused blastodermal cells yields somatic and germline chimeric chickens more successfully.

Regulation of epiblast cell movements by chondroitin sulfate during gastrulation in the chick

Avian gastrulation is dependent on the ingression of outer layer cells into the interior of the embryo by means of a transient structure referred to as the primitive streak. As the growing streak progresses through the central area pellucida of the blastoderm, selective de-epithelialization of epiblast cells results in the initial migratory cells of the primitive mesoderm and endoderm. Here, we have examined the possibility that extracellular matrix molecules of the epiblast basal lamina influence the selection of streak-specific epiblast cells. By using whole embryo culture, we have found that removal of chondroitin sulfate glycosaminoglycans at gastrulation stages leads to defective streak formation. In situ hybridization with streak-specific markers in these embryos reveals ectopic patterns of gene expression, suggesting that differentiation of primitive streak precursors in the pregastrula epiblast is independent of normal streak morphogenesis. In addition, in vitro assays with chondroitin sulfate containing matrices suggest that specific cells of the epiblast are inhibited from joining the streak during gastrulation. Taken together, these results indicate that the presence of chondroitin sulfate in the epiblast basal lamina facilitates the allocation of cells to the primary germ layers by preventing ectopic axis formation.

A modified method of shell windowing for producing somatic or germline chimeras in fertilized chicken eggs

Stage X chick blastoderms following oviposition were accessed via a small window in the egg. Windowing, however, substantially reduces the hatchability of eggs containing early embryos. For example, only 32 of 389 (8.2%) eggs hatched after standard windowing with or without irradiation or injection. Ex ovo culture systems can overcome this problem but are labor intensive. A modification of a standard windowing technique has yielded an average hatch rate of 32% for 892 windowed eggs independent of incubator type, gamma-irradiation, or injection of the embryo. This was a fourfold increase over a standard windowing method. Similar hatch rates were observed using fertile eggs from five chicken lines [Barred Plymouth Rock (BR), Athens-Canadian (AC), Line 0, SPAFAS, and commercial White Leghorns (WL)]. The modification involves covering the egg shell membrane with PBS after grinding away the shell and before piercing the membrane. The window is then sealed by overlaying with fresh shell membrane and cementing it in place once it has dried. The method has been used successfully for the production of somatic and germline chimeras because donor BR blastodermal cells injected into Stage X, gamma-irradiated recipient embryos from WL or AC yielded a hatch of 33.7%, of which 42.3% were feather chimeras. Two of 11 cockerels tested were germline mosaics bearing at least 1% BR sperm. The modified windowing technique may be broadly applicable in emerging technologies in avian transgenesis and development.