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At least 19 recordsLinked to original sources

PubMed1992

Brown adipose tissue as a derivative of mesoderm grafted below the kidney capsule. A model for differentiation of isolated rat mesoderm.

During development, mesoderm differentiates into connective tissue, cartilage, bone, muscle and kidney. In experimental conditions the developmental spectrum of mesoderm grafted below the kidney capsule is reduced so that mostly brown adipose tissue (BAT) appears. Since BAT is a particular tissue with a specific developmental pattern, the structural and functional characteristics of experimentally developed BAT were analyzed in the present study. Mesoderm from nine-day-old rat embryos was grafted below the kidney capsule of adult rats and one month later the BAT-containing tumors were analyzed. The ultrastructural and morphometrical data of BAT-containing tumors were the same as in BAT developed in situ. Tissue-specific mRNA for uncoupling protein (UCP) was expressed in BAT-containing tumors, and immuno-electron microscopical analysis showed that mitochondria of these brown adipocytes contained UCP. Injections of noradrenaline and exposure of BAT-tumor-bearing rats to cold stress increased both the amount of UCP and the expression of UCP mRNA in tumors of BAT; i.e., experimentally developed BAT entirely resembled standard BAT. It is proposed that mesoderm isolated and displaced below the kidney capsule lacks the inductive stimuli of ectoderm and endoderm, and as a result mesoderm can not realize the natural pattern of differentiation. Here, in a new environment, mesoderm is exposed to new stimuli which induce differentiation of mesoderm into BAT, probably through neuro-vascular elements from the medial side of the kidney (BAT area). Thus, although mesoderm contains a wide differentiation capacity, it can differentiate into only one type of tissue, depending on the presence and range of inductive stimuli.

Adipose Tissue, Brown↗

PubMed1991

Xwnt-8, a Xenopus Wnt-1/int-1-related gene responsive to mesoderm-inducing growth factors, may play a role in ventral mesodermal patterning during embryogenesis.

In amphibian embryos, formation of the basic body plan depends on positional differences in the mesoderm. Although peptide growth factors involved in mesoderm induction have tentatively been identified, additional signals are required to generate pattern in this tissue. We have isolated a Xenopus cDNA for a Wnt-1 related gene, designated Xwnt-8, which is activated in response to mesoderm-inducing growth factors. Xwnt-8 transcripts are transiently expressed, being most abundant during gastrulation at which time expression is confined primarily to ventral mesodermal cells. Embryos dorsoanteriorized by exposure to lithium exhibit greatly reduced levels of Xwnt-8 mRNA, supporting a correlation between Xwnt-8 expression and a ventral mesodermal cell fate. Surprisingly, ectopic expression of Xwnt-8 in embryos causes a dorsoanterior-enhanced phenotype. These findings suggest that Xwnt-8 may be a secondary signalling agent which is produced in response to mesoderm-inducing factors and is involved in the early steps of mesodermal patterning.

Animals↗

PubMed1992

Expression of tenascin mRNA in mesoderm during Xenopus laevis embryogenesis: the potential role of mesoderm patterning in tenascin regionalization.

In Xenopus embryos, the extracellular matrix (ECM) protein tenascin (TN) is expressed dorsally in a very restricted pattern. We have studied the spatial and temporal expression of TN mRNA in tailbud-stage embryos by RNAase protection and in situ hybridization using a cDNA probe for Xenopus TN obtained by PCR amplification. We report that TN transcripts are principally expressed in cells dispersed around the neural tube and notochord as well as in myotome and sclerotome cells. No TN mRNA could be detected in lateral plate mesoderm, but expression was detectable beneath tail fin epidermis. In a second series of experiments, we studied the expression of TN mRNA and protein in combinations between animal and vegetal stage-6 blastomeres and in stage-8 blastula animal caps treated with activin A or basic fibroblastic growth factor (b-FGF). Isolated animal cap tissue cultured alone differentiates into epidermis, which expresses neither TN protein nor TN mRNA. TN expression is, however, elicited in response to isolated dorsal vegetal blastomeres and in response to high concentrations of activin, both of which treatments lead to formation of muscle and/or notochord. Low concentrations of activin, and ventral vegetal blastomeres, treatments that induce mesoderm of ventral character, are poor inducers of TN. However, b-FGF, which also induces ventral mesoderm, elicits strong expression. These results indicate that TN regionalization is a complex process, dependent both on the pattern of differentiation of mesodermal tissues and on the agent with which they are induced. The data further show that "ventral mesoderm" induced by low concentrations of activin is distinct from that induced by b-FGF, and imply that activin induces ventral mesoderm of the trunk while b-FGF induces posterior mesoderm of the tailbud.

Animals↗

PubMed1990

Mesodermal cell migration during Xenopus gastrulation.

The adhesive glycoprotein fibronectin (FN), which is a component of the network of extracellular matrix fibrils on the inner surface of the blastocoel roof (BCR), has been proposed to play a major role in directing mesodermal cell migration during amphibian gastrulation. In the first part of this paper, the adhesion of Xenopus mesodermal cells to FN in vitro is examined. Cells from several mesoderm regions, which differ in developmental fate and morphogenetic activity, are able to bind specifically to the RGD cell-binding site of FN. Dorsal mesodermal cell adhesion to FN varies along the anterior-posterior (a-p) axis: adhesion is strongest in the anterior head mesoderm, and gradually decreases posteriorly. This a-p gradient of mesodermal adhesiveness to FN does not change during mesodermal involution, and is reflected in the morphology of mesodermal explants on FN. An a-p strip of mesoderm develops a spreading, leading anterior margin and a compact, retracting posterior end, thus moving slowly and directionally over the FN substrate at about 0.8 micron/min. Although dissociated cells from all levels of the dorsal mesodermal axis adhere to FN, only the anterior, leading prospective head mesoderm cells migrate as single cells on a FN substrate in vitro. Locomotion by means of lamelliform protrusions occurs at an average rate of about 1.5 micron/min. Cells of the more posterior axial mesoderm merely shift position at random without substantial net translocation, and preinvolution mesoderm cells are completely stationary. On the BCR, the in vivo substrate for mesodermal cell migration, dissociated prospective head mesoderm cells spread and migrate as on FN in vitro, at 2.2 microns/min. In the presence of an RGD peptide which inhibits cell-FN interaction, cells remain globular and do not spread. They are still motile, but change direction frequently, which leads to less efficient net translocation. Apparently, interaction with the RGD cell-binding site of FN and concomitant spreading of head mesoderm cells is required for the stabilization of cell locomotion. In contrast to the directional migration of the mesoderm cell population toward the animal pole in the embryo, the pathways of dissociated cells on the BCR are randomly oriented. Coherent explants of migratory mesoderm do not move at all on the BCR, although they translocate on FN in vitro.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acid Sequence↗

PubMed1992

Positional control of mesoderm movement and fate during avian gastrulation and neurulation.

Segments of primitive streak from donor quail embryos at stages of gastrulation and neurulation were transplanted heterotopically and isochronically to primitive streaks of host chick embryos. The subsequent movement and fate of grafted cells was determined using the quail nucleolar marker to define grafted cells. The pattern of movement of grafted cells depended on their new position within the primitive streak, not on their original position. When cells of cranial regions were placed more caudally, they moved to mesodermal subdivisions that were located lateral to those they would have populated if left in their original position. When caudal segments were placed more cranially, they moved to more medial mesodermal subdivisions. Whether the fate of grafted cells corresponded to their original location or their new location depended on both their level of origin and their new position. Cells from heterotopically transplanted Hensen's nodes, which migrated to the somitic and more lateral mesoderm, self-differentiated notochords. Similarly, in some cases, heterotopically transplanted prospective somitic cells, which migrated to lateral plate mesoderm, formed ectopic somites. In other cases, however, grafted cells contributed to the host's somites, intermediate mesoderm, and lateral plate mesoderm. Moreover, prospective somitic cells, which migrated to the extraembryonic lateral plate mesoderm, changed their fate and formed extraembryonic lateral plate mesoderm; and prospective lateral plate mesoderm cells, which migrated to the somitic mesoderm, formed somites as well as intermediate mesoderm and lateral plate mesoderm.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Leg bud mesoderm retains morphogenetic potential to express limb-like characteristics ("limbness") in collagen gel culture.

Recent in situ hybridization studies have correlated expression of potential regulatory genes with pattern formation in limb bud mesoderm (Tabin: Cell 66:199-217, 1991); however, the mechanism(s) controlling their expression in mesoderm and their relevance to the establishment of a limb morphogenetic pattern remain unknown. One likely candidate for regulating patterning events in limb mesoderm is the apical ectodermal ridge, as its removal in ovo results in a graded truncation of limb skeletal elements in the proximal-distal axis dependent upon the time of excision (Rowe and Fallon: J Embryol Exp Morph 68:1-7, 1982). In the present study, we investigate whether the hypothetical imprint of ridge ectoderm is retained in cultured mesoderm. Specifically, we sought to determine if a subpopulation of limb mesoderm that forms in collagen gel culture (Markwald et al: Anat Rec 226:91-107, 1990), retains any expression of "limbness" in the absence of limb ectoderm as characterized by the formation of a predictable number and distribution of limb-like chondrogenic elements in comparison to the temporal and spatial relationships of the in situ proximal, hindlimb skeletal structures. Accordingly, explants of undissociated mesoderm from stage 18-22 chicken leg buds were cultured without ectoderm on collagen gel lattices and the central subpopulation of mesoderm was examined morphologically. We show that this central subset of mesoderm will form chondrogenic cells which were not expressed uniformly throughout the subset, but rather distinct nodules or elements of cartilage were elaborated. Moreover, the number of elements expressed by the central subset increased with the age of the mesoderm at the time of explantation; spatially and temporally, the sequence of elements that formed always proceeded from the proximal, anterior margin of the subset to its distal, posterior border. The shapes of the initial elements (designated I and II) resembled the forms of in situ proximal skeletal structures (girdle and femur-like), whereas more distal elements (III-V) were often fused and without structural similarity to in situ skeletal structures. When cultures were established from the posterior mesoderm of stage 19/20 or 21 mesoblasts, the frequency of element I formation was reduced approximately one-half, whereas formation of more distal elements was unaffected. Conversely, element formation from the central subset established from isolated anterior mesoderm was virtually identical to intact mesoblasts, indicating a capacity to regulate for the loss of mesoderm as occurs in situ (Hampé: Archs Anat Microsc Morph Exp 48:345-378, 1959).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1991

Directional mesoderm cell migration in the Xenopus gastrula.

The movement of the dorsal mesoderm across the blastocoel roof of the Xenopus gastrula is examined. We show that different parts of the mesoderm which can be distinguished by their morphogenetic behavior in the embryo are all able to migrate independently on the inner surface of the blastocoel roof. The direction of mesoderm cell migration is determined by guidance cues in the extracellular matrix of the blastocoel roof and by an intrinsic tissue polarity of the mesoderm. The mesodermal polarity shows the same orientation as the external guidance cues and is strongly expressed in the more posterior mesoderm. The guidance cues of the extracellular matrix are recognized by all parts of the dorsal mesoderm and even by nonmesodermal cells from other regions of the embryo. The extracellular matrix consists of a network of fibronectin-containing fibrils. The adhesiveness of this matrix does not vary along the axis of mesoderm movement, excluding haptotaxis as a guidance mechanism in this system. However, an intact fibronectin fibril structure is necessary for directional mesoderm cell migration. When the assembly of fibronectin into fibrils is inhibited, mesoderm explants still migrate on the amorphous extracellular matrix, but no longer directionally. It is proposed that polarized extracellular matrix fibrils may normally guide the migrating mesoderm to its target region.

Animals↗

PubMed1991

Retinoic acid modifies mesodermal patterning in early Xenopus embryos.

Treatment of early Xenopus embryos with retinoic acid (RA) produces a concentration-dependent series of defects in anterior axial structures that range from small deletions to embryos lacking heads. The graded series of axial defects obtained after RA administration to early embryos appears to result, in part, from actions of RA on embryonic mesoderm. RA modifies the differentiation of anterior dorsal mesoderm from animal cap ectoderm induced by mesoderm-inducing peptide growth factors (PGFs). Concentrations of RA that suppress anterior dorsal mesoderm result in the differentiation of mesoderm of more posterior or ventral character. The suppression of anterior dorsal mesoderm may account for the absence of anterior neural ectoderm after RA treatment. Although RA changes the character of mesoderm, it does not seem to affect mesodermal induction by PGFs or the levels of Xhox3 mRNA induced in the mesoderm by PGFs. RA therefore appears to affect steps downstream from those involved in the initial induction of mesoderm. In experiments to examine the possible physiological role of RA in early Xenopus development, dorsal and ventral ectoderm were found to respond differently to identical concentrations of PGFs. One potential basis for this heterogeneity is the existence of a localized inhibitor, possibly RA, in the early Xenopus embryo. RA could therefore contribute to axial patterning by inhibiting the development of mesoderm of different character induced by PGFs.

Animals↗

PubMed1991

Interactions between mesoderm cells and the extracellular matrix following gastrulation in the chick embryo.

In the gastrulating chick embryo, the mesoderm cells arise from the epiblast layer by ingression through the linear accumulation of cells called the primitive streak. The mesoderm cells emerge from the streak with a fibroblastic morphology and proceed to move away from the mid-line of the embryo using, as a substratum, the basement membrane of the overlying epiblast and the extracellular matrix. We have investigated the roles of fibronectin and laminin as putative substrata for mesoderm cells using complementary in vivo and in vitro methods. We have microinjected agents into the tissue space adjacent to the primitive streak of living embryos and, after further incubation, we have examined the embryos for perturbation of the mesoderm tissue. These agents were: cell-binding regions from fibronectin (RGDS) and laminin (YIGSR), antibodies to these glycoproteins, and a Fab' fragment of the antibody to fibronectin. We find that RGDS, antibody to fibronectin, and the Fab' fragment cause a decrease in the number of mesoderm cells spread on the basement membrane, and a perturbation of cell shape suggesting locomotory impairment. No such influence was seen with YIGSR or antibodies to laminin. These results were extended using in vitro methods in which mesoderm cells were cultured in fibronectin-free medium on fibronectin or laminin in the presence of various agents. These agents were: RGDS; YIGSR; antibodies to fibronectin, fibronectin receptor, laminin and vitronectin; and a Fab' fragment of the fibronectin antiserum. We find that cell attachment and spreading on fibronectin is impaired by RGDS, antiserum to fibronectin, the Fab' fragment of fibronectin antiserum, and antiserum to fibronectin receptor. The results suggest that although the RGDS site in fibronectin is important, it is probably not the only fibronectin cell-binding site involved in mediating the behaviour of the mesoderm cells. Cells growing on laminin were perturbed by YIGSR, RGDS and antibodies to laminin, suggesting that mesoderm cells are able to recognise at least two sites in the laminin molecule. We conclude that the in vivo dependence of mesoderm cells on fibronectin is confirmed, but that although these cells have the ability to recognise sites in laminin as mediators of attachment and spreading, the in vivo role of this molecule in mesoderm morphogenesis is not yet certain.

Analysis of Variance↗

PubMed1977

Scanning electron microscopy of the development of the mesoderm layer in chick embryos.

The distribution of mesoderm, the structure of mesoderm cells and relationship between mesoderm and ectoderm were examined by SEM in embryos at stages 3 to 5. The mesoderm was displayed by removal of the endoderm and by fracturing the embryos through mesoderm containing regions. Within the mesoderm layer four zones could be distinguished by their cell shape and arrangement--the primitive streak, a multilayered compact area around the margins of the area pellucida, multilayered and loosely arranged cells near the primitive streak and a flattened monolayer of cells around the advancing lateral and anterior edges of the mesoderm sheet. Secretion of basement membrane by the ectoderm was seen to precede the arrival of mesoderm cells. This suggests that ectoderm alone can synthesize basement membrane without mesodermal contribution.

Animals↗

PubMed1990

A new homeobox-containing gene, msh-2, is transiently expressed early during mesoderm formation of Drosophila.

Many homeobox-containing genes of Drosophila regulate pathways of differentiation. These proteins probably function as promoter- or enhancer-selective transcription factors. We have isolated a new homeobox-containing gene, msh-2, by means of the polymerase chain reactions (PCR) using redundant primers. msh-2 is specifically expressed in mesodermal primordia during a short time period early in development. It first appears at blastoderm stage just before the ventral invagination of the mesoderm and shortly after twist, a gene required for mesoderm formation, is expressed. During germband elongation all the mesodermal cells in the segmented part of the embryo express msh-2, but soon afterwards msh-2 becomes restricted to the dorsal mesoderm, which includes the primordia for the visceral musculature and the heart. Prior to muscle differentiation, msh-2 expression ceases, except for two rows of cells that will be included in the dorsal vessel. Embryos that are deficient for the chromosomal region, 93C-F, which includes the msh-2 gene, show normal mesoderm invagination and dorsal spreading. However, later in development no visceral muscle and dorsal vessel differentiation can be detected, but some skeletal muscles do form, albeit abnormally. msh-2 expression, except for a patch in the head, is dependent on twist function. On the other hand, snail, another mesoderm determinant, does not appear to be required for msh-2 initiation, but is necessary for the maintenance of msh-2 expression after germband elongation. H2.0, a homeo-box-containing gene specifically expressed in visceral mesoderm, is not transcribed in the mesoderm in 93C-F deficiency embryos.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗

PubMed1975

The function of the ectodermal apical ridge and distinctive characteristics of adjacent distal mesoderm in the avian wing-bud.

Blocks of mesoderm about 0-1 mm in diameter were isolated from various regions of chick wing-buds of stages 17 through 22 and cultured individually, or sometimes in pairs, in microtest plate wells. Cell deaths had occurred after 10 h of culture in those explants that had come from the region associated with the thickest part of the ectodermal ridge, and after 11-12 h in all other mesoderm. When the adjacent ectodermal ridge was left attached to the mesodermal block there were almost no cell deaths for up to 24 h of culture. When the dorsal ectoderm immediately proximal to the apical ridge was left attached, but no ridge was present, cell deaths occurred just as they did in mesoderm with no ectoderm. When a number (usually six) of complete ridges were suspended in a wire basket at the top of a well, cell deaths did not occur in a test mesodermal block at the bottom of the well (six of eight cases). These experiments support previous evivence for a special function of the ectodermal apical ridge in limb morphogenesis, and indicate that there is a chemical messenger. The cells that migrated from distal mesodermal explants (the band up to 0-15 mm from the apical ridge) differed sharply in morphology and behavior from those coming from explants from any more proximal region. Within the proximal mesoderm there was a less striking variation alons the antero-posterior axis. These observations reveal that there is present even at early stages a detailed pattern within the mesoderm of the limb-bud. The particularly striking and distinctive characteristics of that mesoderm closest to the apical ectodermal ridge provide new possibilities for the understanding of the function of the ridge in limb morphogenesis.

Animals↗

PubMed1992

Cell interaction and its role in mesoderm cell migration during Xenopus gastrulation.

In the Xenopus gastrula, the mesoderm moves as a coherent cell aggregate across the blastocoel roof toward the animal pole. We show that the cohesion of the mesoderm is not only mechanically necessary, but that aggregate formation has profound effects on the migratory behavior of mesoderm cells. Whereas isolated mesoderm cells are bi- or multipolar, move stepwise and change their direction of movement frequently, aggregated mesoderm cells migrating on their in vivo substrate appear unipolar and move continuously and persistently. Moreover, only mesoderm cell aggregates, but not single cells, can follow guidance cues present in the extracellular matrix of the blastocoel roof substrate. Thus, the cohesion of the mesodermal cell mass is an essential feature of mesoderm migration during Xenopus gastrulation. We show that the Ca(2+)-dependent cell adhesion molecule U-cadherin is involved in mediating this cohesion.

Animals↗

PubMed1992

Xwnt-8 modifies the character of mesoderm induced by bFGF in isolated Xenopus ectoderm.

In Xenopus, growth factors of the TGF-beta, FGF and Wnt oncogene families have been proposed to play a role in generating embryonic pattern. In this paper we examine potential interactions between the bFGF and Xwnt-8 signaling pathways in the induction and dorsal-ventral patterning of mesoderm. Injection of Xwnt-8 mRNA into 2-cell Xenopus embryos does not induce mesoderm formation in animal cap ectoderm isolated from these embryos at the blastula stage, but alters the response of this tissue to mesoderm induction by bFGF. While animal cap explants isolated from non-injected embryos differentiate to form ventral types of mesoderm and muscle in response to bFGF, explants from Xwnt-8 injected embryos form dorsal mesodermal and neural tissues in response to the same concentration of bFGF, even if the ectoderm is isolated from the prospective ventral sides of embryos or from UV-ventralized animals. Our results support a model whereby dorso-ventral mesodermal patterning can be attained by a single mesoderm inducing agent, possibly bFGF, which is uniformly distributed across the prospective dorsal-ventral axis, and which acts in concert with a dorsally localized signal, possibly a Wnt protein, which either alters the response of ectoderm to induction or modifies the character of mesoderm after its induction.

Animals↗

PubMed1992

Cardiac looping in the chick embryo: the role of the posterior precardiac mesoderm.

Grafts of mesoderm taken from the precardiac region of quail embryos of stages 5-7 were inserted into the precardiac mesoderm of chick embryos of stages 5-7. The experiments were of four types and were code named to indicate the origin and the destination of the graft. QACP: tissue from the anterior end of the quail precardiac area was inserted into the posterior end of the chick precardiac mesoderm; QPCA: tissue from the posterior end of the quail precardiac area was inserted into the anterior end of the chick precardiac mesoderm; QACA: tissue from the anterior end of the quail precardiac area was inserted into the anterior end of the chick precardiac mesoderm; QPCP: tissue from the posterior end of the quail precardiac area was inserted into the posterior end of the chick precardiac mesoderm. In no case was precardiac tissue removed from the host. Three main-types of anomaly were obtained: inverted hearts, in which looping took place to the left rather than to the right; compact hearts, in which no looping occurred, and hearts in which extra tissues or regions were apparent. The incidence of compact hearts was significantly greater with QPCA than with any other category of experiment. When older donors were used (stages 8-9), the incidence of compact hearts fell. No variations in the origin of the graft, nor in its ultimate destination in the host, were found to affect the frequency of any of the anomalies. Sections showed that quail hearts tended to have thicker walls than chick hearts; although quail tissues were often incorporated into the host chick hearts, they retained the histological characteristics of the donors. The fact that no compact hearts resulted from the experiment QACA, or from the mock operations, leads us to conclude that failure to loop in the compact hearts was not due to mechanical trauma caused by the operation, but to some specific difference between grafts taken from the anterior and posterior precardiac mesoderm. The fact that compact hearts were obtained when chick donors were used instead of quails, shows that the effect is not species-specific. We propose that a morphogen is secreted by the posterior end of the precardiac mesoderm and this plays a role in controlling the cessation of looping.

Animals↗

PubMed1990

Differential cytokeratin gene expression reveals early dorsal-ventral regionalization in chick mesoderm.

The induction and spatial patterning of early mesoderm are known to be critical events in the establishment of the vertebrate body plan. However, it has been difficult to define precisely the steps by which mesoderm is initially subdivided into functionally discrete regions. Here we present evidence for a sharply defined distinction between presumptive dorsal and presumptive ventral regions in early chick mesoderm. Northern blot and in situ hybridization analyses reveal that transcripts corresponding to CKse1, a cytokeratin gene expressed during early development, are present at high levels in the presumptive ventral mesoderm, but are greatly reduced or undetectable in the future dorsal region of mesoderm, where the formation of axial structures occurs later in development. This distinction is present even while the mesoderm layer is being formed, and persists during the extensive cellular movements and tissue remodelling associated with morphogenesis. These results point to an early step in which two fundamentally distinct states are established along the presumptive dorsal-ventral axis in the mesoderm, and suggest that determination in this germ layer occurs in a hierarchical manner, rather than by direct specification of individual types of histological differentiation. The differential expression of CKse1 represents the earliest molecular index of dorsoventral regionalization detected thus far in the mesoderm.

Animals↗

PubMed1992

Mesoderm movement and fate during avian gastrulation and neurulation.

Quail/chick transplantation chimeras were constructed during stages of gastrulation and neurulation to follow the subsequent movement and fate of cells of the primitive streak. All grafts were placed solely within the confines of the primitive streak to prevent confusion between cells that had not yet ingressed and those that had already ingressed, and transplanted cells were distinguished from host cells on the basis of a naturally occurring cell marker. Pathways of movement of ingressing cells corresponded to their level of residence within the primitive streak. Cells residing within Hensen's node (the cranial end of the primitive streak) initially migrated mainly cranially, remaining on or near the midline, and then extended caudally along the midline as regression of Hensen's node occurred. Cells residing within the nodus posterior (the caudal end of the primitive streak) migrated caudally. Cells residing at levels of the primitive streak between Hensen's node and the nodus posterior typically migrated bilaterally, confirming that such cells had not already ingressed prior to their transplantation (in which case, they would have migrated unilaterally). Subsets of these cells residing at progressively more caudal levels of the primitive streak migrated incrementally more laterally. Hensen's node contributed cells to the gut endoderm, head mesenchyme, notochord, and median hinge-point (MHP) cells of the neural tube (future floor plate). At younger stages (i.e., stages 3a, 3b) Hensen's node contributed cells to principally the foregut endoderm and head mesenchyme, whereas at older stages (i.e., stages 3c, 3d, 4), it contributed cells to principally the notochord and MHP region. The remaining segments of the cranial half of the primitive streak contributed cells to the various mesodermal subdivisions of the embryo, and the lengths of the segments forming these subdivisions were estimated. The most cranial level of the streak (directly behind Hensen's node) contributed cells to the most medial mesodermal subdivisions (head mesenchyme, somites) and consecutively more caudal levels of the streak contributed cells to sequentially more lateral mesodermal subdivisions (intermediate mesoderm, lateral plate mesoderm). The caudal half of the primitive streak contributed cells to the extraembryonic mesoderm, with the nodus posterior contributing to the most caudal extraembryonic mesoderm, including the blood islands. Our results confirm and extend the previous avian prospective fate maps, increasing our understanding of the movement and fate of cells of the gastrula and neurula stages.

Animals↗

PubMed1992

A positive correlation between permissiveness of mesoderm to neural crest migration and early DRG growth.

The microenvironment created by grafting rostral somitic halves in place of normal somites leads to the formation of nonsegmented peripheral ganglia (Kalcheim and Teillet, 1989; Goldstein and Kalcheim, 1991) and is mitogenic for neural crest (NC) cells that become dorsal root ganglia (DRG) (Goldstein et al., 1990). We have now extended these studies by using three surgical manipulations to determine how additional mesodermal tissues affected DRG growth in chick embryos. The following experimental manipulations were performed: (1) unilateral deletion of epithelial somites, similar deletions followed by replacing the somites with (2) a three-dimensional collagen matrix, or (3) fragments of quail lateral plate mesoderm. When somites were absent or replaced by collagen matrix, ganglia were unsegmented, and their volumes were decreased by 21% and 12%, respectively, compared to contralateral intact DRG. In contrast, when lateral plate mesoderm was transplanted in place of somitic mesoderm, NC cells migrated into the grafted mesoderm and formed unsegmented DRG whose volumes were increased by 62.6% compared to the contralateral ganglia. These results suggest that although DRG precursors do not require sclerotome to begin migration and condensation processes, DRG size is modulated by the properties of the mesoderm. Permissiveness to migration is positively correlated with an increase in DRG volume. This volume increase observed in grafts of lateral plate mesoderm is likely to result from enhanced proliferation of neural crest progenitors, previously demonstrated for DRG cells in rostral somitic grafts.

Animals↗