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D M Gardiner

Publications and source records attributed to D M Gardiner.

9 recordsLinked to original sources
PubMed1992

Mouse limb bud cells respond to retinoic acid in vitro with reduced growth.

Retinoic acid (RA) has dramatic effects on the pattern of developing and regenerating vertebrate limbs. These effects are considered to result from RA-induced changes in the positional identity of limb cells, and involve the formation of extra structures. Whether the growth required to form the supernumerary parts of the pattern is a primary effect of RA treatment or a secondary effect that follows after a change in positional identity is not at present known. In this paper we have investigated the effects of RA treatment on the growth of cells from anterior and posterior halves of mouse limb buds in vitro. We observed that under our culture conditions, limb bud cells treated with 1 nM to 1 microM RA (0.3 ng/ml to 300 ng/ml) continue to grow but do so at a significantly slower rate than control cultures. There is a maximum inhibition of growth (50% of controls) between 10 nM and 100 nM RA, which corresponds to the measured range of concentrations of RA in vivo. Our observation of a significant decrease in growth rate over a wide range of RA concentrations is consistent with comparable reports of growth inhibition for a large number of other cell types in vitro as well as with the observation that exogenous RA inhibits blastemal growth in amphibians during the period of exposure to RA. We propose that the effects of RA on growth, either enhancement in vivo or reduction in vitro, can be seen as consequences of the ability of RA to alter positional identity.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Retinoic acid, local cell-cell interactions, and pattern formation in vertebrate limbs.

Retinoic acid (RA), a derivative of vitamin A, has remarkable effects on developing and regenerating limbs. These effects include teratogenesis, arising from RA's ability to inhibit growth and pattern formation. They also include pattern duplication, arising as a result of the stimulation of additional growth and pattern formation. In this review we present evidence that the diverse effects of RA are consistent with a singular, underlying explanation. We propose that in all cases exogenously applied RA causes the positional information of pattern formation-competent cells to be reset to a value that is posterior-ventral-proximal with respect to the limb. The diversity of outcomes can be seen as a product of the mode of application of exogenous RA (global versus local) coupled with the unifying concept that growth and pattern formation in both limb development and limb regeneration are controlled by local cell-cell interactions, as formulated in the polar coordinate model. We explore the possibility that the major role of endogenous RA in limb development is in the establishment of the limb field rather than as a diffusible morphogen that specifies graded positional information across the limb as previously proposed. Finally, we interpret the results of the recent finding that RA can turn tail regenerates into limbs, as evidence that intercalary interactions may also be involved in the formation of the primary body axis.

Animals↗
PubMed1991

Conversion by retinoic acid of anterior cells into ZPA cells in the chick wing bud.

In recent years there has been considerable interest in the role of retinoic acid (RA) in vertebrate-limb pattern formation. When RA is applied to the anterior of the chick wing bud, a mirror-image duplication of the limb pattern develops that is identical to the pattern resulting from grafts of posterior tissue (zone of polarizing activity, or ZPA). It has been proposed that position along the anterior-posterior axis in the chick limb is specified by a gradient of a diffusible factor produced by the ZPA. The ZPA-mimicking action of RA has led to the hypothesis that exogenously applied RA acts by providing graded spatial information across the anterior-posterior limb axis. An alternative interpretation is that RA changes anterior cells into ZPA cells, which in turn provide the actual pattern-duplicating stimulus; there is already some preliminary evidence that this occurs. A hybrid interpretation has also been suggested whereby ZPA cells are formed in response to RA exposure and then begin to release retinoids that act as graded spatial cues. We have used a functional assay to test anterior chick wing-bud cells for ZPA activity after exposure to RA. The results of our studies indicate that the action of RA is to change anterior cells into ZPA cells. Further, our results indicate that it is unlikely that RA-treated anterior cells then begin producing RA in such a way as to provide a graded positional signal.

Animals↗
PubMed1989

Organization of positional information in the axolotl limb.

We have used the phenomenon of position-dependent growth stimulation, brought about by the confrontation of cells with dissimilar positional values, to reveal the organization of positional information in the center of the upper and lower arms of axolotls. When either humerus or radius was transplanted into either dorsal or posterior positions, extra growth leading to the formation of supernumerary digits occurred following amputation through the graft. However, transplants of humerus or radius into anterior or ventral positions did not lead to the formation of any additional digits. The ulna by contrast was capable of stimulating supernumerary digit formation when transplanted into anterior, posterior, dorsal, or ventral positions. We interpret these results to indicate that the humerus and radius are surrounded by symmetrically arranged anterior and ventral positional values, whereas the ulna is surrounded by a complete asymmetrical set of angular positional values. We use our proposed arrangement for the positional information in the limb center to explain a number of previous experimental findings. In addition, we provide an explanation, in terms of the underlying positional information, for the structural and developmental relationships between the different skeletal elements of the vertebrate limb, and in particular for the anatomical pattern known as Gregory's pyramid.

Ambystoma mexicanum↗
PubMed1989

Compatible limb patterning mechanisms in urodeles and anurans.

We have experimentally tested the similarity of limb pattern-forming mechanisms in urodeles and anurans. To determine whether the mechanisms of limb outgrowth are equivalent, we compared the results of two kinds of reciprocal limb bud grafts between Xenopus and axolotls: contralateral grafts to confront anterior and posterior positions of graft and host, and ipsilateral grafts to align equivalent circumferential positions. Axolotl limb buds grafted to Xenopus hosts are immunologically rejected at a relatively early stage. Prior to rejection, however, experimental (but not control) grafts form supernumerary digits. Xenopus limb buds grafted to axolotl hosts are not rejected within the time frame of the experiment and therefore can be used to test the ability of frog cells to elicit responses from axolotl tissue that are similar to those that are elicited by axolotl tissue itself. When Xenopus buds were grafted to axolotl limb stumps so as to align circumferential positions, the majority of limbs did not form any supernumerary digits. However, in experimental grafts, where anterior and posterior of host and graft were misaligned, supernumerary digits formed at positional discontinuities. These results suggest that Xenopus/axolotl cell interactions result in responses that are similar to axolotl/axolotl cell interactions. Furthermore, axolotl and Xenopus cells can cooperate to build recognizable skeletal elements, despite large differences in cell size and growth rate between the two species. We infer from these results that urodeles and anurans share the same limb pattern-forming mechanisms, including compatible positional signals that allow appropriate localized cellular interactions between the two species. Our results suggest an approach for understanding homology of the tetrapod limb based on experimental cellular interactions.

Ambystoma↗
PubMed1986

The migration of dermal cells during blastema formation in axolotls.

Using the diploid/triploid cell marker in the axolotl (Ambystoma mexicanum) we have examined the movement of cells from the dermis into the early limb blastema. Cells of dermal origin begin to migrate beneath the wound epithelium at about 5 days postamputation, and by 10 days they are widely distributed across the amputation surface. By 15 days, a dense accumulation of blastema cells is present beneath the apical cap, and these cells are preferentially oriented in a circumferential direction. These results are discussed in relation to previous studies showing that the progeny of dermal cells become widely distributed during regeneration, and that cells of dermal origin are a major source of blastema cells. The results are also discussed in relation to ideas about how growth and patterning of the new appendage occur.

Ambystoma mexicanum↗
PubMed1983

Membrane junctions in Xenopus eggs: their distribution suggests a role in calcium regulation.

We have observed the presence of membrane junctions formed between the plasma membrane and cortical endoplasmic reticulum of mature, unactivated eggs of xenopus laevis. The parallel, paired membranes of the junction are separated by a 10-mn gap within which electron-dense material is present. This material occurs in patches with an average center-to-center distance of approximately 30 nm. These junctions are rare in immature (but fully grown) oocytes (approximately 2 percent of the plasma membrane is associated with junctions) and increase dramatically during progesterone-induced maturation. Junctions in the mature, unactivated egg are two to three times more abundant in the animal hemisphere (25-30 percent of the plasma membrane associated with junction) as compared with the vegetal hemisphere (10-15 percent). Junction density decreases rapidly to values characteristic of immature oocytes in response to egg activation. The plasma membrane-ER junctions of xenopus eggs are strikingly similar in structure to membrane junctions in muscle cells thought to be essential in the triggering of intracellular calcium release from the sarcoplasmic reticulum. In addition, the junctions' distinctive, animal-vegetal polarity of distribution, their dramatic appearance during maturation, and their disapperance during activation are correlated with previously documented patterns of calcium-mediated events in anuran eggs. We discuss several lines of evidence supporting the hypothesis that these junctions in xenopus eggs are sites that transduce extracellular events into intracellular calcium release during fertilization and activation of development.

Animals↗
PubMed1978

Cyclic changes in fine structure of the epithelium lining the ovary of the viviparous teleost, Cymatogaster aggregata (Perciformes: Embiotocidae).

The most striking morphological feature of the ovarian epithelium of Cymatogaster is the presence of intercellular dilations during much of the year. These dilations increase markedly in volume during the several months prior to ovulation and fertilization, and decrease in volume during the months of embryogenesis and gestation. The epithelium then returns to its initial, relatively undifferentiated state. The extracellular material within the dilations likely is synthesized in the cells adjacent to or within the dilations. Apparently most of this material is released into the ovarian lumen when the apicolateral margins of adjacent epithelial cells pull apart; possibly it serves as nutrient for developing embryos. In addition to supporting embryogenesis, the ovarian epithelium also apparently is involved in sperm storage. Sperm are maintained within pockets in the ovarian epithelium for the several months between insemination and fertilization. The cells lining the sperm pocket do not develop the intercellular dilations characteristic of most of the ovarian epithelium, and sperm remain associated only with the sperm pocket cells.

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