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Biomedical subjects

J M Slack

Publications and source records attributed to J M Slack.

At least 109 records · Page 6Linked to original sources
PubMed1982

Protein synthesis during limb regeneration in the axolotl.

A study has been made of limb regeneration in the axolotl using two-dimensional gel electrophoresis of proteins labelled with [35S] methionine. In the early stages of regeneration seven proteins are identified which are specific to the mesenchyme and thirteen which are specific to the epidermis. These is very little change in the gel pattern until the onset of overt cytodifferentiation upon which the muscle and cartilage become substantially different both from each other and from the blastemal mesenchyme. The gel pattern of mesenchyme from the larval limb bud is almost identical to that of the blastemal mesenchyme, while the pattern of the limb-bud epidermis differs somewhat from the blastemal epidermis. A careful search has been made for differences which might be associated with 'positional information' by comparing forelimb with hindlimb, proximal with distal and anterior with posterior. The differences which have been found are all associated with differences in visible cellular composition or in rates of differentiation, rather than with position per se. It is concluded that positional codings cannot be detected by this technique. On the basis of biological experiments, six criteria are proposed by which to assess future searches for positional codings.

Ambystoma↗
PubMed1980

Determination and cellular commitment in the embryonic amphibian mesoderm.

In classical embryology, the term 'determination' was used to describe the process by which a region of the embryo became committed to form a particular part of the body and it was understood that there was a particular stage of development at which determination occurred with respect to each decision. Some authors insisted that only an irreversible commitment should be described as determination, but this leaves undefined the nature of the preceding events involved in its establishment. We show here that there may be several stages of commitment for an organ rudiment that can be distinguished from one another by the use of different methods for altering the pathway of development. We have investigated the region of the amphibian mesoderm that is destined to become somitic muscle in the course of normal development and applied to it a variety of isolation and grafting techniques. The results, which show an extensive period of lability within this region, can be used to explain how pattern formation might occur within the dorsoventral axis of the mesoderm.

Ambystoma↗
PubMed1980

Cell commitment and gene expression in the axolotl embryo.

In the axolotl embryo the somitic mesoderm passes through a reversible and then an irreversible phase of commitment with respect to its later differentiation into muscle. We show that the commencement of alpha-actin synthesis and the first appearance of thin myofilaments occur at the the same developmental stage as the transition between these phases. In intact embryos beta-and gamma-actin are made at all stages of development and in all tissues. alpha-actin, however, first appears at the late head process stage and is confined to the somites and tailbud, the regions of the embryo which later form the myotomal muscles. The thin filaments first appear near the myocoel of the somites in association with a novel organelle of mottled appearance and are quite distinct in structure from the epidermal microfilament networks found in embryos of the same stage. Cell fusion occurs somewhat later, although before the appearance of muscle striations in the light microscope. Well formed sarcomeres are not found until some time after the onset of motility. Presumptive somitic mesoderm isolated at a stage before the transition and cultured in a buffered salt solution for six days autonomously begins to synthesize alpha-actin and develop sarcomeres with the normal structure.

Actins↗
PubMed1980

An interaction between dorsal and ventral regions of the marginal zone in early amphibian embryos.

When small explants from early gastrulae of Xenopus laevis are allowed to develop in a buffered salt solution there is a considerable difference between the patterns of differentiation obtained from different dorsoventral levels of the marginal zone. These patterns of differentiation correspond to the fates of the different regions in the course of normal development. They are not altered if several explants of the same type are fused before culture. If a ventral marginal zone explant from Xenopus is cultured in contact with a piece of dorsal marginal zone from the axolotl, it forms structures more dorsal in character than it would in isolation or in normal development. This behaviour is shown only feebly with other regions of the axolotl gastrula. A piece of dorsal marginal zone from Xenopus is not affected in its development by culture in contact with an explant of ventral marginal zone from the axolotl. The dorsalization of ventral marginal zone explants is shown only if there is a large area of direct contact with the dorsal explant and if the pieces remain in contact for a period of 48 h or more. It is proposed that this in vitro interaction is the same as the dorsoventral component of action of the 'organizer' graft discovered by Spermann and Mangold.

Ambystoma↗
PubMed1980

Regulation and potency in the forelimb rudiment of the axolotl embryo.

Anterior, posterior, dorsal and ventral halves of the pre-bud forelimb rudiment of tail-bud axolotl embryos can all give rise to normal limbs after the complementary half has been removed. A histological study of the regulation of the posterior half rudiment showed no requirement for mesodermal healing across the gap, and no proliferative zone. The development of the limb bud on the operated side lags behind that of the control bud for several weeks of larval life. When half limb rudiments and double half limb rudiments were grafted to the head they did not develop unless both the posterior and the dorsal margin were present. Double posterior and double dorsal halves could develop into duplications on this site, single halves formed normal or hypomorphic limbs. When half limb rudiments were grafted to the flank the anterior halves developed into normal limbs and the posterior halves into duplicate limbs. The results are interpreted to indicate that at the pre-bud stage the limb rudiment is a homogeneous group of cells with no internal regional subdivisions. Its regulative behaviour is thus similar to that of early embryos and different from the regeneration behaviour shown by adult organs.

Ambystoma↗
PubMed1978

Regeneration of mirror symmetrical limbs in the axolotl.

Mirrow symmetrical, double posterior limbs were produced by two types of operations on tailbud embryos of the axolotl, Ambystoma mexicanum. When the limbs had grown to maturity, they were amputated and a study was made of the structures which regenerated in their place. The majority of the regenerates were also reduplicated, although a significant minority showed other types of symmetry. Reduplicate symmetry was often conserved following several amputations, but the number of skeletal elements present in each regenerate tended to vary: both expansion and contraction were found, the changes always occurring in the center of the pattern. These results are discussed in terms of the possible mechanisms for pattern formation in the anteroposterior axis during regeneration. We suggest that control of normal regeneration is carried out by a monotonic gradient of positional information which is originally set up to control pattern formation during embryonic development.

Ambystoma↗
PubMed1977

Determination of anteroposterior polarity in the axolotl forelimb by an interaction between limb and flank rudiments.

1. It is shown that the mesoderm in the prospective forelimb-bud of the axolotl embryo is thickened and divided into somatic and splanchnic layers, while that of the flank is thinner and divided. The first sign of the limb-bud itself appears at stage 38. 2. A whole, a half or a third of a limb rudiment can develop into a normal or reduplicated limb when transplanted to the flank. 3. An anterior half of a limb rudiment fails to develop when transplanted to the head but will do so if accompanied by a small piece of flank tissue. 4. Small pieces of tissue from a wide area of the flank will cause reduplication of the forelimb if grafted to the anterior margin of the rudiment. It is shown that the whole of the reduplication is formed from host tissue and has the morphological character of the host. 5. Reduplications have posterior structures arranged symmetrically on both sides of midline. Both muscles and cartilages are duplicated. 6. It is suggested that the same interaction between prospective flank and limb is responsible for the capacity for growth on the head, the induction of reduplications and the formation of the anteroposterior pattern of the limb in normal development. 7. A simple rule is proposed which explains the occurrence of reduplications in classical work on the amphibian limb.

Ambystoma↗
PubMed1977

Control of anteroposterior pattern in the axolotl forelimb by a smoothly graded signal.

(1) It is shown that the number of cartilage elements in an experimentally produced reduplicated limb depends on the width of competent tissue between pieces of flank tissue. (2) Seventy well formed reduplications were examined on the assumption that the difference in the number of elements between them results from small differences in graft position. (3) All the reduplications are symmetrical along their entire length. (4) All possess the most posterior structures at both edges with other elements arranged in between in the correct serial order. (5) The existence of vestigial and of branched elements in the midline suggest that the cases examined come from a potentially continuous series. (6) All these characteristics can readily be explained by assuming the existence of a continuous U-shaped anteroposterior gradient, and of threshold responses by the cells which together control the pattern of cell differentiation. (7) It is suggested that the gradient "deepens" as more distal levels of the limb are laid down. (8) Most but not all of the cases can be arranged in a single series in which proximal and distal levels expand coordinately. (9) Possible mechanisms for the establishment and maintenance of the gradient are discussed.

Ambystoma↗
PubMed1976

Serological identification of Actinomyces using fluorescent antibody techniques.

Species-specific FITC conjugated antiserum can be prepared for each of the five species of Actinomyces and for Arachnia propionica. These serums can be used for rapid and specific identification of pure or mixed cultures of the bacteria and for identification of organisms seen in direct smears of clinical material or in tissue sections. Two serotypes each of A bovis, A odontolyticus, A israelii, A viscosus, and A propionica have been established, and A naeslundii has been tentatively divided into four serotypes.

Actinomyces↗
PubMed1975

Methylation of ribosomal-precursor RNA, synthesized in vitro, by isolated rat-liver nucleoli.

Nucleoli isolated from rat liver were incubated for synthesis of RNA in vitro in the presence or absence of S-adenosyl [3H] methionine. The results obtained indicate that neither the rate of RNA synthesis not the processing of pre-ribosomal RNA was changed if methylation was allowed to take place. The methylation process acts on the RNA most recently synthesized, rather than on the bulk of the RNA already present in the nucleoli. The reaction seems to occur faithfully both quantitatively and qualitatively. It is calculated that 104 mol methyl groups were incorporated per mol of newly synthesized 45-S RNA. Methylation of the ribose rather than the bases predominated. The pattern of alkali-stable oligonucleotides of RNA methylated in vitro was analyzed and found to correspond closely to that of ribosomal RNA labelled in vivo.

Animals↗