John B. Armstrong

The effect of salt on phospholipid fatty acid composition in Escherichia coli K-12

The fatty acid compositions of the three major phospholipids of Escherichia coli K-12; phosphatidylethanolamine, phosphatidylglycerol and cardiolipin; were determined during growth in media differing in NaCl concentration. Significant differences in fatty acid composition of the phospholipids were observed in the stationary phase cultures, but no appreciable differences were found in early exponential cultures.

Development of the mechanoreceptive lateral-line system in the axolotl: placode specification, guidance of migration, and the origin of neuromast polarity

SummaryThe mechanosensory lateral-line system offers a unique opportunity to study a wide variety of developmental phenomena, including cell migration, the origin of polarity, and pattern formation. In this study, we use a series of transplantation experiments to examine some of the factors affecting the origin of the lateral-line placodes, the establishment of sensory organ polarity and placement, and the guidance of cell migration in the Mexican axolotl (Ambystoma mexicanum). We find that placode-forming ectoderm is at least partially specified as early as the beginning of neurulation, and we suggest that this may be a result of early processes involved in neural induction. Furthermore, we find that the migration of the primordia on the body depends on the presence of both the ectoderm and the subjacent mesoderm for guidance. Sensory organ polarity on the body appears to be the result of an interaction between the primordia, which deposit organs of set polarity relative to the direction of migration, and the substrate, which determines the direction of migration. Spacing of the organs is independent of the substrate, and may be due to an intrinsic property of either the primordia or the emerging organs themselves. Finally, we suggest that the lateral-line primordia are guided, as they migrate, by a contact guidance mechanism.

Lateral-line neuromast development in Ambystoma mexicanum and a comparison with Rana pipiens

We have examined the embryonic development of the major neuromast lines of the lateral‐line system in the urodele Ambystoma mexicanum both in vivo (using microsurgical techniques to transplant placodes) and in preserved embryos using scanning electron microscopy (SEM). We have compared this to SEM observations of embryos of the anuran Rana pipiens. We have determined the approximate locations of the lateral‐line placodes in A. mexicanum and the approximate timing of both the migration of the lateral line primordia and the neuromast eruption in both species. We find that, at hatching, all primary neuromasts are present and fully formed in Ambystoma, while migration of the primordia is just beginning in Rana. The neuromast systems in both species are fully formed by the time feeding begins. If neuromast eruption is considered in relation to developmental events other than hatching, fewer differences are found between species, suggesting that hatching is precocious in Rana. We find no evidence of heterochrony to account for the morphological differences observed in these lateral‐line systems. Orthogonal neuromasts on the head, a derived feature of urodeles, appears to be the result of lateral neuromast movement subsequent to the rostral migration of the primordia. This process was not observed in the anuran. In addition, we observe that ciliated epidermal cells disappear from the area immediately around each of the lines and suggest that neuromasts cause the regression of cilia in their immediate vicinity.

Phase transition behaviour of artificial liposomes composed of phosphatidylcholines acylated with cyclopropane fatty acids

Phase transitions of liposomes composed of synthetic phoshatidylcholines acylated with the cyclopropane fatty acids, lactobacillic and dihydrosterculic acid, were studied by differential scanning calorimetry. Transition temperatures were approx. 16 degrees C higher than for phosphatidylcholines acylated with the corresponding unsaturated fatty acids, cis-vaccenic and oleic acid. Though our transition temperatures were all several degrees lower than those determined by Silvius and McElhaney ((1979) Chem. Phys. Lipids 25, 125--134), the increase produced by replacement of the double bond with a cyclopropane ring was the same. We propose that this replacement, through its effect on membrane fluidity, may serve to regulate the activity of membrane-associated processes such as transport.

Experimental evidence for a proteinaceous presegmental wave required for morphogenesis of axolotl mesoderm

Mesoderm of axolotl embryos at various developmental stages was briefly exposed to a calcium-free 0.01% trypsin solution by temporary removal of the epidermis. This treatment was found to disrupt somite segmentation in a localized region and the pronephric duct was unable to migrate through this region. The affected area, consisting of 3.91 +/- 1.04 somites, traveled through the embryo in synchrony with, and 3.55 +/- 0.69-somite widths ahead of segmentation. Trypsinization in the presence of 340 microM calcium resulted in normal duct migration while somite segmentation was still affected. These results demonstrate the existence of a trypsin-sensitive region in the somitic mesoderm and the lateral mesoderm of the duct path that travels in advance of somite segmentation and in synchrony with it. In addition, the trypsin sensitivity of the duct path is calcium dependent whereas that of the somitic mesoderm is not.

Effects of fenitrothion and carbaryl on Xenopus laevis development

When Xenopus laevis embryos were treated with fenitrothion, a substantial proportion failed to survive to hatching only when the concentration approached saturation. However, after exposure to 10 ppm of fenitrothion, most were abnormal and did not survive past feeding stage. Different embryonic stages were not equally sensitive to fenitrothion and the greatest sensitivity was observed at gastrulation. The most common visible abnormalities were altered body shape, including microcephaly, and edema. Internally, abnormalities of the heart, spinal cord and notochord were common. Though carbaryl was toxic at somewhat lower concentrations, carbaryl-induced abnormalities were less severe. In particular, cardiac malformations were less extreme. Recently hatched tadpoles were less sensitive to carbaryl than were the embryos, but were more sensitive to fenitrothion. However, with both compounds, the effects were seen only at concentrations higher than would normally be expected in the environment after spraying.

Progressive patterning precedes somite segmentation in the mexican axolotl (Ambystoma mexicanum)

Beginning at mid-neurulation, a wave of somite segmentation passes down the axolotl body axis in a cephalocaudal direction. At 20 degrees C a somite forms every 2.57 hr. Fate-mapping of the presomitic mesoderm indicates that the primordia for the next few somites occupy nearly the same space that they will after segmentation, but that the remaining somites are densely packed in tip of the tail bud. Brief heat shocks at 37 and 38.5 degrees C reveal that within the first of these two zones, there is a graded sensitivity to the shock, with the primordia closest to the last-formed somite showing the greatest resistance. However, primordia within the densely packed tip (the packing zone) also appear resistant, or have sufficient time to repair the damage. We propose that once cells have left the packing zone, they undergo progressive patterning which renders them increasingly insensitive to the disruptive effects of heat shock, and culminates in rosette formation.

Formation of the pronephros and pronephric duct rudiment in the Mexican axolotl

In the Mexican axolotl (Ambystoma mexicanum), the pronephros begins to form at the four‐somite stage. It is initially continuous with the posterior‐lateral region of somite 2 and the lateral margin of somites 3 and 4. By the seven‐somite stage, the pronephros has become compacted, and the cells are now morphologically distinct from the somitic cells. At this stage, a mass of loosely connected cells, apparently originating from the lateral mesoderm, is seen below somites 4 and 5. By the eight‐somite stage, these presumptive duct cells have migrated dorsally to the duct path and are found below somites 5–7. By the nine‐somite stage they have begun to migrate caudally.

Ionic composition of the plasma of Ambystoma mexicanum

Abstract 1. 1. The ionic composition of Ambystoma mexicanum plasma was found to be: Na + , 94.7 mM; K + , 3.0 mM; Ca 2+ , 1.63mM; Cl − , 84.7mM. The pH was 7.69, and the osmolality 216 mOsm. 2. 2. The composition is similar to that of A. tigrinum and A. gracile , and also to that of several anurans. 3. 3. The suitability of various amphibian tissue culture media as plasma analogs is discussed.