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51.
George F. Weisel 《Journal of morphology》1975,145(2):143-150
The integument of the paddlefish (Polyodon spathula) is unusual as a relatively small amount of mucus is produced by epithelial cells that are not modified into regular mucous gland cells. A thick compact epidermis and dermis compensate for the slight amount of mucus secreted. Paddlefish have a variety of scales formed of concentric bony lamellae containing osteocytes. There are five kinds of scales: dorsal and ventral fulcra on the caudal fin, rhomboidal scales on the caudal lobe, horny denticles over the pectoral girdle, calcareous denticles on the trunk, and anchor-shaped plates on the rostrum. Except for the fulcra, the scales are undoubtedly vestigial. The numerous surface pits on the rostrum, head, operculum, and throat are epithelial invaginations which are not connected to lateral line canals. No nerves lead to the pits. The spherical to cuboidal and often ciliated cells at the base of the pits are considered to be aplasic cells of unformed neuromasts. 相似文献
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A synthesis of adenosine-5′-d (4), and its p.m.r. spectral characteristics, are described. The presence of deuterium in 4 gives rise to a 2:1 mixture of R and S configurations at C-5, thereby permitting specific assignments for the resonances of the residual 5′-protons. From the observed spin-spin coupling between the latter and H-4′, and estimate has been made of the rotamer population of the exocyclic 5′-carbinol group. It is shown that the gauche-gauche rotamer is preponderant (≈70%) and the gauche-trans one of minor importance (≈20%) in aqueous solution, which contrasts markedly with the preference for the latter rotamer exhibited by adenosine in the solid state. 相似文献
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Carex eleocharis leaves contain large intercellular cavities that traverse the length of the leaf above rows of stomata and are roughly constant in size throughout a leaf. Semithin sections (1–2 μm) demonstrate that the substomatal chambers are directly continuous with the intercellular cavities. Leaves of plants inhabiting moist swale regions of the shortgrass steppe, in northeastern Colorado, were found to have larger cavities as compared to leaves of plants inhabiting dry hilltops. Plants collected from a common hilltop site were grown in a controlled environment chamber, and, by manipulating the watering schedule, we obtained water potentials similar to those in the field. Leaves of “well-watered” plants were found to have larger intercellular cavities as compared to “water-stressed” plants. Leaf mesophyll cell sizes did not differ significantly between “well-watered” and “water-stressed” plants, suggesting that cavity size differences are not the result of developmental differences. Leaf cavities were shown to contain gases and to occur along the leaf length above rows of stomata. Additionally, the cavities in unstressed plants were continuous with substomal chambers. It is proposed that the reduction of cavity size is a mechanism to reduce water loss from the leaves during periods of plant water stress. 相似文献
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We observed mixed groups of dark-eyed juncos (Junco hyemalis) and grey-headed juncos (Junco caniceps) at baited sites in northern Arizona during the non-breeding season. In interspecific and inter-racial conflicts, J. caniceps dorsalis was dominant to J. caniceps caniceps and to two races of dark-eyed juncos. Junco caniceps dorsalis also fed significantly faster than any of the other juncos. For both species, feeding rates were approximately the same in large and small mixed-species groups, though in larger groups, individual grey-headed juncos won conflicts at a higher rate and individual dark-eyed juncos lost conflicts at a higher rate. Also, dark-eyed juncos fed at a significantly lower rate in groups comprised mostly of grey-headed juncos than in groups of similar size but composed mostly of conspecifics. Residency times and recapture probabilities were similar for the two species, suggesting little difference in over-winter survival. 相似文献
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Micromolarconcentrations of ATP stimulate biphasic change in transepithelialconductance across CaSki cultures on filters, an acute transientincrease (phase I response; triggered by P2Y2 receptor and mediated by calcium mobilization-dependent cell volume decrease) followed by a slower decrease in permeability (phase II response). Phase II response is mediated byaugmented calcium influx and protein kinase C-dependent increase intight junctional resistance. The objective of the study was todetermine the role of P2X4 receptor as a mediator ofphase II response. Human cervical epithelial cells expressP2X4 receptor mRNA (1.4-, 2.2-, and 4.4-kb isoforms byNorthern blot analysis) and P2X4 protein. Depletion ofvitamin A reversibly downregulated P2X4 receptor mRNA andprotein and ATP-induced calcium influx. Depletion of vitamin Aabrogated phase II response, and the effect could bepartially reversed only with retinoic acid receptor (RAR)-selectiveretinoids but not retinoid X receptor (RXR) agonists. Depletion ofvitamin A also abrogated protein kinase C increase in tight junctionalresistance, and the effect could not be reversed with retinoids.Depletion of vitamin A also abrogated phase I increase inpermeability and reversibly downregulated P2Y2 receptormRNA and ATP-induced calcium mobilization. However, in contrast tophase II response, both RAR and RXR agonists could fullyreverse those effects. These results suggest that phase IIresponse is mediated by a P2X4 receptor mechanism. 相似文献
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