The ultrastructure of the sense organs of some turbellaria rhabdocoela. I. The eyes of Polycystis naegelii Kölliker (Eukalyptorhynchia Polycystididae)

Summary Each pigment-cup eye of Polycystis naegelii consists of two retinal clubs and a single pigmented cell. The latter is divided into two cavities by a septum. Under bright illumination the photoreceptor process appears as a disk containing membranous laminar whorls; under faint illumination the latter are replaced by numerous straight, closely packed, microvilli. This morphological variation is correlated with the intensity of the photoreceptor's exposure to light. The lenticular structures described by previous light microscopists have not been observed.This work was supported by a grant from the Italian National Research Council (C.N.R.)     

Composition of the antennal nerve of Homarus americanus at different points along the flagellum

The antennal flagellum nerve of Homarus americanus was investigated as to structure, number and size of axons, and propagation velocity. Frequency distributions of axon diameters, evaluated at four equidistant levels on the flagellum, ranged with continuity between 0.25 and 14.7 µm, with maximum at 0.5-1.5 µm. Axons 0.5-1.5 µm in diameter were more abundant at the distal level, indicating sensory specialization near the tip. The total axon numbers increased from about 9000 at the distal level to about 45 000 at the base. Axons of different size followed different patterns of increase in number from tip to base; these patterns were examined in relation to structural features of the flagellum, and to hypotheses of association with known or unidentified receptors. Propagation velocities were distributed with continuity, in the range between 3.39 and 0.24 m/s; velocity-diameter correspondences were outlined.     

Episodic corticosterone secretion in the female rat

Studies conducted in several laboratories have shown primate glucocorticoid secretion to occur episodically. In light of the methodological, as well as physiological importance of this finding, the present study was undertaken to determine whether the rat corticosteroid secretion also occurs episodically. Female rats were outfitted with chronic intravenous cannulas, and 1 week later 200 units of heparin were injected through the implanted cannula and blood samples (0.3-0.4 ml) were collected from each rat every 10 min for 3 h during the morning (06.00-10.00 h) or during the afternoon (16.00-19.00 h) (lights on from 05.00 to 19.00 h). Plasma corticosterone levels in cannulated rats showed fluctuations indicative of episodic secretion. The pattern of plasma corticosterone levels was characterized by periodic rapid increases in hormone concentration during both the morning and afternoon sampling periods; the occurrence of these hormonal fluctuations did not have a characteristic frequency. When the data were grouped to obtain single morning and afternoon values, the AM-PM difference was significant (p < 0.005). Collectively, these data suggest that in the rat, adrenal corticosteroids are secreted episodically.     

Analysis of phytoplankton-zooplankton relationships in an oligotrophic lake under natural and manipulated conditions

During August, 1987, we performed a series of Limnocorral experiments in lake La Caldera, a small winter-kill lake in which phytoplankton is strikingly nutrient-limited. The effects of biomanipulation on zooplankton-phytoplankton relationships were assessed by monitoring both individual species and whole-assemblage responses. Two sizes of enclosures were used (15 and 350 litres) and two treatments were assayed: 1) removal of zooplankton by 45 μm filter net and 2) doubling the natural grazing pressure by increasing the zooplankton concentration. Results show the two enclosure types to differ strikingly: flagellates disappeared from the small enclosures, resulting in four- to six-fold changes in chlorophylla concentration and three- to four-fold changes in number of individuals. Most species were grazed (a prey selectivity based on criteria other than size was observed) and their net growth rate increased with zooplankton concentration, causing a net increase in the phytoplankton growth, a stimulatory effect probably through nutrient regeneration that overrides the losses due to grazing.     

Ecoenzyme activity ratios reveal interactive effects of nutrient inputs and UVR in a Mediterranean high-mountain lake

To understand how atmospheric dust deposition and ultraviolet radiation (UVR) can affect remote, freshwater ecosystems through changes in their microbial metabolism, it is important to have tools that allow us detecting alterations and anticipating potential shifts in the functioning of microbial communities. Ecoenzyme activities (EA) are easy to measure and their ratios can be used to assess system microbial metabolism of freshwater bodies, thus evaluating the effects of global change stressors. We carried out an in situ full factorial experiment to determine how the interaction between the addition of C and P, and UVR affect the microbial metabolism of a Mediterranean high-mountain lake. Overall, activities of five ecoenzymes involved in the degradation of C-compounds and in the acquisition of N and P revealed that, under natural conditions, the growth of heterotrophic prokaryotes was dependent on organic compounds released by algae, which is consistent with a higher constraint of bacterial carbon production by C than by P or N, as suggested by EA ratios. Accordingly, the addition of a labile C source did not lead to any significant response of microbial communities, but the addition of P provoked a clear change in the microbial metabolism of the lake, promoting the growth of phytoplankton and leading heterotrophic prokaryotes to be more constrained by P, and to a lesser extent by N, in relation to C. UVR played a secondary role, probably because microbial communities inhabiting high-mountain lakes possess several evolutionary adaptations to high UVR levels. Changes in the microbial metabolism of our model lake under different scenarios of nutrient inputs and UVR can therefore be evaluated by EA ratios.     

Exponential equilibration of genetic circuits using entropy methods

Journal of Mathematical Biology - We analyse a continuum model for genetic circuits based on a partial integro-differential equation initially proposed in Friedman et al. (Phys Rev Lett...     

Skin Responses to Ultraviolet Radiation: Effects of Constitutive Pigmentation,Sex, and Ancestry

Constitutive skin pigmentation and skin responses to ultraviolet radiation were measured on a sample of volunteers (n=250) living in State College, PA, USA. The sample was composed of individuals of European American (n=190), Hispanic (n=45), and East Asian ancestry (n=15). Constitutive pigmentation was measured using the Adjusted Melanin Index (AMI), Erythemal Dose Response (EDR) was measured using the slope of a* at 24 h (Δa*), and Melanogenic Dose–Response (MDR) was measured using ΔAM, the slope of AMI at 7 d. The relationships between constitutive skin pigmentation, EDR, MDR, sex, age, and ancestry were investigated. European Americans showed a lower constitutive pigmentation, had a significantly higher burn response (EDR), and had a significantly lower tanning response (MDR) than Hispanics and East Asians. No significant difference is seen between Hispanics and East Asians for either constitutive pigmentation or EDR. Constitutive pigmentation in females was slightly lower than in males in all three samples, but the difference was not significant. While no differences were observed in MDR between sexes, males had a stronger EDR than females regardless of population or constitutive pigmentation level, and this difference was significant in European Americans and Hispanics. We observed no age‐related differences in any of the populations or measures investigated. We evaluated the relationship between constitutive pigmentation, EDR and MDR. There was a strong inverse correlation between constitutive pigmentation and EDR in the three samples (European Americans, R²=0.176, P < 0.001; Hispanics, R²=0.204, P=0.009; East Asians, R²=0.223, P=0.098) and a strong direct correlation between constitutive pigmentation and MDR in European Americans and Hispanics (European Americans, R²=0.094, P < 0.001; Hispanics, R²=0.164, P=0.012). In other words, persons with lower constitutive pigmentation both burn more and tan less than persons with higher pigmentation. However, after controlling for constitutive pigmentation, EDR and MDR were significantly correlated in European Americans (R²=0.041 P=0.006). Thus, the general observation that persons who burn more tan less is probable because of the common link that these two phenotypes have with constitutive skin pigmentation and, in fact, once pigmentation has been adjusted for, there is a positive correlation between tanning response and burning response in European Americans.     

Fine structure and axonal organization in the buccal ganglia nerves ofAplysia (Mollusca,Gastropoda)

Summary The morphology of theAplysia buccal nerves and connectives has been studied by electron microscopic analysis. In these nerves the fine structure of the elements (connective sheath, glia, axons and their vesicular and cytoplasmatic content) is similar to that of other molluscan nerves. Some features seem to be comparable to other invertebrate groups such as Crustacea and Annelida. The axons have been divided into four classes on the basis of their calibre, and each type has been counted in all the nerves. The number of axons relating to identified buccal neurons is discussed. Finally, some speculations about relationships between buccal ganglia and peripheral regions connected by buccal nerves are proposed.