SYNOPSISTime-resolved studies of network self-organization from homogeneous solutions of the representative biostructural polymer agarose are presented. Solutions are temperature quenched and observed by several techniques. Consistent with previous suggestions by the authors, experiments at concentrations up to about 1.75% w/v provide direct kinetic evidence for the occurrence of at least two distinct processes, leading, in sequence, to selfassembly. These are as follows: ( a ) a liquid-liquid phase separation of the solution occurring via spinodal demixing and resulting in two sets of regions that have, respectively, higher and lower than average concentrations of random-coiled polymers; and ( b ) the subsequent 2 coils + double helix transition and accompanying cross-linking and gelation (due to branching of double helices ), occurring in the high-concentration regions. The size of the high-concentration regions depends upon agarose concentration and quenching temperature, and is in the range from a fraction of micrometers to a few micrometers, in agreement with earlier experiments. Bundling of the double-helical segments is known to follow self-assembly and can be considered as a third step (gel curing). This follows from the thermodynamic instability of the helical segments in the solvent, behaving as a system of rod-like particles connected by more or less flexible joints.The two processes leading in succession to self-assembly are discussed in terms of a phase diagram consistent with available data. Their time scales differ remarkably. A t the end of the first process, all polymers remain random coiled and freely drifting. Much later coil-helix transition is observed, always in coincidence with polymer cross-linking and gelation. The enhancement of concentration of random-coiled polymers in specific regions of the sol caused by spinodal demixing is thus a prerequisite for self-assembly of these biostructural gels in the concentration interval studied. Conceptually, concentration enhancements of this type can provide a new pathway for promotion of functional biomolecular interactions even at very low average concentrations. The mechanism will work identically if the region of instability is reached by varying the polymer concentration (e.g., by biosynthesis), rather than by temperature quenching.
We present a variety of new experiments which concern the self‐assembly of a polymeric network from homogeneous solutions of Agarose, a representative biostructural polysaccharide used for previous studies at our laboratories. They allow deriving a semi‐quantitative phase diagram in the T, C plane. The diagram includes both the spinodal and gelation lines. Below a value of about 2% w/v, concentration is not sufficient for direct gelation; however, quenching of the sol from high temperatures to below the spinodal line initiates the spinodal demixing. The latter generates two sets of regions having respectively, higher‐ and lower‐than‐average polymer concentrations. In the higher‐concentration regions the functional polymer‐polymer interaction (that is, self‐assembly) is favoured. In fact, as in the course of demixing the point representative of higher‐concentration regions reaches the region below the gelation line, gelation is allowed and indeed observed to occur. This evidences the possibly more general role of spinodal demixing (extended to multi‐component systems) as a pathway for specific, local and biofunctional enhancements of concentrations. At very low concentrations (e.g. 10−4 w/v) the kinetics of demixing is still observed to occur, but the set of higher‐concentration regions is no longer percolative. In these conditions, gelation occurs only within each individual region, while the specimen remains free‐running. This illustrates a novel aspect of the notion of non‐gelling concentrations.
SUMMARY:The reproductive cycle of the Norway lobster (Nephrops norvegicus) female was investigated to determine the size and age at onset of sexual maturity (SOM), as a baseline to establish the minimum catchable size. Samples were gathered in four seasonal (Spring 1986 -Winter 1987 trawl surveys on the Italian side of the Strait of Sicily (Central Mediterranean Sea). The relative frequency of females in different stages of maturation and in berried conditions was observed. According to the ovary stages, gonadal maturation is attained during Spring and Summer; berried animals occur almost exclusively in Summer and Autumn, in about 40% of the sample. Length structure changes slightly by location and by season; recruitment to the gear occurs more or less continuously. Gaussian components appear to be highly overlapped; the resolution of the length frequency distribution values leads to the identification of multiple cohorts, five of which were well represented. The population attains 50% of maturity at 30-32 mm CL, corresponding to an age of 5 years. The mean size of the first Gaussian component from the catch (about 22 mm CL) is well below the estimated size at 50% maturity, suggesting a precocious recruitment to the fishery.
The deep-water rose shrimp, Parapenaeus longirostris (Lucas, 1846), is one of the most valuable and heavily exploited demersal species of the Mediterranean bottom trawl fisheries. The basic life traits of this shrimp, in particular its reproductive aspects, are regularly monitored during experimental trawl surveys carried out in the Mediterranean Sea. Gonadic condition and maturity status for estimating the size at onset of sexual maturity are commonly assessed in females, using macroscopic color scales, histologically validated only in a few geographical areas. In this study, histological analyses were performed on rose shrimps collected from a trawl survey carried out in the Strait of Sicily, in order to support the empirical 4-stage macroscopic scale locally employed. Ovaries from females of different sizes, ranging between 16 mm and 40 mm carapace length, were collected and used for microscopic examination of their structure, for oocytes counts and for oocyte diameter frequency distributions; oocytes diameter was measured by imaging analysis. The collected data were also used to estimate other basic vital parameters of the rose shrimp population. The histological observations show a broad correspondence between ovarian development and macroscopic features; therefore, the classifications derived by the empirical scale remain suitable for estimating the maturity parameters.
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