Retinal development of West Australian dhufish, Glaucosoma hebraicum.

An investigation of retinal specializations was carried out in larval and juvenile dhufish, Glaucosoma hebraicum (Glaucosomidae, Teleostei). The development of photoreceptors and formation of the retinal mosaic was followed by light and electron microscopy. At hatching the eye was undifferentiated. Cone photoreceptors were present by day 3 posthatch (dph), when exogenous feeding began. Single and multiple cones were present in a row arrangement from 3 dph to 20 dph, when the first rod nuclei were observed. Between 20 dph and approximately 3 months posthatch (mph), the row arrangement was replaced by a square mosaic of four double cones surrounding a single cone, and the cones increased in size, with the outer segments reaching up to 30 microm in length. During the period of spatial rearrangement, triple cones were often observed. From their first appearance, rod photoreceptors were added rapidly. Investigation of ganglion cell topography in 3-mph fish that had attained the adult-like square photoreceptor mosaic was carried out using retinal wholemounts. The highest densities of neurones in the ganglion cell layer were in temporal retina but no well-defined area centralis was observed. Microspectrophotometric measurements of the visual pigments within the outer segments of the photoreceptors of 3-mph fish revealed double cones with identical absorption spectra in each member of the outer segment, and the wavelength of maximum absorption (lambda(max)) located at 522 nm. Single cones were found to possess a visual pigment with lambda(max) at 460 nm and rods with a lambda(max) of 498 nm. The results imply that the larvae and juveniles are adapted for survival in coastal waters and may be active in relatively low light levels from early stages of development.

Fetal sciatic nerve growth is delayed following repeated maternal injections of corticosteroid in sheep.

AIMS: A single course of prenatal corticosteroid reduces the mortality and morbidity of preterm birth. Repeated courses of prenatal corticosteroids are widely prescribed despite a lack of safety data. Repeated corticosteroids delay myelination in the ovine central nervous system at the time of preterm delivery but with catch-up at term. We aimed to evaluate their effect in the peripheral nervous system. METHODS: Thirty date-mated ewes were administered either saline, a single injection of betamethasone, or four injections of betamethasone between 104 and 124 days' gestation, with delivery on day 125 or 145 (term = 150 days). Sciatic nerves were dissected and fixed in modified Karnovsky's fixative and prepared for light and electron microscopy to determine the proportion of myelinated axons and mean axon diameter. RESULTS: Repeated, but not single, corticosteroid administration resulted in significant decreases in the total cross-sectional and fascicle-containing areas of the sciatic nerve, and in the mean diameter of myelinated and unmyelinated axons. Deficits persisted at term. The proportion of myelinated axons was unaffected. CONCLUSION: Repeated prenatal corticosteroids have the capacity to affect the growth of peripheral nerve axons in sheep. Documentation of their effects in human pregnancy await randomized trials.

Transgressive segregation, adaptation and speciation.

The production of extreme or 'transgressive' phenotypes in segregating hybrid populations has been speculated to contribute to niche divergence of hybrid lineages. Here, we assess the frequency of transgressive segregation in hybrid populations, describe its genetic basis and discuss the factors that best predict its occurrence. From a survey of 171 studies that report phenotypic variation in segregating hybrid populations, we show that transgression is the rule rather than the exception. In fact, 155 of the 171 studies (91%) report at least one transgressive trait, and 44% of 1229 traits examined were transgressive. Transgression occurred most frequently in intraspecific crosses involving inbred, domesticated plant populations, and least frequently in interspecific crosses between outbred, wild animal species. Quantitative genetic studies of plant hybrids consistently point to the action of complementary genes as the primary cause of transgression, although overdominance and epistasis also contribute. Complementary genes appear to be common for most traits, with the possible exception of those with a history of disruptive selection. These results lend credence to the view that hybridization may provide the raw material for rapid adaptation and provide a simple explanation for niche divergence and phenotypic novelty often associated with hybrid lineages.

Ontogenetic changes in the retinal photoreceptor mosaic in a fish, the black bream, Acanthopagrus butcheri.

The morphological development of the photoreceptor mosaic was followed by light and electron microscopy in a specific region of dorsal retina of the black bream, Acanthopagrus butcheri (Sparidae, Teleostei), from hatching to eight weeks of age. The retina was differentiated when the larvae reached a total length of 3 mm (3-5 days posthatch). Single cones, arranged in tightly packed rows, were the only morphologically distinct type of photoreceptor present until the larvae were 6 mm (day 15) in standard length (SL). At this time, the rod nuclei had become differentiated and the ellipsoids of selected cones began to form subsurface cisternae along neighbouring cone membranes. In this way, double, triple, quadruple, and occasionally photoreceptor chains of up to 10 cones were formed. At 8 mm SL, there was little apparent order in the photoreceptor mosaic. However, concomitant with subsequent growth, quadruple and other multiple cone receptors disappeared, with the exception of the triple cones, which gradually reduced in both number and retinal coverage to be restricted to central retina by 15 mm SL (days 40-55). Following this stage, the arrangement of double and single cones peripheral to the region of triple cones in dorsal retina was transformed into the adult pattern of a regular mosaic of four double cones surrounding a single cone. These results demonstrate that an established photoreceptor mosaic of rows of single cones can be reorganised to form a regular square mosaic composed of single and double cones.

Fetal growth retardation, particularly within lymphoid organs, following repeated maternal injections of betamethasone in sheep.

OBJECTIVE: To examine the effect of single or repeated injections of maternally administered corticosteroids on fetal growth in sheep. METHODS: Forty-six date-mated singleton gestation ewes were allocated at random to one of 3 groups: a single, or repeated injections of betamethasone, or a control group which received saline. On days 125 (preterm) or 145 (term) caesarean section delivery was performed. After lambs were killed, measures of size and weight were recorded. Data were analysed using Fishers Exact test and the Student's t-test. RESULTS: Significant betamethasone dose dependent reductions in body and organ weights and biometry were found at preterm and term gestational ages (p < 0.05). There was little catch up growth in those in whom delivery was delayed until term. Thymus, spleen and liver were particularly targeted. CONCLUSION: Repeated injections of betamethasone to the pregnant ewe cause significant reductions in fetal growth with little evidence of catch up by term. The effect of repeated maternal injections of corticosteroids in human pregnancy will await the results of randomized controlled trails.

Repeated prenatal corticosteroids delay myelination in the ovine central nervous system.

Maternal administration of corticosteroids is used to promote lung maturation in human infants considered at risk of preterm delivery [1]. Randomised trials of a single course of corticosteroid treatment have indicated no adverse long-term neurological or cognitive sequelae [2-5]. However, the current trend in many obstetric centres is to use repeated courses in cases where preterm birth has not eventuated, but the risk persists 7 days beyond administration of the original course [6-7]. This practice has not yet been subject to randomised trials of outcome. We have examined the effect of repeated injections of corticosteroids on the development of the optic nerve in prenatal fetal sheep and report a significant delay in the myelination of optic axons. Our results, together with those from other animal studies [8], show that repeated courses of corticosteroids may be detrimental to central nervous system (CNS) development. Clinical practice should balance the known beneficial effects on lung maturation of a single course of corticosteroid against the potential damage to the CNS of repeated courses.

Genetic analysis of mechanisms of aging.

A wide range of genetic models with postponed aging are now available, from selected mice and Drosophilia to mutant Caenorhabditis elegans and Saccharomyces cerevisiae. These systems allow efficient testing of alternative mechanistic hypotheses for aging. Genetic analysis is forging stronger connections between particular alleles and susceptibility to particular 'diseases of aging'; for example, two different genes for Alzheimer disease have been identified.