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Results for “developmental timing”

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At least 109 records · Page 6Linked to original sources
PubMed2003

Developmental timing of exposure to elevated levels of phenylalanine is associated with ADHD symptom expression.

This study addresses attention deficit hyperactivity disorder (ADHD), with a focus on how the timing of a known biological insult affects ADHD symptom expression. The sample consists of children exposed to elevated levels of phenylalanine, either postnatally as in Phenylketonuria (PKU; n = 46) or prenatally as in Maternal PKU (MPKU; n = 15). Non-hyperphenylalaninemic siblings of children with PKU (n = 18) serve as controls. Results indicate that elevated levels of phenylalanine are associated with ADHD symptoms. The manifestations of the symptom expression are dependent on exposure timing: prenatal exposure is associated with a higher likelihood of expressing hyperactive/impulsive symptoms and postnatal exposure is associated with a higher likelihood of expressing inattentive symptoms. This toxicity is dose-dependent and higher levels of phenylalanine appear more detrimental.

Adult↗
PubMed2002

Effects of size and temperature on developmental time.

Body size and temperature are the two most important variables affecting nearly all biological rates and times. The relationship of size and temperature to development is of particular interest, because during ontogeny size changes and temperature often varies. Here we derive a general model, based on first principles of allometry and biochemical kinetics, that predicts the time of ontogenetic development as a function of body mass and temperature. The model fits embryonic development times spanning a wide range of egg sizes and incubation temperatures for birds and aquatic ectotherms (fish, amphibians, aquatic insects and zooplankton). The model also describes nearly 75% of the variation in post-embryonic development among a diverse sample of zooplankton. The remaining variation is partially explained by stoichiometry, specifically the whole-body carbon to phosphorus ratio. Development in other animals at other life stages is also described by this model. These results suggest a general definition of biological time that is approximately invariant and common to all organisms.

Amphibians↗
PubMed1978

A step function model using tooth counts to assess the developmental timing of the dentition.

Step functions representing median attainment ages of successive counts on numbers of permanent teeth present in the mouth were obtained separately for 12 early-emerging teeth (I1, I2 and M1) and for 16 late-emerging teeth (C, Pm1, Pm2, and M2). Displacement of each step function to best fit of individual tooth counts defined the variation in dental maturation for early teeth and for late teeth. Moreover, differences in the displacements of the two-step functions defined individual variations in the duration of the emergence process.

Age Factors↗
PubMed2004

Experience-induced changes of dendritic spine densities in the prefrontal and sensory cortex: correlation with developmental time windows.

The present study provides evidence for the hypothesis that the extent and the direction of experience-induced synaptic changes in cortical areas correlates with time windows of neuronal as well as endocrine development. Repeated brief exposure to maternal separation prior to the stress hyporesponsive period (SHRP) of the hypothalamic-pituitary-adrenal (HPA) axis induced significantly reduced dendritic spine density (-16%) in layer II/III pyramidal neurons of the anterior cingulate cortex (ACd) of 21-day-old rats, whereas separation after termination of the SHRP resulted in increased spine densities (+16%) in this neuron type. In addition, rats of both groups displayed elevated basal plasma levels of corticosterone at this age. Separation during the SHRP (postnatal days 5-7) did not influence spine density in the ACd, and basal corticosterone levels remained unchanged. In contrast, pyramidal neurons in the somatosensory cortex (SSC) displayed significantly enhanced spine densities (up to 52% increase) independent from the time of separation. These results indicate that alterations in the synaptic balance in limbic and sensory cortical regions in response to early emotional experience are region-specific and related to the maturational stage of endocrine and neuronal systems.

Animals↗
PubMed1995

Cellular basis and developmental timing in a size cline of Drosophila melanogaster.

We examined 20 Drosophila melanogaster populations collected from a 2600-km north-south transect in Australia. In laboratory culture at constant temperature and standard larval density, a genetic cline in thorax length and wing area was found, with both traits increasing with latitude. The cline in wing area was based on clines in both cell size and cell number, but was primarily determined by changes in cell number. Body size and larval development time were not associated among populations. We discuss our results in the context of selection processes operating in natural and experimental populations.

Analysis of Variance↗
PubMedDate not supplied

Physeal reconstruction with blocks of cartilage of varying developmental time.

Reconstruction of physeal regions excised from the distal femoral chondroepiphysis was performed in a murine model. The excised regions were replaced with analogous blocks of syngeneic tissue. Vascularity, matrix formation, and cell division were assessed as well as growth. The blocks of tissue did not produce growth consistently, although individual specimens did show some growth. Tritiated thymidine incorporation by the proliferative zone of the transplanted block of tissue was not normal. The blocks of tissue did receive a nutritional supply from the host and did continue matrix formation after transplantation.

Animals↗
PubMed2001

Dicer functions in RNA interference and in synthesis of small RNA involved in developmental timing in C. elegans.

Double-stranded RNAs can suppress expression of homologous genes through an evolutionarily conserved process named RNA interference (RNAi) or post-transcriptional gene silencing (PTGS). One mechanism underlying silencing is degradation of target mRNAs by an RNP complex, which contains approximately 22 nt of siRNAs as guides to substrate selection. A bidentate nuclease called Dicer has been implicated as the protein responsible for siRNA production. Here we characterize the Caenorhabditis elegans ortholog of Dicer (K12H4.8; dcr-1) in vivo and in vitro. dcr-1 mutants show a defect in RNAi. Furthermore, a combination of phenotypic abnormalities and RNA analysis suggests a role for dcr-1 in a regulatory pathway comprised of small temporal RNA (let-7) and its target (e.g., lin-41).

Alleles↗
PubMed1999

Similarity of the C. elegans developmental timing protein LIN-42 to circadian rhythm proteins.

The Caenorhabditis elegans heterochronic genes control the relative timing and sequence of many events during postembryonic development, including the terminal differentiation of the lateral hypodermis, which occurs during the final (fourth) molt. Inactivation of the heterochronic gene lin-42 causes hypodermal terminal differentiation to occur precociously, during the third molt. LIN-42 most closely resembles the Period family of proteins from Drosophila and other organisms, proteins that function in another type of biological timing mechanism: the timing of circadian rhythms. Per mRNA levels oscillate with an approximately 24-hour periodicity. lin-42 mRNA levels also oscillate, but with a faster rhythm; the oscillation occurs relative to the approximately 6-hour molting cycles of postembryonic development.

Alleles↗
PubMed2005

An evoked potential study of the developmental time course of the auditory nerve and brainstem in children using cochlear implants.

Central auditory responses to electrical stimulation from a cochlear implant were studied in 75 pre-lingually deafened children and 11 adults. Electrically evoked auditory brainstem response (EABR) latencies significantly decreased with duration of cochlear implant use and were not significantly affected by the age at implant activation. Significant decreases in early latency waves and interwaves occurred within the first 1-2 months of implant use, whereas longer term changes (6-12 months) were found for eV and eIII-eV, which measure activity in the more rostral brainstem. Comparisons to acoustically evoked auditory brainstem response (ABR) in children with normal hearing suggested shorter interwave EABR latencies, reflecting either distinct neural generators or increased neural synchrony, but similar rates of change in the later latency eV and eIII-eV with time in sound. In sum, normal-like development of the rostral auditory brainstem is promoted by cochlear implant use in children of a wide range of ages.

Adolescent↗