Afferent influences on the development of the brain stem auditory nuclei of the chicken: otocyst ablation

The effects of embryonic deafferentation on the morphological development of the avian cochlear nuclei, n. angularis (NA) and n. magnocellularis (NM), were investigated. The right otocyst was surgically removed from chick embryos at 55 to 60 hours of incubation and the subsequent development of total volume, neuron number, and neuron cross-sectional area were studied with quantitative methods in animals sacrificed at 2-day intervals between embryonic days 9 and 19 and at 28 days posthatching. The development of NA and NM is severely affected by otocyst ablation. Between embryonic days 9 and 19, a large group of NA neurons in the medioventral portion of the nucleus on the operated side moves to an ectopic ventromedial position, while the remainder of this nucleus stays in its normal dorsolateral position. Beginning about day 13 of incubation, the normal increase in the volume of NA and the size of its neurons becomes progressively retarded and 40% of its neurons are lost. The growth of NM is also retarded after day 11 of incubation and the growth of mean neuron size is retarded after day 15. There is a 30% loss of neurons in NM which begins after embryonic day 11. The results indicate the primary cochlear fibers make a critical contribution to the growth and maintenance of their target neurons. The absence of this facilitative influence following otocyst ablation becomes apparent just at the time synapses would normally be formed between the the primary auditory afferents and the brain stem auditory neurons. The abnormal movement of neurons in nucleus angularis to an ectopic position after otocyst ablation suggests that primary auditory afferents may serve to stabilize the position of their target cells within the developing brain.     

Microstructural and thermal stability of a Ti-43AI alloy containing dispersoids of titanium di-boride

Ti-43Al (atomic percent) alloy containing a dispersion of 7 vol pct TiB₂ particles was exposed to various thermal treatments to determine the stability of TiB₂ in an ⇌₂ + β-phase matrix. No new phases were detected at the particle/matrix interfaces even after thermal exposure at 1473 K for 7 days. The absence of an Al peak in the energy dispersive X-ray analysis system (EDS) spectra from TiB₂ particles chemically extracted from the specimens aged at 1473 K for 7 days indicated no diffusion of Al from the matrix to the particles. These results indicate that TiB₂ is stable in an α₂+ β matrix at 1473 K. E. Clevenger, formerly Undergraduate Student, Department of Mechanical and Materials Engineering, Wright State University     

Analysis of Mechanically Induced Reactivity of Boehmite Using Kinetics of Boehmite to <Emphasis Type="Italic">γ-</Emphasis>Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Transformation

This article focuses on the mechanically induced reactivity of boehmite prepared by thermal decomposition of gibbsite. Boehmite, which retained the morphology of gibbsite, was characterized by a specific surface area of 264 m²/g. Mechanical activation (MA) was carried out in a planetary mill up to 240 minutes. The samples were characterized in terms of morphology, characteristic particle diameters, Brunauer Emmett Teller (BET) specific surface area (SSA_BET), microcrystallite dimension (MCD), microstrain (ε) and Fourier transform infrared spectroscopy. The reactivity was construed from the kinetics of thermal transformation of boehmite into γ-Al₂O₃. The transformation observed between 600 K and 900 K (327 °C and 627 °C), manifested itself as two overlapping peaks in the differential thermogravimetric plot. These peaks correspond to two stages of dehydroxylation involving Al₂OH and AlOH groups in succession. The peaks were resolved using Gaussian deconvolution. The reactivity was assessed separately for the two stages by comparing the fraction reacted in MA samples (α) with that of nonactivated sample (α _ref). During both stages, enhanced kinetics, as revealed by α-α _ref plots, indicated an increase in reactivity with MA. The transformation mechanism conformed to n ^th order reaction (f[α] = [1 – α] ⁿ with n = 1.3–1.5 in both stages). Values of n remained similar for the activated and reference samples. Activation energies (E _a) for the first and second dehydroxylation stages were respectively 115 and 300 kJ/mol for the nonactivated sample. E _a for the second stage decreased exponentially to a value of 222 kJ/mol after 240 minutes of milling. An anomalous negative correlation between reactivity and SSA_BET was observed. Reactivity parameters were strongly correlated with MCD and ε. A plausible explanation for the observed correlations is presented.