Initial enamel crystals are not spatially associated with mineralized dentine.

During epithelial-mesenchymal interactions associated with mammalian tooth development, epithelially-derived and mesenchymally-derived extracellular matrix molecules form a discrete dentine-enamel junction. The developmental and molecular processes required to form this junction are not known. To address this problem we designed studies to test the hypothesis that ectodermally-derived epithelial cells synthesize and secrete enamel proteins which function to nucleate and regulate the growth of enamel calcium phosphate crystals. Initial enamel crystals were detected separate from the adjacent dentine. Electron-microprobe analyses revealed that early enamel crystals were octacalciumphosphate or tricalciumphosphate rather than hydroxyapatite. Thereafter, enamel crystals became confluent with the adjacent, albeit significantly smaller hydroxyapatite crystals associated with mineralized dentine. Therefore, we interpret our data to indicate that de novo enamel crystal nucleation and growth are independent from the mineralization processes characterized for dentine. We further argue that gene expression of enamel protein appears to have a constitutive function during early enamel formation and that supramolecular aggregates of amelogenin and enamelin provide the microenvironment for the nucleation and crystal growth of the initial enamel matrix.

Iodide and thiocyanate efflux from brain following injection into rat caudate nucleus.

Mechanisms and pathways of 125I and 35SCN efflux from the brain were investigated in anesthetized rats. Tracers were injected into the caudate nucleus through a guide cannula implanted 1 wk previously and concentrations of isotope in brain and cerebrospinal fluid (CSF) were determined at various times after injection. 125I clearance from the brain followed a single exponential curve. In control rats 36.2% of the 125I remained in the brain 30 min after injection and 60.4% in rats pretreated with perchlorate. Comparable values for 35SCN were 25.8% in control rats, 41.0% with perchlorate, and 39.7% with iodide loading. Estimates of 125I and 35SCN effluxes from the brain via the blood-brain barrier and CSF pathways suggest that greater than 95% of efflux crosses the blood-brain barrier. These results indicate that 1)iodide and thiocyanate are transported across the blood-brain barrier by a common mechanism, and 2) this efflux system is an important factor in the control of the distributions of iodide and thiocyanate in the central nervous system.

Purification, characterization, and attempts at isotopic labeling of mouse interferon.

Under optimal conditions which minimized the accumulation of extraneous proteins, interferon preparations were obtained in L cells containing from 2 x 10(4) to 5 x 10(4) units/mg of protein. The radiolabeled proteins were liberated simultaneously with interferon from cultures exposed to tritiated amino acids after viral stimulation and from corresponding controls, and were subsequently purified with the following results. Chromatography of interferon on carboxymethyl-Sephadex C-25 eliminated selectively unlabeled or poorly labeled proteins, resulting in a greater than sixfold increase in counts per minute per milligram of protein. Similarly purified control material harbored at least 12 times less tritium per milligram of protein than interferon, and the label was more diversely distributed among proteins of different molecular weights. On electrophoresis of interferon in polyacrylamide gels, labeled proteins were reduced further by a factor of at least 10 without loss in titer. Final purification was estimated at greater than 280-fold, representing a calculated specific activity of at least 1.4 x 10(7) units of interferon per milligram of protein.