Revealing crystalline domains in a mollusc shell single-crystalline prism.

Biomineralization integrates complex processes leading to an extraordinary diversity of calcareous biomineral crystalline architectures, in intriguing contrast with the consistent presence of a sub-micrometric granular structure. Hence, gaining access to the crystalline architecture at the mesoscale, that is, over a few granules, is key to building realistic biomineralization scenarios. Here we provide the nanoscale spatial arrangement of the crystalline structure within the 'single-crystalline' prisms of the prismatic layer of a Pinctada margaritifera shell, exploiting three-dimensional X-ray Bragg ptychography microscopy. We reveal the details of the mesocrystalline organization, evidencing a crystalline coherence extending over a few granules. We additionally prove the existence of larger iso-oriented crystalline domains, slightly misoriented with respect to each other, around one unique rotation axis, and whose shapes are correlated with iso-strain domains. The highlighted mesocrystalline properties support recent biomineralization models involving partial fusion of oriented nanoparticle assembly and/or liquid droplet precursors.

Orbital and millennial Antarctic climate variability over the past 800,000 years.

A high-resolution deuterium profile is now available along the entire European Project for Ice Coring in Antarctica Dome C ice core, extending this climate record back to marine isotope stage 20.2, approximately 800,000 years ago. Experiments performed with an atmospheric general circulation model including water isotopes support its temperature interpretation. We assessed the general correspondence between Dansgaard-Oeschger events and their smoothed Antarctic counterparts for this Dome C record, which reveals the presence of such features with similar amplitudes during previous glacial periods. We suggest that the interplay between obliquity and precession accounts for the variable intensity of interglacial periods in ice core records.

Substrate-induced modulation of the Raman scattering signals from self-assembled organic nanometric films.

Raman scattering signals recorded by microscopy from organic self-assembled monolayers (thin nanometric films of calibrated thickness) on silica substrates were found to be much stronger than those obtained from identical films assembled on bulk silicon substrates. This effect, observed in the backscattering geometry, is shown to result from interferences between the direct and reflected beams (including both the excitation and scattered radiation) in front of a smooth reflecting surface. Strong dependence of the effect on the distance between the sampled monolayer and the bulk silicon substrate allows enhancement of the Raman signals of organic monolayer films on silicon by factors up to approximately 70 by using appropriate silica spacers. The dependence of the Raman signal intensity on film thickness was also studied for thicker nanometric films comprising a series of self-assembled organosilane multilayers on bulk silicon and fused silica substrates, and the predicted deviation from linearity in the case of the silicon substrate is experimentally confirmed.

[Topographic distribution of mitoses in the adrenal cortex of the rat]. / Distribution topographique des mitoses dans la cortico-surrénale du rat.

The situation of mitoses in the adrenal glands of the rat has been studied by measuring the distance between each of them (3,061 in the whole) at the surface. The curve of their distribution, logarithmic-normal type, shows that they are the most numerous in germinative zone classically described, but that they appear down to the depth of inner fascicula.

[Circadian variation of the mitotic activity of the adrenal cortex in male rats]. / Variation circadienne de l'activité mitotique du cortex surrénalien chez le rat mâle.

The mitotic activity in the adrenal cortex has been studied during a 24 hours-cycle in male rats that have been sacrificed by perfusion of fixative. A method of calculation of the thickness of the zona germinativa is proposed : it permits to compute mitotic densities in relation to surface, and mitotic index. Mitotic activity seems to be the combination of a relatively low and constant activity occurring from 7 a.m. (local time) to 11 p.m., and an hyperactivity from 0 to 6 a.m. Two maxima are demonstrated : one at 0 h, and another one, more important, at 5 a.m. (local time). Results are analyzed, discussed and compared with others obtained on different organs : each tissue seems to have its own characteristics according to horary and intensity in mitotic activity.

[Numerical arguments in favor of the existence of clone cells in the adenohypophysis]. / Arguments numériques en faveur de l'existence de clônes cellularies dans l'adénohypophyse.

In the rat's anterior pituitary, mitoses occur to be side by side, forming "pairs", the frequency of which is significant all the more as their number is underrated in sections. In 85% of the pairs the mitoses are synchronous. These paired synchronous mitoses are suggestive of the existence of cellular clones in the pituitary.

Variations of mitotic activity in the adenohypophysis of male rats during a 24-hour cycle.

The mitotic activity in the adenohypophysis of male rats during a 24 hours' cycle has been studied at the time of the spring equinox. The animals were killed by perfusion of fixative. The mitotic activity seems to be the combination of two elements: a relatively low and nearly constant activity with two peaks of high activity. The first one takes place at 6 o'clock (local time) with an index reaching 4.25; the second one occurs at 11 o'clock (local time) and its index is 2.09. The afternoon activity is weak with an average index of 0.56 between 1 p.m. and 9 p.m. The results are analyzed, discussed and compared with other results observed in different organs.

[Nycthemeral variations of mitotic activity in the adrenal cortex of the rat]. / Variations nycthémérales de l'activité mitotique dans la cortico-surrénale du rat mâle

Working on histologic sections 7 microns thick in the equatorial part of male rat adrenal cortex, we found two high mitotic activity periods. The first one stays at midnight and shows an average number of 42 mitoses per section; the second one occurs at 5 a.m. with an average number of mitoses of 70,1 per section. If we except these particular points, the mitotic activity is low, the average number of mitoses being 13,2 per section.