Water and lipid MRI of the Xenopus oocyte

Abstract

Oocytes of Xenopus laevis are large, single cells that provide a promising model system for the exploration of the MR biophysics fundamental to more complex living systems. Previous studies have generally employed 2D spin-echo sequences with an image slice thickness greater than the thickness of the cellular volumes of interest. Also, the large cytoplasmic lipid signal has typically been ignored. This study describes separate, highresolution 3D measurements of the water and lipid spin densities, T 1 and T 2 relaxation time constants, and the water apparent diffusion rate constant (ADC) in the Xenopus oocyte without significant partial volume artifacts. The lipid spin-density and values for water MR properties varied monotonically from the vegetal to animal poles, indicating that the border between the poles is not sharply demarcated. The Xenopus laevis oocyte is a well-established cell model used in many branches of modern experimental biology. This versatile cell provides a promising model platform upon which to explore fundamental aspects of the MR biophysics underlying more complex living systems. In particular, the ability to spatially resolve the intracellular compartment allows the direct testing of hypotheses regarding various MR properties of the cytoplasm and nucleus. Recent efforts to integrate confocal microscopy and MR imaging will likely augment the capability of such experiments (1).Aguayo et al. (2) reported the first MR images of the Xenopus oocyte in 1986. Along with intriguing contrast between intracellular regions, the authors described a strong chemical shift artifact from cytoplasmic lipid. Posse and Aue (3) further characterized this lipid signal using a 4D spectroscopic imaging technique. Both studies localized the lipid signal to the cytoplasm, with little or no lipid signal arising from the nucleus. Since these initial studies, several investigators have calculated spin densities and T 2 and/or T 1 relaxation time constants for the oocyte nucleus, animal pole, and vegetal pole in control or altered cells (4 -6). The effect of the unknown, spatially varying lipid spin-density on these calculations, if any, has been largely ignored. Further, these studies have routinely employed 2D spin-echo sequences with slice profiles thicker than the cellular volumes of interest (VOIs), raising concerns regarding partial volume effects. This shortcoming is especially detrimental to studies of the nucleus, which has a diameter of only 300 m.Herein we present separate, high-resolution 3D measurements of the water and lipid spin densities, T 1 and T 2 relaxation time constants, and the water apparent diffusion coefficient (ADC) in the Xenopus oocyte without significant partial volume artifacts. We challenge the conventional binary segmentation of the oocyte cytoplasm into vegetal and animal poles and recognize cylindrical symmetry about the vegetal-animal (V-A) axis. Lipid-specific imaging is demonstrated for which water suppression is achieved via high diffusion weighting in the imaging sequence. We corr...

Two‐dimensional and three‐dimensional time‐lapse microscopic magnetic resonance imaging of Xenopus gastrulation movements using intrinsic tissue‐specific contrast

“…The intrinsic contrast seen in the early embryo is similar to what has been observed in Xenopus oocytes (Sehy et al, 2001). Although not strictly lineage specific, the contrast can be used as a specific marker for tracing morphogenetic movements of different tissue regions through gastrulation.…”

“…This is because tissue water and the lipids exhibit separate proton-NMR frequencies: 4.7 ppm for the tissue water and multiple resonances between 0.5 and 6 ppm for the methine-, methyl-, and methylene groups of the lipids (Sehy et al, 2001). Hence, images formed from the NMR signal of the tissue water are designated "water images," and images formed from the NMR signal of the tissue lipids are designated "fat images."…”

“…The reason for this tissue contrast is the higher yolk content at the vegetal pole (Danilchik and Gerhart, 1987;Hausen and Riebesell, 1991), which results in differential T1, T2, and spin density values in the different embryonic regions (Sehy et al, 2001). It should be noted that, to suppress the fat signal in the water images, we used an initial 90 degrees selective radio frequency (RF) pulse centered around 1.2 ppm with a subsequent dephasing gradient.…”

Simultaneous NMR microimaging of multiple single‐cell samples

“…The particular cell was Xenopus laevis, which has been used for many recent magnetic resonance studies (15)(16)(17)(18)(19)(20).…”

Time‐lapse tracing of mitotic cell divisions in the early Xenopus embryo using microscopic MRI

“…MRI is a noninvasive imaging method, which is not dependent on the optical properties of the tissue and can visualize events deep inside optically opaque specimen. It has no adverse effect on embryo development (Kay et al, 1988), and cell-and tissue-specific magnetic properties in the frog embryo result in image contrast, allowing the specific tissue identification (Sehy et al, 2001;Jacobs et al, 2003). By taking a 2D in vivo time series of a developing Xenopus embryo, we are able to trace the nuclear divisions of a D1-blastomere for five cleavages and record developmentally important parameters of mitosis like the timing as well as the orientation of the divisions.…”