Secondary ion images of the developing rat brain

Abstract

Secondary ion images were obtained from sections of rat brain over a 21 day postnatal period, using the intensity of m/z 184, phosphocholine. When compared with corresponding optical images of similar, but stained sections from the same animal, the secondary ion images appear to reflect less developed brains. During development, myelination occurs after axon extension. Apparently, myelination obscures the source of secondary m/z 184, phosphatidylcholine, from the analyzing ion probe; absenting myelination, secondary ion images show no physiological features. (J Am Soc Mass Spectrom 2004, 15, 1116 -1122) © 2004 American Society for Mass Spectrometry I mages obtained by a variety of methods-optical, secondary electron, nuclear magnetic resonance, etc., are integral to the philosophy of all branches of medicine. Among mass spectral methods, secondary ion mass spectrometry (SIMS) and imaging matrix assisted laser desorption (iMALDI) mass spectrometry are the major players in this arena [1]. These are both emerging methods, and as such, it is necessary to establish that not only are results consistent with known mass spectrometry, but also with known biomedicine.Secondary ions are emitted from tissue at virtually every m/z. We identify secondary ions by tandem mass spectrometry (MS/MS), and map the emission of ions whose structure we know. We then try to interpret the images in the context of known anatomy physiology. For example, the major peak in the secondary ion mass spectrum of almost any collection of animal cells is m/z 184, phosphocholine. Phosphocholine is unique among secondary ions emitted from tissue in that its structure has been verified by MS/MS [2], a method permitting distinction of phosphocholine from other sources of m/z 184 such as epinephrine. The secondary phosphocholine ion derives from the head group of phosphatidylcholine, (PC in the shorthand of biochemists) a major constituent of the animal cell membrane. Most of the outer layer of cell membranes in the brain is either phosphatidylcholine or sphingomyelin [3][4][5], and both compounds yield abundant m/z 184. Thus, detection of phosphocholine is consistent with the chemistry of low dose or static SIMS [6], a surface analytical method, and the fact that most of the surface of a tissue sample is comprised of compounds yielding phosphocholine secondary ions, m/z 184.The utility of SIMS for mapping phoshocholine is based on the observation that emission of phosphocholine secondary ions is heterogeneous across the tissue section surface [7]. Moreover, secondary m/z 184 images of adult rat brain show a direct correlation with optical images of stained tissue sections [8]. Regions of the brain containing a high density of cells such as cerebral cortex or the caudate/putamen yield an intense m/z 184 secondary ion signal, whereas acellular regions such as the corpus callosum or external capsule yield little or no m/z 184 secondary emission. In the adult, these acellular regions of the brain are very dense in cell projections (axons), many of which ar...

Imaging Mass Spectrometry for Drugs and Metabolites

“…While much development has occurred for the application of SIMS to imaging endogenous components, or mostly molecular fragment ions, of tissues, the use of SIMS for drug and metabolite imaging has been limited [36][37][38][39]. Most of the work done related to the pharmaceutical field has been accomplished by atomic imaging of drugs with SIMS ion microscopy [40][41][42][43][44].…”

Imaging mass spectrometry at cellular length scales

“…Consequently, in static SIMS, significantly less fragmentation of the molecular content occurs, which allows the technique to be used in imaging of small organic components [44][45][46] . To enhance the ionization yield for large intact molecular ions by SIMS, different kinds of surface modifications (MALDI matrices [47][48][49][50][51] , silver 52 and gold [53][54][55][56][57] ) as well as the use of polyatomic primary ion beams [58][59][60][61][62][63] have been suggested. Although these methods have been shown to desorb and ionize peptide and proteins from model samples, in direct tissue analysis they are highly biased toward lipids and steroids.…”

Mass Spectrometric Imaging of the Nervous System

“…For example, using high-resolution SIMS, the distribution of 28 CNin a single U-14 C-thymidine-labeled chromosome, as well as that of Ca 2+ , Mg 2+ , Na + , and K + associated with mitotic chromosomes, has been studied [23,24]. There have been many reports of SIMS having provided images of a variety of samples, including neuroblastoma cells [25], oligodendrocytes [17], astrocytes [17], molluscan neurons [2], human glioblastoma cells [26], human melanoma cells [27,28], rat glioma cells [29], molluscan ganglion sections [30,31], cockroach neurohemal organ [30], rat cerebellum [32], and sections of rat [3,[33][34][35], mouse [36,37], and human [38] brains. Localization of elements such as aluminum [38], boron, calcium [29,39], fluorine [26], and sodium and potassium [3,17,28,29] have also been reported using SIMS imaging with cellular or even subcellular resolutions.…”