Magnetic resonance imaging of the rat Harderian gland

Abstract

The intra-orbital lacrimal gland (Harderian gland, or HG) of the female rat was studied by magnetic resonance imaging (MRI) to evaluate whether MRI can be used to visualize the gland in vivo and localized-1H-spectroscopy detect its lipid content. The results were correlated with post-mortem anatomical sections, and with light and electron microscopy. On MRI, HG presented as a mass located between the ocular bulb and the orbit. In strongly T2W sequences the secretory structures had a reduced signal while intraparenchymal connective tissue was visible. T2-quantitative maps values of HG (60.12 +/- 8.15 ms, mean +/- SD) were different from other tissues (i.e. muscular tissue, T2 = 44.79 +/- 3.43 ms and olfactory bulb, T2 = 79.26 +/- 4.25 ms). In contrast-enhanced-MRI, HG had a signal-intensity-drop of 0.074 +/- 0.072 (mean +/- SD), after injection of AMI-25, significantly different from the muscle (0.17 +/- 0.10). Localized MRI spectra gave a large part of the signal originating from fat protons, but with a significant percentage from water protons. At light and electron microscopy the lipid deposition appeared to be composed of low-density material filling a large part of the cytoplasm, and the porphyrin aggregates were easily recognizable. The data demonstrate that an in vivo study of the HG was feasible and that high-field MRI allowed analysis of the gross anatomy detecting the lipid content of the gland.

Fig. 1. MRI of a rat's head, coronal section. (a) T1W image. (b) Proton density image. (c) Lightly T2W image. (d) T2W image. (e) Pre-contrast image. (f) Post-contrast image. E, eyeball; B, brain; H, Harderian gland; M, muscle; T, tongue.

Fig. 2. (a) MRI of a rat's head, coronal section. T2 parametric map. (b–e) Anatomical sections. E, eyeball; B, brain; H, Harderian gland; M, muscle; T, tongue. Arrows mark pigment deposit in HG.

Fig. 3. In the left column (a–e), images of a rat's head are visible. The intersection between horizontal and vertical lines marks the point where localized 1H spectra were acquired. In the right column (a1–e1) the corresponding 1H-spectra are shown (W, water proton peak; F, fat proton peak). If a window is selected in frequencies corresponding to fat proton peak (e1) it is possible to discriminate lipid-rich areas (e). The coloured pixels were superimposed on to an anatomical image. Clear colours indicate areas with richest lipid deposit.

Fig. 4. (a–c) Light microscopy, toluidine-stained semithin sections. White asterisks indicate the connective-tissue septa, while the black asterisks indicate the porphyrin precipitates. (d) Transmission electron microscopy. E, extracellular space; L, lipid pools; N, nucleus; P, porphyrin precipitates. Scale bars = a, 65 µm; b,c, 16 µm (particulars of panel a); d, 2 µm.