M/Z: 751.5154

1 male adult wild-type rat was obtained from Inserm U1085 - Irset Research Institute (University of Rennes1, France). Animals were age 60 days and were reared under ad-lib conditions. Care and handling of all animals complied with EU directive 2010/63/EU on the protection of animals used for scientific purposes. The whole epididymis was excised from each animal immediately post-mortem, loosely wrapped rapidly in an aluminum foil and a 2.5% (w/v) carboxymethylcellulose (CMC) solution was poured to embed the epididymis to preserve their morphology. To remove air bubbles, the filled aluminum molds was gently freezed by depositing it on isopentane or dry ice, then on the nitrogen vapors and finally by progressively dipping the CMC/sample coated with aluminum foil into liquid nitrogen (or only flush with liquid nitrogen). Frozen tissues were stored at -80 °C until use to avoid degradation.

Fig. 2 MALDI-MSI data from the same mouse lung tissue analyzed in Fig. 1. A: Optical image of the post-MSI, H&E-stained tissue section. B–D, F–G: Ion images of (B) m/z 796.6855 ([U13C-DPPC+Na]+), (C) m/z 756.5514 ([PC32:0+Na]+), (D) m/z 765.6079 ([D9-PC32:0+Na]+), (F) m/z 754.5359 ([PC32:1+Na]+), and (G) m/z 763.5923 ([D9-PC32:1+Na]+). E, H: Ratio images of (E) [D9-PC32:0+Na]+:[PC32:0+Na]+ and (H) [D9-PC32:1+Na]+:[PC32:1+Na]+. Part-per-million (ppm) mass errors are indicated in parentheses. All images were visualized using total-ion-current normalization and using hotspot removal (high quantile = 99%). DPPC = PC16:0/16:0. U13C-DPPC, universally 13C-labeled dipalmitoyl PC; PC, phosphatidylcholine; MSI, mass spectrometry imaging; H&E, hematoxylin and eosin. Fig 1-3, Fig S1-S3, S5

Fig 6c Fig. 6 MALDI-MSI of U13C-PC16:0/16:0 acyl chain remodeling. A: Averaged MALDI mass spectrum from lung tissue collected from mice euthanized 12 h after administration of D9-choline and U13C-DPPC–containing Poractant alfa surfactant. The ion at m/z 828.6321 is assigned as the [M+Na]+ ion of 13C24-PC16:0_20:4 formed by acyl remodeling of U13C-PC16:0/16:0. The “NL” value refers to the intensity of the base peak in the full range MS1 spectrum. B: MS/MS spectrum of precursor ions at m/z 828.5 ± 0.5 with fragment ions originating from [13C24-PC16:0_20:4+Na]+ annotated. Part-per-million (ppm) mass errors are provided in parentheses. C, D: MALDI-MSI data of [U13C-DPPC+Na]+ (blue), [PC36:4+Na]+ (green) and [13C24-PC16:0_20:4+Na]+ (red) in lung tissue collected from mice (C) 12 h and (D) 18 h after label administration. All images were visualized using total-ion-current normalization and hotspot removal (high quantile = 99%). MS/MS, tandem mass spectrometry; MSI, mass spectrometry imaging; PC, phosphatidylcholine; U13C-DPPC, universally 13C-labeled dipalmitoyl PC.

Supplementary Figure S8. MALDI-MSI data of mouse lung tissue administered with D9-choline and U 13C-DPPC–containing Poractant alfa surfactant (labels administered 18 h prior to sacrifice). Ion images of (a) m/z 796.6856 ([U13C-DPPC+Na]+), (b) m/z 756.5154 [PC32:0+Na]+ and (c) m/z 765.6079 ([D9-PC32:0+Na]+). (d) Overlay image of [U13C-DPPC+Na]+ (red) and [D9-PC32:0+Na]+ (green). Parts per million (ppm) mass errors are indicated in parentheses. All images were visualised using totalion-current normalisation and using hotspot removal (high quantile = 99%). DPPC = PC16:0/16:0.

Sample information Organism: Macropus giganteus (kangaroo) Organism part: Brain Condition: Wildtype Sample growth conditions: Wild

The human colorectal adenocarcinoma sample was excised during a surgical operation performed at the Imperial College Healthcare NHS Trust. The sample and procedures were carried out in accordance with ethical approval (14/EE/0024).

The human colorectal adenocarcinoma sample was excised during a surgical operation performed at the Imperial College Healthcare NHS Trust. The sample and procedures were carried out in accordance with ethical approval (14/EE/0024).