With oceans warming, marine mammals might face additional physiological challenges to maintaining energy and thermal balance. Seals’ insulation and energy storage depend on blubber thickness. Blubber has a thermal conductivity gradient and metabolic stratification, suggesting it might also be morphologically stratified. However, grey seals alternate fasting and feeding periods and therefore undergo natural periodic changes in blubber thickness. Seal’s energetic reservoir and thermoregulation capacity thus fluctuates annually. In autumn, pregnant females come ashore to give birth, where they will face additional heat dissipation constrains with different air temperatures and humidity than at sea. Females fast during lactation, therefore diminishing fat stores. In contrast, pups triple their size during the three weeks suckling period. The fatter they get, the more likely they are to survive their first year. Females face a trade-off between fattening their pups and keeping enough blubber for their own survival upon returning to sea. Quick adaptation to thermal and energy needs is vital for seals and its balance can be affected by raising temperatures. Better understanding on how seals’ blubber morphology adjusts to body composition changes is needed to predict potential responses to climate warming. Here we investigated how fat cells and vascularity change throughout blubber depth during weight fluctuations at different age classes in wild grey seals. Blubber biopsies from the dorsal flank were collected from mother and pup pairs (n=6) at early and late lactation and at pup’s early weaning. Appropriate Zoletil age dependant dose (Females: 0.01mL/Kg, IM; pups: 0.002 mL/Kg, IV) and Lignol (2mL, SC) were administrated as anaesthetics by experienced licensed personnel. Samples were fixed in formalin, processed and stained with Masson’s Trichrome. Blubber was divided into three depth sections: inner, middle and outer. Five microphotographs of each section were taken, and adipocyte size and vascularity were analysed with Image J. Linear mixed models were used to investigate association with body mass, tissue depth and feeding vs fasting within each age class. Preliminary results showed adipocyte size was positively associated with body mass in both mothers (LME: AIC=493.6; N=6; n=30, p=0.03) and pups (LME: AIC=700.9; N=6; n=46, p<0.01). However, pups’ adipocytes are smaller than their mothers’ (mothers = 4018 ± 898 µm2, pups 3360 ± 1310 µm2), despite pups’ fat percentage being generally higher than mothers at this stage. Vascularity was not significantly associated with any measurements in mothers. However, in pups, vascularity increases at late suckling, particularly in the inner and middle blubber. These results suggest angiogenesis happens alongside adipogenesis, but it is independent from adipocytes hypertrophy. Analysis distinguishing between capillaries vs big blood vessels is underway to investigate the perfusion capacity during extreme fattening and fasting. Based on these results, grey seals’ blubber structure is not morphologically stratified across depth except at early age, when tissue is expanding for first time. Insulation capacity will be affected by the dramatic fatness fluctuations during breeding season, with no apparent morphological changes in vascularity to counteract thermoregulatory constrains. Changing environment is a concern to maintain the delicate trade-off female seals face during breeding season.