The development of the inner layer of backfat in fetal and young pigs.

The development of the innermost layer of backfat was studied histologically and histochemically in fetuses from Yorkshire sows at 45, 60, 65, 75, 85, 90, 105, 110, and 112 d of gestation. Samples were also obtained from fetuses at 110 d of gestation from Ossabaw sows and from sows selected for high (obese) or low (lead) backfat thickness. Fetuses from some sows were decapitated at 45 d of gestation and samples were obtained from these fetuses at 110 d of gestation. The inner layer develops before 75 d of gestation and has stromal-vascular characteristics that clearly distinguish it from the middle layer of backfat. For instance, a well organized and dense collagen matrix immediately surrounds small fat cell clusters in the inner layer. Most fat cell cluster capillaries (in the inner layer) reacted strongly for ATPase histochemically and are histologically immature. The inner layer morphology and cell size was similar in Ossabaw, obese and lean fetuses. The number of PAS (Periodic Acid Schiff) positive cells were dependent on fetal strain. The number was highest in obese fetuses and lowest in Ossabaws. Comparison of decapitated fetuses and weight-matched controls indicated that the inner layer was most affected by decapitation. A decreased organization and amount of collagen was associated with larger fat cell clusters in decapitated fetuses, and most fat cell cluster capillaries reacted little for ATPase histochemically and were histologically mature. The range in inner-layer fat cell diameter was wider in decapitated fetuses (11 to 55 micron) than in control fetuses (11 to 30 micron). These results demonstrate that the characteristics of the inner layer are clearly different from the other layers and that the amount and organization of collagen was inversely associated with the size of fat cell clusters and fat cells and with the maturity of the adipocyte-associated capillaries.