Differentiation of Blood Vessels in the Adipose Tissue of Lean and Obese Fetal Pigs, Studied by Differential Enzyme Histochemistry

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Abstract
Subcutaneous adipose tissue was obtained from fetuses removed from pregnant obese (Ossabaw) and lean (crossbred) sows at three stages of gestation (70, 90, and 110 days). Histochemical analysis for nucleoside phosphatase (NPase), alkaline phosphatase (APase), and NADH tetrazoleum reductase (NADH-TR) was conducted on fresh-frozen cryostat sections. Age- associated changes in NPase and NADH-TR reactions in the arteriolar system were correlated with the morphological development of the medial layer of arterioles and arteries. For instance, a strong NPase reaction in small arterioles was associated temporally with the assumption of a normal smooth muscle cell morphology and arrangement in the medial layer. In the youngest fetuses, strong NADH-TR reactions were only evident in small and presumptive arterioles and venules (associated with fat cells). Little NADH-TR reactivity was evident in larger arterioles and venules in 70-day tissue. Arteries and large arterioles were distinguished from veins and venules (strong reactions vs. weak reactions) with NADH-TR and NPase reactions in the oldest fetuses. In the younger fetuses, the NPase distinction (arterioles vs. veinules) was obvious before NADH-TR distinction. Small adipocyte-associated vessels were APase positive in the youngest fetuses, but APase reactivity was limited to short segments of vessel between arterioles and capillaries in the oldest fetuses. With the following exceptions, all the above observations were independent of fetal strain. In obese fetuses (110 day) small venules and small arterioles were equally reactive for NPase activity. Capillaries in obese fetuses (110 day) were NADH-TR reactive, whereas no activity was evident in capillaries from lean fetuses (110 day). The histology of intimal and medial vessel walls was independent of fetal strain. However, at every fetal age the adventitial layer of large arterioles and the collagenous stroma (around fat cell clusters) was thicker and better organized in obese fetuses. In the present study, differential enzyme histochemistry was utilized to distinguish the differentiation of small adipocyte-associated arterioles from larger arterioles. Furthermore, structural evidence of the arteriolar system clearly precedes metabolic and histological differentiation of tunica media cells. Finally, metabolic and structural traits of the stromal vascular cells of adipose tissue were sensitive to a fetal obese pheno-type.