Adipose tissues in human fetuses with special reference to the associated fascial development

The differentiation of adipocytes and their structured cluster (=lobule) had been classified into stages 1-5. However, there was no information as to whether 1) large lobules are composed of matured adipocytes or not; 2) a stage-dependent organization occurs within a lobule or not; and 3) all lobules at an area develop synchronously or lobules at different stages are mixed. We examined histological sections of 18 sites in the body from 38 human fetuses at approximately 21-40 weeks of gestation. All adipose tissue lobules developed synchronously in the foot dorsum, face, perianal area, retrobulbar area of the orbit, deep neck space, intervertebral canal, mesentery, and perirenal area. However, lobules at different stages were mixed in the anterior abdominal wall, axilla, hand palm and foot sole. Irrespective of whether the lobule comprised of undifferentiated or matured adipocytes, the palm and sole usually contained "heterogeneous lobules" larger than uniform lobules in the dorsum. In the retrobulbar area and colic mesentery, larger adipocytes made a core of the lobule and smaller cells surrounded it. Conversely, the intervertebral canal contained lobules with an unmatured adipocyte-made core. A candidate brown fat, comprising of highly eosinophilic small adipocytes, was found in the axilla and deep neck. Topohistology of adipocytes within a lobule as well as the correlation of lobule size with adipocyte differentiation was different between sites. Fascial compartments completed prior to the adipocyte differentiation in extremities, whereas they enlarged depending on the lobule growth at the other sites. The adipose tissue development showed a region-specific manner.