The epididymal tissue is recognisable in the fetus in the last days of gestation. At this time, only the capillaries are morphologically visible, immersed in the extracellular matrix together with collagen fibres and undifferentiated cells. Postnatally, a profound rearrangement takes place (in rats and mice on days 4-6) marked microanatomically by areas well circumscribed by fibroblast-like cells and by numerous dilated capillaries. Among these vessels a rich cell population is observed, the vast majority of which is made up of preadipocytes at various stages of differentiation from cells barely showing the first signs of differentiation (glycogen and small triglyceride vacuoles) to unilocular cells similar to mature adipocytes in all but size (5-10 ^m in diameter compared with the 70-80 ^m of mature adipocytes) (Fig. 5).
It is important to note that the less differentiated cells are often found in the pericytic position (completely enveloped in a doubling of the basal capillary membrane). The extracellular matrix inside the area circumscribed by the fibroblast-like cells is very rich in collagen fibrils and is clearly different from the one surrounding it, where an amorphous matrix predominates. We designated these areas as vasculo-adipocytic islands and believe them to be the primary sites of adipocyte development in the epididymal depot. The preadipocytes developing in these areas are easily distinguished from the other cell elements (endothelial cells, fibroblasts, mast cells, macrophages) by their characteristic cytoplasm, which contains varying amounts of glycogen and
lipids. Rough endoplasmic reticulum and Golgi apparatus are well represented when lipids are still scarce. Smooth endoplasmic reticulum is variable and is frequently associated with lipid vacuoles. Mitochondria are typically elongated with small, variously oriented cristae.
Vasculo-adipocytic islands are found in the first week of postnatal development, but preadipocytes are recognisable until postnatal weeks 3-6 in the epididymal depots of mice and rats (our unpublished data).
The study of subcutaneous adipose tissue in human fetuses has evidenced morphological developmental features very similar to those of mice and rats reported above. At present, it is impossible to say whether they are stem cells or cells genetically predisposed to differentiate into adipocytes, or to understand whether they originate from the vessels of the tissue itself or at distant sites like bone marrow.
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