YKL-40 secreted from adipose tissue inhibits degradation of type I collagen.
Differentiating white adipose tissue from presumptive and developing fat pads of newborn and young rats was fixed in buffered osmium tetroxide, embedded in Vestopal W, and examined in an electron microscope. Pre-adipose cells were found to be fibroblasts characterized by their spindle shape, long tenuous cytoplasmic extensions, and profuse endoplasmic reticulum. The developmental stages traced from fibroblast to mature adipose cell show a gradual change in cell shape, an accumulation of cytoplasm and non-membrane-bounded lipid, a decrease in the endoplasmic reticulum, and a change in shape of mitochondria. Transitory glycogen appears at mid-differentiation. Numerous smooth-membraned vesicles occur in the cytoplasm throughout differentiation. Pinocytosis is constantly evident. Cells of the mnltilocular stage are shown to differ from brown fat ceils, particularly with respect to cytoplasmic membrane systems and mitochondria. No transport of particulate lipid from the lumen of the capillary to, or within, the adipose cell was detected, nor could any cell organelle be demonstrated to be visibly related to lipid synthesis and/or deposition. I N T R O D U C T I O N The physiology and biochemistry of lipid metabolism in general, and of adipose tissue in particular, are currently under intensive investigation in many laboratories. The recent development of refined techniques for the measurement of lipids has shown adipose tissue to be highly sensitive (both in vivo and in vitro) to a wide variety of stimuli. However, problems of fixation and ultramicrotomy associated with tissues rich in lipids are such that the cytologist's modern tool of electron microscopy has not yet been extensively applied to the study of the fine structure of adipose cells. Various aspects of the ultrastructure of mature adipose cells have been reported (2-4, 8, 9, 14, 16, 19, 20, 24, 30), but it is apparent in many of these studies that the technical problems mentioned above have made interpretations difficult. Although a number of light microscope studies have been made on the maturation of adipose cells, and indeed this process of lipid storage is a routine laboratory exercise in most histology courses, confusion still exists as to whether fat cells always arise from specific, predetermined anlagen, or, in fact, may also arise from fibroblasts. Differences of opinion have existed for nearly a century, and, unfortunately, recent studies have not resolved the problem. In 1909, Bell (5) reviewed the early investigations; these, and more contemporary studies, have been extensively discussed by Tedeschi (28) and Barrnett (3). To our knowledge, no elec-