Cell culture models have been developed to study commitment and subsequent differentiation of preadipocytes into adipocytes. Bone morphogenetic protein 4 commits mesenchymal stem cells to the adipose lineage. Other factors, including Wnt signaling, cell density, and cell shape, play a role in lineage commitment. Following commitment to the adipose lineage, growth-arrested preadipocytes can differentiate to adipocytes by treatment with insulin-like growth factor 1, glucocorticoid and an agent that increases cAMP level. This process is characterized by a rapid and transient increase in CCAAT/enhancer binding protein (C/EBP) beta and synchronous re-entry into the cell cycle. Acquisition of DNA-binding by C/EBPbeta occurs after the transcription factor becomes phosphorylated. The cells enter a growth-arrested state and begin terminal differentiation. C/EBPalpha, peroxisome proliferator-activated receptor gamma, and adipocyte determination, and differentiation-dependent factor 1 coordinate the expression of genes that create and maintain the adipocyte phenotype.

We have analysed the expression of keratins in the epidermis of normal human palm and sole skin (ridged skin) using immunohistochemistry and in situ hybridization. The epidermis of human ridged skin expresses a more complex pattern of keratins than thin skin, which is probably due to the greater stress that ridged skin has to withstand. In addition to keratin K9, we document specific expression patterns of keratins K6, K16 and K17 which are suggestive of regional adaptations of this epidermis to a high cell turnover rate. In particular, the sequestered location of nests of K17-positive cells at the bottom of the deep primary epidermal ridges supports the notion of functional heterogeneity of basal cells and suggests that the K17-positive sites may include stem cells. Expression of K6 and K16 in some basal and most suprabasal keratinocytes is compatible with a constitutively high proliferative activity of normal ridged epidermis, but may also reflect different physical properties of the suprabasal cells, in contrast with regions expressing K9. The distinct labelling patterns observed in primary and secondary epidermal ridges as well as epidermal layers above dermal papillae suggest the existence of local microenvironmental niches leading to differences in keratinocyte differentiation.

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Swiss mouse 3T3-C2 fibroblasts, grown to confluence in monolayer culture, are shown to fuse when exposed to electric fields. Exposure to five repetitive electric pulses of about 1 kilovolt per centimeter with a duration of 50 microseconds caused approximately 20 percent of the cells to become fused (multinucleate) when 1 millimolar magnesium was present in the medium. The effects of minimum thresholds of field strength, pulse duration, and number of pulses were determined. Cell disruption was observed when the electric field exceeded 2.0 kilovolts per centimeter or the pulse was of longer duration than 120 microseconds.

Transit of newly synthesized triacyl[3H]-glycerol through organelles of the secretory system leading to assembly into nascent very low density lipoproteins (VLDLs) or to cytoplasmic storage was investigated in chick hepatocytes. Cells in monolayer culture were pulse-labeled with [2-3H]glycerol, and after different periods of chase with unlabeled glycerol, the movement of triacyl[3H~glycerol through the endoplasmic reticulum (ER) and Golgi and the incorporation into nascent VLDL and cytoplasmic triacylglycerol-rich vesi- Very low density lipoprotein (VLDL), secreted by the hepatocyte, is a major carrier of triacylglycerol in the bloodstream of higher animals. The VLDL particle consists of a neutral hydrophobic core of triacylglycerol and cholesterol ester surrounded by an amphipathic shell of phospholipid, cholesterol, and specific apolipoproteins. Each ofthese components is synthesized by a distinct membrane-bound enzyme system associated with the endoplasmic reticulum (ER) (1, 2). The apolipoproteins, like other secretory proteins, are synthesized and cotranslationally translocated through the membrane of the ER into its lumen (3,4). Since the lipid components of VLDL are synthesized by enzymes situated on the cytoplasmic face of the ER (2), the question is raised as to how these lipids cross the ER membrane to gain access to the lumenal apolipoproteins with which they must assemble? Also unknown is the site within the secretory system at which this assembly process occurs.Recently we determined that three apolipoproteins (apoB, apoII, and apoA-I) of nascent VLDL move through the ER of the chick hepatocyte at markedly different rates (5). In each case transit through the Golgi was found to be ratelimiting for the overall process of apolipoprotein secretion. Thus, it can be concluded that these apolipoproteins do not associate with each other or with the same particle during movement through the ER; apoB and apoll are hydrophobic and difficult to dissociate from the triacylglycerol core of VLDL once this association has occurred (6, 7). It would be expected, therefore, that upon binding to the nascent triacylglycerol particle, these apolipoproteins would remain tightly bound to this lipid during the remainder of their transit through the secretory system. Taken together, these observations and the fact that the apolipoproteins traverse the Golgi at comparably slow rates (5) Cells were scraped into SH buffer containing protease inhibitors (5) and then were homogenized by 10 passages through the ball-bearing homogenizer (5) described by Balch and Rothman (10). One-half milliliter of 0.22 M sucrose was layered over the cell homogenate, and the sample was centrifuged for 10 min at 10,000 x g. Nuclei and debris from the cell homogenate were in the pellet, whereas cytoplasmic triacylglycerol-rich vesicles (TGRVs; see ref. 11) were in the Abbreviations: VLDL, very low density lipoprotein; ER, endoplasmic reticulum; TGRVs, triacylglycerol-rich vesicles; apoB, apoll, and apoA-I, apolipoproteins B, II, and A-I.

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