Transmission electron microscopy (TEM) studies reveal that early lipoblasts have an elongated configuration, multiple cytoplasmic processes, and abundant endoplasmic reticulum and Golgi apparatus. As lipoblastic differentiation begins, vesicles increase in number, with a corresponding decrease in rough-surfaced endoplasmic reticulum (rER). Small lipid inclusions appear at one pole of the cytoplasm. Pinocytotic vesicles and an external lamina also appear. The presence of an external lamina is a feature that further distinguishes adipocytes from proper connective tissue cells.
Midstage lipoblasts become ovoid as lipid accumulation changes the cell dimensions.
With continued development, the early lipoblasts assume an oval configuration. The most characteristic feature at this stage is an extensive concentration of vesicles and small lipid droplets around the nucleus and extending toward both poles of the cell. Glycogen particles appear at the periphery of the lipid droplets, and pinocytotic vesicles and basal lamina become more apparent. These cells are designated midstage lipoblasts.
TABLE Summary of Molecules Synthesized and Secreted by Adipose Tissue and Their Functions 9.1 Molecule Major Function or Effect Acylation-stimulating Influences the rate of triglyceride synthesis in adipose tissue protein (ASP) Adiponectin, adipocyte Stimulates fatty-acid oxidation complement–related Decreases plasma triglycerides and glucose concentrations and increases insulin protein (ACRP30) sensitivity in cells Plays a role in the pathogenesis of familial combined hyperlipidemia Correlated with insulin resistance and hyperinsulinemia Adipophilin Serves as a specific marker for lipid accumulation in cells Adipsin Serine proteinase that regulates adipose tissue metabolism by facilitating fatty-acid  storage and stimulating triglyceride synthesis Angiotensinogen (AGE) ACG is the precursor of vasoactive angiotensin II (AngII) that regulates blood pressure and Angiotensin II and electrolyte levels in the serum and is also involved in the metabolism and (AngII) differentiation of adipose tissue During development, AngII inhibits differentiation of lipoblasts; in mature adipocytes it regulates lipid storage Insulinlike growth factor I Stimulates proliferation of a wide variety of cells and mediates many of the effects (IGF-I) of growth hormone Interleukin 6 (IL-6) Interacts with cells of immune system and regulates glucose and lipid metabolism Decreases activity of adipose tissue in cancer and other wasting disorders Leptin Regulates appetite and body energy expenditure Signals to the brain about body fat stores Increases formation of new vessels (angiogenesis) Involved in blood pressure control by regulating vascular tone Potent inhibitor of bone formation Plasminogen activator Inhibits the fibrinolytic system inhibitor-1 (PAI-1) Elevated levels are associated with increased formation of blood clots Prostaglandins I2 and Helps regulate inflammation, blood clotting, ovulation, menstruation, F2, (PGI2 and PGF2) and acid secretion Resistin Increases insulin resistance Linked to obesity and to type 2 diabetes Transforming growth Regulates a wide variety of biologic responses, including proliferation, differentiation, factor (TGF-) apoptosis, and development Tumor necrosis factor Interferes with insulin receptor signaling and is a possible cause of development (TNF-) of insulin resistance in obesity (Modified from Frbeck G, Gomez-Ambrosi J, Muruzabal FJ, Burrell MA. The adipocyte: a model for integration of endocrine and metabolic signaling in energy metabolism regulation. Am J Physiol Endocrinol Metab 2001;280:E827–E847.)
The mature adipocyte is characterized by a single, large lipid inclusion surrounded by a thin rim of cytoplasm.