04-Adipose Tissue
How are triglycerides absorbed from foods in the digestive tract and released into the circulation? How are triglycerides released into the circulatory system by hepatocytes?
Chylomicrons (Gr. chylos, juice + micros, small) are particles of variable size, up to 1200 nm in diameter, formed from ingested lipids in epithelial cells lining the small intestine and transported in the blood and lymph. They consist of a core containing mainly triglycerides, surrounded by a stabilizing monolayer of phospholipids, cholesterol, and several apolipoproteins.
Are there any other types of cells present in white adipose tissue beside adipocytes? What type of fibers in the extracellular matrix support the individual adipocytes?
Fibroblasts, macrophages, and other cells typically comprise about half the total cell number in white adipose tissue. Reticular fibers form a fine interwoven network that supports individual fat cells and binds them together
What types of hormones and cytokines are secreted by adipose tissue? What is the hypothesized role of excess adipose tissue in human disease, and is any particular location of excess adipose tissue especially a problem?
Hormonal activity of white adipocytes themselves includes production of the 16-kDa polypeptide hormone leptin (Gr. leptos, thin), a "satiety factor" with target cells in the hypothalamus, other brain regions, and peripheral organs which helps regulate the appetite under normal conditions and participates in regulating the formation of new adipose tissue. other cytokines and other factors with paracrine and autocrine activity, including many proinflammatory cytokines. It is not clear whether these are produced by adipocytes or other cells of the tissue such as macrophages or fibroblasts. With its increased amounts of white adipose tissue, obesity is characterized by a state of chronic mild inflammation. Proinflammatory factors released from visceral fat are being investigated for links to the inflammation-related disorders associated with obesity, such as diabetes and heart disease
What is the mechanism for the uptake of triglycerides from the blood and into the adipocyte? What hormone plays a role?
In adipose tissue both chylomicrons and VLDLs are hydrolyzed at the luminal surfaces of blood capillaries by lipoprotein lipase, an enzyme synthesized by the adipocytes and transferred to the capillary cell membrane. Free fatty acids then enter the adipocytes by both active transport and diffusion. Within the adipocytes, the fatty acids combine with glycerol phosphate, supplied by glucose metabolism, to again form triglycerides, which are then deposited in the growing lipid droplet. Insulin stimulates glucose uptake by adipocytes and accelerates its conversion into triglycerides, and the production of lipoprotein lipase.
What is the sequence by which the different types of adipocytes develop?
Like other connective tissue, skeletal and muscle cells, adipocytes develop from mesenchymal stem cells. Adipose development first produces preadipocytes, which look rather like larger fibroblasts with cytoplasmic lipid droplets. white adipocytes develop together with a smaller population of cells termed beige adipocytes, which remain within white adipose tissue and have histological and metabolic features generally intermediate between white and brown adipocytes. With adaptation to cold temperatures beige adipocytes change reversibly, forming many more small lipid droplets, adopting a gene expression profile more like that of brown fat, and begin to release heat
What is the mechanism for the release of fatty acids from the adipocyte and into the blood, and what is the role of norepinephrine in this process? How are fatty acids transported in the blood?
Norepinephrine released in the adrenal gland and by postganglionic sympathetic nerves in adipose tissue activates a hormone-sensitive lipase that breaks down triglycerides at the surface of the stored lipid droplets (Figure 6-2). This lipase activity is also stimulated by growth hormone (GH) from the pituitary gland. The free fatty acids diffuse across the membranes of the adipocyte and the capillary endothelium, and bind the protein albumin in blood for transport throughout the body. The more water-soluble glycerol remains free in blood and is taken up by the liver. Insulin inhibits the hormone-sensitive lipase, reducing fatty acid release, and also stimulates enzymes for lipid synthesis. Besides insulin and GH, other peptide hormones also cooperate in regulating lipid synthesis and mobilization in adipocytes.
What type of energy storage molecule does the adipose cell contain, what is its chemical structure, and what advantage does this molecule have over glycogen or glucose? In addition to energy storage, what other functions does adipose tissue perform?
They contain lipids. storage depots for neutral fats, chiefly triglycerides (long-chain fatty acyl esters of glycerol), adipocytes function as key regulators of the body's overall energy metabolism, the advantage it has over glucose is that it can produce more ATP per molecule and it's 2x the caloric density. Adipose also serves as a cushion for organs. fat conducts heat poorly and provides thermal insulation for the body.
What other organ synthesizes lipids?
VLDLs are smaller complexes (30-80 nm, providing a greater surface-to-volume ratio), of similar lipid and protein composition to chylomicrons, but are synthesized from lipids in liver cells. Levels of circulating lipoproteins are routinely measured in clinical tests for blood lipids, after fasting to allow depletion of chylomicrons. Varying levels of apoproteins and triglycerides in the complexes allow their categorization according to density, from VLDL to high-density lipoprotein (HDL
What is the mechanism for obesity in infants versus the mechanism in adults?
adult-onset obesity mainly involves Adipose Tissue increasing the size of existing adipocytes (hypertrophy). Childhood obesity, in contrast, often involves increases in both adipocyte size and numbers due to the differentiation of more preadipocytes from mesenchymal cells (hyperplasia). Weight loss after dietary changes is due to reductions in adipocyte volume, but not their overall number.
In what locations is white adipose tissue most abundant?
cells are typically found isolated or in small groups within loose or dense irregular connective tissue but occur in large aggregates in adipose tissue or "fat" in many organs and body regions.
How do the adipocytes of brown adipose tissue appear with respect to their stored triglyceride and nucleus? Why is this tissue brown when viewed with the naked eye? Where do you find it in humans? What is its function, and what is the mechanism by which it carries out this function?
located mainly in the back, neck, and shoulders, but it is greatly reduced during childhood and adolescence. In adults it is found only in scattered areas, especially around the kidneys, adrenal glands, aorta, and mediastinum. The color of brown fat is due to both the very abundant mitochondria (containing cytochrome pigment) scattered among the lipid droplets of the fat cells and the large number of blood capillaries in this tissue. Brown adipocytes contain many small lipid inclusions and are therefore called multilocular (Figure 6-3). The small lipid droplets, abundant mitochondria, and rich vasculature all help mediate this tissue's principal function of heat production and warming the blood. Heat production in brown adipocytes is greater than that of other cells because their inner mitochondrial membranes have greatly upregulated levels of the transmembrane protein uncoupling protein-1 (UCP1) or thermogenin.
In what form is triglyceride stored in the adipocyte, and what molecules immediately surround it? Where are lipid droplets formed in the cell?
stored in lipid droplet, single phospholipid around it with proteins. White adipocytes can store triglycerides derived from three sources: 1) Dietary fats brought to the cells via the circulation as chylomicrons 2) Lipids synthesized in the liver and transported in blood with very-low-density lipoproteins (VLDLs), 3)Free fatty acids and glycerol synthesized by the adipocytes
How do the adipocytes of white adipose tissue appear in histological sections in terms of their nucleus, triglyceride, and cytoplasm? What extracellular layer immediately surrounds them?Why do adipocytes look empty in histological sections?
unilocular. Because lipid is removed from cells by xylene or other solvents used in routine histological techniques, unilocular adipocytes are often empty in standard light microscopy. The cells are sometimes said to have a signet-ring appearance, with the lipid droplet displacing and flattening the nucleus against the cell membrane. They are surrounded by thin external lamina containing type IV collagen. Most cytoplasmic organelles in a white adipocyte are near the peripheral nucleus, including mitochondria, a small Golgi apparatus, a few cisternae of RER, and free polyribosomes. The thin, submembranous layer of cytoplasm surrounding the lipid droplet contains cisternae of smooth ER (SER) and pinocytotic vesicles
What is the histological composition of adipose tissue?
white adipose tissue, the more common type specialized for fat storage, consists of cells each containing one large cytoplasmic droplet of whitish-yellow fat. Brown adipose tissue contains cells with multiple lipid droplets interspersed among abundant mitochondria, which helps give this tissue a darker appearance. unlike other cells of connective tissue proper, are individually surrounded by a thin external lamina containing type IV collagen.
