Connective and Adipose Tissue

• Adipose tissue is a type of connective tissue studied by Seda Karabulut, PhD.
• Other tissue types like epithelium, muscle, and nerve consist mainly of cells.
• Connective tissue's major component is the extracellular matrix (ECM).
• The extracellular matrix comprise protein fibers, such as collagens and elastic fibers, combined with ground substance.

Structure of Connective Tissue

• Connective tissue contains cells, fibers, and ground substance.
• Cells include fibroblasts, macrophages, mast cells, plasma cells, lymphocytes, leukocytes, and adipose cells.
• Types of fibers in connective tissue are reticular, elastic, and collagen.
• The ground substance contains macromolecules and multiadhesive glycoproteins.

About Adipose Tissue

• Connective tissue in which adipocytes or fat cells are predominant is called adipose tissue.
• These large cells occur in small groups within loose or dense connective tissue, but can form large masses like adipose tissue or fat in body regions.
• Adipose tissue location is throughout the human body and represents normally 15%-20% of men body weight, somewhat more in women.
• Besides storing neutral fats (triglycerides), adipocytes regulate the body's overall energy metabolism.
• Obesity, diabetes, and heart ailment links mean adipocytes and tissues are a major area of medical research.
• Energy balance, nutrient transport, blood pressure, insulin sensitivity, glucose homeostasis, angiogenesis, lipid metabolism, and vascular hemostasis are key for the study of adipose tissue.
• Adipocytes are metabolically active and respond to nervous and hormonal stimuli.
• Adipose tissue releases hormones and other substances, which marks it as an important endocrine tissue.
• PAI-1, Adipisn, Adiponectin, Resistin,Leptin, and Angiotensin are released by adipose tissue.
• Adipose tissue insulates the body, cushions other tissues/organs, and helps shape the body surface, acting as shock absorbers in the soles and palms.

Types of Adipose Tissue

• Types of adipose tissue exist with different locations, structures, appearances, and pathologic characteristics.
• White adipose tissue is the more common type, that contain a large droplet of whitish-yellow fat to their cytoplasm, once fully developed
• Brown adipose tissue contains multiple lipid droplets interspersed among abundant mitochondria, giving the cells a darker appearance.
• Both types of adipose tissue have a substantial blood supply.

Adipose Tissue Processes

• Lipolysis and Lipogenesis are key processes in adipose tissue.
• Adipocytes of white tissue store energy long term, appear spherical when isolated but polyhedral within closely packed in situ.
• White adipose tissue cells are very large, ranging between 50 and 150 µm in diameter
• Within the cells are single, huge droplet of lipid that fills almost entire cell.
• White adipocytes are unilocular because triglycerides are stored in only one large droplet
• These cells have a signet-ring appearance, since their lipid droplet displaces and flattens the nucleus.
• The membrane and thin cytoplasm rim shrink, collapse, or rupture upon removal of the triglycerides - distorting the tissue structure.
• Removed lipid is due to xylene or other solvents used in routine histological techniques, unilocular adipocytes appear empty in standard light microscope preparations.
• The cytoplasm of a white adipocyte surrounds nucleus and carries mitochondria, a small Golgi apparatus, some RER cisternae, and free polyribosomes.
• Most adipocytes, notably immature cells, have minute lipid droplets aside from any single large droplet seen with the light microscope.
• The lipid droplet-cytoplasm interface is reinforced by intermediate filaments of vimentin.
• Adipocytes are surrounded by a thin external lamina containing type IV collagen, instead of other connective tissue cells.
• White adipose tissue is subdivided into incomplete lobules by connective tissue partitions, with a vascular bed and nerve network.
• White tissue contains fibroblasts and macrophages, which account for about half the cell total.
• Reticular fibers support individual fat cells by forming an interwoven network that interconnects them.

Adipose Tissue Facts

• Almost all adipose tissue in adults is unilocular and found within many organs throughout the body.
• Tissue distribution varies in childhood and adult life and relates in part to sex hormones
• White adipose tissue, when dissected fresh, shows its color based on someone's diet, which ranges from white to yellow based on dissolved carotenoids levels in the lipid.
• When stimulated by nerves or hormones, stored lipids are mobilized inside adipocytes causing fatty acids and glycerol to go inside the bloodstream.
• The released fatty acids travels across the membranes of the adipocyte and the capillary endothelium by binding to protein albumin carriers in blood.
• Liberated glycerol remains free inside the body and goes to the liver.
• White adipose tissue functions serve as an important endocrine organ.
• Adipocytes are the sole source of leptin which is "satiety factor" and targets their cells inside the hypothalamus and also other organs.
• Leptin helps regulate appetite in normal conditions and plays a key role in the formation of new adipose tissue.
• Variations in gene expression have been noted for visceral deposits (in the abdomen) and subcutaneous deposits of white fat, even though white adipose tissue appears histologically similar.
• Differences may indicate medical risks with obesity.
• It is well-researched that increased visceral adipose tissue increases the risk of diabetes and cardiovascular diseases, but not heightened subcutaneous fat.
• Lipids are mobilized rather uniformly in all body parts, regarding body needs with few exceptions
• Adipose tissue in palms, soles, and retro-orbital fat pads resists even long periods of starvation.
• During starvation, cells of unilocular adipocytes lose nearly all their fat and become polyhedral or spindle shapes carrying some small lipid droplets.

Histogenesis of White Adipose Tissue

• Adipocytes, like the fiber-producing cells of connective tissue - differentiate from embryonic mesenchymal cells.
• The differentiation is first seen with the appearance of preadipocytes.
• These cells appear the same as fibroblasts but accumulate lipid droplets in the cytoplasm.
• Lipid accumulations are isolated at first, but fuse and form single huge droplet, which is typical of cells in unilocular adipose tissue.
• Excessive fatty tissue formation, which brings to obesity, happens when energy intake goes further beyond energy expenditure - a very increasingly common matter in sedentary lifestyles.
• Adult-onset obesity relates mainly to the raised size of existing adipocytes (hypertrophic obesity), even though adipocytes are able to differentiate from mesenchymal stem cells at any point.
• Childhood obesity is because of the increased rate of adipocyte size and numbers due to the differentiation of more preadipocytes from mesenchymal stem cells, which is hyperplastic obesity.
• Hypertrophy (SIZE) is true to Adults and children, while Hyperplasia (NUMBER) is related to several things
• Hyperplasia is related to the patient's Last-Trimester during Pregnancy ,the diet choices of the Mother, the first year of their life and whether 9 to 13yr or more
• Hyperplasia is also relevant to obese patients

Details about Brown Adipose Tissue

• Brown adipose tissue accounts for 2% to 5% of the newborn weight, it resides mainly in the back, neck, and shoulders
• Levels are reduced more during childhood and adolescence.
• In adults, this tissue is is scattered areas located especially around the kidneys, adrenal glands, aorta, and mediastinum.
• Brown or brown adipose tissue gains its color from abundant mitochondria which has cytochrome pigment spread among the lipid droplets of the fat cells, also from the blood capillaries within.
• Brown fat adipocytes contain small lipid inclusions, meaning it is multilocular.
• These small lipid droplets, abundant mitochondria, all mediate this tissue's function of heat production.
• Brown adipose tissue cells appear polygonal and they area smaller than white adipocytes
• Cytoplasm contains many lipid droplets of varying dimensions alongside nuclei are located centrally
• Brown fat adipocytes are often closely packed around large capillaries.
• Tissue is subdivided by connective tissue partitions into lobules that are delineated well compared to the ones found in white adipose tissue.
• Cells of this tissue directly receive sympathetic innervation.
• Main function of multilocular adipose cells is to create heat via nonshivering thermogenesis (generation of body heat in warm-blooded animals).
• The physiology of multilocular adipose tissue is best understood from studies of the tissue in hibernating species.
• When animals conclude hibernation, nerve impulses release norepinephrine - this also applies to newborn humans exposed to cooler environments than inside the uterus.
• As in white fat, this neurotransmitter activates adipocyte hormone-sensitive lipase and promotes triglyceride hydrolysis into fatty acids and glycerol.
• Unlike the white fat process, liberated fatty acids of multilocular adipocytes aren't released, they are metabolized for increased in oxygen consumption and heat production.
• This process warms the locally circulating blood and distributes heat throughout the body.
• Heat output increases in these cells since there are more thermogenin, or thermouncoupling protein (UCP-1) in a cell's mitochondria in its inside membrane.
• Thermogenin enables protons to backflow after being transported through without engaging with ATP-synthetase complexes.
• Proton flow dissipates energy as heat, which heats up the blood.

Histogenesis of Brown Adipose Tissue

• Brown adipose tissue forms with embryonic mesenchyme, its cells emerge more quickly than white fat during fetal development. These cells grow as growing preadipocytes.
• In humans, the quantity of brown fat peaks relative to overall body weight at the time a baby is born. Its job is to provide heat, which means thermogenesis, then gets slowly removed through cell death from apoptosis as the patient enters childhood.
• Adults with leaner builds contain more brown fat and more activity of the fat.
• Number of brown adipocytes grows as a result of cold adaptation for adults, and appears in areas which typically include groups of multilocular cells surrounded by white adipose tissue.
• This can indicate proliferation and differentiation of modern adipocytes, also alteration into already differentiated white adipoctyes.
• Besides stirring up thermogenic activity, autonomic nerves boost brown adipose tissue differentiation and forestall cell death (apoptosis) around older brown fatt cells