Chapter 6. Adipose Tissue
Unlike other connective tissue cells, adipocytes are surrounded by what?
-by a thin external lamina containing type IV collagen
What other cells are found in white adipose tissue and in what abundance?
-Fibroblasts, macrophages, and other cells make up about half the total number of cells.
What is the lipid droplet-cytoplasm interface reinforced by?
-reinforced by intermediate filaments of vimentin
How does white adipose tissue subdivided into incomplete lobules?
-by partitions of connective tissue containing a vascular bed and nerve network
How do the cells of brown adipose tissue compare those of white adipose tissue?
-cells are polygonal and generally smaller than cells of white adipose tissue -cytoplasm contains a great number of lipid droplets of various sizes -nuclei are often centrally located
What happens to most unilocular adipocytes during starvation?
-most unilocular adipocytes lose nearly all their fat and become polyhedral or spindle-shaped cells with only very small lipid droplets.
Of adipose tissue, how much does brown adipose tissue make up and where is it found?
- constitutes 2% to 5% of the newborn body weight, 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
How are adipocytes arranged in proximity to one another and to other structures in the tissue?
-Adipocytes of brown fat are often closely packed around large capillaries. The tissue is subdivided by partitions of connective tissue into lobules that are better delineated than the lobules of white adipose tissue
What is adipose tissue like in adults vs developing children? What is the reason for this difference?
-Almost all adipose tissue in adults is unilocular and it is found in and around many organs throughout the body -distribution of this tissue changes significantly through childhood and adult life and is partly regulated by sex hormones, which control adipose deposition in the breasts and thighs
Why is it that unilocular adipocytes are often empty in standard light microscope preparations? What other appearances in structure are seen in these cells due to this reason?
-Because lipid is removed from cells by xylene or other solvents used in routine histological techniques -cells are sometimes said to have a signet-ring appearance, with the lipid droplet displacing and flattening the nucleus against the cell membrane -This membrane and the thin rim of cytoplasm that remains after removal of the stored triglycerides may shrink, collapse, or rupture, distorting the tissue structure.
Describe the histogenesis of brown adipose tissue.
-Brown adipose tissue also develops from embryonic mesenchyme, with cells developing as growing preadipocytes, and emerges earlier than white fat during fetal development -In humans the amount of brown fat is maximal relative to body weight at birth, when thermogenesis is most needed and partially disappears by apoptosis and involution during childhood
How do free fatty acids enter the adipocytes and what subsequently happens to them? What organelles actively participate in these events?
-Free fatty acids enter the adipocyte by both active transport and diffusion -Within the adipocyte, the fatty acids combine with glycerol phosphate, supplied by glucose metabolism, to again form triglycerides, which are then deposited in the growing lipid droplet -Mitochondria and SER participate actively in the process of lipid uptake and storage
What are some observations seen in regards to brown adipose tissue histogenesis and function in adults?
-In adults the amount and activity of brown fat are higher in lean individuals. -The number of brown adipocytes increases during cold adaptation in adults, usually appearing as clusters of multilocular cells in white adipose tissue -This may represent proliferation and differentiation of new adipocytes from preexisting progenitor cells or a change in the differentiated state of the white adipocytes.
What is the ECM of white adipose tissue composed of and what is the function of these components?
-Reticular fibers form a fine interwoven network that supports individual fat cells and binds them together. -The microvasculature between adipocytes may not always be apparent in tissue sections.
What do adipocytes differentiate from and what process do they go through to mature?
-adipocytes differentiate from embryonic mesenchymal cells -differentiation is first seen with the appearance of preadipocytes -preadipocytes have the appearance of fibroblasts but accumulate lipid droplets in their cytoplasm - Lipid accumulations are isolated from one another at first but soon fuse to form the single large droplet characteristic of cells in unilocular adipose tissue
Development of new adipocytes continues postnatally, mainly where? And why in this location?
-around small blood vessels, where undifferentiated mesenchymal cells are fairly abundant.
How is heat production increased in adipocytes of brown adipose tissue?
-because the mitochondria have in their inner membrane much greater levels of a transmembrane protein called thermogenin or uncoupling protein (UCP-1) -Thermogenin permits the backflow of protons previously transported to the intermembranous space without passing through the ATP-synthetase complexes -Consequently, the energy generated by proton flow dissipates as heat to warm the blood.
1. What happens to chylomicrons and VLDLs in adipose tissue?
-both chylomicrons and VLDL are hydrolyzed at the luminal surfaces of blood capillaries by lipoprotein lipase, an enzyme synthesized by the adipocyte and transferred to the capillary cell membrane
How are the adipocytes of brown adipose tissue innervated?
-cells of this tissue receive direct sympathetic innervation.
Under TEM, what is observed in most adipocytes of white adipose tissue? What adipocytes is this especially seen?
-contain minute lipid droplets in addition to the single large droplet seen with the light microscope -especially immature cells (not seen in the picture here though)
What organelles are found in adipocytes of white adipose issue and what kind of abundance of these organelles is typically observed?
-contains 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
Triglycerides stored by cells of white adipose tissue can be derived from what sources?
-dietary fats brought to adipocytes as circulating chylomicrons -from triglycerides synthesized in the liver and transported as very-low-density lipoproteins (VLDLs) - by the local synthesis of free fatty acids and glycerol from glucose
How does white adipose tissue differ based on location?
-differences in gene expression have been noted between visceral deposits (in the abdomen) and subcutaneous deposits of white fat -well established that increased visceral adipose tissue raises the risk of diabetes and cardiovascular disease whereas increased subcutaneous fat does not -release of visceral fat products directly to the portal circulation and liver may also influence the medical importance of this form of obesity
Why is brown adipose tissue have a brown color to it? What is the function of the parts that make it brown?
-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 -Adipocytes of brown fat contain many small lipid inclusions and are therefore called multilocular -The many small lipid droplets, abundant mitochondria, and rich vasculature all help mediate this tissue's principal function of heat production
What is the main function of brown adipose tissue and how is this function achieved?
-main function of the multilocular adipose cells is to produce heat by nonshivering thermogenesis -nerve impulses liberate norepinephrine into brown adipose tissue -neurotransmitter activates the hormone-sensitive lipase of adipocytes, promoting hydrolysis of triglycerides to fatty acids and glycerol. -unlike the process in white fat, liberated fatty acids of multilocular adipocytes are not released but are quickly metabolized, with a consequent increase in oxygen consumption and heat production -This raises the temperature within the tissue and warms the locally circulating blood, which then distributes the heat throughout the body
Relating to obesity, what is typically the problem as it relates to leptin?
-not a problem with production of sufficient amounts of leptin -but target cells are not responsive due apparently to insufficient or defective receptors or postreceptor signal transduction
Besides the innervation of thermogenesis in brown adipose tissue, what other function do autonomic nerves play?
-promote brown adipocyte differentiation -prevent apoptosis in mature brown fat cells
What endocrine function does white adipose tissue have?
-sole source of the 16-kDa polypeptide hormone leptin (Gr. leptos, thin), a "satiety factor" with target cells in the hypothalamus and other organs Leptin -helps regulate the appetite under normal conditions -participates in regulating the formation of new adipose tissue
White adipocytes are called __________ because the triglycerides are stored in this single large droplet.
-unilocular
How does a person's diet effect the color of freshly dissected adipose tissue?
-varying from white to yellow with the amount of carotenoids dissolved in the lipid
What else is secreted in white adipose tissue besides leptin? What is the function of these secretions? What is the medical significance of this?
-white adipose tissue secretes numerous other cytokines and other factors with paracrine and autocrine activity, including many proinflammatory cytokines -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.
Two properties of triglycerides explain their selection as the preferred form of nutrient storage, what are these two properties?
1. Since water insoluble, they have no adverse osmotic effects on cells 2. caloric density of triglycerides (9.3 kcal/g) is twice that of proteins or carbohydrates
Other than energy storage, what other roles does adipose tissue play?
1. Thermal insulation - poor conductor of heat 2. Cushion and keep some organs in place
What are the two types of adipose tissue? Generally, how are they distinguished? What is a characteristic that both share (other than lipid storage)?
1. White adipose tissue -more common type -composed of cells that, when completely developed, contain one very large droplet of whitish-yellow fat in their cytoplasm 2. Brown adipose tissue -contains cells with multiple lipid droplets interspersed among abundant mitochondria, which give these cells a darker appearance. -Both: have rich blood supply
1. What is the general structure of chylomicrons? 2. Where are they typically formed? 3. What is another structure like that of a chylomicron but differs to a degree? Describe them. How are they catagorized?
1. consist of a central core, composed mainly of triglycerides and a small quantity of cholesterol esters, surrounded by a stabilizing monolayer of apolipoproteins, cholesterol, and phospholipids 2. formed in intestinal epithelial cells and transported in blood plasma and lymph 3. Lipoproteins are also complexes of lipids and proteins, but are generally smaller than chylomicrons (providing a greater surface-to-volume ratio) -have much higher levels of lipoproteins, cholesterol, and phospholipids in the surface layer -categorized by density due to varying levels of surface apolipoprotein, from VLDLs to HDLs.
What are two different types of obesity observed based off whether an individual is an adult or child?
1. hypertrophic obesity -adult-onset obesity -mainly involves increased size of existing adipocytes, though adipocytes can differentiate from mesenchymal stem cells throughout life 2. hyperplastic obesity - seen in childhood obesity -can involve increases in both adipocyte size and numbers due to differentiation of more preadipocytes from mesenchymal stem cells
What hormones play a role in regulation of lipid metabolism in adipocytes?
Insulin -fatty acid synthesis from glucose is accelerated, which also stimulates the uptake of glucose into adipocytes and increases the synthesis of lipoprotein lipase -inhibits the hormone-sensitive lipase, reducing fatty acid release -also stimulates enzymes for lipid synthesis Norepinephrine -released by postganglionic sympathetic nerves in adipose tissue -activates hormone-sensitive lipase that breaks down triglycerides at the surface of the stored lipid droplets
What happens to the components of triglycerides after they are broken down by hormone-sensitive lipase?
free fatty acids -diffuse across the membranes of the adipocyte and the capillary endothelium, and bind the carrier protein albumin in blood for transport throughout the body glycerol -water-soluble glycerol remains free and is taken up by the liver Glucagon and growth hormone -promote triglyceride breakdown and release of fatty acids.
