Chapter 5 - Adipose Tissue
Histogenesis of adipose tissue : - In relation to adipose tissue, what are the progenitor cells (differentiated from mesenchymal cells) of adipocytes called? - What do brown adipocytes and white adipocytes differentiate from? - Give the definition of beige adipocytes.
- Progenitor cells of adipocytes are called preadipocytes. - Brown adipocytes are differentiated from preadipocytes located in paraxial embryonic mesoderm while the white are differentiated from preadipocytes located in lateral embryonic mesoderm. - Beige adipocytes : adipocytes having cytoplasmic features and gene expression patterns of both white and brown adipocytes, and remain in white adipose tissue.
What are the main functions of adipocytes?
- Storage depots for neutral fats (chiefly triglycerides) - Regulators of the body's overall energy metabolism - Providing thermal insulation for the body - Helping to keep some organs in place (by filling spaces between other tissues) - Shape to body surface (subcutaneous layers of adipose tissues) - Cushioning regions subject to repeated mechanical stress such as palms, heels and toe pads
What 2 properties make triglyceride lipids be the preferred form of nutrient storage?
- Water insolubility - Caloric density of triglycerides (9.3 kcal/g)
What are the 3 sources are triglycerides derived from?
1. Chylomicrons : dietary fats brought to the cells via the circulation 2. Very low-density lipoproteins (VLDLs) : lipids synthesized in the liver and transported in blood 3. free fatty acids and glycerol : synthesized by the adipocytes
What week of gestation does white adipose tissue begin to accumulate?
14th week
How many percent of the body weight in men does adipose tissue normally represent?
15 - 20%
How many percent of newborn body weight does brown adipose tissue constitute?
2% - 5%
What makes the color of brown fat tissues?
Abundance in mitochondria (containing cytochrome pigment) and large number of blood capillaries in the tissue
Explain the involvement of adult- and children-onset obesity.
Adult-onset obesity involves increasing the size of existing adipocytes (hypertrophy) Children-onset obesity involves increasing in both adipocytes size and numbers due to the differentiation of preadipocytes from mesenchymal cells (hypertrophy + hyperplasia)
MCQ : What are the relatively large particles formed in the intestinal epithelial cells and rich in ingested lipids? A. Fatty acids B. Chylomicrons C. Glycerols D. Very low-density lipoproteins E. Adipocytes
B. Chylomicrons
Why is heat production in brown adipocytes greater than that of other cells? How can brown adipocytes produce heat?
Because their inner mitochondrial membranes have uncoupling protein-1 (UCP1) or thermogenin. In the presence of fatty acids, UCP1 permits the flow of protons from intermembranous space to the matrix without passing through ATP synthetase complexes. Instead of producing ATP, the energy associated with this proton dissipates as heat.
MCQ : Fully differentiated white adipocytes are large cells, typically having diameters of approximately what size? A. 5 µm B. 10 µm C. 100 µm D. 500 µm E. 1000 µm
C. 100 µm
MCQ : White adipocytes are derived developmentally from what precursor cells? A. Monocytes B. Fibroblasts C. Mesenchymal cells D. Brown adipocytes E. Mast cells
C. Mesenchymal cells
MCQ : What substance, released from the adrenal gland and some autonomic neurons, increases lipolytic activity in white adipocytes? A. Leptin B. Insulin C. Norepinephrine D. Glycogen E. Triglyceride
C. Norepinephrine
MCQ : Ten days after birth a full-term newborn boy develops firm, erythematous nodules, and plaques over his trunk, arms, buttocks, thighs, and cheeks. His mother's pregnancy was complicated by placenta previa and his airway was cleared of aspirated meconium immediately after birth. A biopsy of subcutaneous tissue shows necrosis within the brown adipose tissue. What metabolic activity is liable to be affected in this patient? A. Export of fatty acids from fat B. Thermal insulation C. Oxidation of fatty acids for thermogenesis D. Activation of the adenylate cyclase system E. Initiation of shivering
C. Oxidation of fatty acids for thermogenesis
What is the diameter of chylomicron? How is it formed? What is it made up of?
Chylomicron has up to 1200 nm in diameters. It is formed from ingested lipids in epithelial cells lining the small intestine, and then transported by blood and lymph. Chylomicron is made up of a core containing mainly triglyceride, monolayer of phospholipid, cholesterol and several apolipoproteins
Explain briefly how metabolism of chylomicrons and VLDL occurs.
Chylomicrons (from ingested lipid in epithelial lining of small intestine) and VLDLs (synthesized from lipid in liver cells) are transported through blood and lymph to adipose tissues. Because they are big in size, thus have to be hydrolyzed by lipoprotein lipase in luminal surface of capillaries before entering the tissues. Hydrolysis of chylomicrons and VLDLs yields free fatty acids and glycerol, which then enter the tissues through diffusion or active transport from capillary endothelia. Inside the adipocytes, fatty acids and glycerol phosphate (reesterified), forming triglycerides that are stored in the lipid droplet until needed. Norepinephrine from nerve endings stimulates the cyclic AMP (cAMP) system, which activates hormone-sensitive lipase to hydrolyse stored triglycerides to free fatty acids and glycerol. These substances then diffuse into the capillary, where the fatty acids bind albumin for transport throughout the body for use as an energy source. And more water-soluble glycerol remains free in blood and is taken up by the liver.
MCQ : Important target cells of leptin are found in which organ? A. Small intestine B. White adipose tissue C. Large intestine D. Hypothalamus E. Brown adipose tissue
D. Hypothalamus
MCQ : The hormone-sensitive lipase in the cells of adipose tissue acts primarily on what substrate? A. Glucose B. Free fatty acids C. Glycerol D. Triglycerides E. Very low-density lipoproteins
D. Triglycerides
MCQ : A 44-year-old African-American woman visits her family physician for a physical examination at the urging of her husband. She has no current complaints and is taking no medications. She is allergic to erythromycin. She works as a software developer and lives with her 52-year-old husband and 12-year-old daughter. she is a nonsmoker and drinks an occasional glass of wine when she and her husband go out to dinner. She is involved in no regular exercise. Her mother is 66 and suffers from type II diabetes, hyperlipidemia, and hypertension and had a myocardial infarction last year. The patient's father died of a stroke last year at the age of 72. On examination, the patient's blood pressure is 155/100 mm Hg, pulse 84, weight 215 lb (increased from 180 lb 3 years ago), and height 5 ft. 7 in. In this patient, during the period of weight gain which one of the following responses would be most likely in her white fat? A. Increased synthesis of growth hormone B. Decreased synthesis of leptin C. Increased release of chylomicrons to the blood D. Decreased conversion of beige adipocytes to unilocular white adipocytes E. Increased incorporation of fatty acids into triglyceride
E. Increased incorporation of fatty acids into triglyceride
MCQ : What is the most important form of lipid storage in both white and brown adipocytes? A. Free fatty acids B. Cholesterol C. Chylomicrons D. Glycerol E. Triglycerides
E. Triglycerides
Lipomas and liposarcomas are tumors made of what type of adipocytes? Which tumor is benign (or malignant)? What type of adipocytes are hibernomas (fetal lipomas) made of?
Lipomas and liposarcomas are made of unilocular adipocytes. Lipomas are benign while liposarcomas are malignant tumors. Hibernomas are made of brown fat
What enzyme hydrolyzes chylomicron and VLDL? Where does the hydrolysis occur?
Lipoprotein lipase. The reaction occurs in luminal surface of capillaries.
What function is white adipose tissue specialized for?
Long-term energy storage
What is the brown adipocyte also called? Why? What function is it specialized for?
Multilocular adipocyte. Because it contains many lipid inclusions. It is specialized for producing heat to warm the blood.
What hormones stimulate lipolysis?
Norepinephrine (released in the adrenal gland and by postganglionic sympathetic nerves in adipose tissue activates a hormone-sensitive lipase) and GH (Growth hormone released from pituitary gland)
What are the target cells of the 16-kDa polypeptide hormone leptin? Where is leptin released from? What are functions of leptin?
Target cells : hypothalamus, other regions and peripheral organs Leptin is released from white adipocytes. Functions : - Regulating appetite under normal condition as a "satiety factor" - Participating in forming new adipose tissue
What is the diameter of VLDL? Where (or what) is it synthesized from?
VLDL has the diameter of 30 - 80 nm. It is synthesized from lipids in liver cells.
What is the sequence of white adipocytes and brown adipocytes after being directly activated by norepinephrine (neurotransmitter)?
White adipocyte : activating hormone-sensitive lipase to hydrolyze the stored triglycerides into fatty acids and glycerol Brown adipocyte : activating quick metabolism of liberated fatty acids with an increase in O2 and heat production to warm the locally circulating blood, promoting brown adipocyte differentiation and preventing apoptosis in mature brown fat cells.
What is diameter of a white adipose tissue? What is white adipocyte also called, why?
White adipose tissue has the diameter of 50 - 150 µm White adipocyte is also called unilocular adipocyte because it contains a huge single droplet of triglycerides.