Adipose Tissue

Functions of adipose tissue

storage depots for neutral fats

regulator of metabolic homeostasis

insulator

structure supporter

releases large number of hormones

What type of tissue is adipose tissue?

Loose connective

What is the source of our energy bank of fats?

Food

How does adipose tissue act as an insulator?

Protects the body from thermogenic stimuli, such as the cold, and releases heat

How does adipose tissue act as a structure supporter?

It encapsulates organs like the kidneys to keep them in place; same for eye

T/F: Adipose tissue is considered endocrine

True. It releases hormones in the blood by itself, such as leptin

What are the 3 sources of stored lipids?

1. chylomicrons in intestine

2. triglycerides produced in liver

3. fatty acids synthesized locally

What is adipose tissue made up of?

1. cell types (adipocyte, preadipocyte, vascular endothelial cells, immune cells, fibroblasts)

2. ECM (reticular fibers made from type III collagen and other fibrillar proteins)

What are the 3 types of adipocytes?

1. white

2. brown

3. beige

From which mesoderm layer is white adipocyte development from?

Lateral plate mesoderm

If adipose progenitor cells decrease with age, how do people get fat?

Hypertrophy

Existing cells increase in size (no new cells)

White vs brown adipocyte histologically

White: unilocular, signet-ring appearance with nucleus to the side, spherical (polyhedral when closely packed)

Brown: multilocular, polygonal, smaller cells, closely packed, nucleus in center

White vs brown adipocyte differences

White adipocytes are unilocular and are directly involved in lipid storage and the production of hormones. They are also widespread in the body and form about 20% body weight in adults.

Brown adipocytes are multilocular, involved in thermogenesis and regulation of nutrient homeostasis, and are organized interscapular, axillary, cervical, paravertebral. They comprise up to 5% of newborn bodyweight but around 0.5% in adults

BROWN ADIPOCYTES HAVE MANY MITOCHONDRIA AND BLOOD CAPILLARIES WHILE WHITE ADIPOCYTES DO NOT.

Browning

White adipocytes gain gene expressions of brown adipocytes, forming beige adipocytes. The large lipid droplet is broken down to small lipid droplets and triglycerides to fatty acids that generate heat.

What organelles are in white adipocyte tissue?

Golgi complex, mitochondria, small amount of RER, free ribosomes, peripheral nucleus

SER and pinocytotic vesicles in thin rim of cytoplasm

Caveolae

invaginations of the cell membrane that aid in lipid trafficking, allowing fatty acid transport through the membrane

relies on caveolin protein

How many lipid droplets are in white adipocyte tissue?

1 (unilocular)

How is white adipose tissue distributed in the body?

Widely! Subcutaneous fat (under skin), intra-abdominal fat (visceral), around major joints like knees, and in palms and soles

What enzymes are responsible for lipid storage and mobilization?

Storage: lipoprotein lipase

Mobilization: hormone-sensitive lipase

Lipoprotein lipase

Breaks down chylomicron and VLDL from triglycerides into fatty acids so they can pass through caveolae, from capillary into adipocyte. Free fatty acids and glycerol are released.

Hormone-sensitive lipase

Norepinephrine (neurohormonal effect) activates adenyl cyclase and then cAMP which activates hormone-sensitive lipase to break down triglycerides at surface of stored droplets (lipolytic influence) into fatty acids and glycerol. Glycerol goes to the liver via blood capillaries and FFAs used for energy

Lipolytic effect

Breaking down triglycerides into free fatty acids and glycerol

Often involves ACTH growth hormone or thyroid hormones

Lipogenic influence

Enzymes for lipid synthesis stimulated by insulin release, which inhibits hormone-sensitive lipase

What is the role of blood capillaries (endothelial cells) for white adipose tissue?

In the capillary endothelial cells, chylomicrons and VLDLs from triglycerides are broken down, releasing free fatty acids and glycerol. Free fatty acids can diffuse from capillaries to adipocytes via caveolae.

What mesoderm layer is brown adipose tissue derived from?

Paraxial mesoderm

Brown adipose tissue function

Adaptive thermogenesis

Regulation of nutrient homeostasis

Cellular characteristics of brown adipose tissue

Abundant mitochondria, polygonal, smaller than white, small lipid droplets and central spherical nucleus, closely packed, large number of blood capillaries

What do mitochondria do in brown adipose tissue?

Where thermogenesis occurs. Free fatty acids bind to UCP-1 and trigger the flow of H+ from intermembranous space to matrix. This process DOES NOT require ATP (ATP-independent) and generates heat.

2 reasons for brown color in brown adipose tissue

1. Abundant mitochondria

2. Abundance of blood capillaries

Thermogenesis

Convert chemical energy to heat. In brown and beige adipocytes, liberated fatty acids are quickly metabolized creating an increase in O2 consumption and heat production, raising the temperature within the tissue

What protein is responsible for heat production within the mitochondria?

Transmembrane uncoupling protein-1 (UCP-1)

aka Thermogenin

How does UCP-1 protein generate heat?

Fatty acids bind to UCP-1 and induce a conformational change. Then, UCP-1 permits flow of protons from intermembranous space to matrix without passing through ATP synthetase complexes. This process is ATP-independent!

Leptin

Hormone released by white adipose tissue that serves as a gauge for energy reserves and directs the CNS to adjust food intake and energy expenditure. Leptin levels correlated positively to amount of body fat.

Obesity is maintained by __________________.

Leptin resistance. Inflammation from obesity inhibits leptin from binding to receptors in the hypothalamus, or mutations occur in receptors.

Obesity characterized by state of chronic mild inflammation

Leptin's role in homeostasis

Increased leptin suppresses appetite and stimulates lipid mobilization, while decreased leptin stimulates hunger and encourages lipid storage. Leptin creates the feeling of fullness.

What receptor does leptin interact with?

ObRs/LEPR in hypothalamus