Adipose Tissue Notes

Adipose tissue is composed of adipocytes.
Two main types:
- White adipose tissue (unilocular)
- Brown adipose tissue (multilocular)
White adipocytes are generally unilocular, serve as an energy storage site, synthesize and store triglycerides, and function as an endocrine organ.
Brown adipocytes are multilocular, smaller in appearance, store many droplets of fat, and have a primary role in heat production; they contain numerous mitochondria.
Endocrine function: Adipose tissue secretes leptin, a hormone that binds to receptors in the brain (hypothalamus) to signal satiety.

Fat comes from circulating lipoproteins:
- Chylomicrons (circulating fats from the intestines)
- Very low-density lipoproteins (VLDLs) from the liver
Fat synthesis within adipocytes occurs from glucose; triglycerides are stored in droplets.
Mobilization of fat is regulated by the autonomic nervous system and hormones; lipases break down triglycerides during lipolysis.
Core players in lipolysis:
- Hormone-sensitive lipase (HSL) activated by cAMP
- During mobilization, triglycerides are hydrolyzed into free fatty acids (FFAs) and glycerol.
- FFAs are released into capillaries and bound to albumin for transport to tissues.
- Glycerol can be released and transported to the liver for gluconeogenesis or glycerol-3-phosphate serves as a backbone for triglyceride synthesis.
Lipoprotein lipase (LPL) acts on chylomicrons and VLDLs at the capillary endothelium to hydrolyze circulating triglycerides, releasing FFAs for uptake by tissues.
Chylomicrons contain apolipoproteins (e.g., B-48); VLDLs contain other apolipoproteins; core lipid components include triglycerides and cholesteryl esters; phospholipids are also present in the particle surface.
That lipid processing forms the basis for transport of dietary and endogenous fats throughout the body.

Short-Term (daily) regulation:
- Ghrelin stimulates appetite (produced by the stomach), promoting feeding and energy intake.
- Peptide YY (PYY) acts as an appetite suppressant, contributing to meal termination.
Long-Term regulation of body weight involves two key hormones:
- Leptin (secreted by adipose tissue itself), informs the brain about energy stores and suppresses appetite when stores are sufficient.
- Insulin (from the pancreas) also participates in long-term energy storage and adipose tissue metabolism.

Adipose tissue is composed of various cell types involved in development and storage:
- Fibroblast
- Lipoblast
- Mesenchymal cell
Lipoblasts differentiate into adipocytes; adipocytes can be:
- Multilocular adipocytes (brown fat)
- Unilocular adipocytes (white fat)

White adipose tissue is unilocular:
- The nucleus and cytoplasm are displaced toward the cell membrane due to a large central lipid droplet.
- Primary role: energy storage site; synthesis and storage of triglycerides.
Distribution and development:
- At birth, adipose tissue is more uniformly distributed.
- Sex hormones lead to redistribution of adipose tissue later in life.
Subcutaneous adipose tissue is located in the subcutaneous compartment; overall distribution includes subcutaneous and visceral compartments.
The anatomical arrangement in the subcutaneous region includes loose skin and skin ligaments; deep fascia is nearby; subcutaneous tissue is relatively abundant and tightly confined in some regions.

Unilocular adipocytes feature a single large lipid droplet with peripheral nucleus.
They function as energy storage and are involved in lipid synthesis and storage of triglycerides.
In newborns, distribution is more uniform; hormonal changes (e.g., sex hormones) drive redistribution with age.

Brown adipose tissue is multilocular:
- Contains multiple small lipid droplets per cell (multilocular appearance).
- Cells have central nuclei.
- Rich in mitochondria.
Primary function is heat production through metabolic energy expenditure.
Brown fat is represented by brown adipocytes with many mitochondria to support thermogenesis.

White adipose tissue is distributed in subcutaneous and visceral compartments, each with distinct physiological implications.
Subcutaneous tissue sits under the skin; there are skin ligaments and deep fascia in the surrounding architecture.

Adipose tissue consists of adipocytes and has two main types:
- White adipose tissue (unilocular): energy storage and endocrine function (leptin).
- Brown adipose tissue (multilocular): heat production with many mitochondria.
Fat sources and mobilization involve dietary lipids via chylomicrons, hepatic lipids via VLDLs, lipolysis by hormone-sensitive lipase, and lipoprotein lipase activity on chylomicrons and VLDLs.
Transport of free fatty acids is primarily via albumin in the bloodstream.
Glycerol-3-phosphate serves as the backbone for triglyceride synthesis; glycerol released during lipolysis can be used or transported to the liver.
Regulation of appetite and energy balance involves short-term signals (ghrelin, peptide YY) and long-term signals (leptin, insulin).
White adipose tissue remodeling is influenced by developmental stage and sex hormones, with redistribution from birth onward.
Brown adipose tissue contributes to thermogenesis through multilocular adipocytes with numerous mitochondria, supporting heat generation.
The anatomical distribution of adipose tissue includes subcutaneous and visceral compartments, with structural features in the subcutaneous region such as skin ligaments and deep fascia.