Adipocyte Characteristics
Adipocytes are cells with specialized properties that develop from connective tissue stem cells. Their primary function is to store neutral fats.
In loose fibrous connective tissue, they may appear singly or in small groups, often located near blood vessels. When adipocytes form massive aggregates, they constitute true adipose tissue.
With standard hematoxylin and eosin (H&E) staining, fat is dissolved by alcohols and xylene during slide preparation. Consequently, the cell appears empty, with only a thin rim of cytoplasm visible. To preserve lipids and view them under a microscope, special techniques are required: treatment with osmic acid or staining with Sudan III.
White Adipose Tissue
This tissue consists of aggregates of white adipocytes, which structurally and functionally do not differ from the fat cells of loose connective tissue.
Location:
- Subcutaneous tissue (hypodermis).
- Omentum (omentum).
- Fat capsules of internal organs (e.g., kidneys).
- Diaphysis of long bones (as yellow bone marrow).
Structure: A white adipocyte has a signet ring appearance. Almost the entire intracellular space is occupied by a single giant lipid droplet. This droplet pushes the nucleus to the periphery, flattening it. Mitochondria are sparse in these cells.
Functions:
- Energy reservoir: Storage of lipids.
- Metabolic: During stress or starvation, lipolysis is triggered by hormones (adrenaline, glucagon, growth hormone). Fat is broken down into glycerol and fatty acids, which enter the bloodstream to fuel muscles, the heart, and the liver. The size of the lipid droplet decreases accordingly.
- Thermal insulation: Passive protection against heat loss.
- Cushioning: Mechanical protection of organs.
- Endocrine: The tissue produces leptin (the satiety hormone) and estrogens (particularly important for men and postmenopausal women).
Brown Adipose Tissue
This tissue type is most developed in newborns (found in the neck, behind the sternum, between the scapulae, and around the renal hila). In adults, only small remnants persist.
Differences between brown and white adipocytes:
- Instead of a single large lipid droplet, the cell contains many small droplets (multilocular structure).
- The nucleus is rounded and located in the center rather than the periphery.
- The cytoplasm contains a massive number of mitochondria. Their cytochromes impart the characteristic brown color to the tissue.
Function: The primary role of brown tissue is active heat production to warm the blood and adjacent organs. During lipid oxidation in mitochondria, energy is not stored as ATP but is directly dissipated as heat. Unlike white tissue, which acts as a passive insulating layer, brown tissue functions like an active heater.