Features of adipocytes
Adipocytes are cells with special properties that develop from connective tissue stem cells. Their main task is to store neutral fats.
In loose fibrous connective tissue, they can occur singly or in small groups, often located near blood vessels. When adipocytes form massive clusters, they constitute true adipose tissue.
With standard hematoxylin and eosin staining, fat is washed out by alcohols and xylene during slide preparation. As a result, the cell appears empty, with only a thin rim of cytoplasm visible. To preserve lipids and view them under a microscope, special methods are used: treatment with osmic acid or staining with Sudan III.
White adipose tissue
This is a cluster of white adipocytes, which in structure and function are no different from the fat cells of loose connective tissue.
Location:
- Subcutaneous adipose tissue (hypodermis).
- Omentum (omentum).
- Fat capsules of internal organs (e.g., kidneys).
- Diaphysis of long bones (as yellow bone marrow).
Structure: A white adipocyte is shaped like a signet ring. Almost the entire internal space of the cell is occupied by a single giant fat droplet, which pushes the nucleus to the periphery and flattens it. Mitochondria are scarce in such cells.
Functions:
- Energy depot: lipid storage.
- Metabolic function: during stress or fasting, hormones (adrenaline, glucagon, somatotropin) trigger lipolysis. 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 fat droplet decreases in the process.
- Thermal insulation: passive protection against heat loss.
- Cushioning: mechanical protection of organs.
- Endocrine function: the tissue produces leptin (the satiety hormone) and estrogens (especially important for men and postmenopausal women).
Brown Adipose Tissue
This type of tissue is most developed in newborns (located in the neck, behind the sternum, between the shoulder blades, and in the renal hilum). In adults, only small patches remain.
Differences between brown and white adipocytes:
- Instead of one large fat droplet, the cell contains many small ones (multilocular structure).
- The nucleus is rounded and located in the center rather than at the periphery.
- The cytoplasm contains a huge number of mitochondria. Their cytochromes give the tissue its specific brown hue.
Function: The main task of brown tissue is active heat production to warm the blood and neighboring organs. During fat oxidation in mitochondria, energy is not stored as ATP but is immediately dissipated as heat. Unlike white tissue, which simply retains heat as an insulator, brown tissue acts like a real heater.
Briefly: answers to topical questions
Where is brown adipose tissue localized in newborns and adults?
In newborns, brown adipose tissue is located in the neck, scapular region, behind the sternum, in the renal hilum, and at the roots of the lungs. In adults, the amount of this tissue decreases significantly (making up 0.1–0.2% of body weight) with only remnants persisting, primarily localized in the upper back under the shoulder blades.
From which embryonic structures and progenitor cells do adipocytes develop?
Adipocytes are of mesenchymal origin, differentiating from connective tissue stem cells. Bone marrow stromal adipocytes are indicated to likely share common precursors with reticular cells.
Which protein uncouples tissue respiration and oxidative phosphorylation in brown adipose tissue?
The uncoupling of tissue respiration and oxidative phosphorylation is mediated by a mitochondrial inner membrane protein — thermogenin (UCP1).
This protein acts as a physiological uncoupler that creates a channel for protons to return from the intermembrane space into the matrix, bypassing ATP synthase. As a result, the energy of the electrochemical gradient is not stored as ATP but is dissipated as heat, providing non-shivering thermogenesis.
To remember the difference in thermoregulation: White tissue is a "down jacket" (retains heat, passive protection). Brown tissue is a "heater" (generates heat itself).
Class and exam questions
Why do adipocytes appear empty after hematoxylin and eosin staining?
Standard tissue processing uses alcohols and xylene, which dissolve fat. As a result, only the cell membrane, a thin rim of cytoplasm, and a displaced nucleus remain.
What is the visual difference between a white adipocyte and a brown adipocyte?
A white adipocyte contains one large fat droplet, with a flattened nucleus pushed to the edge (signet ring shape). A brown adipocyte contains many small fat droplets, with its nucleus located in the center.
What causes the brown color in brown adipose tissue?
The specific color is due to the presence of a large number of mitochondria containing iron-bearing proteins called cytochromes.
Which hormones are secreted by white adipose tissue?
It secretes the peptide hormone leptin, which regulates the sensation of satiety, as well as female sex hormones, estrogens.
What else to study on this topic
- Mechanism of lipolysis induced by adrenaline and glucagon
- Synthesis and role of the hormone leptin
- The process of adipocyte differentiation from mesenchyme
- Histochemical reactions for lipids (osmic acid, Sudan III)
- The role of adipose tissue in estrogen metabolism
Master the entire topic in the "Histology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Adipose tissue" and 3,485 more pages on the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 137 video lessons on the subject
Other topics in the section "Proper connective tissues. Connective tissues with special properties"
Free video lessons: histologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.