M01.10.01 Adipose tissue

Adipose (Fat) Normal adult fat cells. Medium power microscopic

Normal adult fat cells, also known as adipocytes, are specialized cells responsible for storing energy in the form of fat. They play a vital role in metabolism, energy storage, and insulation. Under the microscope at medium power, these cells have a distinct appearance that aids in the identification and understanding of their function in the human body.

Microscopic Appearance of Adipocytes

1. Shape and Size:
   • Shape: Adipocytes are typically round or oval.
   • Size: They are relatively large cells compared to other cell types, ranging from 30 to 100 micrometers in diameter.
2. Cytoplasmic Features:
   • Large Central Lipid Droplet: Each adipocyte has a single, large lipid droplet that occupies most of the cytoplasm.
   • Cytoplasm Compression: The lipid droplet pushes the cytoplasm and other organelles toward the cell periphery, giving the cell a “signet-ring” appearance.
3. Nucleus:
   • Location: Due to the lipid droplet’s size, the nucleus is displaced to the cell edge.
   • Shape: The nucleus appears small and flattened against the cell membrane.
4. Surrounding Matrix:
   • Connective Tissue Septa: Adipocytes are organized into clusters within the connective tissue matrix. Thin fibrous septa may separate clusters of cells, often containing blood vessels and fibroblasts.

Staining Characteristics

• H&E Staining (Hematoxylin and Eosin):
  • Lipids within adipocytes are not stained by H&E since they are usually dissolved during tissue processing.
  • Adipocytes appear as empty, clear spaces due to the absence of lipids.
  • The cell membrane and peripheral cytoplasm may be faint pink, while the nucleus stains dark purple.

Functions of Adipocytes

• Energy Storage: Adipocytes store triglycerides, which are a crucial energy source for the body.
• Insulation: Fat tissue helps insulate the body to maintain a stable internal temperature.
• Endocrine Role: Adipocytes secrete various hormones like leptin, which regulates appetite and energy balance.

Clinical Relevance

Understanding the microscopic structure of adipocytes is crucial, as abnormal fat cell morphology or distribution is associated with metabolic disorders like obesity, diabetes, and lipodystrophy.

Key Points to Remember

• Adipocytes are large cells with a single, central lipid droplet.
• The nucleus is displaced to the cell periphery, giving a signet-ring appearance.
• In H&E staining, they appear as clear spaces due to lipid dissolution during processing.

Recommended Reference

• Mescher, A. L. (2018). Junqueira’s Basic Histology: Text and Atlas (15th ed.). McGraw-Hill Education.

Adipose (Fat) Normal adult fat cells

Brown adipocytes, commonly known as brown fat cells, are specialized cells involved in thermogenesis (heat production) in adults, unlike white adipocytes, which primarily store energy as fat, brown adipocytes play a vital role in regulating body temperature, particularly through non-shivering thermogenesis.

Histological Characteristics

General Appearance

Under the microscope at medium power magnification:

• Brown adipocytes appear polygonal and contain multiple lipid droplets within the cytoplasm. This contrasts with white adipocytes, which contain a single large lipid droplet.
• These cells are smaller than white adipocytes.

Nucleus

• The nucleus of brown adipocytes is typically round and centrally located. This central positioning is distinctive, as white adipocytes often have nuclei pushed to the periphery due to a large, single lipid droplet.

Cytoplasm

• Brown adipocytes are rich in the cytoplasm, which has a foamy appearance because of the multiple small lipid droplets.

Mitochondria

• A hallmark feature of brown adipocytes is their high mitochondrial content. Mitochondria are densely packed and give the cells a brownish tint due to the iron content in cytochromes, which play a key role in the process of thermogenesis.

Function

The primary role of brown adipocytes is to generate heat. This thermogenic function is facilitated by a unique protein known as uncoupling protein 1 (UCP1), located in the inner mitochondrial membrane. UCP1 allows the protons generated by the electron transport chain to bypass ATP synthase, producing heat instead of ATP.

Clinical Relevance

• Brown adipose tissue (BAT) is found in small deposits in adults, primarily around the neck and shoulder blades. It is more prominent in newborns and hibernating animals.
• The presence and activity of brown adipocytes are associated with improved metabolic health, as BAT is involved in burning calories.
• In medical research, activation of brown adipocytes is being explored as a therapeutic approach to combat obesity and metabolic syndrome.

Points to Remember

• Location: BAT is primarily located in the interscapular region, neck, and around large blood vessels.
• Function: Non-shivering thermogenesis via UCP1.
• Microscopic Appearance: Multilocular lipid droplets, central nucleus, high mitochondrial content, and brownish cytoplasmic tint.