Soft tissue
General
Histology-white adipose


Topic Completed: 1 August 2012

Minor changes: 22 March 2021

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PubMed Search: White fat [title]

Vijay Shankar, M.D.
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Cite this page: Shankar V. Histology-white adipose. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/softtissueadiposewhitefat.html. Accessed May 7th, 2021.
Terminology
  • Also called White Adipose Tissue (WAT)
  • Cells are called adipocytes
  • See also Brown adipose tissue (topics are being merged)
Embryology
  • Stage I: prior to 14 weeks of gestation, loose spindle cells and ground substance are present
  • Stage II: aggregates of mesenchymal cells condense around proliferating primitive blood vessels
  • Stage III: capillaries proliferate into rich network, preadipocytes become stellate and organize into lobules
  • Stage IV: fine lipid vacuoles develop within cytoplasm
  • Stage V: adipocytes accumulate within rich capillary network and perilobular mesenchyme condenses at periphery of lobule to form fibrovascular septa at week 24 (Early Hum Dev 1983;8:1)
  • Brown fat has similar development, but contains large mitochondria with uncoupling protein 1 (OMIM 113730); white fat progenitor cells can also induce brown adipogenesis through mir 16a signaling pathway (PLoS Biol 2012;10:e1001314)
  • Brown fat deposits are well established by month five of gestation, particularly in posterior cervical, axillary, suprailiac and perirenal regions
  • Adipocyte development is closely associated with angiogenesis: (a) fat appears first in well vascularized regions; (b) adipocytes synthesize lipoprotein lipase (transports serum triglycerides into adipocytes), which is transferred to luminal surface of capillary endothelium; (c) vasculature functions as a progenitor niche and may provide signals for adipocyte development (Science 2008;322:583)
  • Fat development is controlled by CHOP gene, which mediates differentiation of fibroblasts into adipocytes and growth arrest of terminally differentiated adipocytes; translocation of CHOP in myxoid liposarcomas may remove normal inhibition on CHOP
  • At birth, amount of adipose tissue increases for next 10 years with overall growth; at puberty, adipocytes increase in size and number
  • Brown fat is widely distributed in children in interscapular region, around neck vessels and muscles, around mediastinal structures, near lung hila and around abdominal viscera
  • Some adipocytes may originate from neural crest (Development 2007;134:2283)
Sites
  • Subcutaneous, mediastinum, abdomen, retroperitoneum
Pathophysiology
  • Main function is energy storage as lipid, which is mobilized to other tissues depending on metabolic needs
  • Triglycerides circulate in blood as chylomicrons; the enzyme lipoprotein lipase, produced by adipocytes and present on luminal surface of endothelium, converts triglycerides to free fatty acids; free fatty acids are taken up by adipocytes and converted to triacylglycerol, which is stored within cytoplasmic lipid droplet of adipocytes
  • Adipocytes also secrete leptin and IL6, but their effects in humans are not well defined
  • Stromal cells also convert androstenedione to estrone, the main source of estrogen in men and postmenopausal women
Gross description
  • Homogeneous, yellow, greasy surface, faint fibrous septa
Microscopic (histologic) description
  • Uniform large spherical cells up to 120 microns; lipid droplet pushes nucleus to periphery and deforms it to a thin crescent
  • Cytoplasm usually has single, clearly delimited vacuole
  • Cells are clear because cytoplasmic fat dissolves during tissue processing
  • Nucleus may have central vacuole or Lochkern
  • Very thin membranes are present between cells
Microscopic (histologic) images

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White adipose tissue and vasculature

White adipose tissue

Fat emboli in lung-Oil Red O stain

Mature adipose tissue

Positive stains
  • Vimentin (nonspecific), S100 (primarily for mature adipocytes), calretinin (also stains lipoma and liposarcoma) (Hum Pathol 2006;37:312)
  • Oil red O and Sudan Black stain neutral fat (must use frozen tissue because tissue processing removes fat); Oil red O also stains most carcinomas
  • Nile blue sulfate stains neutral fat pink-red, and stains fatty acids / phospholipids blue
  • Basement membrane is highlighted by PAS and reticulin
Electron microscopy description
  • Pre-adipocytes - spindle cells with abundant endoplasmic reticulum and small spherical mitochondria
  • Mature adipocyte - large lipid droplet flattens nucleus against cytoplasmic membrane, variable pinocytotic vesicles; capillaries are close to adipocyte basement membrane
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