Electron Tomography Revels that Milk Lipids Originate from Endoplasmic Reticulum Domains with Novel Structural Features

  • Mark S. Ladinsky
  • Gonzalo A. Mardones
  • David J. Orlicky
  • Kathryn E. Howell
  • James L. McManamanEmail author


Lipid droplets (LD) are dynamically-regulated organelles that originate from the endoplasmic reticulum (ER), and function in the storage, trafficking and metabolism of neutral lipids. In mammary epithelial cells (MEC) of lactating animals, intact LD are secreted intact into milk to form milk lipids by a novel apocrine mechanism. The secretion of intact LD and the relatively large amounts of lipid secreted by lactating MEC increase demands on the cellular processes responsible for lipid synthesis and LD formation. As yet these processes are poorly defined due to limited understanding of LD-ER interactions. To overcome these limitations, we used rapid-freezing and freeze-substitution methods in conjunction with 3D electron tomography and high resolution immunolocalization to define interactions between LD with ER in MEC of pregnant and lactating rats. Using these approaches, we identified distinct ER domains that contribute to lipid droplet formation and stabilization and which possess unique features previously unrecognized or not fully appreciated. Our results show nascent lipid droplets within the ER lumen and the association of both forming and mature droplets with structurally unique regions of ER cisternae, characterized by the presence of perilipin-2, a protein implicated in lipid droplet formation, and enzymes involved in lipid synthesis. These data demonstrate that milk lipids originate from LD-ER domains with novel structural features and suggest a mechanism for initial droplet formation in the ER lumen and subsequent maturation of the droplets in association with ER cisternae.


Mammary gland Lipid droplet Endoplasmic reticulum domain Electron microscopy Tomography Differentiation 



Acyl-CoA cholesterol acyltransferase-1


Acyl-CoA diacylglycerol acyltransferase


Electron microscopy


Endoplasmic reticulum


Lipid droplets


Mammary epithelial cell


Milk fat globules


Nile red




Pregnancy day 5


Pregnancy day 10


Protein disulfide isomerase







We thank Dr. Robert Farese Jr., Harvard School of Public Health for the antibodies against Dgat1 and Dgat2 and Dr. T.Y. Chang of Dartmouth Medical School for the antibodies Acat1. This work was supported by NIH grants PO1GM61306 (KEH); 5PO1HD038129 (KEH); 2PO1HD038129 (JLM); and 2R01HD45965 and R01HD093729 (JLM).


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© Springer Science+Business Media, LLC, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Boulder Laboratory for 3D Electron Microscopy of CellsUniversity of ColoradoBoulderUSA
  2. 2.Division of BiologyCalifornia Institute of TechnologyPasadenaUSA
  3. 3.Department of Cell & Developmental BiologyUniversity of Colorado Anschutz Medical CampusAuroraUSA
  4. 4.Instituto de FisiologiaUniversidad Austral de ChileValdivaChile
  5. 5.Department of PathologyUniversity of Colorado Anschutz Medical CampusAuroraUSA
  6. 6.Division of Reproductive SciencesUniversity of Colorado Anschutz Medical CampusAuroraUSA

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