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Dev Biol. 2015 Jul 1;403(1):30-42. doi: 10.1016/j.ydbio.2015.03.017. Epub 2015 Apr 24.

Macrophages engulf endothelial cell membrane particles preceding pupillary membrane capillary regression.

Author information

1
Department of Molecular Physiology and Biophysics, Baylor College of Medicine, Houston, TX, United States; Integrative Molecular and Biomedical Sciences Graduate Program, Baylor College of Medicine, Houston, TX, United States.
2
Department of Molecular Physiology and Biophysics, Baylor College of Medicine, Houston, TX, United States.
3
College of Optometry, University of Houston, Houston, TX, United States.
4
Department of Molecular Physiology and Biophysics, Baylor College of Medicine, Houston, TX, United States; Program in Developmental Biology, Baylor College of Medicine, Houston, TX, United States; Integrative Molecular and Biomedical Sciences Graduate Program, Baylor College of Medicine, Houston, TX, United States; Cardiovascular Research Institute, Baylor College of Medicine, Houston, TX, United States. Electronic address: mdickins@bcm.edu.

Abstract

Programmed capillary regression and remodeling are essential developmental processes. However, the cellular and molecular mechanisms that regulate vessel regression are only the beginning to be understood. Here, using in vivo, dynamic, confocal imaging of mouse transgenic reporters as well as static confocal and electron microscopy, we studied the embryonic development and postnatal regression of the transient mouse pupillary membrane (PM) vasculature. This approach allowed us to directly observe the precise temporal sequence of cellular events preceding and during the elimination of the PM from the mouse eye. Imaging of Tcf/Lef-H2B::GFP Wnt-reporter mice uncovered that, unlike the hyaloid vasculature of the posterior eye, a PM endothelial cell (EC) Wnt/β-catenin response is unlikely to be part of the regression mechanism. Live imaging of EC and macrophage dynamics revealed highly active Csf1r-GFP+ macrophages making direct contact with the Flk1-myr::mCherry+ vessel surface and with membrane protrusions or filopodia extending from the ECs. Flk1-myr::mCherry+ EC membrane particles were observed on and around ECs as well as within macrophages. Electron microscopy studies confirmed that they were in phagosomes within macrophages, indicating that the macrophages engulfed the membrane particles. Interestingly, EC plasma membrane uptake by PM macrophages did not correlate with apoptosis and was found shortly after vessel formation at mid-gestation stages in the embryo; long before vessel regression begins during postnatal development. Additionally, genetic ablation of macrophages showed that EC membrane particles were still shed in the absence of macrophages suggesting that macrophages do not induce the formation or release of EC microparticles. These studies have uncovered a novel event during programmed capillary regression in which resident macrophages scavenge endothelial cell microparticles released from the PM vessels. This finding suggests that there may be an initial disruption in vessel homeostasis embryonically as the PM forms that may underlie its ultimate regression postnatally.

KEYWORDS:

Endothelial cells; Eye; Macrophages; Pupillary membrane

PMID:
25912686
PMCID:
PMC4469537
DOI:
10.1016/j.ydbio.2015.03.017
[Indexed for MEDLINE]
Free PMC Article

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