Phagosome

Table of Contents

Definition

noun
plural: phagosomes
phag·o·some, ˈfæg əˌsoʊm
(cell biology) A vesicle that forms by pinching off from the cell membrane of a cell to contain the ingested particulate

Details

Overview

Phagocytosis is the process of engulfing and ingestion of particles by the cell or a phagocyte. In mammals, a phagocyte pertains to the immune cells specializing in the engulfing and destroying foreign particles, as well as in removing waste particles and cell debris. Examples of phagocytes are macrophages, neutrophils, and dendritic cells. These cells phagocytose the target particulate that needs to be degraded by forming a phagosome (a food vacuole) that can fuse with the lysosome to form a phagolysosome.

Characteristics

A phagosome is a vesicle that forms within a phagocyte. It contains foreign particle that has been captured by phagocytosis. It forms when a phagocyte engulfs a particulate that needs to be destroyed, surrounds it with its cell membrane, and then pinches off as a vesicle. The resulting vesicle is termed phagosome. It fuses with the lysosome, which is another cytoplasmic structure that is characterized by containing a wide range of digestive enzymes and reactive oxygen species. The digestive enzymes and ROS play a role in the degradation of particles and killing of viruses and bacteria. The phagosome can fuse with the lysosome through its membrane proteins. There are instances as well when a phagosome forms inside a non-phagocytic cell that engulfs smaller particles.

Phagosome vs. Endosome

A phagosome is different from an endosome, which is another vesicle. Both of them can fuse with the lysosome to have their contents degraded. The endosome, though, originates from the Golgi apparatus, particularly from the trans-Golgi network. Nevertheless, the late endosome may also arise from the phagosomes of the phagocytic pathway apart from the maturing early endosome of the endocytic pathway.¹ Since phagosomes may form from a phagocyte ingesting a whole bacterium or a senescent cell they are therefore relatively larger than endosomes.

Biological functions

Phagosome is a vesicle to contain pathogens and particulates inside. The contents would therefore be restricted inside the phagosome. The phagosome moves along the microtubule of the cell’s cytoskeleton to fuse with certain endosomes and/or with the lysosome to have its contents processed for disposal. In this regard, the phagosome is involved in immune protection against invading pathogens as well as in the removal of senescent and apoptotic cells and cellular debris to essentially maintain homeostasis.
In amoeba, a phagosome is a food vacuole that forms when the organism ingests a food material. Here, the phagosome acts as a site for digesting food particles.

Common biological reactions

Phagocytosis is an important defense mechanism against infection by microorganisms (e.g. bacteria) and the process of removing cell debris (e.g. dead tissue cells) and other foreign bodies. The phagocytic pathway begins in the recognition or identification of a foreign body or a worn-out cellular particle by the phagocyte. Next, the phagocyte engulfs the target particulate. It surrounds the particulate with its cell membrane. Then, the nascent vesicle pinches off from the cell membrane of the phagocyte. The nascent phagosome then undergoes maturation to become late phagosome that can fuse with the lysosome. Maturation of the nascent phagosome into a late phagosome entails a drop of pH (from 6.5 to 4) via its vacuolar proton pumps. A mature phagosome can also be characterized by having protein markers (e.g. RAB7) and hydrolytic enzymes. A phagosome that fuses with a lysosome becomes a phagolysosome where the engulfed material is eventually digested or degraded and then either released extracellularly via exocytosis, or released intracellularly to undergo further processing. In a neutrophil, another mechanism to destroy pathogen inside the phagosome is by oxidative burst. In this method, the neutrophil’s granules (containing NADPH oxidase and myeloperoxidase) fuse with the phagosome and produce toxic oxygen and chlorine derivatives. As for the dendritic cells, pathogen destruction is relatively less intense than the macrophages and the neutrophils. The dendritic cells form phagosomes that are only slightly acidic and less hydrolytic. As such, the pathogen is not fully degraded. Rather, protein fragments from the partially-degraded pathogen are transported to the Major Histocompatibility Complex so that they could be used for antigen presentation on the cell surface of the dendritic cell so that another group of immune cells, the lymphocytes, would be able to identify them.

Stoorvogel, W., Strous, G. J., Geuze, H. J., Oorschot, V., & Schwartz, A. L. (May 1991). “Late endosomes derive from early endosomes by maturation”. Cell. 65 (3): 417–27.