Microglia and Ependymal Cells

Microglia are the resident immune cells of the central nervous system, small ramified cells that survey the brain parenchyma and respond to injury and infection. Ependymal cells are the epithelial-like cells that line the ventricles of the brain and the central canal of the spinal cord, where they interface with cerebrospinal fluid. Both are glial elements but differ markedly in origin and function from the macroglia.

Definition

Microglia are the resident mononuclear phagocyte (immune) cells of the central nervous system, while ependymal cells are the ciliated epithelial-like glial cells that line the cerebral ventricles and the central canal of the spinal cord.

Scope

This topic covers the histology and core functions of two distinct glial populations: microglia (their ramified surveillant morphology, immune origin, and activation) and ependymal cells (their ciliated columnar lining of the ventricular system and relationship to cerebrospinal fluid). It is reference-educational and not clinical guidance.

Core questions

• What is the embryological origin of microglia, and how does it differ from that of macroglia?
• What does the ramified morphology of resting microglia reflect functionally?
• How are ependymal cells arranged and what is their relationship to cerebrospinal fluid?
• How do microglia change morphology when activated?

Key concepts

• Microglia
• Ramified (surveillant) and activated microglia
• Yolk-sac (myeloid) origin of microglia
• Resident immune surveillance
• Ependymal cells
• Cilia and microvilli of ependyma
• Cerebrospinal fluid interface

Mechanisms

Microglia arise from yolk-sac myeloid progenitors and populate the brain early in development, distinguishing them from the neuroectoderm-derived macroglia (Prinz & Priller, 2014). In the healthy brain their fine processes are continuously extended and retracted to survey the surrounding tissue (Nimmerjahn et al., 2005); upon injury or infection they retract their processes, adopt an amoeboid form, and act as phagocytes and immune effectors (Kettenmann et al., 2011). Ependymal cells form a single layer of cuboidal to columnar cells lining the ventricular system; many bear motile cilia and microvilli at their apical surface and help move and exchange cerebrospinal fluid (Del Bigio, 2010).

Clinical relevance

Microglial activation is a hallmark of central nervous system inflammation and neurodegeneration, and ependymal integrity is relevant to hydrocephalus and cerebrospinal fluid dynamics. This entry describes normal histology and cell biology for educational reference and is not a basis for diagnosis or treatment.

History

Río-Hortega identified microglia as a distinct glial population in the 1920s and recognized their phagocytic, immune-like behavior, a view confirmed by modern lineage tracing that places their origin in the embryonic yolk sac (Prinz & Priller, 2014). In vivo two-photon imaging by Nimmerjahn and colleagues (2005) overturned the notion that resting microglia are quiescent, showing instead constant surveillance of the parenchyma. Ependyma has long been described in classical histology as the ciliated lining of the ventricular system.

Key figures

• Pío del Río-Hortega
• Helmut Kettenmann

Seminal works

• nimmerjahn-2005
• prinz-2014
• kettenmann-2011

Frequently asked questions

Are microglia true glial cells?

Microglia are classed with the glia of the central nervous system, but unlike astrocytes, oligodendrocytes, and ependyma they originate from myeloid (yolk-sac) precursors rather than from neuroectoderm and function as the brain's resident immune cells.

What is the function of cilia on ependymal cells?

Many ependymal cells bear motile cilia at their apical surface that beat to help circulate cerebrospinal fluid through the ventricular system.