Type I pneumocyte
Pneumocytus typus I
Also known as: type I alveolar cell, squamous alveolar cell, AT1 cell
Extremely thin, broad squamous epithelial cells that cover ≈95% of the alveolar surface and form the epithelial part of the blood–gas barrier.
Overview
Type I cells are few in number (≈8% of all lung cells, about a third of alveolar epithelial cells) but enormous: a single cell spreads its attenuated cytoplasm — in places only ≈0.1–0.2 µm thick — over ≈5000 µm² and often into adjacent alveoli. Organelles are clustered around the nucleus, leaving the thin cytoplasmic plates for diffusion.
Neighbouring type I cells and type II cells are joined by tight junctions that make the alveolar epithelium very impermeable to solutes and protein, preventing flooding. Type I cells cannot divide; after injury they are replaced by proliferation and differentiation of type II cells. They also contribute to fluid and ion transport (aquaporin-5, Na+ channels).
- Location
Lining the alveolar walls.
Anatomy
- Histology
Simple squamous epithelium; flattened nucleus bulging into the alveolus; very thin cytoplasmic extensions; tight junctions; shares a fused basement membrane with capillary endothelium on the thin side of the septum.
Function
- Form the thin epithelial layer of the respiratory membrane for gas diffusion.
- Provide a tight, low-permeability barrier against fluid and protein leakage.
- Participate in alveolar fluid clearance.
Clinical significance
Vulnerability to injury
Type I cells are easily damaged by oxygen toxicity, inhaled toxins, sepsis and viral infection (e.g. influenza, SARS-CoV-2); their loss with denuded basement membrane and hyaline membranes is the hallmark of diffuse alveolar damage in ARDS.
Compiled from standard anatomy and physiology references — see the Respiratory System sources. For education only; not medical advice.