Difference Between Type 1 and Type 2 Pneumocytes

The key difference between type 1 and type 2 pneumocytes is that type 1 pneumocytes are thin and flattened alveolar cells that are responsible for the gas exchange between alveoli and capillaries, while type 2 pneumocytes are cuboidal alveolar cells that are responsible for the secretion of pulmonary surfactants that reduce surface tension in the alveoli.

Pneumocytes are the surface epithelial cells of the alveoli. They are also called alveolar cells. These cells line the alveoli and are present in the majority of the inner surface of the lungs. There are two types of pneumocytes as type 1 and type 2 pneumocytes. More than 95% of the alveoli surface is lined by type 1 pneumocytes. They are flattened, thin and large cells. Type 2 pneumocytes are small cuboidal cells that do not cover much of the surface area of alveoli. They contain secretory organelles which secrete pulmonary surfactants to reduce surface tension in the alveoli.

CONTENTS

1. Overview and Key Difference
2. What are Type 1 Pneumocytes
3. What are Type 2 Pneumocytes
4. Similarities Between Type 1 and Type 2 Pneumocytes
5. Side by Side Comparison – Type 1 vs Type 2 Pneumocytes in Tabular Form
6. Summary

What are Type 1 Pneumocytes?

Type 1 pneumocytes are one of the two types of pneumocytes found in the alveolar wall. They are flattened alveolar cells that cover more than 95% of the surface area of alveoli. These cells participate in the process of gas exchange between alveoli and capillaries. In fact, they form part of the barrier across which gas exchange happens.

Type 1 pneumocytes are extremely thin cells. Hence, they minimize the diffusion distance for respiratory gases. In order to prevent the leakage of tissue fluid into the alveolar air space, type 1 pneumocytes are connected by occluding junctions.

What are Type 2 Pneumocytes?

Type 2 pneumocytes are a type of alveolar cells that are cuboidal in shape. They cover a relatively less surface area (about 5%) of alveoli in comparison to type 1 cells. Type 2 pneumocytes are responsible for producing pulmonary surfactants in order to reduce the surface tension in the alveoli. Therefore, type 2 cells contain secretory organelles full of granules (lamellar bodies) to produce these surfactants.

Compared to type 1 cells, type 2 cells are smaller. However, they are the most numerous cells in the alveoli. Type 2 pneumocytes can proliferate and differentiate into type 1 cells to compensate for damaged cells.

What are the Similarities Between Type 1 and Type 2 Pneumocytes?

• Type 1 and type 2 pneumocytes are two types of pneumocytes or alveolar cells.
• They are epithelial cells.
• They are present in the alveolar wall.

What is the Difference Between Type 1 and Type 2 Pneumocytes?

Type 1 pneumocytes are extremely thin, flattened epithelial cells lining the alveoli, while type 2 pneumocytes are small cuboidal epithelial cells that contain secretory organelles. Moreover, functionally, type 1 pneumocytes are responsible for the process of gas exchange between alveoli and capillaries, while type 2 pneumocytes are responsible for the secretion of pulmonary surfactants in order to reduce surface tension. So, this is the key difference between type 1 and type 2 pneumocytes.

The below infographic lists more differences between type 1 and type 2 pneumocytes in tabular form for side by side comparison.

Summary – Type 1 vs Type 2 Pneumocytes

Pneumocytes are epithelial cells lining the alveoli in the lung. There are two types as type 1 and type 2 pneumocytes. Type 1 pneumocytes are thin flattened cells that are responsible for the gas exchange between alveoli and capillaries. Type 2 pneumocytes are smaller cells that are cuboidal in shape. They are responsible for the secretion of pulmonary surfactants in order to reduce the surface tension in the alveoli. Type 2 cells are the most numerous cells in the lung. Most importantly, type 2 pneumocytes can proliferate and differentiate into type 1 cells to compensate for damaged cells. Thus, this is the summary of the difference between type 1 and type 2 pneumocytes.

Reference:

1. “Pulmonary Alveolus.” Wikipedia, Wikimedia Foundation, 18 Mar. 2021, Available here.
2. Brandt, Josiah P. “Histology, Alveolar Cells.” StatPearls [Internet]., U.S. National Library of Medicine, 29 Apr. 2020, Available here.

Image Courtesy:

1. “Cross section of an alveolus and capillaries showing diffusion of gases” By Delmalani18 – Own work (CC BY-SA 4.0) via Commons Wikimedia

2. “Marked atypia of type 2 pneumocytes associated with Busulfan therapy” By Yale Rosen (CC BY-SA 2.0) via Flickr