Difference between Type 1 and Type 2 Pneumocytes

Pneumocytes are alveolar cells that are found on the surface of alveoli in the lungs. There are two types of cells that cover the alveoli: type 1 pneumocytes and type 2 pneumocytes. They are present in a ratio of 1:2. Let us discuss them in detail.

Type 1 Pneumocytes

Type 1 pneumocytes are flat and thin (0.1-0.2 µm thick) squamous cells that cover almost 95% of the alveolar surface. They have long extensions (2300 µm) that aid in covering the alveolar surface. The pneumocytes are connected to each other by tight junctions. During inspiration and expiration, the flat extensions overlap each other.

They have a small nucleus with a highly branched cytoplasm. The distribution of cell organelles is sparse. It is formed by mitotic division and transformation of type 2 pneumocyte cells. The type 1 cells are involved in gaseous exchange between the alveoli and capillaries. The thinness of these cells allows fast diffusion of gases.

The type 1 cells cannot replicate at the time of toxicity, so the type 2 cells differentiate to form type 1 cells, and compensate for the damage.

Type 2 Pneumocytes

Type 2 pneumocytes are another type of alveolar cells that are cubic in shape. They have a granular appearance and abundant cell organelles. They have secretory granules called lamellar bodies. The lamellar bodies secrete surfactants (proteins) that reduce the alveolar surface tension. The surfactant is made up of phospholipids majorly. The coating of these lipids in alveoli is important, without which the alveoli may collapse. The surfactant proteins start to secrete at about 25 weeks of gestation.

The nucleus is large with a prominent nucleolus. The cells are characterised by the presence of microvilli on their surface. The major functions of the type 2 cells include: secretion of surfactants to reduce surface tension, movement of water and transepithelial ions, and regeneration of the alveolar epithelium (by differentiating into type 1 cells) in case of injury. They are also called the “caretakers” of the alveoli.

Type 1 vs Type 2 Pneumocytes

Type 1 Pneumocytes	Type 2 Pneumocytes
Definition
Type 1 pneumocytes are alveolar cells that line the alveolar surface.	Type 2 pneumocytes are alveolar cells that secrete surfactant proteins to reduce surface tension.
Shape of the cells
Type 1 pneumocytes are flat and thin.	Type 2 pneumocytes are cubic in shape.
Distribution of Organelles
The distribution of organelles is sparse in type 1 pneumocytes.	The organelles are heavily populated in type 2 pneumocytes.
Shape of the Nuclei
The nucleus is small.	The nucleus is large.
Occurrence
They occur less in the alveoli as compared to type 2 cells.	They occur more in the alveoli. The ratio of type 1 cells to type 2 cells is 1:2.
Surface Area Coverage
It covers almost 95% of the alveolar surface.	Though they occur more often, they are less involved in lining the alveolar surface.
Size of the Cell
Comparatively larger than type 2 cells.	The cells are smaller than type 1 cells.
Function
The type 1 cells form a lining on the alveolar surface, and fasten the exchange of gases between alveoli and capillaries.	It secretes surfactant proteins that reduce surface tension of the alveolar surface. It can convert into type 1 cells, and regenerate the alveolar surface at the time of injury.
Presence of Secretory Granules
No such presence.	Secretory granules called lamellar bodies are present.
Junction between Cells
The adjacent type 1 cells are connected by tight or occluding junctions that prevent leakage of fluid into the alveolar space.	No such connection is found.

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Frequently Asked Questions on

What is the anatomical difference between Type 1 and Type 2 alveolar cells?

Type 1 alveolar cells have a small nucleus with sparsely populated cell organelles, whereas type 2 cells have a large nucleus with heavily populated cell organelles.

Can type 1 pneumocytes regenerate?

No, type 1 pneumocytes cannot regenerate. At the time of injury, type 2 cells are transformed into type 1 cells.

What does surfactant do in the lungs?

Surfactants lower the surface tension on the alveolar surface. This helps in easy reinflation and deflation of alveoli and also prevents its collapse.

What would happen to your lungs without surfactant?

Without the presence of surfactants, the alveolar sacs will stick together (because of high surface tension), and as a result the alveoli will collapse.

What is the main active ingredient in lung surfactants?

The lung surfactants are majorly (70-80%) made of phospholipids.