The key difference between type 1 and type 2 alveolar cells is that type 1 alveolar cells do not have secretory organelles, while type 2 alveolar cells have secretory organelles.
Alveoli are located in the respiratory bronchioles as pockets and extend from their lumens. The bronchioles extend for considerable lengths and become increasingly alveolated with branches of alveolar ducts. These are deeply lined with alveoli. Each duct opens to five or six alveolar sacs. The pulmonary alveolus makes up the functional tissue of the lungs of mammals. The alveolus comprises a simple squamous epithelial layer and an extracellular matrix surrounded by capillaries. The membrane also has many layers of lining fluid that contain surfactants. Alveolar cells consist of three types of cells, and they are type 1 cell, type 2 cell, and phagocytic cell. Type 1 alveolar cells are also known as type 1 pneumocytes, while type 2 alveolar cells are also known as type 2 pneumocytes.
CONTENTS
1. Overview and Key Difference
2. What are Type 1 Alveolar Cells
3. What are Type 2 Alveolar Cells
4. Similarities – Type 1 and Type 2 Alveolar Cells
5. Type 1 vs Type 2 Alveolar Cells in Tabular Form
6. Summary – Type 1 and Type 2 Alveolar Cells
What are Type 1 Alveolar Cells?
Type 1 alveolar cells are complex branched cells with a variety of thin cytoplasmic plates that represent the gaseous exchange surface in the alveolus. They cover a vast area of the alveolar surface. These cells cover capillaries in the alveolar walls. They consist of a central nucleus and a large, thin cytoplasm. The cytoplasm contains a few mitochondria and other organelles closer to the central nucleus. Thin and flattened type 1 alveolar cells are important components of the air-blood barrier. One type 1 cell extends into more than one alveolus. They house the major gas exchange surface of the alveolus and are essential for the maintenance of the permeability barrier function of the alveolar membrane.
The progenitors of type 1 alveolar cells are type 1 pneumocytes, and they aid in the production of surfactants and homeostasis. The most commonly used markers in the detection of type 1 alveolar cells are podoplanin (T1α) and aquaporin5 (Aqua5). Podoplanin is a plasma membrane protein and is the best marker. It is expressed only in type 1 alveolar cells in the lung. Type 1 alveolar cells also consist of caveolae. Caveolae are plasma membrane structures that mediate the transport of materials across a cell and are also signaling mediators. Claudins are transmembrane proteins that contribute to the tight junctions of the alveolar epithelium. They are most prominent in type 1 alveolar cells.
What are Type 2 Alveolar Cells?
Type 2 alveolar cells are also known as defenders of the alveoli since they secrete surfactants, mainly in order to keep the alveolar surface free of fluid. They have a simple epithelial lining, cuboidal in shape, and much smaller. These cells are present in the alveolar wall and contain secretory organelles called lamellar bodies. Phospholipids are stored in these lamellar bodies. They fuse with the cell membrane and help in the secretion of pulmonary surfactant. The differentiation of type 2 cells starts at about 24-26 weeks of gestation. These differentiated cells produce a pulmonary surfactant, which is a lipoprotein substance required for the function of the lungs by controlling surface tension in the alveoli.
A fluid coating is present to facilitate the gaseous exchange between blood and alveolar air, and type 2 cells are found in the blood-air barrier. Type 2 cells are essential to maintain homeostasis in the alveolar region of the lung. These alveolar cells also function as stem cells possessing the ability to self-renew and differentiate into type 1 cells. Type 2 cells are also capable of cellular division and give rise to both type 1 and 2 alveolar cells when the lung is damaged. The human gene MUC1 acts as a marker in the detection of type 2 alveolar cells in lung cancer.
What are the Similarities Between Type 1 and Type 2 Alveolar Cells?
- Type 1 and type 2 alveolar cells are present in alveolar cells.
- They are lined by simple squamous epithelial cells.
- Both facilitate the secretion of pulmonary surfactant.
- They help in homeostasis in the alveolar region.
- Both act as progenitor cells.
What is the Difference Between Type 1 and Type 2 Alveolar Cells?
Type 1 alveolar cells do not have secretory organelles, while type 2 alveolar cells have secretory organelles. Thus, this is the key difference between type 1 and type 2 alveolar cells. Type 1 cells cover more than 95 % of the alveolar surface, while type 2 cells occupy around 5 % of the alveolar surface area. Moreover, type 1 cells are squamous or flattened whereas type 2 cells are cuboidal in shape.
The below infographic presents the differences between type 1 and type 2 alveolar cells in tabular form for side-by-side comparison.
Summary – Type 1 vs Type 2 Alveolar Cells
The pulmonary alveolus makes up the functional tissue of the lungs of mammals. Alveolar cells are of three types: type 1 cell, type 2 cell, and phagocytic cell. Type 1 alveolar cells do not have secretory organelles, while type 2 alveolar cells have secretory organelles. Type 1 alveolar cells are complex branched cells with a variety of thin cytoplasmic plates that represent the gaseous exchange surface in the alveolus. They cover about 95% of the alveolar surface. Type 2 alveolar cells are also known as defenders of the alveoli since they mainly secrete surfactant to keep the alveolar surface free of fluid. They cover about 5-7% of the alveolar surface. So, this summarizes the difference between type 1 and type 2 alveolar cells.
Reference:
1. “Alveolar Type I Cells.” An Overview | ScienceDirect Topics.
2. “Alveolar Type II Cells.” An Overview | ScienceDirect Topics.
Image Courtesy:
1. “An annotated diagram of an alveolus” By Katherinebutler1331 – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “2310 Structures of the Respiratory Zone” By OpenStax College – Anatomy & Physiology, Connexions Website, Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia