**Mucous cells (Goblet cells): Unicellular gland**

**Definition**
Mucous cells, also known as goblet cells, are specialized epithelial cells found in the respiratory and digestive tracts. These cells are responsible for producing and secreting mucus, a thick and slimy substance that helps protect and lubricate the epithelial surfaces.

**Unicellular Gland**
Goblet cells are classified as unicellular glands because they are individual cells that function as secretory units. Unlike multicellular glands, which are composed of multiple cells organized into a glandular structure, goblet cells work independently. They are scattered among the other cells of the epithelium, and their shape and appearance make them easily distinguishable.

**Structure and Function**
Goblet cells have a characteristic goblet or wineglass shape, with a bulge at the base and a narrow apical portion. The apical portion of the cell contains numerous secretory granules filled with mucin, a glycoprotein that gives mucus its slimy consistency. When stimulated, these granules fuse with the cell membrane and release their contents into the lumen of the respiratory or digestive tract.

The secretion of mucus by goblet cells serves several important functions. It acts as a protective barrier that helps trap and remove foreign particles, such as dust, bacteria, and viruses, from the respiratory and digestive systems. Mucus also provides lubrication, facilitating the movement of food and waste materials through the digestive tract and aiding in the passage of air through the respiratory system.

**Location**
In the respiratory tract, goblet cells are primarily found in the epithelial lining of the trachea, bronchi, and bronchioles. They play a crucial role in the production of the respiratory mucus, which helps to humidify the air, trap inhaled particles, and facilitate the movement of mucus towards the throat for removal.

In the digestive system, goblet cells are present in the epithelial lining of the stomach, small intestine, and large intestine. They produce mucus that protects the lining of these organs from the acidic environment of the stomach, aids in the lubrication of food for easier digestion and absorption, and helps facilitate the passage of waste materials through the intestines.

**Conclusion**
Mucous cells or goblet cells are classified as unicellular glands because they function as individual secretory units. They produce and secrete mucus in the respiratory and digestive tracts, providing protection and lubrication to these systems.