Parietal cells

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Have a thorough look at stomach under the microscope.

Parietal cells also known as oxyntic cell, are large pear shaped cells. They are found in the neck of the gastric glands among the mucous (neck) cells. While they are typically more prevalent in the upper and middle sections of the neck, they can also be found in the base of the gland.  

The parietal cells are occasionally binuclear cells and display a somewhat triangular shape in cross-sections. Their apex faces the gland's lumen, while the base rests on the basement membrane. The nucleus is rounded, and the cytoplasm exhibits eosinophilic staining. 

Under transmission electron microscope (TEM), parietal cells have an intricate intracellular canalicular network, connecting with the gland's lumen. A canaliculus is a deep infolding that serves to increase surface area. 

Abundant microvilli extend from the canalicular surface, alongside a tubulovesicular membrane system in the nearby cytoplasm. In a cell undergoing active secretion, microvilli in the canaliculi multiply, while the tubulovesicular system diminishes or vanishes. The membranes of this system act as a reservoir for proton pumps which are important for acid production.

The canaliculi network secretes hydrochloric acid (HCl) by active transport into the stomach. The enzyme hydrogen potassium ATPase (H+/K+ ATPase) is unique to the parietal cells and pumps the hydrogen ions out of the cell into the canaliculi in exchange for potassium ions. As a result of the cellular export of hydrogen ions, the gastric lumen is maintained as a highly acidic environment. Mitochondria provide the substantial energy required for acid secretion.

The parietal cells also release intrinsic factor, a glycoprotein that binds with vitamin B12 (cobalamin) in the stomach and duodenum. Intrinsic factor is crucial for the absorption of vitamin B12 in the ileum. A lack of intrinsic factor can lead to malabsoprtion of vitamin B12 and pernicious anaemia.