Schematic illustration of a goblet cell in close up, illustrating different internal structures of the cell.
Transverse section of a villus, from the human intestine. X 350.
a. Basement membrane, here somewhat shrunken away from the epithelium.
c. Columnar epithelium.
d. Its striated border.
e. Goblet cells.
f. Leucocytes in epithelium.
f’. Leucocytes below epithelium.
g. Blood vessels.
h. Muscle cells cut across.
A goblet cell is a glandular simple columnar epithelial cell whose function is to secrete gel-forming mucins, the major components of mucus. The goblet cells mainly use the merocrine method of secretion, secreting vesicles into a duct, but may use apocrine methods, budding off their secretions, when under stress.
The goblet cell is highly polarised with the nucleus and other organelles concentrated at the base of the cell. The remainder of the cell's cytoplasm is occupied by membrane-bound secretory granules containing mucin. The goblet shape is due to the mucus laden granules in the apical part expanding, causing that part of the cell to balloon. The apical plasma membrane projects microvilli to give an increased surface area for secretion.
Goblet cells are found scattered among the epithelial lining of organs, such as the intestinal and respiratory tracts. They are found inside the trachea, bronchi, and larger bronchioles in the respiratory tract, small intestines, the large intestine, and conjunctiva in the upper eyelid. Goblet cells are a source of mucus in tears and secrete different types of mucins onto the ocular surface, especially in the conjunctiva. In the lacrimal glands, mucus is synthetized by acinar cells instead.
Goblet cells are modified simple columnar epithelial cells, having a height of four times that of their width. The cytoplasm of goblet cells tends to be displaced toward the basal end of the cell body by the large mucin granules, which accumulate near the apical surface of the cell along the Golgi apparatus, which lies between the granules and the nucleus. This gives the basal part of the cell a basophilic staining because of nucleic acids within the nucleus and rough endoplasmic reticulum staining with hematoxylin. Mucin within the granules stains pale in routine histology sections, primarly because these carbohydrate-rich proteins are washed out in the preparation of microscopy samples. However, they stain easily with the PAS staining method, which colours them magenta.
In mucicarmine stains, deep red mucin is found within goblet cell bodies. Goblet cells can be seen in the examples below as the larger, more pale cells.
The main role of goblet cells is to secrete mucus in order to protect the mucous membranes where they are found. Goblet cells accomplish this by secreting mucins, large glycoproteins formed mostly by carbohydrates. The gel-like properties of mucins are given by its glycans (bound carbohydrates) attracting relatively large quantities of water. On the inner surface of the human intestine, it forms a 200 µm thick layer (less in other animals) that lubricates and protects the wall of the organ. Distinct forms of mucin are produced in different organs: while MUC2 is prevalent in the intestine, MUC5AC and MUC5B are the main forms found in the human airway. Mucins are stored in granules inside the goblet cells before being released to the lumen of the organ. Secretion may be stimulated by irritants such as dust and smoke, especially in the airway. Other stimuli are microbes such as viruses and bacteria.
Role in oral tolerance
Oral tolerance is the process by which the immune system is prevented from responding to antigen derived from food products, as peptides from food may pass into the bloodstream via the gut, which would in theory lead to an immune response. A paper published in Nature in 2012 has shed some light on the process and implicated goblet cells as having a role in the process. It was known that CD103-expressing dendritic cells of the lamina propria had a role to play in the induction of oral tolerance (potentially by inducing the differentiation of regulatory T cells), and this paper suggests that the goblet cells act to preferentially deliver antigen to these CD103+ dendritic cells.
Goblet cell carcinoids are a class of rare tumors that form as a result of an excessive proliferation of both goblet and neuroendocrine cells. The majority of these tumors arise in the appendix and may present symptoms similar to the much more common acute appendicitis. The main treatment for localized goblet cells tumors is removal of the appendix, and sometimes removal of the right hemicolon is also performed. Disseminated tumors may require treatment with chemotherapy in addition to surgery.
The term goblet refers to the cell's goblet-like shape. The apical portion is shaped like a cup, as it is distended by abundant mucinogen granules; its basal portion is shaped like a stem, as it is narrow for lack of these granules.
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