MAIN FEATURESS OF THE GLANDULAR EPITHELIA
The glandular or secretory epithelia are a division of the epithelial tissue that is specialized in production and/or transport of diverse types of ions and molecules followed by their excretion.
Secretion is being used here in its broadest sense, which includes:
- synthesis of new molecules from smaller precursors – e.g., protein synthesis.
- modification of preexisting molecules – e.g., secretion of steroids.
- transport of ions – e.g., secretion of sweat and part of the saliva.
Being a division of the epithelial tissue, the glandular or secretory cells have many of the main characteristics of the epithelial cells already explained in Chapter 2, among which stand out the following:
- Great proximity and adhesion between cells.
- Small amount of extracellular matrix between the cells.
- Cell polarity.
- Cells supported by a basal lamina that is surrounded by diverse amounts of connective tissue.
Arrangement of the secretory cells:
The glandular epithelial cells are usually organized in single cell layers or as differently sized clusters associated in three-dimensional arrangements.
Secreting epithelia
The division of the epithelial tissue into two categories does not however mean that lining epithelia do not also secrete. Two examples:
– The goblet cells of the respiratory epithelium were shown in the previous chapter to be secretory cells inserted among the cells of a lining epithelium. Goblet cells are also present in large amounts in the epithelium of the intestinal mucosa, forming tubular glands.
– Most cells of the simple epithelium that lines the stomach cavity are also secretory.
How are secretory cells organized?
They can assume several types of arrangements:
- As single cells (such as the goblet cells) placed among the other cells of a lining epithelium, may be considered as unicellular glands.
- Placed in a lining epithelium that is also secretory (such as the covering epithelium of the stomach mucosa).
- As multicellular glands, variously sized associations of secretory cells that constitute three-dimensional structures or entire organs.