Gastric glands of submammalian vertebrates are formed by one single cell type known as the oxyntopeptic cell. This cell secretes both hydrochloric acid and pepsinogen. In mammals, this cell differentiates into an acid secreting cell and a pepsinogen secreting one. In the elasmobranch fishHexanchus griseuswe observed, by means of histological studies at the light-and electron-microscopic levels, two different cell types for the secretion of acid and zymogen. This organization represents an evolutionary divergence in a primitive animal, i.e., the appearance of a feature that is acquired much later in evolution, in mammals.
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