The neoplastic glands of this adenocarcinoma demonstrate less differentiation than normal glands, although they still resemble glands, but more irregular in size and shape. One aspect of differentiation is shown in the left panel with a PAS stain, highlighting red-staining mucin in vacuoles in neoplastic cell cytoplasm, characteristic of a neoplasm derived from glandular epithelium.
Note characteristics of a neoplasm. There are small vessels (angiogenesis factors are secreted by neoplastic cells) to support the tumor. There is a stroma with connective tissue and extracellular matrix to provide a supporting framework for tumor growth. Malignant neoplastic cells are not robust, and tend to become necrotic, and may express tumor antigens, and both of these characteristics can incite an inflammatory response.
In general, less differentiation of a neoplasm means a greater likelihood of malignant behavior. This is the basis for histologic grading of malignant neoplasms. The less differentiated, the higher the grade, and the more aggressive the malignant neoplasm is likely to be. (Benign neoplasms aren't graded, since by definition they are well-differentiated.)