There are two main groups of testicular tumours. The first are called seminomas and derive from the cells of the seminiferous epithelium. However, a common primordial germ cell lineage with the other main type of testicular tumour: teratoma, is proven by a common molecular abnormality viz. the presence of an isochromosome on the short arm of chromosome 12 (called 12p). To the naked eye, these seminomas have a grey and uniform appearance and under the microscope appear as sheets of quite large and similar malignant cells, special staining for alkaline phosphatase.
Teratomas are a mixed set of germ cell tumours and may contain different elements of different primitive embryonic structures and tissues. Five specific sub-entities are recognised: Teratoma differentiated is the most benign of the group, having an admixture of cell types within the tumour on microscopic examination but few cells in any dividing mode. Malignant teratoma undifferentiated is the converse with many cells in division and often evidence of small vessel invasion within the tumour when examined microscopically. In between these two extremes lies the subgroup: malignant teratoma intermediate which contains elements of both the aforementioned tumour characteristics. The last two groups are of greatest interest as they truly declare the embryonic or germ cell origin of these tumours.
The Malignant teratoma trophoblastic (also called choriocarcinoma) contains elements that are recognisable as being of placental trophoblast origin microscopically and it is these that secrete the marker HCG.
The Yolk Sac Tumour (sometimes refereed to as endodermal sinus tumour) contains tissue that is microscopically like that from the primitive yolk sac and the secretion of the embryonic origin AFP is by these elements. Many teratomas contain different proportions of all the above elements, and therefore secrete HCG or AFP or both in different amounts. Nevertheless, the level of these markers at presentation is a fairly reliable marker by which to serially to assess the response of he cancer to therapy.
