The following discussion and classification is taken from Adami and McCrae:

To properly understand the formation of the chorio-epithelioma it will be necessary to review briefly the origin of the fetal placenta. This structure, arising from chorionic villi, finally develops a vascular mesoblastic core covered by epiblast. The outer cells of the fetal chorion erode into the mucous membranes of the uterus. Normally these cells, when they have penetrated into the sinuses, have done their work, and the outer layer becomes inactive, fuses, and forms the syncytium. Sometimes in cases of abortion these placental changes, usually complete by the time of full term, have not yet occurred, and when the immature fetus is expelled, there remain chorionic cells which have not degenerated and are still actively growing. These constitute the evil agent; they continue their growth in the uterus and form a neoplasm.

Placental Mole

In some instances the fetus may die, be absorbed, and leave the placenta and membranes grafted upon the uterus in the form of an irregular fleshy mass, prone to hemorrhage and to putrefaction. On the other hand, the chorionic villi, being nourished by maternal blood, may grow actively and absorb fluid so that a villus becomes a vesicle or a series of vesicles. These may be large or small, due to distention by an edematous mucoid fluid. These vesicles may eventually take up as much room as a full-term fetus. Such an edematous mass of vesicles is known as a hydatid mole.

Chorio-Epithelioma

Such a hydatid mole as above described may pass beyond the usual growth and fill the uterine sinuses with polypoid masses, the so-called destructive placental polyp. The outer surface of the villus, consisting of fetal epiderm, becomes fused to form syncytium, which is made of deeply staining cells whose bodies have fused, the mass thus being multinuclear. Below the syncytial layer the cells of Langhans' layer remain unfused, individual, and less deeply staining. The syncytium possesses erosive and phagocytic properties, and it is these masses of cells that tend to be swept away in the blood of the maternal sinuses and to be deposited in the capillaries of the lung and elsewhere.

Chorio-Epithelioma Malignum

This term is applied to a neoplasm that is entirely cellular, formed of large actively vegetative cells growing entirely within the vessels, not requiring an individual blood-supply by vessels of its own, not cap-sulated, liable to induce hemorrhage by erosion of the vessel walls, and very readily tending to have particles carried away to grow elsewhere. Such a tumor generally occurs shortly after an interrupted pregnancy, but may not occur for years after an abortion with no intervening pregnancy. Microscopically, various cells are seen. Very large ones containing many large nuclei, rich in chromatin, are formed by direct division. Others, much smaller, with single well-formed nuclei. Some that resemble lymphocytes, and all kinds of forms resembling the above types more or less closely. In places there may be found long narrow strands of protoplasm containing nuclei, but showing no division into individual cells, syncytium. Clinically, these tumors show great variations in malignancy, although microscopically their structure may be similar.

Teratoma

Under this heading are included those tumors which have a tendency to the formation not only of irregular cell masses, but also of fully formed organs, such as brain, teeth, skin, hair, bone, or secreting glands. Such growths may be due to the development of two germinal areas on one germinal vesicle, giving rise to double monsters, one of which undergoes inclusion in the other - fetal inclusion. They may result from the displacement of totipotential cells - those capable of giving origin to an individual - which become included in the growing organism. These cells may develop early and grow elaborately, giving rise to inclusions recognizable at birth. They may lie latent and at a subsequent time grow actively as abdominal inclusions, teratoma of the genital glands, and certain mixed tumors.

Dermoid cysts, ovarian dermoids are the most common of the teratoma. The cyst cavity is lined by squamous epithelium in which are found sweat and sebaceous glands. Within the cavity is usually a varying amount of fatty material in which are masses of hair. In the wall of the cyst are found masses of bone to which teeth, usually but poorly formed, are attached. In some instances the extremities and genitalia have been seen.

Fig. 67. - Teeth from Ovarian Dermoid (From Coplin after Roberts).

An irregular branching piece of bone contained in a dermoid cyst of the ovary, in which are implanted well-formed teeth: 1, 1, Bony mass; 2, a tooth resembling a canine of the first dentition; 3, 3, 3, teeth resembling molars.

Somewhat similar growths may be found in those parts of the body where fetal clefts have united and in the median fissures of the body.

There is another type, the sporadic teratoma, which grow in regions bearing no relationship to the fissures, to the poles of the body, or to the generative glands, as in the anterior mediastinum and the abdomen. These are probably due to the development of a misplaced totipotential cell. They generally consist of tissues from all three germinal layers. Sometimes the tissues are of adult appearance and of limited growth. More frequently they appear about puberty, grow rapidly, and tend to form secondary tumors.

Cysts

A cyst is a collection of a fluid or semifluid substance contained within a connective-tissue wall lined by epithelium or endothelium. The contained material may be serous, mucous, or purulent if infection has occurred.

Cysts may be either single or multilocular. The latter when divided into numerous compartments by fibrous partitions. These division walls may break down and convert a multilocular into a simple cyst.

Cysts may be divided into the following:

1. Retention cysts, resulting from an obstruction to the outflow of the secretion of a gland.

2. Exudation cysts, those formed by an increase of fluid in a closed cavity, as in the tunica vaginalis.

3. Necrotic or liquefaction cysts result from the breaking down of the central portion of solid tumors.

4. Parasitic cysts may occur on account of an inflammatory reaction around the parasite, or may be formed directly by it in its development.

5. Dermoid cysts belong to the teratomata, where they are described.

6. Cystoma, a cyst of neoplastic formation. They most frequently occur in the ovary and are multilocular.