A cyst is a cavity having a defined wall composed of connective tissue lined with epithelium, and containing more or less fluid contents.
The modes of formation of cysts are somewhat various, but they all imply the progressive accumulation within the sac of material which has generally the characters of glandular secretion, but may be simply serous. Hence, with few exceptions, cysts arise in connection with secreting structures, and these may be the normal glands or else structures which in themselves are of pathological origin. In the former case there is the retention and accumulation of the normal secretion, hence the name Retention cyst, and in the latter there is an abnormal tissue producing secretion, which accumulates. Besides these, which are the chief modes of formation, we have subordinate groups arising in some cases by dropsy of existing structures, in others by softenings of the tissues, more especially of tumours, and in others by the transformation of an irregular space, such as a collection of blood has formed, into a defined cavity or cavities containing fluid.
The cysts thus form a somewhat heterogeneous group, and only some of them have the characters of tumours in the sense of forming independent growths. Those arising by softening are entirely subordinate, and are not to be regarded as tumour formations. On the other hand, those arising from accumulation of secretion or dropsical fluid in normal structures may or may not have the characters of tumours. Where there is a progressive increase implying a new-formation of tissue so as to form the wall of the enlarging sac, the characters assume very much those of a tumour. In the case of cysts formed from some new-formed pathological tissue, the characters of a tumour are complete and it may be proper to reserve the name Cystoma for them.
These arise from glands by retention of their secretion owing to obstruction or obliteration of the ducts. Examples of this on a large scale are furnished by cases in which a main duct is obstructed. Thus obstruction of the ureter may lead to the conversion of the kidney into a large cystic cavity (hydronephrosis). Obstruction of the cystic duct may cause the gall bladder to form a large sac; and closure of the orifice of the vermiform appendage may lead to the formation of a large cyst. (See further under their respective headings.) On a smaller scale we find in the kidney, liver, and mamma, multiple cysts formed by accumulation of secretion in the tubules and ducts. In the kidney the cysts grow to such an extent that we have a cystic transformation of the kidney (see Fig. 93).
Fig. 93. - Cystic transformation of kidney shown in section. The other kidney was similarly affected.
Mucous cysts also form an important group of retention cysts. They occur in situations where mucous glands are present, their chief sites being the nostrils and communicating cavities, the upper surface of the epiglottis, the larynx, the oesophagus, and in connection with the glands of Cowper and Bartolin.
The origin and mode of formation of the mucous cyst has been carefully studied by Recklinghausen. The cyst is formed, not out of the gland, but from its duct, and, as the gland persists, its secretion is thrown into the cyst. This is shown in Fig. 94 copied from Recklinghausen's paper. It would thus appear that the persistence of the gland is an essential element in the formation of the cyst. In the enlargement of the cyst the power which mucin has of swelling up and absorbing water is of some consequence. Before a regular cyst forms, the orifice of the duct is obstructed, usually by an inflammation around it. But on account of the peculiarity of mucin just noticed, a small temporary cyst may form without any considerable obstruction of the duct. If a quantity of mucin is discharged into the duct it may swell so much as to be unable at once to escape from the orifice and so form a small cyst, which afterwards discharges. In this way cysts often form in the mucous membrane of the mouth.
Fig. 94 - Formation of mucous cysts; a, the persistent mucous gland tissue; b, the dilated duct, x 300. (Recklinghausen.
Cysts arising in mucous polypi have a similar mode of formation to that just described. The obstruction of the orifices is here the more likely, as the polypus itself usually originates in connection with a chronic inflammation of the mucous membrane.
Such cysts may be from dropsy, as in the ovary, where we often have many cysts from accumulation of fluid in Graafian vesicles (see under Diseases of the Ovaries). Again, serous cysts form by dilatation of lymphatics and of serous spaces in the connective tissue. Another example is afforded by the thyroid gland, where the colloid degeneration in the saccules may lead to the formation of cysts, which enlarge both by accumulation of their contents and coalescence of neighbouring ones.
These are cysts which warrant the name of cystoma as their walls are not formed from normal tissue, but are themselves of pathological origin. The wall of the cyst is formed of tissue like that of the skin, and the contents are epidermis and sebaceous matter. The simplest of them are found in and under the skin, and they arise from inclusion-of foetal rudiments of the skin. Thus, as already mentioned, the imperfect closure of the branchial clefts may give rise to a dermoid cyst in the neck. Dermoid cysts in the scalp constituting wens are very simple in structure, whilst those of the face are more complex, often containing hairs. Still more complex are the dermoid cysts of the ovary, which arise apparently from the ovum, and usually contain, besides skin structures, bone, cartilage, teeth, etc. Such complex -cysts may even be classed among the Teratomas. Similar cysts have been met with, though very rarely, in the testicles, brain, orbit, lung, peritoneum, and elsewhere.
Cysts of the ovary frequently arise from glandular tissue (see Fig. 95). These tumours will be fully described in the section on diseases of the ovaries. The gland tissue has no ducts, and as the epithelium produces a mucous or other secretion there is great distension resulting in the formation of cysts. Sometimes the gland tissue is rather in the form of villous or papillary processes, the cysts forming by the union of these processes and accumulation in their recesses.
Fig. 95. - From a colloid ovarian cystoma. Gland-like tissue and the beginning of cysts, x 70.
Certain parasitic animals (the Taeniae or tapeworms) in one phase of their development in different species of animals, occur in the form of cysts (so-called bladder worms). Around these parasites the tissues of the animal form sacs composed of connective tissue. The most important example of this is the so-called hydatid cyst which occurs in connection with the taenia echinococcus and sometimes attains large dimensions.
Cysts of subordinate importance are formed chiefly as a result of haemorrhage or of softening. Extravasation cysts result from the changes which may occur after haemorrhage. This is exemplified in the case of the apoplectic cyst where the blood-clot and softened brain substance are replaced by connective tissue containing fluid in its meshes. Such cysts may occur from softening of the brain without haemorrhage.
Cysts in tumours sometimes originate by dilatation of glandular structures and hence are very common in adenomas. They also occur not infrequently from softening of the tumour tissue. This is especially the case in large tumours and in those whose tissue is already comparatively soft. All forms of tumours when they grow to a large size are liable to have cavities in their central parts clue to softening. On the other hand the soft sarcomas often contain cysts without attaining to large dimensions. These latter cysts often contain blood, and indeed the tumour may present little more than the characters of a cyst filled with blood. Blood-cysts are apparently for the most part sarcomas which have undergone this process. (Godlee).
Virchow, Geschwulste, i., Lecture ix.; Butlin, Internal;. Ency. of Surg., iv., 655; Recklinghausen, Virch. Arch., lxxxiv., 425; Godlee, Path. Trans., xxvii., 270; Bland Sutton, On tumours, 1893.