The squamous tissues of the vagina and cervix are designated colposcopically as the original or native squamous epithelium, the columnar is called the original or native columnar epithelium. Native squamous epithelium shows little variation from the subject to subject. Under the colposcope, it is uniformly pale pink and translucent and frequently exhibits a feathery vascular arrangement. Native columnar epithelium is unmistakable colposcopically. It is readily identified by its intense red hue and multiple grapes like projections or villi. Each villus possesses a single looped capillary which is sometimes seen colposcopically, especially with use of the green filter. Columnar epithelium, in contrast to squamous epithelium, usually fails to stain with iodine.

The colposcopic examination is directed primarily toward investigating the tissue in the area where original squamous and original columnar epithelia come together. At this squamous columnar interface, columnar epithelium is gradually transformed into squamous epithelium by a process of metaplasia. This dynamic area of change is known as the transformation zone. The transformation zone is the principal focus of colposcopic interest. A clear concept of the transformation zone is essential for understanding the origin and development of cervical neoplasia. It is within this location specifically that preclinical squamous cancer and epithelial dysplasias are believed to develop. The junction between original columnar and original squamous epithelium is transitory.

Presumably as a result of high plasma estrogen concentration and low pH conditions of the vagina, the original columnar epithelium becomes replaced by metaplastic squamous epithelium. The original squamocolumnar junction is therefore converted into a squamo-squamo junction of the original and metaplastic squamous tissue. A new squamocolumnar junction occurs between the original columnar epithelium and transformed areas. This junction may then be subject to a similar transformation process at a later time. The process of metaplasia is multifocal. Metaplastic squamous epithelium is often identified at the outer edge of columnar epithelium adjacent to squamous tissue. It also frequently develops as discrete islands with the columnar tissue. The many metaplastic foci gradually widen, coalesce, and eventually join peripheral component. Since abnormal epithelium can arise from metaplastic tissue, multifocal areas of metaplasia can obviously give rise to multifocal atypical change. The normal physiologic transition from columnar to squamous epithelium occurs throughout the female lifetime. It is most active at the three phases of life. First is the fetal existence and neonatal period, second is the menarche, and third is the first pregnancy. The periods of the most active metaplasia appear to correspond to the times at which high levels of estrogen are present. Estrogen stimulation enhances cervical eversion and allows a maximum amount of columnar tissue to be exposed to the vaginal environment. Estrogen promotes an acid pH in the vagina.

This acidity appears to initiate the metaplastic process. During fetal existence, menarche, and first pregnancy, metaplasia is active in its early and immature form. Metaplasia at other times is minimal. Active metaplasia in the fetus takes place late in pregnancy, primarily from 28 weeks to term and has been attributed to the influence of maternal steroids. After birth, metaplasia slackens, probably as a result o the diminished level of plasma steroids and neutral vaginal secretion in the neonate. With the surge of estrogen that occurs at the menarche and during the first pregnancy, the transformation process is reactivated. Patients taking oral contraceptives, although often displaying large cervical eversions, demonstrates remarkably slow process of metaplasia. This is probably due to the protection afforded by the increased amount of cervical mucus.

Metaplasia can be promoted by the use of contraceptive jellies because of their markedly acid pH. Complete transformation from columnar to squamous epithelium occurs over a period of many years. The process of metaplasia may be divided into three stages. First is the early metaplasia, second is the well developed metaplasia, and third is the fully developed metaplasia like gland openings, nabothian follicles, typical vascular patterns.

The process of metaplasia result in the development of a new squamous epithelium which is, at first, immature. When it reaches maturity it does not seem subject to cancer formation. Mature metaplasia is a permanent change. By contrast, the initial process of metaplasia is vulnerable to genetic change. New squamous cell populations with acquired neoplastic potential can arise from active immature metaplasia. Epithelia which exhibit morphologic characteristics of precursors of squamous cancer are the principal subjects of interest to the colposcopist. The earliest colposcopic changes of metaplasia involve loss of translucency of the tips of the villi of columnar epithelium along with increase in clarity and demarcation of vascular structures. Colposcopically, the tips of the villli stand out as individual opaque structures against a background of red color. The red color is created by the columnar tissue which remains within the clefts between villi.

Histologically, the tall columnar cells at the tips of the villi are replaced by several layers of low cuboidal cells. The metaplastic squamous epithelium originates from multipotential stromal cells that are found subjacent to the columnar epithelium. These so called activated stromal cells or columnar reserve cells become visible or at least recognizable only at the moment that the metaplastic process becomes manifest. These cells then become apparent and increase in number before they differentiate. They develop a less densely staining nucleus and increased cytoplasm, are drawn into long process, and transform into squamous cells.

All metaplasia takes place within the transformation zone. The transformation zone can be quite extensive or very limited. Its colposcopic picture is extremely variable according to the extent of the squamous metaplasia. Sometimes, only a few tongues of squamous metaplasia or isolated areas of squamous metaplasia are visible in the columnar epithelium. Alternatively, the original columnar epithelium can be almost completely covered by metaplastic squamous tissue. Immature metaplastic squamous epithelium consists of six to eight cell thicknesses of undifferentiated surface cells which undergo gradual process of maturation. After acetic acid is applied to immature metaplastic squamous epithelium, it becomes more opaque than the original squamous epithelium, possibly as a result of the increase nuclear to cytoplasmic ratio exhibited by immature squamous cells. Undifferentiated metaplastic epithelium can be difficult to distinguish from dysplasia or carcinoma in situ histologically. Immature metaplastic tissue is nonglycogenatic and characteristically iodine negative. As a result, staining techniques are not adequate to distinguish early metaplastic from dysplastic tissue. It must be emphasized that the characteristics of the transformation zone are bets recognized by its colposcopic appearance. As the metaplastic process advances, a multilayered undifferentiated sheet of epithelial cells is gradually formed. The activated stromal cells which cap the villi rapidly divide and extend into the clefts between adjacent villi. The tips of the villi coalesce; fusion of the opposed surfaces of villi occurs. Colposcopically, although individual opaque villi appear fused, minute humps are present on the tissue surface which represents the tips of the old villi. This tissue also is not stained by iodine.

The fully end stage of the metaplastic process represents the near obliteration of the original villous structures. A smooth surface of multilayered, increasingly differentiated epithelium is produced. The vascular supply remains similar to that of the original squamous epithelium, mature metaplastic tissue exhibits both network and hairpin capillaries of varying number and pattern. A third type of terminal vessel, not identified in normal squamous tissue, is often seen in typical normal or mature transformation zones. This vessel runs parallel with the tissue surface and is strikingly large. It divides dichotomously into a network of delicate capillaries of normal or even increased intercapillary distance. Such vessels are associated with nabothian cysts. Even in mature transformation zones, the coalescence of villi or papillae is rarely complete. Islands of columnar tissue often remain in the deep clefts within the stroma below the metaplastic epithelium. Such areas can have an outlet to the surface which is surrounded by a narrow ban of heaped up squamous epithelium. Mucus is secreted and expelled through these small gland openings.

Under the colposcope, these gland openings appear characteristically as dark red craters encircled by a dense white border. If areas of columnar tissue have no communication with the tissue surface, retention cysts or nabothian cysts develop. Under the colposcope, these cysts are yellowish and translucent and are elevated above the level of surrounding epithelium. Large branching blood vessels transverse their surface. The ultimate maturity of the metaplastic epithelium is its development into a fully differentiated squamous tissue distinguishable only with difficulty from the native squamous variety.

Histologically, it is almost impossible to differentiate between original squamous and mature metaplastic squamous epithelium. Both stain the iodine. Colposcopically, columnar remnants like nabothian follicles and gland openings serve to define the distal margin of the transformation zone, the original border between native squamous and metaplastic squamous epithelium. As a result of the influence of unknown mutagenic agents, areas of metaplastic epithelium within the transformation zone can acquire neoplastic potential. In these areas, columnar epithelium undergoes a process of atypical metaplasia and atypical transformation zone is created. An atypical transformation zone is readily identified colposcopically. Abnormal metaplastic epithelium is characterized by the appearance within the transformation zone of keratosis termed as leukoplakia, white epithelium, punctuation, mosaic structure, and abnormal blood vessels. These atypical colposcopic tissue patterns may occur singly or in combination. They may be unifocal or multi focal and almost invariably are sharply delineated from surrounding normal tissue. Their lateral margins rarely, if ever, extend beyond the original squamocolumnar junction onto the native squamous epithelium. With exception of keratosis, none of the lesions peculiar to the atypical transformation zone can be seen with the naked eye prior to the application of acetic acid. Areas of punctuation, mosaic structure, and abnormal blood vessels can sometimes be observed without the magnification provided by the colposcope. It should be emphasized that the colposcopic variations of atypia are found primarily within the transformation zone but can be present also in the vagina and on the vulva. Moreover, the same colposcopic findings that often signal a neoplastic change may occur in non neoplastic conditions such as normal metaplasia, infection, inflammation, and regeneration and repair following trauma, cautery, or cryosurgery.

The presence of an abnormal transformation zone, though highly suggestive, does not prove that neoplasia exists. The white focal lesion constitutes the most common appearance of the atypical transformation zone. White epithelium exhibits a flat smooth surface which is level with the surrounding normal tissue. It displays either no visible terminal vessels or only minimally developed vascular structures. Without acetic acid, the white lesion is not evident colposcopically and cannot be distinguished from adjacent normal epithelium. Whiteness of the abnormal epithelium presumably is related to the nuclear predominance of atypical cells. Light is reflected from atypical cells that have dense, hyperchromatic nuclei and scantly cytoplasm, but it is readily transmitted through surrounding normal cells with abundant cytoplasm and homogeneous nuclei. The significant of a white lesion is related to the intensity of its whiteness as well as the sharpness of its borders. The whiter and more distinct the lesion, the greater is its histologic significance. Dysplasia or even in situ carcinoma with high nuclear density and low transparency can produce this appearance. As columnar epithelium undergoes atypical metaplasia, the vascular patterns of punctuation and mosaic structure develop. During atypical metaplasia, papillae do not coalesce or fuse with each other as they do during the normal metaplastic process. Ultimately, a flat sheet of stroma covered by uniformly distributed metaplastic squamous epithelium is not produced. Instead the vascular network within each villus persists and undergoes marked proliferation. The central vascular network of these remaining stromal papillae features blood vessels loops which extend close to the surface of the overlying epithelium. Consequently, the blood supply of the surface epithelium is greater in atypical than in normal transformation zones. Punctuation and mosaic patterns are reflections of this abnormal developmental process.

Colposcopically in very early forms atypical metaplasia, after the application of acetic acid, the stromal papillae appear as reddish fields surrounded by white strands of metaplastic epithelium. This appearance has been called reverse mosaic. The red islands represent the tops of the stromal papillae with underlying superficial vascularity and the white strands are produced by a piling up of atypical metaplastic squamous cells which invade and fill surrounding clefts and folds. Since connective tissue papillae become thinner and cleft areas widen as the atypical metaplastic process advances.