The ectocervix of the adult female is lined by a nonkeratizing squamous epithelium which shows cyclical changes in response to local steroid hormonal levels [1]. ER and PR are present in the epithelium and stromal fibroblasts. The endocervix is covered by a mucinous columnar epithelium which also invaginates to line the cleft-like structures in the stroma (endocervical glands). The glandular clefts are arranged in a collateral tunnellike pattern and usually penetrate less than 5 mm from the surface (but can be as deep as 1 cm). In contrast to the marked cyclic morphological changes in the ectocervical squamous cells, the columnar cells manifest minimal cytological changes. Instead, it shows dramatic variations in the production of secretion during the menstrual cycle. Importantly, mitotic activity is extremely rare in benign nongravid columnar epithelium. The subepithelial microvessel network of the endocervix is better developed than that of the ectocervix and the glandular structures largely shun from larger vessels and nerve bundles [2]. Furthermore, the endocervical stroma has twice as many stromal cells as does the ectocervical counterpart. The stromal cell from the two regions has different immunohistochemical features with the former showing reactivity for SMA and the latter for desmin. Moreover, the normal stroma also contains a dense network of CD34+ fibrocytes which are more predominantly located in the subepithelial and perivascular areas. In deep locations of the stroma, smooth muscle fibers are present and they are usually surrounded by CD34+ stromal cells [3].

The cervix plays important roles in the reproductive function. Not only is it involved in providing a port of entry for the sperm and a chemical barrier to infectious microorganism; the cervix also contributes to the creation of a safe haven for the implanting embryo and subsequently the growing fetus and a passageway for the ripe product of conception. The dramatic changes in the process of remodeling and ripening are effected through steroid hormones and complex epithelial–stromal interactions. It is only during the late phase of pregnancy that the endocervical glands are fully developed and manifest as lobules of tightly packed glands [4–6]. The destruction and rearrangement of collage fibers and other changes in the extracellular matrix apparently provide space for this remarkable glandular expansion. During the subsequent postpartum months, the hyperplastic glands involute gradually reverting to the baseline collateral clefts. Defective involution of some of the hyperplastic glands might result in diffuse laminar glandular hyperplasia.

Squamous cell carcinoma of the cervix develops from cervical intraepithelial dysplasia with clear HPV involvement. Well-differentiated squamous cell carcinoma is characterized by abundant keratin pearls and individual cell keratinization and a chronically inflamed stroma. Four common criteria used in the evaluation of invasion are (1) desmoplasia, (2) paradoxical maturation (excessive cytoplasmic keratinization with conspicuous nucleoli), (3) loss of cell polarity at the basal layer, and (4) and the blurring of the epithelial–stromal interface.

However, the evaluation of stromal invasion is frequently compromised by tangential sectioning, cautery effect, obscuring inflammation at the tumor border and other inflammatory process, and even glandular involvement by metaplasia or dysplasia. In order to avoid overcalling metaplasia or dysplasia involving the glandular clefts, attention should be directed to the smooth contour of the tumor nests, their cytological features in comparison to those of the surface dysplastic epithelium, and polarity of tumor cells. In difficult cases, immunostainings for CD34 and SMA hold the promise for making the distinction [7, 8]. The stromal cells of invasive squamous carcinoma lack reactivity for CD34. SMA stain lights up haphazardly arranged myofibroblasts (desmoplasia). To differentiate pseudoepitheliomatous hyperplasia from well-differentiated squamous cell carcinoma would require the appreciation of the underlying inflammatory process and shape of the invasive components and their connection to the surface and shape (jagged pointed tips and a broad base connected to the surface). In addition to the panel of p53, MMP-1, and Ki67 which is used in other squamous locations, p16 can be used [9, 10].

It has been increasingly realized that there are two pathways for cervical glandular carcinogenesis. The vast majority of endocervical adenocarcinomas are HPV related and progress through the dysplasia to carcinoma in situ progression pathway. The second pathway is independent of HPV infection and follows the pathway from lobular endocervical glandular hyperplasia to minimal deviation adenocarcinoma and mucinous adenocarcinoma [12, 13].

It is generally known that invasion should be suspected if there are nonlobular exuberant glandular growth in the form of extensive budding, confluency, cribriform, and exophytic papillary projections. However, the adenocarcinoma in situ can have complex papillary formation and even cribriform formation, and some benign glandular lesions are not lobular. Deep location, inflammatory, and desmoplasia of the stroma and presence adjacent to large vessels and nerve fibers are some important features of stromal invasion. Nevertheless, they are not always present. For instance, endocervical adenocarcinoma can also invade in expansile pattern without eliciting salient desmoplastic reaction. Some other promising indicators of stromal invasion include increased periglandular expression of SMA-positive myofibroblasts and stromal expression of CD44 and tenascin [14, 15] (Fig. 6.6).

The depth of glands is a very important index for the minimal deviation adenocarcinoma because it lacks exuberant epithelial growth and contains only focal cytological atypia. Most of the benign glands are confined to the inner 1/3 of the cervical wall. If the questionable glands extend more than 1 mm deeper than the adjacent benign ones, they are considered deep enough. This “deep concept” is not to be confused with those benign lesions usually located in the outer layer of the cervical wall. As discussed in the differential diagnosis section, some benign lesions can also be deep and even transmural.

To differentiate from those benign mimics of adenocarcinoma, a panel of immunostaining can be very helpful. The panel includes CEA, Ki67, and p16. Benign lesions usually show low mitotic activity and are negative for CEA and p16 [16, 17].

Another common dilemma in gynecological pathology is to distinguish a primary cervical adenocarcinoma from uterine extensions since several variants (such as villoglandular, endometrioid, and even mucinous) can be seen in both locations. Clues to a cervical origin include the presence of cervical dysplasia and lack of benign squamous morules and stromal foamy cells. Endocervical adenocarcinoma cells are positive of CEA and negative for ER, PR, and vimentin (the cells at the invasive front can be vimentin positive due to epithelial– mesenchymal transition) [16, 17]. Apparently, the HPV infection messes up the epithelial–stromal coupling through the steroid hormones with the resultant loss of ER and PR expressions [18–20]. Endometrial carcinoma is ER, PR, and vimentin positive and negative for CEA.

The uterus is an important organ in the reproductive system in that it is the actual site where a new life is formed and develops to term. In order to accommodate the inception, growth, and subsequent expulsion of the new life, the three major structural components of the uterus are brought under the coordinated influence of steroid hormones [27].

Endometrial carcinoma develops from endometrial hyperplasia with PTEN mutation and PAX2 downregulation. The tipping point is reached when stromal invasion is evident. In this specialized organ where the epithelial and stromal components are derived from the same stem cells, stromal invasion manifests in three different forms [34]. They are (1) confluent glandular growth (or back-to-back glands) with no evident intervening stroma, (2) complex papillary or villoglandular structures, and (3) altered stroma.

Confluent glandular growth pattern manifests as complex branching or cribriform formations so that individual glands are surrounded by minimal or nil uterine stromal component. Complex papillary and villous glands are defined as branching papillary or villous structures with a fibrovascular core. Those intraglandular papillation and delicate papillae which are commonly present in metaplastic and hyperplastic processes should be excluded. To avoid overcalls, it is essential that the integrity of the specimen be not compromised.

In the evaluation of stromal alteration as evidence for stromal invasion, the shape and arrangement of the collagen-producing stromal cells are important features. They are more elongated, wavy, and eosinophilic than the normal stromal cells and are arranged in parallel fascicles compressed by collagen fibers. Caution should be exercised so that this stromal alteration is not confused with the fibrous stroma of endometrial polyps and tissue from the lower uterine segment. Endometrial polyps seem to be monoclonal stromal proliferation with secondary epithelial growth which is usually mild and seems undisturbed by stromal component. The stromal proliferation is frequently hyalinized and contains thick-walled blood vessels. The lower uterine segment stroma, particularly that of the inferior portion, is very fibrous. However, glandular proliferation in the form of glandular crowding and branching is lacking.

The grading of endometrial carcinoma employs a different set of criteria from those of most other adenocarcinomas. Well-differentiated carcinoma is defined as a tumor which contains less 5 % solid areas and lacks high-grade nuclei. In a case where the nuclei are markedly enlarged, hyperchromatic with prominent nucleoli, a higher grade is rendered (moderately differentiated, grade 2). Moderately differentiated carcinoma typically contains solid growth involving between 5 and 50 % of the tumor. Presence of marked nuclear atypia upgrades the tumor to grade 3 (poorly differentiated). Therefore, tumor necrosis and mitotic rate are left out of the grading system. In the evaluation of solid masses, it is very important that the squamous nests and sheets be excluded.

Similar to the nonneoplastic endometrial epithelium, the transformed epithelial cells can also manifest remarkable plasticity and the tumor shows frequent squamous, mucinous, ciliated, and even secretory and sex cord differentiation. Awareness of this important phenomenon avoids mistaking the differentiated components as benign tissue or confusing them as high carcinomas (such as clear cell carcinoma and serous cell carcinoma).

Finally, endometrial carcinoma, particularly in the well differentiated, can undergo dedifferentiation [35, 36]. Together with those entities showing sex cord differentiation (with spindle cells and hyalinized stroma) or containing benign chondroid and osteoid stroma, it needs to be differentiated from malignant mesodermal mixed tumor (MMMT). Malignant mesodermal mixed tumor (MMMT) contains separable high-grade epithelial and mesenchymal components.

Uterine stromal sarcomatous cells overexpress Bcl-2 probably as a result of chromosomal rearrangement, and they typically show low mitotic activity and negligible tumor cell necrosis and/or apoptosis. In contrast to the parameters used in the evaluation of uterine leiomyomatous lesions, muscle or vascular invasion becomes the yardstick in the differentiation between a benign stromal nodule and stromal sarcoma. Typically, myometrial invasion is in the form of tentacles, but may also present as an intramyometrial nodule or rarely diffuse myometrial thickening with no evident tumor mass formation. While uterine stromal cells develop a close functional/histological relationship with arterioles, sarcomatous cells tend to invade lymphatics and veins.

Uterine stromal lesions frequently show divergent differentiations, and unfamiliarity with this important feature could lead to erroneous diagnoses. For instance, frequent sex cord elements, glandular structures, and heterologous elements can be seen. Stromal hyalinization and smooth muscle differentiation can also be present resulting in a diagnosis of mixed stromal–smooth muscle tumor.

Adenosarcoma and adenofibroma can also show sex cord differentiation and heterologous elements. They can, however, be differentiated from low-grade endometrial stromal sarcoma with glandular differentiation by the presence of their leaflike growth pattern. Adenosarcoma differs from adenofibroma in that it has periglandular condensation of stromal cells which have increased cellularity, mitotic activity, and cytological atypia [37]. This is in stark contrast to mammary phyllodes tumors in which periglandular stromal cell condensation is not a sign of malignancy. Instead, malignant phyllodes tumor is characterized by marked stromal overgrowth. Only a small fraction of adenosarcomas share this feature.

The uterine smooth muscle tissue (myometrium) is programmed to respond to steroid hormones in order to provide a muscular conglomerate for labor. During pregnancy, the uterine smooth muscle tissue undergoes tremendous increase in mass through cellular hyperplasia and hypertrophy. Understandably, there should be multiple protective mechanisms to keep this extraordinary growth potential in check in nongravid conditions. Commonly encountered uterine leiomyomas are characterized by a variety of simple genetic changes involving multiple pathways (mechanisms) [38]. The multiple involved pathways might account for the many variants of uterine leiomyoma. Some of them show concerning features and they include mitotically active, cellular, atypical (with bizarre nuclei), dissecting, vascular invasion and even metastasizing variants.

The predominant view holds that most leiomyosarcomas arise de novo and they contain far more complex cytogenetic changes than leiomyomas. These complex chromosomal abnormalities lead to genomic instability and knockdown of the protective mechanisms, allowing aggressive growth, recurrence, and frequent metastasis.