Carcinoma in situ (CIS) of the testis is the pre-invasive stage of type II testicular germ cell tumours (TGCTs) of adolescents and adults. These tumours are the most frequently diagnosed cancer in Caucasian adolescents and young adults. In dysgenetic gonads, the precursor of type II GCTs can be either CIS or a lesion known as gonadoblastoma (GB). CIS/GB originates from a primordial germ cell (PGC)/gonocyte, ie an embryonic cell. CIS can be cured by local low-dose irradiation, with limited side effects on hormonal function. Therefore, strategies for early diagnosis of CIS are essential. Various markers are informative to diagnose CIS in adult testis by immunohistochemistry, including c-KIT, PLAP, AP-2gamma, NANOG, and POU5F1 (OCT3/4). OCT3/4 is the most informative and consistent in presence and expression level, resulting in intense nuclear staining. In the case of maturational delay of germ cells, frequently present in gonads of individuals at risk for type II (T)GCTs, use of these markers can result in overdiagnosis of malignant germ cells. This demonstrates the need for a more specific diagnostic marker to distinguish malignant germ cells from germ cells showing maturation delay. Here we report the novel finding that immunohistochemical detection of stem cell factor (SCF), the c-KIT ligand, is informative in this context. This was demonstrated in over 400 cases of normal (fetal, neonatal, infantile, and adult) and pathological gonads, as well as TGCT-derived cell lines, specifically in cases of CIS and GB. Both membrane-bound and soluble SCF were expressed, suggestive of an autocrine loop. SCF immunohistochemistry can be a valuable diagnostic tool, in addition to OCT3/4, to screen for precursor lesions of TGCTs, especially in patients with germ cell maturation delay.

Certain patients with disorders of sex development (DSD), who bear Y chromosome material in their karyotype, are at increased risk for the development of type II germ cell tumors (GCT), which arise from early fetal germ cells. DSD gonads frequently harbor immature germ cells which express early fetal germ cell markers. Some of them (e.g. OCT3/4 and NANOG) seem to be of pathogenetic relevance in GCT development providing cells with the ability of pluripotency, proliferation and apoptosis suppression. Also TSPY (testis-specific protein Y-encoded), the main candidate for the so-called gonadoblastoma locus on Y chromosome, is overexpressed in germ cells of DSD patients and possibly contributes to their survival and proliferation. Nowadays, the use of immunohistochemical methods is highly relevant in identifying DSD gonads at risk. The risk for GCT development varies. While the prevalence of GCT is 15% in patients with partial androgen insensitivity, it may reach more than 30% in patients with gonadal dysgenesis. Patients with complete androgen insensitivity and ovotesticular DSD develop malignancies in 0.8% and 2.6% of cases, respectively. However, these data may be biased for various reasons. To better estimate the risk in individual groups of DSD, further investigations on large patient series are needed.

The transcription factors SOX9 and FOXL2 are required for male and female mammalian gonadal development. We have used specific antibodies to investigate the role of these key proteins in disorders of sex development (DSD), specifically inter-sex states. In normal gonads, SOX9 was found to be restricted to the presence of (pre-)Sertoli cells, while FOXL2 was found in granulosa cells, and in stromal cells interpreted as early ovarian stroma. Both proteins were found within a single patient, when testicular and ovarian development was present; and within the same gonad, when both differentiation lineages were identified, as in ovotesticular DSD (ie hermaphrodite). Especially SOX9 was informative to support the presence of early testicular development (ie seminiferous tubules), expected based on morphological criteria only. In a limited number of DSD cases, FOXL2 was found within reasonably well-developed seminiferous tubules, but double staining demonstrated that it was never strongly co-expressed with SOX9 in the same cell. All seminiferous tubules containing carcinoma in situ (CIS), the malignant counterpart of a primordial germ cell, ie the precursor of type II germ cell tumours of the testis, seminomas and non-seminomas, showed the presence of SOX9 and not FOXL2. In contrast, gonadoblastomas (GBs), the precursor of the same type of cancer, in a dysgenetic gonad, showed expression of FOXL2 and no, or only very low, SOX9 expression. These findings indicate that gonadal differentiation, ie testicular or ovarian, determines the morphology of the precursor of type II germ cell tumours, CIS or GB, respectively. We show that in DSD patients, the formation of either ovarian or/and testicular development can be visualized using FOXL2 and SOX9 expression, respectively. In addition, it initiates a novel way to study the role of the supportive cells in the development of either CIS or GB.

Malignant germ cell tumor (GCT) formation is a well-known complication in the management of patients with a disorder of sex development (DSD). DSDs are defined as congenital conditions in which development of chromosomal, gonadal, or anatomical sex is atypical. DSD patients in whom the karyotype – at least at the gonadal level – contains (a part of) the Y chromosome are at increased risk for neoplastic transformation of germ cells, leading to the development of the so-called ‘type II germ cell tumors’. However, tumor risk in the various forms of DSD varies considerably between the different diagnostic groups. This contribution integrates our actual knowledge on the pathophysiology of tumor development in DSDs, recent findings on gonadal (mal)development in DSD patients, and possible correlations between the patient’s phenotype and his/her risk for germ cell tumor development.

Aspects of the biopsy of the testis from the pathologist's point of view are discussed. Direct enzyme-histochemical staining for alkaline phosphatase (dAP) on frozen sections of biopsies taken during operation is a useful diagnostic tool to aid surgeons in testis-sparing surgery. Biopsy of the contralateral testis for the diagnosis of carcinoma in situ (CIS) in patients with a testicular germ cell tumour is not standard of care in most countries because of the high rate of negative biopsies. Based on risk factors for germ cell tumours, i.p. microlithiasis, a patient population is defined in which the rate of CIS in the contralateral biopsy is about 25%. It is reiterated that the diagnosis of CIS in testicular biopsies requires expertise, and should not be carried out without immunohistochemistry for markers for CIS. As OCT3/4 is increasingly used as marker, it is important to be aware that it may be false-negative in biopsies fixed in Bouin's or Stieve's fixative. Preliminary results are presented on a series of biopsies from cryptorchid testes in infants and children allowing the definition of morphological and immunohistochemical criteria for delayed maturation of gonocytes and pre-CIS.