A common thread in the constellation of features found in this case is the failure of structures normally bilaterally positioned in the term neonate to separate from the midline during development. This is seen, for example, here in the failure of the brain to establish lateral hemispheres and in the globes to become separate eyes. The retained fusion of bilateral structures may be regarded as a defect in the establishment of polarity. In view of the increasing appreciation of embryonic induction and Hedgehog signaling in these processes, we wanted to explore whether such mechanisms could be extended to the tissue level allowing for an explanation of the ocular changes seen in human cyclopia.
The autopsy findings are consistent with the diagnosis of Patau's syndrome. This conclusion is confirmed by the cytogenetic demonstration of trisomy 13 in the patient. The etiology of trisomy 13 is unknown. The clinical presentation is typical except for the young age of the mother (19 years). Trisomies tend to occur with greater frequency with advanced age [31
]. The age effect is not as great with trisomy 13 compared to 18 and 21 [32
]. Trisomy 13 is more common in females [33
]. It should be noted that in contrast to trisomy 21, the risk for recurrence of trisomies 18 and 13 is considered to be very low [34
]. The phenotype in this case represents a more severe end of the spectrum with the manifestation of holosproencephaly and cyclopia.
In theory, cyclopia could result from the fusion of two originally separate eyes, or from the failure of a single primordium to separate during development. Microdissection studies have shown that removal of the prechordal mesoderm leads to the formation of a single retina in chick embryos and Xenopus
]. For example, removal of the prechordal plate results in fusion of the forebrain as well as the retina [36
]. Furthermore, the prechordal plate expresses Shh and was able to rescue the cyclopic phenotype in transplantation studies. Interestingly, transplantation of the prechordal plate to the vicinity of the optic cup suppresses the expression of pax6
in the retina. We can conclude that there is a single retinal morphogenetic field that resolves into two retina primordial. This is accomplished by a prechordal-plate signal that suppresses retinal formation in the medial region of the field [36
]. This model is consistent with that suggested more than 75 years ago by Adelmann (1929) [37
In the present case, a disruption of the normal sequence of development appears to have occurred after lens induction, but before the formation of the cornea and angle structures, suggesting an interruption of neural crest migration. Normally, the corneal stroma, endothelium, anterior iris, and trabecular meshwork are formed by neural crest cells entering the optic-cup lip. Although the reason for the defective neural crest migration is unknown, it is interesting that inhibition of Shh in vivo
results in neural crest cell death [10
]. The absence of the corneal endothelium, which promotes formation of the anterior chamber, could explain the lack of separation of the lens from the cornea [38
Insight into the role that abnormal Hedgehog signaling may have in the histogenesis of retinal dysplasia has come from studies of Ptch
mutations cause Gorlin syndrome, an autosomal dominant disorder characterized by dental, skeletal and radiographic abnormalities including falx calcification, bifid/fused ribs and altered vertebral segmentation, and a predisposition to tumor development including early-onset basal cell carcinomas [39
]. Ocular abnormalities are often present as were in the first patient described by Dr Gorlin [40
]. These findings include microphthalmia, coloboma, cataract, inappropriate retinal myelination, and retinoschisis [45
]. It is therefore intriguing that the Hedgehog pathway has not only a role in establishing separate eyes, but also in the development of the retina itself. This is further supported by studies in zebrafish indicating that signals from the prechordal mesoderm can effect retinal neurogenesis [51
Our finding that Shh is found in the cytoplasm of the retinal-ganglion cells in humans is consistent with recent studies showing that its mRNA is expressed in the ganglion cells of other vertebrates [52
]. Our finding of Shh positive cells in the inner nuclear layer is consistent with its known expression in zebra fish amacrine cells [55
]. The significance of these observations is that Shh secretion by the ganglion cells appears to be critical to orchestrating the normal organization of the developing neural retina [52
]. This notion is supported by the fact that ptch
are expressed in retinal neuroblasts, and astrocyte precursor cells in the optic nerve [62
]. Retinal-ganglion cell derived Shh expression is required for Hedgehog target gene induction in the retina and optic nerve [56
Notwithstanding the potential importance of the ganglion cells in these processes, it should be pointed out that the system is complex with other local sources and targets of Hh
]. In mouse, Ihh
is expressed by RPE, and recombinant Hh protein promotes photoreceptor differentiation in vitro [57
]. In zebrafish, Shh
are both expressed by RPE, and Hh
signaling knockdown results in reduced photoreceptor differentiation, and retinal cell death [65
]. Finally, the RPE itself may need Hh
signaling in order to properly differentiate [67
To address the question of the role of the Hedgehog pathway and the ptch
receptor in particular on the development of the mammalian retina, Black et al (2004) [68
] studied ocular development in mice heterozygotic for disruption of the ptch
]. The retinas of PtchlacZ +/- mice exhibit abnormal cell cycle regulation, culminating in photoreceptor dysplasia and Müller cell gliosis. Interestingly, the PtchlacZ +/- mice also show vitreoretinal abnormalities resembling those found in patients with Gorlin syndrome and dysplastic rosette formation.
The histopathology of cyclopia appears to be more complex than what may have been previously appreciated. In fact, the terms "cyclopia" and "synophthalmia" do not reflect the histopathology or pathophysiology. True cyclopia, i.e., a single eye, is vanishingly rare, with most cases showing two fused eyes (synophthalmia). However, synophthalmia does not accurately reflect the mechanism resulting in the fusion of the eyes. That is, the fusion is not the result of two eyes coming together, but rather the failure of the eyes to form separately during development. According to the animal studies reviewed above, the defect is a failure of the Shh pathway to inhibit competence in the central eye field. The result is an incomplete separation of the eyes during development. At the tissue level, the complexity extends to a failure of the angle structures to form, and the retina to achieve its orientation toward the RPE.
Dysplastic rosettes are laminated retina failing to establish a polarized orientation toward the RPE resulting is the formation of tubules. The finding, which is not considered to be neoplastic or preneoplastic, is often associated with trisomy 13, but may be seen in other clinical [28
] and experimental settings. The rosettes should be distinguished from the various rosettes seen in retinoblastoma which do not contain Müller cells [30
]. The rosettes found in the dysplastic retina are fundamentally different than those of retinoblastoma, in that unlike their neoplastic counterpart they are composed of a variety of differentiated cell types.
The mechanism responsible for the histogenesis of retinal dysplasia is far from clear. The application of the term "retinal dysplasia" to describe a "maldevelopment of the retina" and early descriptions of its rosettes have been reviewed by Lahav et al. (1973) [28
]. It has been proposed that the rosettes may represent an abortive attempt at regeneration [75
], or an abnormality in programmed cell death [73
]. Although the mechanism for the formation of the rosettes remains unknown, it is interesting that retinal dysplasia including rosette-like formations are found in Patch
+/- mice and in conditional ablation of Shh
]. In the present case, offering a mechanism for the histogenesis of the retinal dysplasia should be done with caution. That is, we cannot with certainty establish a cause-effect relationship between the absence of Shh expression and the formation of the rosettes. For example, it is possible that the lack of Shh staining could simply be a secondary effect due, for example, to the absence of the cells producing the protein. Thus, although alternative models should be taken into account, our findings, taken together with the available literature, suggest the possibility that the dysplastic rosettes in human cyclopia result from a direct or indirect failure of Hedgehog signaling.