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| Year : 1999 | Volume
: 47
| Issue : 3 | Page : 178-81 |
Ultrastructural changes in medullomyoblastoma. Similarities with foetal rhabdomyoma.
Dastur DK, Manghani DK
Department of Neuropathology, Medical Research Centre, Bombay Hospital, Mumbai, 400020, India.
Correspondence Address:
Department of Neuropathology, Medical Research Centre, Bombay Hospital, Mumbai, 400020, India.
The light and electronmicroscopic changes are described in two cases of medullomyoblastoma, and compared with the changes seen in a case of foetal rhabdomyoma. The medullomyoblastomas in two children aged 8 and 5 years, consisted predominantly of classical type of medulloblastoma cells, along with few to many 'strap cells' or 'myoid cells' which, on closer examination, showed clear cross striations, consistent with muscle fibres or myofibrils. The primitive myoid cells were similar to those encountered in larger numbers in a post-auricular rhabdomyoma, possibly of foetal origin in a 40 day old infant. The four pathogenetic mechanisms i.e. (i) an embryonal stage of myofibrillar differentiation; (ii) a malformative factor; (iii) a teratoid factor on account of the presence of mesenchyme derived striated muscle tissue in the obviously predominant ectodermal medulloblastoma; and (iv) metaplasia of the vascular smooth muscle cells in the medullomyoblastoma, are discussed.
How to cite this article:
Dastur D K, Manghani D K. Ultrastructural changes in medullomyoblastoma. Similarities with foetal rhabdomyoma. Neurol India 1999;47:178 |
This very interesting and unusual entity appears to represent a differentiation of the typical medulloblastoma cells to myoid and myofibrillar elements with cross striations, well described by Russell and Rubinstein[1] and Burger and Scheithauer.[2] Among the earlier reports of the light and electronmicroscopic changes of cerebellar medullomyoblastoma, appear to be those of Walter and Brucher[3] and Duinkerke et al.[4] Adults are rarely affected.[5]
In our material of 179 medulloblastomas for all ages (out of a total of 5000 intracranial space occupying lesions). 108 were in children. Of these, the two medullomyoblastomas being described here were in male children aged 8 years and 5 years respectively.
Representative portions of the formalin-fixed specimen were processed and embedded in paraffin, and 6mm thick sections were cut. Haematoxylin-eosin, Picro-Mallory, Gomori's reticulin, phosphotungistic acid haematoxylin (PTAH) stains were carried out. The tissue was cut into small pieces and fixed in chilled 4% glutaraldehyde in Millonig's phosphate buffer (pH 7.4). It was then treated with 1% osmium tetroxide in Millonig's buffer, dehydrated in a graded series of alcohol, followed by propylene oxide, embedded in araldite and cut on a Reichert OMU2 ultramicrotome. Semithin (1mm thick) sections were examined, and then thin (silver-grey) sections were cut and stained with uranyl acetate and lead citrate and examined and photographed on a Philips 410 electronmicroscope.
Clinically both patients presented with signs of posterior fossa intracranial space occupying lesions (ICSOL), viz. headache and vomiting. The second child also complained of giddiness and manifested marked nystagmus to the left. A CT scan of the second child gave a clearer evidence of a midline mass involving the vermis and the IVth ventricle. Surgically this was confirmed and the tumour was found adherent to the medulla oblongata also.
On gross examination both tumours were greyish-white in colour and soft to feel, with a small area of necrosis in the specimen of the second case. Histopathologic examination of paraffin sections and semithin araldite sections showed a cellular tumour consistent with a classical type of medulloblastoma (Inset, [Figure 1]), the tumour cells having dense circular or irregular nuclei and very scanty cytoplasm, with occasional mitotic figures (arrow, [Figure 2]). Among these cells, the most striking feature was the presence of strands or `straps' of parallel running mesenchymal tissue. Closer examination, even of routine H&E stained sections, revealed sharp cross striations in many of these strands, confirming them to be striated muscle fibres [Figure 1], and both the tumours to be a `medullomyoblastoma'. Both the specimens evidenced some astrocytic and/or oligodendroglial differentiation.
In PTAH stained sections, the tumour in the second case showed a highly pleomorphic appearance with the presence of thin parallel running strands, some with well formed cross striations [Figure 3]. Also present were bulbous cells with short thick processes which suggested the development of myotubes (arrows, [Figure 3]). In this tumour there were large circular or ovoid cells with peripherally disposed two or more nuclei (*,[Figure 3]). Oil immersion examination revealed clear cross striations in different planes, in the cytoplasm (*Inset, [Figure 3]). Most of the medulloblastoma cells were positive for neuron specific enolase (NSE) using an appropriate antibody marker [Figure 4].
Electronmicroscopic study of this tumour showed the typical medulloblastoma cells with large indented nuclei and scanty cytoplasm [Figure 5]. Among these cells were dark strands with parallel running myofibrils with abortive Z-lines (arrow, Inset, [Figure 5]).
Another noteworthy feature was the proximity of tumour cells bearing myofibrils (arrows, [Figure 6]) to small blood vessels (*[Figure 6]). On electronmicroscopy such cells showed clear actin and myosin filaments (Inset, [Figure 6]).
Fine structural examination of the medullomyoblastoma has been reported.[6],[7] Medullomyoblastomas occur most frequently in male children as in present two cases. This has been the observation of other authors as well.[6],[7],[8] The myoblastic element including frankly striated muscle fibres, also called strap cells, can occur in medulloblastomas of both the classical and desmoplastic varieties.[2] The medullomyoblastoma often combine a monomorphic small cell neuroectodermal element with foci of rhabdomyoblastic differentiation. Densely cellular neuroectodermal tumours with myoblastic differentiation, but arising from the cerebrum, have also been described.[9]
A point of interest was the similarity of the fine structural changes in these medullomyoblastomas, with the electronmicroscopic findings earlier reported by us on the tumour in a 40 day old female infant. This was a postauricular rhabdomyoma, probably foetal in origin.[10] This is a very rare tumour occurring at different sites and reported in infants, with the largest series of 9 cases by Dehner et al.[11] On the basis of a close resemblance of these muscle cells to developing myoblasts at 6-10 week stage of embryonic life, Dehner et al[11] called these tumours `foetal rhabdomyomas'. In their cases, as in those published by a few others,[12] including our own,[10] we were impressed by the immature mesenchymal cells differentiating into cells showing increasing morphologic features of striated muscle fibres. In these infantile tumours, unlike the medullomyoblastomas, there was no other neoplasm beyond the developing rhabdomyoma.
The stages of formation of the myofibrils in this case are illustrated in [Figure 7]. Developing myotube or rhabdomyoblast with a growth cone at one end (arrow, [Figure 7a]) were seen amidst parallel running strands of myofibrils. [Figure 7b] shows the cell on the right bearing irregular clumps of osmiophilic ill-defined myofibrillar material (*); and on the left a cell with a dense aggregate of mature parallel running myofibrils and a basement membrane. The inset in [Figure 7b] shows a few but obviously maturer myofibrils with clear A and I bands, the latter often bearing Z-lines (arrows). Thus this case of foeta rhabdomyoma suggests how even in medullomyoblastomas the mesenchymal tissue could be the source of these developing muscle fibres. This tumour might represent an earlier embryonal stage of myofibrillar differentiation than the well formed myofibres with clear cross striations, as seen in our current two cases of medullomyoblastoma, where too an embryonal origin has to be kept in mind. Embryonal tissues are known to display a wider potential for differentiation than adult tissues.[1],[13] Haltia et al[14] reported 90% of muscle fibres in the normal new born rats to be of the foetal type, containing myotubules only; 10% being larger type-I fibres. They found that only in the final stage of development there were recognisable A and I bands and Z-lines (comparable to inset in [Figure 7b] of our present paper).
While overt teratomatous features were not present in either of our medullomyoblastoma cases, unlike those of Banerjee and Kak[15] and of Choudhury et al,[16] a `teratoid' element is introduced into the argument about the genesis of the medullomyoblastoma, by the presence of mesenchyme-derived striated muscle tissue in the obviously predominant ectodermal medulloblastomatous tumour. Another pathogenetic mechanism considered by Russell and Rubinstein1 is of a malformative factor coexisting with the rhabdomyoblastic differentiation in the genesis of medullomyoblastoma.
The occasional presence of these aberrant striated muscle fibres in the vicinity of small blood vessels in the medullomyoblastoma [Figure 6], and in the foetal rhabdomyoma [Figure 7a], has raised the possibility of metaplasia of the vascular smooth muscle cells leading to the formation of striated muscle, as first suggested by Marinesco and Goldstein,[17] and recently endorsed by Walter and Brucher[3] and Russell and Rubinstein.[1]
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