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Journal of the Anatomical Society of India

Microscopic Anatomy of Inferior Medullary Velum Of Cerebellum

Author(s): Arora, N.K.

Vol. 51, No. 1 (2002-01 - 2002-06)

Department of Anatomy, Government Medical College, Chandigarh INDIA.

Abstract

A study of the inferior medullary velum (IMV) has been done using two routine histological staining techniques viz. Weigert's haematoxylin and eosin, and PTAH. The study brings out the salient features of IMV in the form of definite layers of neuroglia, white matter and nerve cells underneath the ependyma. Cellular layer consists of rounded cells, with usual nerve processes passing out from them, similar to the cells of granular layer of cerebellar cortex. Deeper is a layer of large, irregular elongated cells with indistinct nuclei and hazy cellular outline, present as a single row of solitary cells. Each of these cells appears at the periphery of a clear space. No Purkinje cells are present. Deep to the above is a layer of white matter of the cerebellum. The piamater forming the tela choroidea of the fourth ventricle does not cover the surface of IMV but gets attached to its anterior free border. The ependymal layer is made up of a single layer of cells varying from squamous to cuboidal type. Cilia are not seen anywhere.

Key words: Ependyma; cerebellar cortex; Purkinje cells; stratum granulosum.

Introduction:

Each Inferior medullary velum (IMV) stretches between the inferior cerebellar peduncle laterally and nodule and pyramid medially. It forms lateral part of the floor of fastigial recess of fourth ventricle and the roof of tonsillar fossa. The free anterior margin of IMV is continuous with the double layer of piamater of tela choroidea of fourth ventricle. A hairpin has been placed deep to it in Figure I showing free anterior margin as well as translucent nature of the velum. The present study has been undertaken because of sketchy descriptions in the textbooks of anatomy. It is merely described as "an exceedingly thin sheet, entirely devoid of nervous tissue and formed by the ventricular ependyma" (Davies and Davies 1962) or as "a thin sheet of white matter" (McMinn, 1990) Textbooks of histology do not give any description of the IMV as such.

Material and methods :

Inferior medullary vela from fresh cadavers were removed and fixed in 10% neutral formalin. Material was also removed from formalin-fixed cadavers and further fixed in 10% neutral formalin. Sixteen specimens were studied from eight cerebellums, and were dehydrated in ascending grades of alcohol, cleared in xylene, given two changes of ceresin wax and finally embedded in the same. Serial sections were cut at 7 mm thickness, mounted and fixed to slides and dried in the incubator at 39°C for 24 hours. The slides were stained by :

  1. Weigert's iron haematoxylin and eosin,
  2. Phosphotungstic and haematoxylin (PTAH) and
  3. Ranson's silver technique.

Results:

The microscopic appearance of the IMV as seen in the present study was quite at variance with the few descriptions available in the standard textbooks of anatomy.

Transverse sections of IMV stained with PTAH and Weigert's haematoxylin showed the following well defined layers (Fig.2) :

  1. On the surface, there was a layer of ependyma, consisting of a single layer of cells varying from flat squamous (Fig.3) to cuboidal shape (Fig.4) in different specimens though in specimens from the same cadaver, the cells were of the same type on both the sides.
  2. Deep to this was a layer consisting of nerve fibres running more or less transversely (since the sections have been cut in a coronal plane). These fibres were more densely packed in the deeper stratum, whereas there were wide spaces between them on the superficial side (Fig. 3). Small stellate cells scattered here and there were more conspicuous in the superficial stratum of this layer.
  3. Deep to the nerve fibre layer was a thick layer of nerve cells. These cells showed the typical features of stratum granulosum of cerebellar cortex with some minor differences. Firstly, the neuroglial network over which these cells were placed was easily visible and the cells were not so compactly packed as in cerebellar cortex. Secondly, there was a wider variation in the morphology of the cells as against their uniform nature in the cerebellar cortex. These cells varied from rounded to stellate in shape and were distinctly multipolar at places. Each cell showed the characteristic feature of a dark staining basophilic compact nucleus having very little cytoplasm around it.

A peculiar feature of this layer was the presence of large cells about 20 to 30 mm in size. Even under low magnification, they were very conspicuous (Fig. 2). Each cell had a hazy outline and the nucleus was not seen distinctly. These cells showed the following characteristics.

  1. They were located mainly at the junctional zone with deeper white stratum, but at times were present at other places also.
  2. These cells appeared as a rather elongated mass of cytoplasm with indistinct processes. Also, there was a considerable variation in their outline varying from round (less often) to oval (usual) and even quite elongated at times.
  3. Another feature was their location to one side of a clear and distinct pericellular space.

At first sight, these cells gave an impression of attenuated Purkinje cells, but on closer examination, their morphology and specially their location in the junctional area between the granular layer and deeper white matter ruled them out as Purkinje cells. These large cells, mainly arranged in a single layer were never in groups and were separate from one another. In very few instances, however, such cells were also seen among the deeply staining small cells of molecular layer. Figures 5 and 6 show the above features under high power.

  1. The deepest layer resembled white matter of cerebellum. This layer showed eosinophilic nerve fibres with Schwann cells around them. The outer surface of IMV was covered by a single layer of flattened cells (Fig. 2).

No Purkinje cells were seen in between the superficial nervous layer and the subjacent molecular layer and Purkinje cells are such a prominent feature of any section across cerebellar cortex that it is impossible to miss them.

Ranson's silver technique did not give any positive results.

Discussion:

No specific work on the histology of IMV has been reported. Most of the standard textbooks of histology have only cared to mention the general histology of cerebellar cortex with no specific reference to IMV. Standard textbooks of neuroanatomy have, however, given a detailed account of the histology of the cerebellar cortex but no reference could be found in these books, specifically about the histology of IMV.

Even the cursory description of IMV in most books differs widely in the concept of this layer. Williams et al (1995) in Gray's Anatomy, describes it as follows: "over most of its extent it consists of an exceedingly thin sheet, entirely devoid of nervous tissue and formed by ventricular ependyma "(Davies and Davies, 1962) and the description is essentially the same in its later editions (Williams et al., 1989 and Williams et al., 1995). Last's Anatomy simply describes it as "a sheet of white matter" (McMinn, 1990) Even this sketchy reference has been deleted in its latest edition (Sinnatamby, 1999). In face of such sparse descriptions, the present work does bring to light many interesting observations on the histology of this small membranous part of the cerebellum.

The ependyma covering IMV varies from simple epithelial layer of cuboidal to flattened squamous cells.

The presence of neuroglial layer immediately deep to the ependyma is also understandable, though its clear cut stratification into a superficial very loosely knit layer and a deeper stratum where fibres are more compact, is a point not made out earlier. In this layer, there are scattered cells seen quite distinctly in PTAH stain.

An interesting finding of this study is the presence of a rich cellular layer deep to the neuroglial layer. Majority of the cells in this layer are more or less like the cells of the granular layer of the cerebellar cortex.

Another important feature is not only the complete absence of Purkinje cells but also the presence of large cell of a hazy appearance with nucleus not seen and invariably located in the vicinity of a clear space. In fact it is not even possible to say if these large masses are cells-since nuclear morphology is nowhere visible-or something akin to "Eosin bodies" seen as a usual feature of the cerebellar cortex in general, but the "bodies" seen in the present study take up basophilic stain both in Weigert's haemotoxylin and eosin, and PTAH stains. This layer of cells has not been described in any earlier work since all have considered the IMV to be either a thin ependymal sheet supplemented by some neuroglia (and after all ependyma is but a part of neuroglia itself) or just a layer of "white matter".

References:

  1. Davies, D.V. and Davies, F.: Gray's Anatomy. 33rd Edn; Longman London. p.1006. (1962)
  2. McMinn, R.M.H.: Last's Anatomy: Regional and Applied. 8th Edn; Churchill Livingstone. Edinburgh. p.618. (1990)
  3. Sinnatamby, C.S.: Last's Anatomy: Regional and Applied. 10th Edn; Churchill Livingstone. Edinburgh. (1999)
  4. Williams, P.L., Warwick, R., Dyson, M. and Bannister, L.H.: Gray's Anatomy. 37th Edn; Churchill Livingstone. Edinburgh. p.963. (1989)
  5. Willams, P.L., Bannister, L.H., Berry, M.M., Collins, P. Dyson, M., Dussek, J.E. and Ferguson, M.W.J.: Gray's Anatomy. 38th Edn; Churchill Livingstone. Edinburgh. p.1032. (1995)

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Fig. 1 Dissection of IMV. A hairpin has been placed deep to it.

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Fig. 2 Photomicrograph showing layers of IMV. Epd, ependyma; NF, nerve fibres; LNC, nuclear cell layer; LC, large cell; LWm, layer of white matter. PTAH, x 100.

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Fig. 3 Photomicrograph showing layers of IMV. SqC, simple squamous cells of ependyma; StC, stellate cell; NF, nerve fibres; LNC, nuclear cell layer. Weigert's iron haematoxylin, x 400.

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Fig. 4 Photomicrograph showing layers of IMV. CuC, simple cuboidal cells of ependyma; NF, nerve fibres; LNC, nuclear cell layer. Weigert's iron haematoxylin, x 400.

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Fig. 5 Photomicrograph of IMV. LNC, nuclear cell layer; PcS, pericellular space; LC, large cell. PTAH, x 400.

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Fig. 6 Photomicrograph of IMV. LNC, nuclear cell layer; PcS, pericellular space; LC, large cell. Weigert's iron haematoxylin, x 400.

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