Journal of Rawalpindi Medical College (JRMC); 2015;19(1):67-70

Original Article

Morphology of Developing Human Occipital Squama

Saadia Muzafar, Mamoona Hamid, Nabeela Akhter Department of Anatomy, Rawalpindi Medical College, Rawalpindi.

Abstract bones known as interparietal bones or Inca bones (Type 1-V). Background: To study the limits and ossification of developing human occipital bone. Methods: In this descriptive study of the Introduction development of Occipital squama, 30 foetal skulls Occipital squama has two parts cartilaginous supra- were selected. They were grouped as AG and Am. occipital and membranous interparietal. There is some Gross examination was carried on 15 foetal skulls controversy in literature concerning the limits and (Group AG ) with a pre-natal age of 8-20 weeks, A ossification of the membranous part of occipital strip was cut 1.5 cm above the lambdoid suture and squama, known as interparietal, in man. Most authors carried along the line of suture till the foramen have stated that portion above the superior nuchal magnum. After examination of un-stained lines ossifies in membrane. 1-4 Others have an opinion specimens under dissecting microscope, the that area above the highest nuchal line is membranous specimens were stained with Alizarin Red -S and in origin. 5-8 Regarding the area between the two Toluidine blue method for gross staining of calcium. nuchal lines, (also known as lamella triangularis or Fifteen foetal skulls of (Group Am) were selected intermediate segment) most of the researchers believe for microscopy and circular strip approximately 3mm that it is membranous in origin, but it never separates 2 was cut in a plate like fashion in radius of 5mm from cartilaginous supra-occipital. Hence supra- with external occipital protuberance as a centre of occipital part of occipital squama has a dual origin.5-12 cutting area. It was confirmed that they did not have There are different views among the anatomists any congenital or artificial deformity. The regarding the ossification centres of this region. Some specimens were fixed in 10% neutral buffered have observed that membranous part of occipital formalin, decalcified in 2% nitric acid, dehydrated squama ossified from two centres. 5-7,13 Some think with graded alcohols cleared in inhibisol and that three ossification centres are involved 1-4,15-16. embedded in paramat. The sections were placed There is also a view that sometimes additional centres vertically in the block with the side facing lambdoid suture anteriorly and the side facing foramen appear at the upper part of lambdoid region leading to 5-8,13-16 magnum posteriorly. Sections were cut at 500µm formation of pre-interparietal bone. These interval and stained with haematoxylin and eosin. observations were mainly performed on adult skulls. The process of ossification in occipital squama was After examining the foetal skulls Srivastava et al studied regarding limits, time and type of (1992) concluded that intra- membranous part of ossification centres . occipital squama ossified from three centres1 (Figure Results: Occipital squama was seen to consist of 1). two parts, supra-occipital and interparietal. Supra- The membranous part develop above the superior occipital has dual origin. The upper part of nuchal lines from three pairs of centres, intermediate cartilaginous supra-occipital between superior and segment (that is the part of supra-occipital) is formed highest nuchal lines known as intermediate segment from first pair. The interparietal part develops above or Torus Occipitalis, ossified intra-membranously the highest nuchal line from two pairs of centres. from two nuclei. The interparietal part (also intra- Second pair (having medial and lateral nuclei) forms membranous in origin) ossifies above the highest lateral plate and is separated from intermediate nuchal line from two pairs of nuclei forming medial segment by lateral fissure. Above it third pair having and lateral plates respectively. upper and lower nuclei forms medial plate of Conclusion: The defect in the fusion of ossifying interparietal part of occipital squama. The two parts centres results in variation in developmental of medial plate are separated from each other by process:leading to formation of one or more separate median fissure: that is later obliterated after 12 weeks.

67 Journal of Rawalpindi Medical College (JRMC); 2015;19(1):67-70

paramat. The sections were placed vertically in the block with the side facing lambdoid suture anteriorly and the side facing foramen magnum posteriorly. Sections were cut at 500µm interval and stained with haematoxylin and eosin. The process of ossification in occipital squama was studied regarding limits, time and type of ossification centres 18.

Figure 1: Diagrammatic representation of centres and their Results nuclei in the membranous part of the occipital bone above The process of ossification of occipital squama was the supra-occipital (SO) bone. Paired centres of the first studied under dissecting microscope. It was seen intermediate segment (IS), medial and lateral nuclei of the that supra-occipital bone under the superior nuchal 2nd pair of centres (II), and upper and lower nuclei of the line was flexible and had smooth appearance and 3rd pair of centres (III). The intermediate segment is separated from the lateral plate by the lateral fissure (LF) lacked in bony trabecalue pointing to its cartilaginous origin, in contrast, above it, intra- membranous part Srivaslave HC negated the concept of previous till highest nuchal line had a rough network of bony researchers regarding pre- interparietal bones, trabeculae (Figure 2). according to whom they are formed by additional After staining by Alizarin -Red S and microscopy it fourth pair of nuclei.13,14, 21 When upper nuclei of third was further confirmed that the supra-occipital part of pair join with each other and fail to join with the rest occipital squama has dual origin (Figure 3). On each of bone it forms interparietal or Inca bone that is the side of midline, these centres were membranous in part of interparietal and is not a separate entity.1-6 origin, extended laterally and fused with each other Interparietals should not to be confused with the and then with supra-occipital. These canters formed sutural bones developing from their own ossification intermediate segment or lamella triangular, which lies centres and found outside the limits of the between superior and highest nuchal line. A second interparietal part of occipital squama. pair appeared above them, each having two nuclei, forming the lateral plate of interparietal part (Figure Material and Methods 4). Lateral plate was separated from intermediate In this descriptive study of the development of segment by lateral fissure. The remaining medial portion of interparietal Occipital squama, 30 foetal skulls were grouped as AG ossified from third pair, having two centres each, and Am. Gross examination was carried on 15 foetal upper and lower. No additional centres were seen skulls (Group AG ) with a pre-natal age of 8-20 weeks, A strip was cut 1.5 cm above the lambdoid suture and anterior to third pair (Figure 5).Microscopic studies carried along the line of suture till the foramen showed intra-cartilaginous ossification below the magnum. After examination of un-stained specimens superior nuchal line and intra- membranous origin under dissecting microscope, the specimens were above it (Fig 6-8). stained with Alizarin Red -S and Toluidine blue method for gross staining of calcium.17 Fifteen foetal skulls of (Group Am) were selected for microscopy and circular strip approximately 3mm was cut in a plate like fashion in radius of 5mm2 with external occipital protuberance as a centre of cutting area. The specimens were taken from different hospitals affiliated with Rawalpindi Medical College, Rawalpindi. It was confirmed that they did not have any congenital or artificial deformity. The specimens Fig 2: Eexternal aspect of an early 3rd month fetus. Supra- were fixed in 10% neutral buffered formalin, occipital (SO), ossification centres. (1&2.), medial fissure decalcified in 2% nitric acid, dehydrated with graded (MF), lateral fissure (LF), interparietal (IP), supra-occipital alcohols cleared in inhibisol and embedded in (SO), intermediate segment (IS).

68 Journal of Rawalpindi Medical College (JRMC); 2015;19(1):67-70

Figure 3 : External aspect of skull in 9-10 weeks fetus showing supra-occipital (SO, ossification centres. (1&2) Figure 8: Ossification centre in intra-membranous ossification in a 9-12 week embryo, above the superior nuchal line. Osteoblast (dark blue arrow), noncalcified matrix (yellow arrow), calcified matrix) ( light blue arrow), trapped osteocytes (green arrow), mesenchyme (red arrow): X1040, H&E Stain.

Discussion

Figure 4 :External aspect of a 10-12 weeks fetal skull. There is a disparity in opinions amongst various Lateral medial (LM 1&2), supra-occipital bone (S.O), researchers. It is due to the difference in number, age, intermediate segment (IS). race or methodology . The views of Srivastava (1992) are consistent with this study as we belong to the same part of world and used same methodology.1 On the other hand our views differ from Matsumura because of different ethnic background and methodology. 15-16 Most of the researchers agreed that interparietal or Inca bones are accessory ossicles

Figure 5: External aspect of occipital squama of a 12-16 formed due to ossification failure. We followed the weeks fetus. showing Intermediate segment (IS), lateral previous nomenclature and criteria for classification of plate (LP), lateral fissure (LF), lower nuclei of medial plate Inca bones or interparietal bones (1-V). 14 (M L), upper nuclei of medial plate (1&2 , supra-occipitals Regarding the origin of the Inca bone (first (S.O). of 12-16 weeks embryo stained with Alizarin red S observed in Inca tribe of south Chile) it was concluded method (MF), Lateral fissure (LF), interparietal (IP), supra- occipital (SO), intermediate segment (IS). that the complete division of membranous and cartilaginous part of occipital bone by a transverse suture extending between the two lambdoid sutures at the level of highest nuchal line above the external occipital protuberance, presenting as Os Inca Totum (Type111 ).11,19 On the contrary there is difference in opinion regarding pre-interparietal bone. Lot of anatomists took interest in this rare bone and Figure 6: An Alizarin red S method stained specimen of 12 controversial issue .as Inca bones can serve as a non wk fetus showing medial fissure (MF), lateral fissure (LF), metric cranial variant for separation of populations.20 interparietal (IP), supra-occipital (SO), intermediate We agree with Srivastava that Matsumura and various segment (IS). other researchers were misinterpreting Type V Inca bone as pre interparietal.13,15,16 In further support of this view it was seen that various researchers calculated the frequency of pre-interparietal bone more than interparietal (10%) to be more than interparietal (3%). This concept was rejected as it is not possible that a rare centre can give a higher frequency than true interparietal bone. 5,6,8,21 Later Figure 7: Intra cartilaginous ossification in a 9 week fetus. Paul himself negated his previous view. 5,6 He agreed Osteoblast (yellow arrow), non-calcified matrix (red arrow), calcified cartilage (green arrow) Light blue arrow that the when upper nuclei of third pair failed to fuse indicates the area seen under high power ): X1040, H&E with the rest of bone, it formed separate bone and Stain. thus it was wrongly interpreted by researchers as

69 Journal of Rawalpindi Medical College (JRMC); 2015;19(1):67-70 separate pre- interparietal bone, which is a 5. Pal GP, Tamankar BP, Routal RV, Bhagwat SS. The ossification of 11,19,22,23 the membranous part of squamous occipital bone in man. J Anat. misnomer. Due to this misconception of 1984; 138(2):259-66. confusing sutural bones as Inca or pre-interparietal 6. Pal GP. Variations of interparietal bone in man. J Anat. bones, it was assumed that any separate bone 1987;152:205 8.21. 7. Ranke J, Cited by Schafer, E A, Symington J, Bryce T.. In: Quains developing as a separate ossification centre, in the Elements of Anatomy.11th ed. London: Longman Green &Co, lambdoid suture but outside the limits of interparietal, 1913; pp. 53-55. should be considered as sutural or wormian bones; as 8. Murlimanju BV, Prabhu LV, Paul MT. Variant morphogenesis of squamous part of occipital bone in human skulls. J Morphol they have their own specific shape, size, territories and Sci. 2010;27(3-4):139-41. time of appearance. (after 6 foetal month). Their shape 9. Sahib SH, Mavishetter GF,, Thomas ST, Prasanna ST. The incidence of interparietal bones in the adult skulls of South is not regular but oval or round and are surrounded India. J Biomed Sci Res. 2010;(2):86-88. by connective tissue only. 4-6,14 10. Bhanu PS, Sankar KD. Interparietal and pre-interparietal bones Several other researchers used morphometery to in the population of south coastal Andhra Pradesh, India. Folia Morphol. 2011;70,(3):185-90. clear the confusion between the three types of bones. 11. Udipi S. Interparietal (Inca) bone, a case report. IJAV. Their results indicated that actual distance between 2011;4:90-92. lambda and external occipital protuberance is in the 12. Da Mata RJ, Da Mata RF, Ferriera TAA. Analysis of bone variations of the occipital bone in Man. Int J Morphol. range of 7- 8.3 cm. 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