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EQUINE VETERINARY EDUCATION / AE / july 2022


Fig 2a shows a suture with mild-to-moderate interdigitation – the angles at which the suture changes direction are between approximately 90° and 145°. In comparison, Fig 2b shows a suture with tight interdigitation; the two marked suture sections run essentially parallel to one another after a short section in which the suture changes course, corresponding to an approximately 180° turn.


Results


Histologically, the sutures can be identified in the photomicrographs as structures interdigitated with surrounding bone that are composed of connective tissue (Fig 3). The sutures contain mostly fibrocytes and fibroblasts, but also some osteoblasts as well as blood vessels (Fig 4). The sutures are wider where large vessels are found within the sutures and these vessels are often concentrated in the central portion of the width of the suture. Many of the larger vessels have a diameter of approximately 200 lm. The sutures course between two adjacent skull bones, joining them together with varying levels of interdigitation (Fig 3). The interdigitation of the sutures is being evaluated on a


two-dimensional plane in the photomicrographs, when in reality the sutures interdigitate three-dimensionally. For this reason, the plane of intersection of the suture becomes relevant in the interpretation of the degree of interdigitation. Ideally, the photomicrographs show the suture along its length at one depth – this was approximated in several samples and is shown in Fig 2. However, this was not always possible to achieve macroscopically, and some samples appear to transect the suture tangentially, resulting in a nonlinear view of the suture and making an evaluation of its interdigitation difficult (Fig 5). Table 1 summarises the results of representative photomicrographs in each of the age categories and sutures.


CT


100 µm


Fig 4: Sutura maxillolacrimalis 1-day-old foal, HE stain. The suture is composed of connective tissue (CT)with the main cells being fibrocytes and fibroblasts (arrowheads). The suture border of this foal is lined with osteoblasts (arrows). In addition, blood vessels can be seen throughout (circled).


There are several features that can be used to mark


the maturation of the suture as the age of the horse increases. The connective tissue is loose in the 1-day-old and 5-day-old foals and appears denser with a lower cellularity in the adult horses (Fig 6). Osteoblasts, which are indicators of new bone formation, are present in a higher concentration in the 1-day-old and 5-day-old foals and rarely found in the adult horses (Fig 7). The borders of the suture are identified by the intersection of fibrous connective tissue with bone. In the foals and 2-year-old horse, these borders have a high cellularity and walls of osteoblasts and/or fibrocytes and fibroblasts are present on the suture side of the border. In adult horses, the borders are composed of connective tissue interspersed with


240 µm a c


b


100 µm


Fig 3: Sutura maxillolacrimalis 5-day-old foal, HE stain. Identifiable features in a suture: bundles of vessels including arterial, venous, lymphatic vessels and nerves (circled), (a) additional smaller vessels concentrated in the central portion of the width of the suture, (b) bone – in this suture woven bone with medullary cavities, (c) connective tissue – in this photomicrograph the connective tissue is highly cellular. Also shown here is an example of a representative measurement of suture width (double sided arrow).


© 2020 EVJ Ltd 500 µm


Fig 5: Sutura maxillolacrimalis 9-year-old horse, HE stain. It is not possible to identify a clear course of the suture, and there are several projections (arrows) from the largest suture sections that suggest the suture has been transected on more than one plane, making the interdigitation difficult to impossible to identify using this sample.


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