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EQUINE VETERINARY EDUCATION
Equine vet. Educ. (2022) 34 (7) 362-368 doi: 10.1111/eve.13436
Original Article
A histological evaluation of facial suture lines in six horses aged 1 day to 9 years L. Geitzenauer†
, H. Geyer‡,A.E.F€ urst†, L. Klein† and M. A. Jackson†*
†Equine Department, Vetsuisse Faculty, University of Zurich, Zurich; and ‡Institute of Veterinary Anatomy, Vetsuisse Faculty, University of Zurich, Zurich, Switzerland *Corresponding author email:
mjackson@vetclinics.uzh.ch
Keywords: horse; head sutures; histology; skull; patency
Summary Craniofacial sutures are fibrous connections between the flat bones in the skull, allowing a small amount of movement to absorb strain and load as long as they are patent. In the present study, we examined the histological appearance of facial suture lines in horses of different ages. The internasal, nasofrontal and maxillolacrimal sutures were macroscopically identified and extracted from the skulls of six horses aged 1 day (two horses), 5 days, 2, 6 and 9 years and prepared into histological samples. The suture lines were then examined for width, form, cell density and morphology, vascularisation and morphology of the surrounding tissue and compared based on the age of the horse. Although macroscopically the sutures became difficult to identify in the 6- and 9-year-old horses, histologically they were clearly visible in all samples, indicating that the sutures were patent and consisted of connective tissue in all analysed horses. The sutures became subjectively narrower with lower cell density in older horses as compared to young horses, and there was a noticeable maturation from woven to cancellous bone surrounding the sutures. Vascular structures were present in all sutures regardless of age or location. In conclusion, this study raises awareness of the histological suture conformation in neonatal and adult horses and their persistence into adulthood. A better understanding of the histological structure of facial sutures and their continued patency into adulthood can aide in the interpretation of radiographs and computed tomography images of the head and in improving treatment methods and outcomes for horses affected with suture exostosis. In addition, if biopsies are taken, clinicians will have a basis on which to determine which cells are related to the disease process and which are normally occurring.
Introduction
Craniofacial sutures are fibrous connective tissue articulations found between neighbouring flat bones in the skull (Jaslow 1990; Nickel et al. 2004; Rice 2008). Sutures not only function as connections between adjacent bones but also are primary sites of osteogenesis in the skull (Rice 2008). Sutures do not have a cartilaginous precursors such as bone growth centres but rather function as intramembranous bone growth sites (Opperman 2000). A small amount of movement occurs as long as they remain patent – meaning the interdigitated bone is separated by connective tissue – which is important during birth (Rice 2008) as well as for load absorption during mastication, as researched using miniature swine (Rafferty and Herring 1999; Herring et al. 2001). In humans, the cranial
© 2020 EVJ Ltd sutures begin to close in the second to third decade of life,
whereas the facial sutures remain patent and separated by fibrous connective tissue for much longer, fusing only in the seventh or eighth decade of life (Opperman 2000; Rice 2008). The reason for this difference is that the calvaria of the mammal has its most active period of growth during embryogenesis and shortly after birth, whereas the facial skeleton does not experience the majority of its rapid growth until adolescence (Rice 2008). It is not known at what age the facial sutures close
histologically in horses, but it stands to reason that this cannot happen until rapid facial growth has been completed (Jaslow 1990; Dixon 2014). The nasofrontal suture is recorded as closing (at least radiographically) in horses at 1 year of age (Butler et al. 2017). Suture line exostosis in horses causes nonpainful, usually
bilateral swelling in the dorsal aspect of the equine head (Dixon 2014; Klein et al. 2014; Butler et al. 2017). Radiographs commonly show a proliferative periosteal (and endosteal) reaction and new bone growth around the suture lines characterised by a radiolucent defect and some degree of soft tissue swelling (Wyn-Jones 1985; Gibbs and Lane 1987). Several case studies have been published focusing on diagnosis, imaging and treatment options of suture exostosis in horses (Carslake 2009; Manso-Dıaz and Taeymans 2012; Dixon 2014; Klein et al. 2014; Klein et al. 2019). However, there is still a distinct lack of information on the histological morphology of equine facial sutures. Therefore, the aim of the present study was to examine facial sutures in horses aged 1 day to 9 years in order to identify histological features of normal facial sutures. This information can then be used to better understand the disease process and hopefully optimise treatment options and outcomes. Moreover, the understanding of histological morphology of facial sutures could help in the interpretation of radiographs and computed tomography images of the equine head. Based on the research of Rice (2008) and Rafferty and Herring
(1999), we hypothesised that the sutures would contain fibrocytes/fibroblasts, interdigitation varying with age and anatomic location as well as osteoblasts in areas where new bone growth is expected, at least in the samples of the foals.
Materials and methods
Samples from six horses ranging in age from 1 day to 9 years were taken from the Sutura internasalis, Sutura nasofrontalis and Sutura maxillolacrimalis (Fig 1). Since samples were taken only over a period of 2 months, the age groups were not
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