Título primero Disposiciones generales

Sutural development and composition varies not only between different sutures but also within the same suture over time (Cohen Jr, 1993). This is mainly because sutures form an intermediate structure somewhere between bony and soft tissues. Consequently, although there have been numerous microscopic descriptions of sutural morphogenesis and anatomy, the details are often conflicting (Fürtwangler et al. 1985).

As the developing cranial bones approach each other, three distinct layers can be identified. On the dorsal surface lies the ectocranial periosteum, beneath which is a central cambial layer. This is a site of very active osteogenesis and the fibrous capsule which surrounds it must expand to keep pace with the growing bone (Pritchard et al. 1956). On the ventral surface of the cambial layer lies the dura mater which forms the remainder of the fibrous membrane. The dura mater is a dense cellular layer formed from the ectomeninx, the non-vascularised portion of the meninges (Tyler, 1983). The dura mater and the periosteal layer which form the fibrous membrane do not encapsulate the margins of the cranial bones as they do in the facial region. Instead they merge slightly ahead of the advancing bones and the single membrane crosses the presumptive sutural region parallel to the plane of the bones.

In addition to vertical stratification of the mineralised areas, there are three morphological zones evident in cross-section (Fig. 1.5). The inner zone is mineralised and formed fi*om highly differentiated and packed bone, a midzone of inner osteoid accumulation follows and a zone of undifferentiated cells is located peripherally. This zone of cellular differentiation which precedes ossification has been termed the osteogenic induction front (OIF) (Delashaw et al. 1989; Johansen and Hall, 1982).

As the brain grows and expands, the calvarial bones continue to grow along their margins by deposition of osteoid tissue. Osteoid becomes mineralised and

transformed into bone and these bone fi^onts eventually meet in the midline at the suture site (Delashaw et al. 1989). As the OIF proceeds through the membrane, the width of the undelaminated ectomeninx between the bones decreases until the cambial layers of the bones are almost touching. The remaining portion of ectomeninx slightly separates allowing a slightly looser structure to form. Although at this point it would appear that the cambial layers could cross the suture and fuse, this is prevented by the appearance of fibrous capsules which separate the two bones. It is proposed by Pritchard and co-workers that this capsule has developed from peripheral cambial cells which are transformed into fibroblasts. Collagen fibres are deposited around the fibroblasts and sit sagittally, at light angles to the uniting layers of the bones. The structure of the suture at this stage consists of five intervening layers - the first cambial layer and the first fibrous capsule, followed by the loose middle zone and the second fibrous capsule and cambial layer. It also has two uniting layers comprised of fibrous laminae which form as the bone fronts approach each other (Pritchard et al.

1956).

It should be pointed out here that although Pritchard and co-workers have described a five layer structure, the number of sutural layers varies fi*om one

(Weinmaim and Sicher, 1955) to three (Moss, 1954) to seven layers (Enlow, 1990; Wagemans et al. 1988) and many investigators simply describe the structure of a suture as "highly variable" (Persson, 1973). Obviously during sutural maturation the number of layers alters becoming highly laminated during development before stabilising in adult life.

The bony edges of a differentiated suture exhibit two forms macroscopically. These have been widely described as end to end (butt, flat or plane) or

overlapping (bevelled or squamous) sutures. The shape of these sutures is determined by the plane of passage of the OIF through the loose connective tissue of the skull (Wagemans et al. 1988). It is generally held that end-to-end sutures develop in the midline region of the skull. For example in the human, the sagittal and midpalatal sutures are of the end-to-end type (Cohen Jr, 1993) as are the metopic and intemasal in the rat (Koskinen, 1977). All other sutures are of the overlapping type and include the coronal and frontozygomatic (Kokich,

1976; Koskinen, 1977). It has been proposed that midline sutures may be of the end-to-end type because the biomechanical forces on either side of this region are quite likely to be equal in magnitude. Sutures such as the coronal which are located away from the midline are subjected to wide differences in force

magnitude which results in an overlapping phenotype.

There appears to be no differentiative activity of cells or fibres to indicate the future location of sutural development (Johansen and Hall, 1982; Wagemans et al. 1988). Markens (1975) however reported the appearance of a characteristic fusiform blastema in humans and rats. He proposed that this temporary structure indicated the site of and the relative vertical position of the bony edges forming the coronal suture (Johansen and Hall, 1982; Markens, 1975; Wagemans et al.

1988). Although the presence of this blastema is not disputed, it appears to be unimportant in determining relative bone position within the suture. Johansen

Anterior Metopic

unmineralised bone matrix mineralised (osteoid) and differentiated

bone osteoblasts Coronal Sagittal Lambdoid periosteum dura mater suture: undifferentiated proliferating osteogenic stem cells Posterior

Figure 1.5 Normal cranial suture development

(A) Dorsal view of a child’s skull, showing the position of the major sutures. Coronal craniosynostosis leads to a short, broad skull; conversely, sagittal synostosis results in a long narrow skull. (B) Diagrammatic cross-section through a coronal suture. The skull bones overlap slightly. OIF, osteogenic induction front. Adapted from Wilkie (1997).

and Hall have indicated that the blastema does not appear until considerably after the initial overlap of the frontal and parietal bones in mice (Johansen and Hall, 1982; Wagemans et al. 1988). This would suggest that at least in this, and likely in other species, the location of the cranial suture is defined long before the growth of the bones and before it is visually identifiable histologically.