Foramen Magnum Ontogeny inHomo sapiens: A Functional Matrix Perspective

Abstract

Historically, the foramen magnum (FM) has been an integral component of studies on skull ontogeny and evolutionary transformations of cranial form. Although this foramen has been considered a single entity, we hypothesize that it comprises two functional matrices, a ventral matrix and a dorsal matrix. In general, the ventral matrix is related to locomotor functions, whereas the dorsal matrix is related to neurological functions and fluid flow dynamics. To test our hypothesis, we used a large ontogenetic sample of modern human crania (seventh fetal month to adult) to (1) delineate bony size and shape ontogeny for both the foramen and its dorsal and ventral units; (2) delineate the role of synchondroses in the observed growth patterns and rates; and (3) explore the relationship between FM and cranial size, shape, and growth. Detailed growth patterns and rates are established for the bony FM and its ventral and dorsal skeletal units. These data are supplemented by literature and observational data on embryonic and fetal FM ontogeny, soft tissue relationships, anomalous/pathological extremes of size, and craniocervical anatomy and locomotor functions. The hypothesis that the FM is composed of a ventral and a dorsal functional matrix is supported by observed ontogenetic differences between ventral and dorsal skeletal units, as well as by the soft tissue anatomy of these matrices. Further documentation of these matrices has the potential to significantly enhance our understanding of the ontogenetic and evolutionary transformations of skull base morphology. Anat Rec, 294:199-216, 2011. V V C 2010 Wiley-Liss, Inc.Key words: foramen magnum; ontogeny; human variation; functional matricesThe basicranium forms in a zone of interaction between structures with neural, skeletomotor, respiratory, auditory, masticatory, digestive, and visual functions. Because of the complex structural-functional relationships, the morphology of the basicranium reflects compromises resulting from the competing demands of multiple soft tissue units that are responding to essential functions.During basicranial development, the structures traversing the endocranial-ectocranial boundary become encircled by cartilage and, eventually, by bone. The resulting bony foramina of the basicranium respond in Additional Supporting Information may be found in the online version of this article.

Morphometric Analysis of the Foramen Magnum

“…The first classifies the foramen magnum as either: egg-shaped; round; oval; tetragonal; pentagonal; hexagonal; or, irregular (Murshed et al, 2003). The other, less common system classifies the foramen magnum into one of the following categories: circular; two semicircles; heart-like; wide oval; bi-rounded oval; ventrally wide oval; bi-pointed oval; or, dorsally convergent oval (Richards & Jabbour, 2011).…”

Morphometrics of foramen magnum in African four-toed hedgehog (Atelerix albiventris)

“…It is laterally bordered by the occipital condyles [17], and is of variable shapes and forms [1]. FM serves as an important landmark for studies on skull and cranial evolutionary modifications [21], as sex determinants in forensic analysis [7], plays a role in determining ancestry of individuals [5] and has clinical implication in diverse fields of medicine [9]. Irregularities of the dimensions of FM and occipital bone are linked in human and animal to different neurological conditions [11].…”

Beyond the functional matrix hypothesis: a network null model of human skull growth for the formation of bone articulations

“…; Kikuchi, ) and some morphological features of the head (Festa et al. ; Richards & Jabbour, ), in particular the integration between brain growth and skull shape (Moss, ; Fields et al. ; Richtsmeier et al.…”

“…1). This hypothesis has been broadly used to explain many craniofacial disorders (Mulliken et al 1989;Breitsprecher et al 2002;Kikuchi, 2005) and some morphological features of the head (Festa et al 2010;Richards & Jabbour, 2011), in particular the integration between brain growth and skull shape (Moss, 1975;Fields et al 1978;Richtsmeier et al 2006;Lieberman, 2011a;Richtsmeier & Flaherty, 2013). Other epigenetic factors that affect the formation of sutures include hormonal signals (Karsenty, 1999) and biomechanical mechanisms (Shwartz et al 2012;Khonsari et al 2013).…”