fetal skull

Chapter 30 The fetal skull

Development of the fetal skull

As the fetus develops in utero, the mesenchyme layer surrounding the brain starts to ossify, forming the various bones of the fetal skull. This process is called intramembranous ossification and begins between 4 to 8 weeks of gestation. The initial development of the skull occurs from this intramembranous structure, derived from neural crest cells and mesoderm. The intramembranous structure is divided into two major components, the neurocranium, which forms the protective case of the skull, and the viscerocranium, forming the bones of the face.

The neurocranium can be subdivided into the chondrocranium and the dermatocranium. The chondrocranium (cartilaginous part) is formed by the fusion of cartilages, and following ossification becomes the occipital, temporal, sphenoid and ethmoid bones. The dermatocranium (membranous part) is thought to arise from the external dermal scales developed to protect the brain. This lies under the superficial layers of the skin, covering and protecting the dorsal section of the brain, giving rise to the parietal and frontal bones.

The earliest visible signs of development can be seen on ultrasonography at about 4 weeks’ gestation with calcification of the membranes and the development of the occiput. This becomes easier to determine from approximately 8 weeks, when intramembranous ossification is more prominent. At 12 weeks, the outline of the individual bones become evident (Moore & Persaud 2007, Sadler 2009) (see Fig. 30.1). Ossification of the bones continues throughout pregnancy with individual bones ossifying from their centre. At term, the bones of the skull are thin and pliable, enabling some movement of bones to take place during labour. The two frontal bones have usually united by term.

The skull (Fig. 30.3)

The fetal skull is a complex structure consisting of 29 irregular flat bones with 22 of these paired symmetrically: 8 bones form the cranium, 14 the face, and 7 the base. Knowledge of the fetal skull in the antenatal period enables a midwife to assess the size of the fetal head in relation to the size of the pelvis, assess engagement of the fetal skull in the pelvis (Dietz & Lanzarone 2005). It also helps inform reviews of ultrasonography and pelvimetry reports.

Jun 18, 2016 | Posted by in MIDWIFERY | Comments Off on fetal skull

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