Embryology of the Sellar Region

Holding the key position at the center of the skull, the sella turcica ("Turkish saddle") is used as a reference point for many lesions, which may be described as sellar, suprasellar or parasellar. The close proximity of many important structures explains the often striking and characteristic clinical presentation of pathology in this compact region.

The cartilaginous neurocranium is the basal region of the developing skull. In its earliest stage of development, it exists as a narrow condensation of mesenchyme forming a plate that links the anterior rim of the foramen magnum with the most anterior part of the skull. Central to the development of this region is the formation of the body of the sphenoid bone. Subsequently, cartilaginous plates develop on either side of the developing sphenoid body to form the wings and complete the development of the middle cranial fossa.

The greater wings of the sphenoid form the majority of the middle cranial fossa. The lesser wings start at the anterior clinoid processes, passing laterally to become the sphenoid ridge of the pterion. Between the two is the superior orbital fissure. By the middle of the third month of gestation, the skull base is a unified mass of cartilage known as the "chondrocranium".

Subsequent ossification takes place in several centers.

The sella develops as a depression in the body of the sphenoid and is lined with dura and houses the pituitary gland. It is roofed over by the diaphragma, which transmits the infundibu-lum or pituitary stalk. On either side of the sphenoid bone the cavernous sinuses, made from folded dura, transmit the carotid artery, the maxillary division of the trigeminal nerve, and cranial nerves III, IV and VI.

The cavernous sinuses receive blood from the petrosal and sphenoparietal sinuses in addition to local veins draining the sella. They also interconnect with each other, which explains why petrosal venous sampling seldom localizes the side of a pituitary microadenoma. The posterior articulation of the sphenoid body is with the clivus at the spheno-occipital synchondrosis. Above the sella are situated the optic nerves, chiasm, third ventricle and hypothalamus.

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