G E N E T I C I M P L I C A T I O N S

With the current interest in the human genome, the field is growing exponentially and numerous studies are exploring and identifying genes expressed by ocular structures and the mechanisms by which cellular characteristics and processes are governed by those genes. The PAX6 gene is considered the master control gene and is necessary for normal development of ocular structures.76,77 An increase in PAX6 in mice is associated with multiple lens defects, including abnormal fiber shape and fiber-to-fiber and fiber-to-cell interactions.53 The PAX6 gene is expressed in the corneal and conjunctival epithelium and may regulate and maintain cell structure; it may also have a role in the proliferation and maintenance of corneal and conjunctival stem cells.76,77

Myriad speculations surround genes and the proteins they encode. Some interesting theories concerning ocular structures include: clusterin might be the factor essential for preserving the nonkeratinized state of corneal epithelial cells and may also provide some protection against apoptosis;78 the gene, ALDH3, may provide some protection against UVR damage to corneal epithelium;78 there may be a connection between atherosclerosis and drusen formed in AMD through the same or similar extracellular matrix genes;79 and genes identified in the aqueous outflow tissues are usually associated with lymphatic tissue, perhaps suggesting additional function for the trabecular meshwork.79

In addition to providing further information about embryologic development, gene expression profiling can further explain cellular physiology as well as pathophysiology affecting ocular structures. Identifying and understanding the genetic regulation of normal cellular process brings us closer to understanding, treating, and possibly preventing ocular disease and dysfunction.

Marfan’s syndrome is a disease affecting connective tissue structures throughout the body. Several genetic mutations have been identified, which lead to faulty production or a reduction in the secretion or assembly of fibrillin molecules.80 Marfan’s related disorders are also associated with abnormalities in extracellular matrix formation. An individual with Marfan’s often has tall and thin body stature and can be at increased risk for aortic enlargement. Ocular structures can be affected; high myopia is often present and can increase the risk of retinal detachment; zonular malformation or dysfunction can result in a dislocated lens.

The gene called RAX is thought to be a major factor in the early stages of ocular development and mutations in RAX have been identified as causative in some cases of anophthalmia.81 Mutations in the PAX2 gene have been implicated in papillorenal syndrome, which is characterized by renal hypoplasia and optic nerve head coloboma.82

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The skull can be divided into two parts: the cranium and the face. The cranium consists of two parietal bones, the occipital bone, two temporal bones, the sphenoid bone, and the ethmoid bone. The face is made up of two maxillary bones, two nasal bones, the vomer, two inferior conchae, two lacrimal bones, two palatine bones, two zygomatic bones, and the mandible. The single frontal bone is a part of both the cranium and the face.

Generally, the bones of the skull unite at sutures that form immovable joints. The exception is the movable temporomandibular joint, which attaches the mandible to the temporal bones. Air-filled cavities called sinuses are contained within several of the bones.

After a brief description of the bones of the skull, this chapter presents a more detailed presentation of the orbital bones. The reader is advised to have a skull available for reference while reading this chapter, particularly for distinguishing the relationships and articulations between bones and identifying foramina and fissures.

B O N E S O F T H E C R A N I U M

The paired parietal bones form the roof and sides of the cranium (Figure 8-1). The parietal bones articulate with each other at the midline in the sagittal suture, with the occipital bone posteriorly in the lambdoid suture, and with the frontal bone anteriorly at the coronal suture. The parietal bone articulates inferiorly with the temporal bone and the greater wing of the sphenoid bone.

The occipital bone forms the posterior aspect of the skull and posterior floor of the cranial cavity. A prominence, the external occipital protuberance, or inion, is found on the external surface at the posterior midline (Figure 8-2). The large foramen magnum is found in the inferior aspect of the occipital bone. The inner surface of the bone forms the posterior cranial fossa, in which there are depressions where the lobes of the cerebellum lie. Figure 8-3 shows the inner aspect of the cranial floor. The occipital bone articulates with the temporal bones, parietal bones, and sphenoid bone.

Clinical Comment: Inion

THE INION,  located just outer to the posterior pole of the occipital cortex, is a useful landmark in the placement of the electrodes used to record a visual-evoked response (VER). This electrodiagnostic test records responses

from the visual cortex. Clinical applications include the determination of visual acuity in a patient unable to respond to the typical eye chart and the assessment of impulse conduction in the patient with suspected multiple sclerosis.

B O N E S O F T H E F A C E

The single frontal bone forms the forehead and articulates with the nasal bones, maxillae, and zygomatic bones in formation of the face (see Figure 8-4). The sutures joining adjacent bones of the face generally are named according to the names of the two bones that are connected (e.g., the suture between the frontal bone and the zygomatic bone is the frontozygomatic suture).

The two maxillae, or maxillary bones, form the upper jaw, the hard palate, the lateral walls of the nasal cavity, and the floor of both orbits (see Figure 8-4). Each maxillary bone articulates with the frontal, nasal, lacrimal, ethmoid, sphenoid, palatine, and zygomatic bones. That portion of the maxillary bone forming the cheek contains the maxillary sinus.

Each of the temporal bones is composed of two portions: a large, flat plate, the squamous portion; and a thickened, wedge-shaped area, the petrous portion. The squamous portion forms the side of the cranium and articulates with the parietal bone and the sphenoid bone. An anterior projection, the zygomatic process, articulates with the zygomatic bone (see Figure 8-1). The petrous portion extends within the cranium and houses the middle and inner ear structures. The mastoid process and styloid process project from the inferior aspect, and between these two processes is the stylomastoid foramen, through which the facial nerve exits the skull. The petrous portion articulates with the occipital bone in the floor of the skull. The carotid canal runs superiorly and anteriorly through the petrous portion and provides an entrance for the internal carotid artery into the cranial cavity (Figure 8-3).

Sagittal suture

Lambda

Temporal bone

External

occipital protuberance (inion)

Maxilla

Parietal bone

Lambdoidal suture

Occipital bone

Asterion

Emissary foramen

Styloid process

Pterygoid hamulus

Mandible (interior surface)

FIGURE 8-2

Posterior view of skull. (From Mathers LH, Chase RA, Dolph J,

et al: Clinical anatomy principles, St Louis, 1996, Mosby.)

FIGURE 8-4

Coronal suture

Frontal bone

Nasal bones

Temporal fossa

Zygomatic bone

Ramus of mandible

Angle of mandible

Mental foramen

The single frontal bone forms the anterior portion of the cranium, anterior floor of the cranial cavity, and superior part of the face (Figure 8-4). At the top of the skull, the frontal bone articulates with the parietal bones; inferiorly it articulates with the sphenoid bone, ethmoid bone, and lacrimal bones. Inferoanteriorly, it articulates with the nasal bones, maxillary bones, and zygomatic bones. The inner surface of the cranial cavity portion of the frontal bone forms the anterior cranial fossa (see Figure 8-3), in which the frontal lobes of the cerebral hemispheres lie. The frontal sinuses are located within the anterior portion of the frontal bone.

The sphenoid bone is a single bone, the body of which lies in the midline and articulates with the occipital bone and the temporal bones to form the base of the cranium (see Figure 8-3). The sphenoid bone joins the zygomatic bones to form the lateral walls of the orbits. Anteriorly and inferiorly the sphenoid bone articulates with the maxillary and palatine bones, superiorly with the parietal bones, and anteriorly and superiorly with the ethmoid and frontal

bones. The depression on the superior cranial surface of the body of the sphenoid bone, the hypophyseal fossa (or the sella turcica), houses the pituitary gland; a portion of the body is hollow, forming the sphenoid sinus cavity.

Two pairs of wings project from the body of the sphenoid bone. The lesser wings project from the anterior aspect of the body and are more superior and smaller than the greater wings (see Figure 8-3). The lesser wings are attached to the body by small roots or struts. The gap between the lesser wing and the sphenoid body forms the optic foramen (canal) through which the optic nerve exits the orbit. The lesser wings articulate with the frontal and ethmoid bones.

The greater wings project from the lateral aspects of the body and articulate with the frontal bone, the parietal bones, squamous portions of the temporal bones, and the zygomatic bones. The pterygoid process projects from the base of the greater wing and articulates with the vertical stem of the palatine bone; each contributes to a shallow depression, the pterygopalatine fossa. Three important foramina are located in