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SKULL

NEUROCRANIUM

Cartilaginous Neurocranium

The parachordal plate becomes the basioccipital on the ventral aspect of the medulla oblongata. The hypoglossal nerve (XII) emerges from the skull through the hypoglossal canal in its caudolateral portion that arises from the occipital sclerotomes. These sclerotomes extend lateral to the hypoglossal canal as the exoccipital that forms the lateral boundary of the foramen magnum at the brain-spinal cord junction. The exoccipital blends with the supraoccipital that closes the foramen magnum dorsally. Most of the definitive occipital bone will arise from the basi-, ex- and supra-occipital.

The basi- and exoccipital join the otic capsule laterally, which will become the petrous part of the temporal bone in the adult. Cranial nerves IX and X and the spinal accessory nerve leave the skull along with the internal jugular vein through the jugular foramen between the exoccipital and otic capsule. Cranial nerves VII and VIII leave the cranial cavity through the internal auditory meatus between the basioccipital and otic capsule. The petrous part of the temporal bone houses the entire membranous labyrinth. The endolymphatic duct will emerge from the bone through a small slit near the fourth brain ventricle.

The hypophyseal cartilages fuse around the hypophysis to form the body of the sphenoid bone containing the hypophyseal fossa (sella turcica). The rostral end of the basioccipital joins with the body of the sphenoid bone. The alisphenoid will become the greater wing of the sphenoid bone through which the maxillary (V2) and mandibular (V3) nerves pass from the skull by way of the foramen rotundum and foramen ovale, respectively. The alisphenoid joins the capsule as the alicochlear commissure that passes lateral to the internal carotid artery. The orbitosphenoid expands as a plate supporting part of the cerebral hemispheres. It will become the lesser wing of the sphenoid bone containing the optic canal through which the optic nerve (II) passes to the eye. The oculomotor (III), trochlear (IV), ophthalamic (V1) and abducens (VI) nerves will enter the orbit through the superior orbital fissure between the alisphenoid and orbitosphenoid.

The trabecular cartilage and nasal capsule will give rise to the ethmoid bone and most of the nasal septal cartilage. The fascicles of the olfactory nerve (I) course through the cribriform plate of the ethmoid to the olfactory mucosa in the roof of the nasal cavity.

Membranous Neurocranium

The outer part of the ectomeninx becomes an osteogenic membrane that covers the dorsal and lateral surfaces of the brain. The osteogenic membrane grows as the cerebral hemispheres enlarge. It gives rise to the frontal, parietal, squamous temporal and interparietal occipital bones of the adult. These bones are separated by thin planes of connective tissue called sutures. At birth a wide suture area called the anterior fontanelle is located in the midline at the junction of the frontal and parietal bones. The anterior fontanelle disappears during the first few years of life.

VISCEROCRANIUM

The malleus and incus ear ossicles develop in the dorsal segment of the first arch cartilage. The anterior ligament of the malleus and sphenomandibular ligament are adult remnants of its intermediate segment. The mandible is laid down lateral to the ventral segment, which eventually disappears.

The stapes ear ossicle develops in the dorsal segment of the second arch cartilage. The ear ossicles are the first bones to fully ossify and reach their adult size. This occurs around the fourth month. The styloid process and stylohyoid ligament are remnants of its intermediate segment. The lesser horn and body (cranial part) of the hyoid bone develop from its ventral segment after it fuses in the midline with its counterpart on the opposite side.

The third arch cartilage also fuses in the midline with its counterpart on the opposite side and with the median part of the fused second arch cartilages. It gives rise to the greater horn and body (caudal part) of the hyoid bone.

The fourth, fifth and sixth arch cartilages never become distinct cartilaginous bars. Instead the laryngeal condensations that represent them give rise directly to the laryngeal cartilages. The thyroid, arytenoid, corniculate and cuneiform are thought to be derived from the fourth and fifth cartilages. The cricoid supposedly represents the sixth cartilage.

POSTCRANIAL PART OF AXIAL SKELETON

VERTEBRAL COLUMN

The notochord degenerates in the area of the centrum but expands in the intervertebral disc to form the nucleus pulposus.

The neural arch fuses with its counterpart on the opposite side dorsal to the spinal cord. Together they produce a spinous process flanked by a lamina on each vertebra.

The S-1 to S-5 sclerotome segments fuse into one bone, the sacrum. The first segment extends laterally as a wing shaped process called the ala. The lateral border of the sacrum articulates with the pelvis at the sacroiliac joint.

RIBS AND STERNUM

The long, proximal segment of each rib primordium begins ossifying to produce a rib. The short distal segment remains cartilaginous forming a costal cartilage. All but the last two costal cartilages attach directly or indirectly to the sternum.

The sternal bar and episternal cartilage will eventually fuse with their counterpart on the opposite side to produce the body and manubrium of the sternum, respectively. Initially the sternum is cranial to its definitive level because of the large liver preventing its caudal displacement.

APPENDICULAR SKELETON

The clavicle becomes a prominent, well-ossified bone that attaches the scapula (acromion process) to the manubrium of the sternum.

The spine of the scapula becomes recognizable. The head of the humerus articulates with the scapula at the glenoid fossa.

The pelvic bone is represented by three pairs of cartilages: ilium, ischium and pubis. The pubic cartilages are separated in the midline by the symphysis pubis. All of the cartilages enter into the formation of the acetabular fossa where the head of the femur articulates.

Most of the bones of the extremities can be identified and are mainly cartilagenous. Before birth the majority of the primary ossification centers occur. The bones of the upper extremity are humerus (arm), radius and ulna (forearm), carpals (wrist), metacarpals (palm) and phalanges (fingers). The bones of the lower extremity are femur (thigh), patella (knee), tibia and fibula (leg), tarsals (ankle), metatarsals (sole) and phalanges (toes).

Source: Atlas of Human Embryos.