SKULL DEVELOPMENT

The skull has two distinct portions: the neuro cranium, surrounding the brain and special sense organs, and the viscerocranium, surrounding the lower face and jaws.

Each part derives from different mesenchymal populations and by different methods.

1. The neurocranium develops from paraxial mesenchyme in the head, i. e. the first five somites and the unsegmented somitomeres rostral to the first somite, and from ectoderm via the neural crest. The basal portion of the skull is similar in structure and development to the vertebral column and is preformed in cartilage.

2. The viscerocranium derives from ectoderm via invaginated head neural crest which streams into the developing arches forming all the connective tissue elements of the face. Bones of the viscerocranium form in the main from membranous ossification but there are cartilage models in each arch.

NEUROCRANIUM DEVELOPMENT

The bones of the skull are developed in the mesenchyme which surrounds the cerebral vesicles but, before the osseous state is reached, the cranium passes through blastemal and cartilaginous stages. The chondrocranium is incomplete, because not all parts pass through a phase of chondrification, the remainder comprising the masenchymatous, blastemal desmocranium. Thus, most of the cranial vault and limited parts of its base are not preformed in cartilage. The mesenchymatous (membranous) and cartilaginous parts of the skull develop together and complement each other in forming the complete cranium, some of whose bones are composite structures derived from both sources. All elements pass first through a mesenchymatous phase.

THE BLASTEMAL SKULL (DESMOCRANIUM) begins to appear at the end of firstmonth as a condensation and thickening of the mesenchyme which surrounds the developing brain, forming localized masses. The first masses evident are in the occipital region, outlining the basilar (ventral) part of the occipital bone. These form an occipital plate which extends: laterally, by two extensions on each side, which grow and spread to complete a foramen around each hypoglossal nerve

forwards, dorsal to the pharynx, to reach the primordium of the hypophysis, thus establishing the clivus of the cranial base and the dorsum sellae of the future sphenoid bone. ventrally and rostrally to surround the developing stalk of the hypophysis and to form the anlage (primordium) of the ethmoid bone and of the nasal septum, between the two halves of the nasal cavity.

The ventral occipital plate is traversed obliquely by the notochord, which it is at first near its dorsal surface and then is lying ventrally, where it comes into close relationship to the epithelium of the dorsal wall of the pharynx. It then re-enters the cranial base and run rostrally to end just caudal to the hypophysis. the lateral process of the occipital plate fuse with the otic capsules which cover the otocysts (auditory vesicles), leaving a wide hiatus through which the internal jugular vein and the gossopharyngeal, vagus and accesory nerves pass. the mesenchyme around the developing hypophyseal stalk, which is forming the rudiment of the postsphenoid part of the sphenoid bone, spreads out laterally to form the future greater wings of the sphenoid bone. the smaller processes rostral to hypophyseal stalk indicate the sites of the lesser wings of the sphenoid, and other condensations reach the sides of the nasal cavity and also blend with the still mesenchymatous septum.

THE CHONDRIFICATION PROCESS begin in the seventh week just in the BASAL REGIONS OF THE SKULL. These are initially preformed in cartilage. This occurs primarily in three regions, named: parachordal region, situated caudally, in relation to the notochord hypophyseal region, intermediately, in relation to the hypophysis interorbitonasal region, situated rostrally, between the orbits and the nasal cavities.

1. The parachordal cartilage or basal plate is developed from paraxial mesenchyme related to the cranial end of the notochord and the first five (occipital) somites. Caudally it exhibits traces of four primitive segments separated by the roots of the hypoglossal nerves. This cartilaginous mass contributes to the base of occipital bone named basioccipital. Later, extensions grow around the cranial end of the spinal cord and form the boundaries of the foramen magnum.

The otic capsule is formed from three different sources: the first somite, a portion of paraxial mesenchyme and neural crest mesenchyme. Otic capsules appear around the developing internal ears, or otic vesicles, and form the petrous and mastoid portions of the temporal bone. These cartilages fuse medially with the parachordal plate, but laterally and dorsally with parietal plate.

2. The hypophyseal cartilage ossifies to form the postsphenoid part of the sphenoid bone (until the seventh or eight month in utero the sphenoid body has a presphenoidal part, anterior to the tuberculum selae, with which the lesser wings are continuous, and a postsphenoidal part, comprising the sella turcica and dorsum sellae, and integral with the greater wings and pterygoid processes). It derives from both paraxial mesenchyme and neural crest. The paraxial mesenchyme contributes to the caudal part of the sella turcica, forming each side of the rostral end of the notochord, whereas the neural crest forms the more rostral portion of the sella turcica and the region named the prechordal skeleton.

3. The interorbitonasal cartilage or trabecular cartilage (trabeculae cranii) is a bilateral structure developing from two centres of chondrification. The trabecula cranii and the ethmoid complex are of neural crest origin. The trabecular plate gives laterally two cartilaginous extensions on each side:- a posterior one or alisphenoid, which represents the future root of the greater wing of the sphenoid bone- an anterior one or orbitosphenoid, which represents the future lesser wing of the sphenoid bone.

The trabecular plate between the two posterior cartilaginous extensions become basisphenoid or postsphenoid, but the trabecular plate between the two anterior cartilaginous extensions become presphenoid. The two trabeculae fused extend rostrally to ethmoidal region where, they unite with the nasal capsules.

In the human embryo, cranial chondrification begins in the second month.

Cartilaginous foci appear in the occipital plate, one on each side of the notochord (parachordal cartilages). These later fuse at the end of the seventh week surrounding the notochord, which oblique transit through the region. The cartilaginous posterior part of the sphenoid is formed from two hypophyseal centres, flanking the stalk of the hypophysis and uniting at first behind, then in front, enclosing a craniopharyngeal canal containing the hypophyseal diverticulum. The canal is usually obliterated by the third month.

The otic capsules, presphenoid, bases of the greater wings and lesser wings of the sphenoid, and finally the nasal capsules, in turn become chondrified.

The nasal capsules develop around the nasal sacs and contribute to the formation of the ethmoid bone. The whole nasal capsule is well developed by the end of the third month, consisting of a common median septal part (the interorbitonasal septum) and two lateral regions. The free caudal borders of the lateral regions incurve to form the interior nasal conchae, which ossify during the fifth month and become separate elements.

Posteriorly, each lateral part of the nasal capsule becomes ossified as the ethmoidal labyrinth, bearing on its medial surface ridges, the future middle and superior conchae.

Part of the rest of the capsule remains cartilaginous as the septal and alar cartilages of the nose.

The bones of the cranial base which are thus preformed in cartilage are: the occipital (excepting the upper part of its squama, above the highest nuchal lines), the petromastoid part of the temporal bone, the body, lesser wings and roots of the greater wings of the sphenoid and the ethmoid. These constitute the cartilaginous part of the neurocranium. Except for the orbital plates of the frontal and the lateral parts of the greater sphenoidal wings, the base of the skull is preformed in cartilage.

OSSIFICATION commences before the chondrocranium has fully developed, and as this change extends, bone overtakes cartilage until little of the chondrocranium remains. Parts of it still exist at birth and small regions remain cartilaginous in the adult skull. At birth unossified chondrocranium still persists at: the alae, lateral nasal and septum of the nose; the spheno-ethmoidal junction; the spheno-occipital and sphenopetrous junctions;the apex of the petrous bone (foramen lacerum) and also between ossifying elements of the sphenoid bone and between elements of the occipital bone.

VAULT OR UPPER REGIONS OF THE SKULL first appear about thirtieth day. They consist of curved plates of mesenchyme at the sides of the skull and gradually extend cranially to blend with each other. They also extend towards and reach the base of the skull, which will become part of the chondrocranium. The mesenchymatous (membranous) neurocranium, corresponding to the cranial vault, is not preformed in cartilage. Its elements, described as dermal bones, are: the frontal bones, the parietals, the squamous parts of the temporal bones and the upper (interparietal) part of the occipital squama. The frontal, parietal and squamosal bones are formed from neural crest. The sutures of the calvarian and facial bones are made up of crest cells.

The dermal bones are formed by the initiation of a wave of osteodifferentiation moving radially from ossification centres within the desmocranial mesenchyme. When adjacent bones meet, proliferation of the osteogenic front ceases and sutures are induced to form.

Once sutures are formed and the fibrous desmocranium is replaced by mineralized cranial bone, a second phase of development occurs in which growth of cranial bones occurs at the sutural margins. Such growth forms most of the calvaria.

Four centres of ossification appear at the embryo of 25 mm length, on each side of the calvaria:

1. The frontal bone is ossified in fibrous mesenchyme from two primary centres appearing in the eighth week in utero, one near each frontal tuber. At birth the bone consists of two halves which may remain separate, a metopic suture persisting.

2. Each parietal bone is ossified from two centres, appearing one above the other near its tuberosity at about the seventh week in utero in dense mesenchyme. These unite early.

3. The squama of temporal bone is ossified in a sheet of condensed mesenchyme from a single centre near the zygomatic roots, appearing in the seventh or eighth week in utero,

4. A common but oversimplified account of occipital ossification states that above the highest nuchal lines the squama is developed in a fibrous membrane and ossified from two centres, one on each side from about the second fetal month. This part may remain separate as the interparietal bone. The rest is prefomed in cartilage.

Other bones which are also ossified in membrane are: ossification of the lacrimal bones is from a centre appearing at about the welfth week in mesenchyme around the nasal capsule ossification of the nasal bones is from a centre which appears early in the third month in mesenchyme overlying the cartilaginous anterior part of the nasal capsule ossification of the vomer is from two centres appear in eighth week, the vomer being ossified in strata of connective tissue covering the septal cartilage on each aspect in its posteroinferior part greater wing of the sphenoid bone (except the wing s root), spreading into the lateral pterygoid plate and also the medial pterygoid plates are ossified in membrane.

The stages in the development of the skull.A to C are wiew of the base of the developing as skull (as viewed superiorly)

Sex weeks showing the various cartilages that will fuse to form the chondrocranium

Seven weeks, after fusion of some of the paired cartilages

12 weeks, showing the cartilaginous base of the skull or chondrocranium fotmrd by the fusionof various cartilages

20 weeks, indicating the derivation of he bones of the fetal skull

During fetal life, the flat bones of the calvaria are separated by dense connective tissue membranes that constitute fibrous joints called sutures. Six large fibrous areas where several sutures meet, called fontanelles, are present, two median (anterior and posterior) and two lateral pairs (sphenoidal and mastoid), situated at the angles of the parietal bones.

1. The anterior fontanelle, the largest, is at the junction of the sagittal, coronal, and frontal sutures, hence rhomboid, and about 4 cm anteroposterior and 2.5 cm in transverse dimensions.

2. The posterior fontanelle, at the junction of sagittal and lambdoid sutures, is hence triangular. 3. The sphenoidal (anterolateral) and mastoid (posterolateral) fontanelles are small, irregular and at sphenoidal and mastoid angles of the parietal bones.

The softness of the bones and their loose connections at the sutures enable the calvaria to undergo changes of shape, called molding during birth. During molding of the fetal skull, the frontal bone becomes flat, the occipital bone is drawn out, and one parietal bone slightly overrides the other. The bony and cartilaginous base of the skull is deformed little during birth. Within a day or so after birth, the shape of the calvaria returns to normal.