The development of skull is a blend of the morphogenesis and growth of three main skull entities. These skull entities are composed of the following :
Neurocranium The Face The Masticatory Apparatus
The Neurocranium comprises of the following : Vault of the Skull or Calvaria or Desmocranium. Cranial Base or Chondrocranium. www.indiandentalacademy.com
The Face (Orthognathofacial Complex) Also known as Splanchnocranium or Viscerocranium forms the visual, olfactory, respiratory aditus and oromasticatory apparatus. The Masticatory Apparatus is composed of the jaw bones,their joints and musculature and the teeth.
The cranial base is shared by both neurocranial & facial elements. The masticatory apparatus is composed of both facial & dental elements. Thus skull is a mosaic of individual components, each of which grows to the proper extent & in the proper direction.
Three general patterns of development of congenital craniofacial defects have been observed : Hypoplastic defects may improve with â€œcatch-upâ€? growth minimizing the defect. The defective pattern of growth is maintained throughout childhood, so that malformation is retained to some degree in the adult. The developmental derangement worsens with age,severity becomes greater in adulthood.
Differentiation & growth of chondrocranium appear to be strongly genetically determined and subject to minimal environmental influence. Growth of desmocranium and splanchnocranium subject to minimal genetic determination but strongly influenced by local environmental factors. The calvaria grows most rapidly in response to early expanding brain, followed by nasal airway system and lastly, the masticatory system.
The mesenchyme that gives rise to the vault of the neurocranium is arranged first as a capsular membrane around the developing brain. The membrane is composed of two layers : Inner Endomeninx (Neural crest origin) Outer Ectomeninx (Paraxial mesodermal & Neural crest origin) www.indiandentalacademy.com
The duramater and its septa show distinctly organized fibre bundles closely related and strongly attached to the sutural systems. The adult form of the neurocranium is the end result of the preferential direction of the forces set up by growth of the brain.
In somite period embryo, the neural tubeâ€™s covering duramater and its surface ectoderm are in contact. Transient maintainance of this contact during development causes a dural projection, that extends into the future frontonasal area. www.indiandentalacademy.com
As the nasal capsules surround the dural projection, the resulting midline canal forms the basis of the Foramen Caecum.
Ossification of the intramembranous calvarial bones depend upon the presence of brain. In its absence, no bony calvaria forms. The condition is known as Anencephaly.
Several primary and secondary ossification centers develop in the outer layer of the ectomeninx, to form individual bones. It gives rise to major portions of the Frontal, Parietal, Sphenoid, Temporal & Occipital bones. Neural Crest provides the mesenchyme forming the Lacrimal, Nasal, Zygomatic, Maxillary and Mandibular bones. www.indiandentalacademy.com
A pair of frontal bones appears from single primary ossification centers at the 8th week post conception. Three pairs of secondary centers appear later. Fusion between these centers is completed at 6 to 7 months post conception.
At birth, frontal bones are separated by frontal or metopic suture; synostotic fusion of this suture usually starts about the 2nd year & unites into a single bone by 7 years. The frontal suture persists into adulthood in 10-15% of skulls. www.indiandentalacademy.com
Two parietal bones arise from two primary ossification centers for each bone that appear in the parietal eminence in 8th week post conception. Delayed ossification may result in sagittal fontanelle at birth. www.indiandentalacademy.com
The supranuchal squamous portion of the occipital bone ossifies intramembranously from two centers, one on each side, appearing the 8th week post conception. Rest of the occipital bone ossifies endochondrally. www.indiandentalacademy.com
The squamous portion ossifies intramembranously from a single center appearing at the root of zygoma at the 8th week post conception. The tympanic ring ossifies intramembranously from four centers appearing in the 3rd month after conception. www.indiandentalacademy.com
If any unusual ossification center develop between the individual calvarial bones, their independent existence is recognizable as small sutural or wormian bones. The earliest centers of ossification first appear during 7th & 8th weeks post conception, but ossification is not completed well after birth. The mesenchyme between the bones develops fibres to form syndesmotic articulations. www.indiandentalacademy.com
At birth, the individual calvarial bones are separated by sutures of variable width & fontanelles. Six fontanelles are present : Anterior, Posterior, Posterolateral and Anterolateral. www.indiandentalacademy.com
At birth, neurocranium has achieved 25% of its ultimate growth, completes 50% by 6 months, 75% by 2 years and by 10 years 95% of neurocranial growth is completed. By 10 years, facial skeleton has achieved only 65% of its total growth. In postnatal life, neurocranium increases 4-5 times in volume whereas facial skeleton increases 8-10 times its volume at birth. www.indiandentalacademy.com
The expanding brain exerts separating tensional forces upon the bone sutures, thereby secondarily stimulating sutural bone growth. The brain acts in this context as a â€œ Functional Matrixâ€? in determining the extent of neurocranial bone growth. www.indiandentalacademy.com
In mid-gestational period (4-5 months), average size of head is 18cm., which nearly doubles to average of 33cm.at birth. During the 1st year, it reaches an average of 46cm.and then slows. Head circumference reaches 49 cm.at 2 years and only 50 cm.at 3 years. The increase between the age of 3 years and adulthood is only about 6 cm.
It is a combination of : SUTURAL GROWTH SURFACE APPOSITION & RESORPTION CENTRIFUGAL DISPLACEMENT
By 4 years of age, lamellar compaction of cancellous trabeculae forms the inner & outer tables of the cranial bones. The inner table is primarily related to brain & intracranial pressures. The outer table is more responsive to extracranial muscular & buttressing forces. www.indiandentalacademy.com
The two cortical plates are not completely independent. The thickening in the region of glabella results from separation of inner & outer tables with the invasion of the frontal sinus. Only external plate is remodeled, as the internal plate becomes stable at 6-7 years of age. www.indiandentalacademy.com
Inner aspect of Frontal bone can be used as a stable reference point from age 7 years onward. www.indiandentalacademy.com
Defects in closure of Foramen Caecum www.indiandentalacademy.com
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