LECTURE 4 SKULL
NEUROCRANIUM VISCEROCRANIUM VISCEROCRANIUM VISCEROCRANIUM
CRANIUM NEUROCRANIUM (protective case around brain) VISCEROCRANIUM (skeleton of face) NASOMAXILLARY COMPLEX MANDIBLE (DESMOCRANIUM) CRANIAL VAULT (CHONDROCRANIUM) CRANIAL BASE
NEUROCRANIUM (CRANIAL VAULT) Houses and protect the brain FRONTAL OCCIPITAL SPHENOID UNPAIRED BONES ETHMOID PARIETAL TEMPORAL PAIRED BONES
Skull base (external) Skull base (internal)
VISCEROCRANIUM (FACE & JAW) MANDIBLE VOMER UNPAIRED BONES MAXILLA ZYGOMATIC NASAL LACRIMAL PAIRED BONES PALATINE INFERIOR CONCHA
NEUROCRANIUM MEMBRANOUS CARTILAGINOUS NEURAL CREST CELLS & PARAXIAL MESODERM Prechordal chondrocranium NEURAL CREST CELLS Chordal chondrocranium OCCIPITAL SCLEROTOMES PARAXIAL MESODERM
The human skull consists of three components: (1) the membranous neurocranium, which constitutes the flat bones of the skull, (2) the cartilaginous neurocranium or chondrocranium which forms the majority of the skull base, and (3) the viscerocranium or facial skeleton. 8
The basicranium develops primarily from cartilage precursors, with a small component from membranous bone. The development of the cartilaginous skull base begins around the 40th day of gestation, with the conversion of mesenchyme into cartilage. Occipital sclerotomal mesenchyme concentrates around the notochord and extends cephalically forming the floor of skull
The Parachordal Cartilages and Early Chondrogenesis The chondrocranium begins to form when the collections of mesenchyme accumulating around and in front of the notochord condense into cartilage. These chondrification centers, termed the parachordal cartilages, form early in the seventh week adjacent to the rostral end of the notochord and contribute to the creation of the basal plate. The parachordal cartilage fuse with the sclerotomes arising from the occipital somites surrounding the neural tube.
CARTILAGINOUS NEUROCRANIUM Cartilages that lie in front of rostral end of notochord arise from neural crest cells forming prechordal chondrocranium Cartilages posterior to this end arise from Occipital sclerotomes forming Chordal chondrocranium
Dorsal view of the chondrocranium, or base of the skull, in the adult showing bones formed by endochondral ossification. Bones that form rostral to the rostral half of the sella turcica arise from neural crest and constitute the prechordal (in front of the notochord) chondrocranium (blue). Those forming posterior to this landmark arise from paraxial mesoderm (chordal chondrocranium) (red).
Base of the skull All cartilages fuse Ossify by Endochondral ossification
MEMBRANOUS NEUROCRANIUM Mesenchyme from these sources invests the brain and undergoes membranous ossification Needle like bony spicules radiate from primary ossification centre towards periphery
With growth during fetal and postnatal life, membranous bones enlarge Apposition on outer surface Osteoclastic resorption from inside
kjjkkjhihihk VISCEROCRANIUM MEMBRANOUS CARTILAGINOUS Intramembranous ossification in maxillary and mandibular prominence of first pharyngeal arch Neural crest cells squamous temporal, maxillary, zygomatic bone, mandible Bones and connective tissue of craniofacial structures
Embryo at 4-5 weeks (Lateral view) By 4th week of development, bars of mesenchymal tissue separated by deep clefts. 5 th branchial (pharyngeal) arches appear along the lateral wall of pharyngeal gut. At the end of 4th week center of face is formed by stomodeum surrounded by 1 st pair of branchial (pharyngeal) arches
BRANCHIAL (PHARYNGEAL) ARCHES The first pharyngeal arch Consists of: Dorsal portion - maxillary process Ventral portion - mandibular process Mesenchyme of maxillary process gives rise to Premaxilla Maxilla Zygomatic bone Part of temporal bone (through membranous ossification) Mandible is formed by membranous ossification of mesenchyme surrounding Meckel s cartilage. The first pair of pharyngeal arches plays a major role in facial development
Face develops from 5 prominences that surround the stomatodeum - Frontonasal - Paired maxillary processes. - Paired mandibular processes.
Early in the 4 th week, five primordial swellings consisting primarily of neural crest-derived mesenchyme appear around the stomodeum and play an important role in the development of face 1 Frontonasal prominence 2 Maxillary prominences 2 Mandibular prominences Stomodeum
The single frontonasal prominence ventral to the forebrain The paired maxillary prominences develop from the cranial part of first pharyngeal arch The paired mandibular prominences develop from the caudal part of first pharyngeal arch Lateral view
By the end of 4 th week, bilateral oval-shaped ectodermal thickenings called nasal placodes appear on each side of the lower part of the frontonasal prominence Nasal placodes are primordia of the nose and nasal cavities. Frontonasal prominence
Frontonasal prominence formed by proliferation of mesenchyme ventral to the forebrain. It forms - Lateral optic diverticula eyes - Forehead (between the eyes) - Nasal placodes
Mesenchyme proliferates around the placodes producing medial and lateral nasal prominences Lateral nasal prominence separated from maxillary process by nasolacrimal groove
Mesenchymal cells proliferate at the margin of the placodes and produce horse-shoe shaped swellings around these. The sides of these swellings are called medial and lateral nasal prominences The placodes now lie in the floor of a depression called nasal pits Each lateral nasal prominence is separated from the maxillary swelling by nasolacrimal groove
The maxillary prominences continue to increase in size and: Laterally, merge with the mandibular prominences to form the cheek Medially, compress the medial nasal prominences toward the midline and finally fuses with these to form the upper lip. The upper lip is formed by the two medial nasal prominences & the two maxillary prominences
The medial nasal swellings enlarge, grow medially and merge with each other in the midline to form the intermaxillary segment Human embryo: 7 weeks
At 42 days embryo Differentiation of structers depends on epithelial and mesenchymal interactions
Changes in Face during Fetal period Mainly result from changes in the proportion & relative positioning of facial structures In early fetal period the nose is flat and mandible underdeveloped. They attain their characteristic form during fetal period The enlargement of brain results in the formation of a prominent forehead Eyes initially appear on each side of frontonasal prominence move medially Ears first appear on lower portion of lower jaw, grow in upper direction to the level of the eyes
Facial clefts Failure of the embryonic facial prominences to fuse properly May be unilateral or bilateral May involve: Lips only: Cleft lip Palate only: Cleft palate Lip & palate: Cleft lip & palate Region of nasolacrimal groove: Facial clefts Lead to difficulty in breathing feeding sucking swallowing & speech
Median cleft lip: results from failure of the medial nasal prominences to merge and form the intermaxillary segments
Bilateral cleft lip: results due to failure of maxillary prominences to meet and unite with the medial nasal prominences on both sides
Unilateral cleft lip: result from failure of the maxillary prominence to merge with the medial nasal prominence on the affected side
Oblique facial cleft: results from failure of the maxillary prominence to fuse with the lateral nasal prominence
Cleft lip coupled with clefts of the anterior palate or entire palate.
Oblique facial cleft Cleft palate leaves the nasal and oral cavities connected & results in nursing problem for the new born May be: Anterior/posterior to incisive foramen Unilateral/bilateral Isolated/associated with cleft lips Cleft lip, cleft jaw & cleft palate
Absence or hypoplasia of the bone of the cranium
PRENATAL CHANGES OF THE SKULL
SKULL of the NEWBORN
Functions of fontanelles: 1) alignment of fluctuations in intracranial pressure; 2) displacement of the bones; 3) sources of apposition of bone growth; 4) amortization. TIMINGS OF CLOSURE: ANTERIOR FONTANELLE= 18 months of age POSTERIOR FONTANELLE = 1 to 2 months of age
Post Natal Development of skull
Features of new born skull proportion (brain / front offices 8/1); incompleteness of ossification (bone fragments); sutures are not formed; a number of cartilage at the base of the skull; fontanelles; the lack of buttresses; pneumatic sinuses are not formed mandibular angle-150 degree (120 in adults); alveolar bone of the upper jaw and alveolar part of the mandible are not developed; large orbit.
Factors of formation of the cranial vault: growing brain intracranial pressure growing brain action of soft tissue The growth mechanism of the skull. The arrows show the movement of the bones of the associated with the expansion of the brain capsule. Zone regional bone growth are marked in black. Osteogenesis in sutures is compensatory and increases the surface moving bones.
Growth of cranial base Cranial base grows post-natally by complex interaction between the following three growth processes. Extensive cortical drift and remodelling Elongation at synchondrosis Sutural growth
Factors of formation of the skull base Inside surface Outer surface: brain development; the development of the sense organs; the formation of dentoalveolar apparatus; dura mater organic interactions muscle traction (support function);
Factors of formation of the facial skull support for the visceral organs; development of the sense organs, primarily the nose of the capsule; the formation of dentoalveolar apparatus; action of of soft tissue.
Connections of bones of a skull Sutures
Connections of bones of a skull SYNCHODROSIS
Connections of bones of a skull Temporomandibular joint
PARANASAL SINUSES
Functions of paranasal sinuses reduction in weight of the skull to reach equilibrium of the head; means for increasing the volume of the head; participate in breathing; resonator function; participation in the sense of smell (in animals); protection and isolation of the brain and the eye socket from vibration; protection of the nasal cavity by heat leakage; tools for growth and development of the skull;