Embryology 2 Notes PDF

Lecture two Embryology of the head and neck Development of the face ,palate and pharyngeal arches Mesenchyme formation of the head region is derived from paraxial and lateral plate mesoderm,neural crest and thickened regions of ectoderm known as ectodermal placodes. Paraxial mesoderm forms most...

Lecture two Embryology of the head and neck Development of the face ,palate and pharyngeal arches Mesenchyme formation of the head region is derived from paraxial and lateral plate mesoderm,neural crest and thickened regions of ectoderm known as ectodermal placodes. Paraxial mesoderm forms most of the cell walls and floor of the brain case, all voluntary muscles of the craniofacial region, the dermis and connective tissues in the dorsal region of the head and the meninges caudal to the prosencephalon. Lateral plate mesoderm forms the laryngeal cartilage (arytenoids and cricoids) and connective tissue in this region. Neural crest cells originate in the neuroectoderm of the for-,mid-and hindbrain regions and migrate ventrally into the pharyngeal arches and rostrally around the forebrain and optic cup into the facial region. In these locations they form the midfacial and pharyngeal arch skeletal structures and all other tissues in these regions. The posterior and anterior cranial bases are derived from distinct embryologic origins and grow independently--the anterior cranial base solely from the neural crest, the posterior cranial base from the paraxial mesoderm. Migration of neural crest cells figure The most typical feature in the development of the head and neck is formed by the branchial or more correctly the pharyngeal arches. They appear in weeks 4and5 of development and largely contribute to the characteristic external appearance of the embryo. Initially they consist of bars of mesenchyme separated by deep clefts, at the same time outpocketings along the lateral wall of the pharyngeal gut the so called pharyngeal pouches penetrate the surrounding mesenchyme but do not establish a communication. The pharyngeal arches not only contribute to the formation of the head and neck but also play a pivotal role in the formation of the face. At the end of the fourth week the center of the face is formed by the stomedeum surrounded by the first pair of pharyngeal arches that contribute five prominences 1. Two mandibular prominences caudal to the stomadeum 2. Two maxillary prominences lateral to the stomadeum 3. One frontonasal prominence canial to the stomadeum The development of the face is further complemented by the formation of the nasal prominences. Light blue: frontonasal, yellow maxillary, red mandibular prominences Pharyngeal arches: Each pharyngeal arch is made up of a core of mesenchyme covered by surface ectoderm on the inside and endodermal epithelium on the outside. In addition to mesenchyme derived from the paraxial and lateral plate mesoderm the core of each arch is receives a large number of neural crest cells which contribute heavily to the skeletal components of the face. The original mesoderm of the arch mainly gives rise to the muscles of the face and neck. Each pharyngeal arch is characterized by its own muscular component and accompanying nerve and artery. Wherever the muscles migrate they take the nerve with them. The first pharyngeal arch Consists of a dorsal portion the maxillary process, a ventral portion the mandibular process which contains Meckels cartilage (this cartilage disappears except for two portions which go on to form the malleus and incus). The mesenchyme of the of the maxillary process gives rise to the premaxilla, maxilla, zygomatic bone and squamous part of the temporal bone through membranous ossification Similarly the mandible is formed by membranous ossification of mesenchyme surrounding Meckels cartilage. The rest of the derivitaves of the arches are clearly noted in table 1. Second pharyngeal arch The cartilage of the second arch (Reichert's cartilage) gives rise to most of the skeletal structures of the second arch (see table 1.) Third arch see table 1. Fourth and sixth arch: the cartilaginous components of these arches fuse to form the cricoid , thryoid, arytenoids, corniculate and cuneiform cartilages of the pharynx. The muscle nerve and blood supply however are separate (see table 1.) The fifth arch doesn't seem to develop in humans.  Table 1. Pharyngeal Arch Derivatives Structures derived from Arches Arch Nerve Muscles Skeletal Artery Mandible,maxilla Sphenomandibular mastication Ligament 1 (temporalis, maxillary (remnants of trigeminal (V) masseter, medial (maxillary/mandibular) Meckels (terminal branches) pterygoid, lateral cartilage):malleus, pterygoid) incus facial expression stapes, styloid stapedial ( buccinator, process, lesser (embryonic) platysma, cornu of hyoid, 2 stapedius, upper part of body corticotympanic (hyoid) facial (VII) stylohyoid, of hyoid bone (adult) digastric Reichert's cartilage posterior belly) greater cornu of common carotid, glossopharynge Stylopharyngeus hyoid, lower part of internal carotid 3 al (IX) body of hyoid bone (root) intrinsic muscles thyroid, cricoid, 4 - aortic arch, right superior of larynx, arytenoid, subclavian laryngeal and 4 and 6 recurrent pharynx; levator corniculate and 6 - ductus arteriosus, laryngeal branch palati cuneform cartilages pulmonary (roots) of vagus (X) Structures derived from Pouches POUCH Overall Structure Specific Structures tympanic membrane, tympanic cavity, tubotympanic recess 1 mastoid antrum, auditory tube crypts of palatine tonsil, lymphatic nodules intratonsillar cleft 2 of palatine tonsil inferior parathyroid gland, (dorsal wing) 3 thymus gland(ventral) superior parathyroid gland, 4 ultimobranchial body 5 The face : At the end of the fourth week the facial prominences, consisting primarily of neural crest derived mesenchyme and mainly formed by the first pair of pharyngeal arches, appear. As previously mentioned the maxillary prominences can be distinguished lateral and the mandibular prominences caudal to the stomadeum. The frontonasal prominence ,formed by the proliferation of of mesenchyme ventral to the brain vesicles constitutes the upper border of the stomadeum. On both sides of the frontonasal prominences are local thickenings of the surface ectoderm, the nasal placodes, which form under the influence of the ventral portion of the forebrain. During week 5, the nasal placodes invaginate to form the nasal pits.this creates a ridge of tissue that surrounds each pit and form the nasal prominences. the prominences on the outer edge of the pits are the lateral nasal prominences and those on the inner edge are the medial nasal prominences. During the following two weeks , the maxillary prominences continue to increase in size. Simultaneously, they grow in the medial direction,compressing the medial nasal prominences toward the midline. Subsequently the cleft between the medial nasal prominence and the maxillary prominences is over bridged and the two fuse.Hence the upper lip is formed by those two prominences,with no contribution from the lateral nasal prominences. The lower lip and mandible are formed from the merging of the mandibular prominences across the midline. Initially the lateral nasal and maxillary prominences are separated by the nasolacrimal groove. The ectoderm in the floor of the groove forms a solid cord that then detaches from the overlying ectoderm,after canalization it becomes the nasolacrimal duct and its upper part widens to form the lacrimal sac. Once this occurs these prominences merge. The duct runs to the nasal cavity. The maxillary prominences then enlarge to form the cheeks and maxillae proper. The nose is formed from five facial prominences, the frontonasal gives rise to the bridge of the nose,the merged mesial prominences provide the crest and tip and the lateral nasal prominences form the alae of the nose. The intermaxillary segment and the primary palate As the medial prominences merge their merger goes quite deep. The structure formed by the two prominences is known as the intermaxillary segment. This segment is composed of: A labial component which forms the philtrum of the upper lip.1 An upper maxillary component which carries the central and lateral incisors.2 A palatal component, which forms which forms the triangular primary palate.3 Cranially the intermaxillary segment is continuous with the nasal septum. The secondary palate The main part of the definitive palate is formed by the two shelves emanating from the maxillary prominences. These palatine shelves appear in week six and are directed obliquely downwards on either side of the tongue. In week 7 the shelves ascend to attain a horizontal position above the tongue and fuse with each other forming the secondary palate. Anteriorly the shelves fuse with the triangular primary palate (premaxilla) and the incisive foramen is considered the midline landmark between both palates. At this time the nasal septum is growing and fuses with the cephalic aspect of the new palate. Failure of fusion of the primary and or the secondary palates lead to various clefts in the lip and palate region (see pharyngeal arch abnormalities below) Development of the tongue The tongue appears in embryos approximately at week four in the form of two lateral lingual swellings and one medial swelling(tuberculum impar). These three swellings originate from the first pharyngeal arch. A second median swelling,tho copula or hypobranchial eminence is formed by the mesoderm of the second, third and part of the fourth arches.( some believe they are two entities that eventually override). Finally a third median swelling formed by the posterior part of the fourth arch ,marks the development of the epiglottis.immediately behind this swelling is the laryngeal orifice, which is flanked by the arytenoids swellings As a result of growth of the lateral swellings,they overgrow the tuberculum impar and merge with eachother, thus forming the anterior two thirds (body) of the tongue. Since the mucosa of the anterior two thirds of the tongue originates from the first pharyngeal arch it is innervated by the manibular branch of the trigeminal nerve. The posterior part of the tongue (root) originates from the second ,third and part of the fourth pharyngeal arch. Since the adult innervations of this part of the tongue is supplied by the glossopharyngeal nerve, it seems likely that the tissue of the third arch has overgrown that of the second. The extreme posterior part of the tongue and epiglottis are innervated by the superior laryngeal nerve, indicating their development from the fourth arch. Some of the tongue muscles probably differentiate in situ, but most are derived from myoblasts originating in the occipital somites. This is supported by the fact that that tongue musculature is innervated by the hypoglossal nerve. Taste to the anterior 2/3 of the tongue is supplied by the chorda tympani a branch of the facial nerve (pretremetic nerve) which hitchhikes on the lingual branch of the trigeminal nerve to supply this sensation to the tongue. Taste to the posterior part of the tongue is supplied by the glossopharyngeal nerve. Thyroid gland The thyroid appears to be an epithilial proliferation in the floor of the pharynx between the tuberculum impar and the copula, at a point later indicated by the foramen caecum. The thyroid descends in front of the pharyngeal gut as a bilobed diverticulum. During this migration the gland remains connected to the tongue by a narrow thyroglossal duct. This duct later disappears. With further development the thyroid gland descends in front of the hyoid bone and the laryngeal cartilage. It then reaches its final destination ventral to the trachea at week 7. At this stage it has acquired an isthmus and two lateral lobes. The thyroid begins to function at the end of the third month. Abnormalities: Thyroglossal cyst and fistula A thyroglossal cyst can appear at any point along the migratory path of the gland ,but it always close to the midline of the neck. If the cyst communicates with the outside it forms a fistula these fistulas can be primary(at birth) or secondary to cyst rupture. Aberrant thyroid tissue may also be found along the path and is subject to the same diseases of the gland itself, this is commonly found just behind the foramen caecum. Abnormalities First Arch Syndromes 2 major types, both result in extensive facial abnormalites Treacher Collins Syndrome: Occurs in about 1 in 50,000 live births. The inheritance pattern of Treacher Collins syndrome is usually autosomal dominant, but there is an autosomal recessive gene that causes less than 2% of Treacher Collins cases. Signs and symptoms: abnormally formed ears small or absent ear canals hearing loss dental problems down-slanting eyes notched lower eyelids missing eyelashes small jaw cleft palate Pierre Robin Sequence: PRS is a condition with several clinical features: a small lower jaw (micrognathia), displacement of the tongue toward the back of the oral cavity (glossoptosis) and, often but not always, an opening in the roof of the mouth (cleft palate). PRS was named after Dr. Pierre Robin, a French dental surgeon who first observed its features during the early 20th century. While the precise cause is not fully clear, the current belief is that multiple contributing factors lead to sequential physical changes within the oral cavity, which ultimately leads to airway obstruction. For this reason, breathing problems are common manifestations of PRS. Feeding problems are also common, since the oral cavity also serves as a conduit to the gastrointestinal tract. Infant with Pierre Robin Sequence DiGeorge Syndrome absence of thymus and parathyroid glands 3rd and 4th pouch do not form disturbance of cervical neural crest migration Cleft lip and palate : Occurs 1: 700 live births Occurs more in males 300+ different abnormalities Different cleft forms and extent upper lip and ant. maxilla hard and soft palate Anterior and posterior clefts should be treated as separate entities. Maternal teratogenic Effects Teratogens are agents that can disturb the development of the embryo or fetus. Examples include : 1. Retinoic Acid present in skin ointments 1988 associated with facial developmental abnormalities of the face, skull, cardiovascular eg (fallots tetralogy), nervous system and thymic abnormalities. 2. Alcohol Fetal Alcohol Syndrome is caused by the mothers use of alcohol during the period of early development (week 3+) facial and neurological abnormalities lowered ears, small face, mild+ retardation Microcephaly - leads to small head circumference Short Palpebral fissure - opening of eye Epicanthal folds - fold of skin at inside of corner of eye Flat midface Low nasal bridge Indistinct Philtrum - vertical grooves between nose and mouth Thin upper lip Micrognathia - small jaw This is a brief account of the devolopment of some structures in the head and neck and some of the anomalies associated with it. Recommended reading Langmans medical embryology www.indiana.edu/~anat550/hnanim/index.html http://www.indiana.edu/~anat550/hnanim/face/face.html http://www.embryology.ch/anglais/hdisqueembry/triderm07.htmlhttp://www.med.uc.edu/ embryology/chapter1/animations/contents.htm http://www.med.unc.edu/embryo_images/unit-hednk/hednk_htms/hednktoc.htm GOOD LUCK TO ALL !!!