Essential Clinical Oral Biology

 
 
Wiley-Blackwell (Verlag)
  • erschienen am 1. Februar 2016
  • |
  • 184 Seiten
 
E-Book | ePUB mit Adobe DRM | Systemvoraussetzungen
978-1-118-93966-6 (ISBN)
 
Essential Clinical Oral Biology is an accessible guide to oral biology, introducing the scientific knowledge necessary to succeed in clinical practice.
* Student-friendly layout with clinical photographs throughout
* Each chapter has clearly defined key topics and learning objectives
* Covers the essentials: what you need to know and why
* Companion website featuring interactive MCQs, teaching presentations and downloadable images
1. Auflage
  • Englisch
  • New York
  • |
  • Großbritannien
John Wiley & Sons
  • 23,80 MB
978-1-118-93966-6 (9781118939666)
1118939662 (1118939662)
weitere Ausgaben werden ermittelt
Stephen Creanor is Clinical Professor of Oral Sciences at Plymouth University Peninsula Schools of Medicine and Dentistry, Plymouth, UK. Stephen has extensive experience of teaching the scientific basis of clinical dentistry and has published widely on the subject. As well as regularly undertaking basic science and oral biology teaching, Stephen is also an active clinician.
Contributors, xxx
Preface, xxx
About the companion website, xxx
1 An Introduction to the Human Dentition
Stephen Creanor
2 Oral Embryology
Stuart McDonald
3 Tooth Development
Stephen Creanor and Kamran Ali
4 Enamel
Paul Anderson and Stephen Creanor
5 The Dentine-Pulp Complex
Stephen Creanor, Christopher Tredwin and Taher Elgalaid
6 Cementum
Stephen Creanor
7 Alveolar Bone
Stephen Creanor and Kamran Ali
8 The Periodontal Ligament (PDL)
Vehid Salih and Svetislav Zaric
9 Oral Mucosa
Kamran Ali
10 The Gingiva
Ewen McColl and Stephen Creanor
11 Tooth eruption and development of the occlusion
Declan Millett and Stephen Creanor
12 The Salivary Glands
Stephen Creanor and Kamran Ali
13 Saliva
Stephen Creanor and Kamran Ali
14 Maxillary Sinus
Stuart McDonald, Stephen Creanor and Kamran Ali
15 The Temporomandibular Joint
Stuart McDonald and Stephen Creanor
16 The Tongue
Stuart McDonald and Stephen Creanor
17 Lymph Nodes of the Head and Neck and the Tonsils
Stuart McDonald
18 Dental Plaque and Calculus
Louise Belfield
19 Dental Caries - the Biological Basis
Stephen Creanor
20 Introduction to Periodontal Disease
Gerry Linden and Lewis Winning
Index, xxx

Chapter 2
Oral Embryology


Stuart McDonald

Key Topics


  • Primitive pharynx and its derivatives
  • Development of the tongue
  • Development of the face
  • Development of the palate
  • Clinical considerations

Learning Objectives


  • To understand the anatomical layout of the primitive pharynx
  • To have an overview of the derivatives of the primitive pharynx
  • To be able to describe the formation of the tongue, face and palate
  • To be able to explain the embryological basis of cleft lip and palate

The Primitive Pharynx


In the fifth week of embryonic life, shortly after the closure of the neural tube, the embryo starts to show signs of development of the primitive pharynx. This is the stage of development in which the embryo shows pharyngeal arches which would form the gills in fish but which in mammals and humans are transformed for other purposes. Technically, on each side, human embryos have six pharyngeal arches but only four are visible on the outer aspect of the pharynx (Figure 2.1). This is because the fifth is rudimentary and the tissues of the sixth are buried in the substance of the future neck region. On first forming, each pharyngeal arch has a cartilage skeleton and its own muscles, nerve and blood supplies. For students, understanding of the muscles of the pharyngeal arches and their innervation is particularly useful as it helps explain the motor distribution of certain cranial nerves (see below). In recent years, it has become clear that many components of the head and neck are derived from neural crest cells (Le Douarin et al., 2007) and that the developmental processes are under complex genetic control (Berkowitz, 2013).

Figure 2.1 Scanning electron micrograph of the primitive pharynx showing four pharyngeal arches. Sheep embryo: 9?mm crown-rump length.

(Source: Dr S. W. McDonald. Reproduced with permission of Dr McDonald)

Most of the cartilage skeleton of the first pharyngeal arch is known as Meckel's cartilage. The mandible forms alongside the ventral part of the cartilage which itself involutes. The most dorsal part of the cartilage becomes the malleus, one of the ossicles of the middle ear. The sphenomandibular ligament, a fibrous band running from the spine of the sphenoid bone on the base of the skull to the lingula of the mandible, the bony projection adjacent to the mandibular foramen, is the remnant of the perichondrium of Meckel's cartilage. The dorsal extremity of the first arch skeleton is a separate cartilage called the quadrate and forms another of the ossicles of the middle ear, the incus. The second arch cartilage, Reichert's cartilage, forms the stapes of the middle ear, the styloid process of the temporal bone and the lesser horn and upper part of the body of the hyoid bone. The third arch cartilage forms the greater horn and lower part of the body of the hyoid bone. The fourth and sixth arches form the cartilages of the larynx: the thyroid, cricoid and arytenoid cartilages.

In mammals and humans, the muscles, which in fish would move the gill cartilages, are transformed for other uses with some of them being translocated to parts of the head remote from the mouth and pharynx. Thus, the muscles of the first arch become the masseter, temporalis and pterygoid muscles, the anterior belly of digastric and mylohyoid, the tensor veli palatini muscle and the tensor tympani muscle. All are supplied by the mandibular division of the trigeminal nerve, the motor nerve of the first pharyngeal arch. The second pharyngeal arch gives the muscles of facial expression, all supplied by the facial nerve which is the nerve of the second arch, along with other muscles with the same innervation: posterior belly of digastric, stylohyoid, stapedius. Only one muscle forms from the third arch. This is the stylopharyngeus muscle, a small muscle of the pharynx, supplied by the nerve of the third arch, the glossopharyngeal nerve. The other muscles of the pharynx and larynx form from the muscles of the fourth and six arches and the vagus nerve is the nerve of both these arches.

The arteries of the pharyngeal arches are also transformed to other uses by contributing to components of the arch of the aorta, the carotid arteries, the pulmonary trunk and its branches.

Between the pharyngeal arches are the pharyngeal clefts (Figure 2.1) on the external surface of the future neck region and the pharyngeal pouches on the internal surface of the primitive pharynx. Each cleft and pouch is numbered from the arch anterior to it. The first pharyngeal cleft and the internal pouch thus lie posterior to the first arch. The first cleft is later transformed to become the external acoustic meatus, and the first pouch becomes the middle ear and auditory tube. The closing membrane that initially separates the first cleft from its corresponding pouch becomes the eardrum, the tympanic membrane. The second pouch is represented by the recess occupied by the palatine tonsil, the tonsillar fossa. The other pharyngeal clefts and pouches with their closing membranes are effaced and become incorporated into the tissues that form the side of the neck.

In the fifth week, it soon becomes apparent that the first pharyngeal arch of each side has two parts: a rostral maxillary process and a caudal mandibular process (Figure 2.2). Here it is necessary to introduce the terms rostral and caudal. Caudal is obvious; it means towards the tail end of the embryo. 'Rostral' is a useful expression. We might say that the upper limb bud is cranial or on the head side of the lower limb bud. However, when considering structures in the head, we cannot say that the maxillary process is cranial to the mandibular process, but express this relation by saying it is rostral to it. The mandibular process is larger than the maxillary process and the mandibular processes of the two sides meet across the ventral aspect of the primitive pharynx. The mandibular process will form the lower lip, chin, and lower jaw. The maxillary processes do not meet across the midline and the tissue that initially lies between them is known as the frontonasal process (Figure 2.2). The two maxillary processes, the frontonasal process and the right and left mandibular processes, bound the entrance into the primitive pharynx. This entrance thus forms a primitive mouth called the stomatodeum or the stomodeum.

Figure 2.2 Scanning electron micrograph of the face of a sheep embryo in the fifth week. The two maxillary processes, the frontonasal process and the right and left mandibular processes, bound the entrance into the primitive pharynx, the stomatodeum.

(Source: Dr S. W. McDonald. Reproduced with permission of Dr McDonald)

Development of the Tongue


The tongue forms from the floor of the primitive pharynx and comes about by the coalescence of four low swellings (Figure 2.3). A rounded elevation that crosses the midline close to the region of the mandibular processes is called the tuberculum impar and will form much of the anterior two-thirds of the tongue. Anterolateral to the tuberculum impar, the most anterior parts of the tongue form from a pair of lateral lingual swellings, low elevations that soon blend with each other and with the tuberculum impar. The posterior part of the tongue forms from the posterior part of the floor of the primitive pharynx between the tuberculum impar and the laryngeal inlet. This region is known as the copula or hypobranchial eminence. In the midline of the primordium of the tongue, at the site where the developing tuberculum impar and hypobranchial eminence meet, a bud of endoderm gives rise to the epithelial cells of the thyroid gland that produces thyroid hormone. The developing thyroid gland separates from the tongue and is drawn down to its definitive position as the head of the embryo grows away from the heart by the seventh week. The foramen caecum of the definitive tongue represents the site from which the thyroid bud arose. Initially an epithelial duct, the thyroglossal duct, connects the thyroid gland with the tongue, but it soon becomes solid and involutes completely.

Figure 2.3 Diagram of the embryonic swellings that contribute to the primitive tongue.

(Source: Dr S. W. McDonald. Reproduced with permission of Dr McDonald)

The sensory innervation of the mucosa of the oral cavity, pharynx and larynx also reflects the development. Each pharyngeal arch is associated with a particular cranial nerve: first arch - CNV, second arch - CNVII, third arch - CNIX, fourth and sixth arches - CNX. In addition to supplying muscles, these cranial nerves also supply the sensory innervation to the mucosa in the vicinity of the corresponding arch. Thus, the oral cavity and the anterior two-thirds of the tongue, formed from the first arch, come to be supplied by the trigeminal nerves. The maxillary nerve gives sensation to structures formed in association with the maxillary processes such as the nasal cavity and palate while more caudal regions are supplied by the mandibular nerves, the nerves of the mandibular processes. The posterior third of the tongue and the oropharynx receive their sensation via the glossopharyngeal nerves and the interior of the larynx and the laryngopharynx receive from the vagus nerves. The facial nerve, the nerve of the second arch, does not, however,...

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