Seminar on HUMAN EMBRYOLOGY for postgraduates

Seminar on Human Embryology.

1. HUMAN EMBRYOLOGY Dr. Neha Nainoor 1st year PG Dept of OMFS

2. CONTENTS Development of Mouth Development of the Tongue Development of Maxilla Development of Mandible Development of the Temporomandibular Joint Development of Facial Muscles Development of the Salivary Glands

3. DEVELOPMENT OF THE MOUTH Bidermal in development. Derived partly from the stomodeum (ectodermal) and partly from the cranial part of the foregut (endodermal). Epithelial lining: partly ectodermal and partly endodermal and the demarcartion between the two is buccopharyngeal membrane. The buccopharyngeal membrane disappears by 4th week.

4. PRIMITIVE ORAL CAVITY STOMATODEUM NASAL PART ORAL PART Ectodermal Derivatives Endodermal Derivatives Epithelium lining inside of lips, cheeks and palate Teeth and gums Epithelium of Tongue, Floor and Soft palate Palatoglossal and Palatopharyngeal folds

5. Alveololabial sulcus is ectodermal. Alveololingual sulcus is endodermal. Floor of the mouth: In this region, the mandibular processes take part in the formation of three structure. Lower lip and lower part of cheeks Lower jaw Tongue DEFINITIVE ORAL CAVITY

6. Roof of the mouth: Formed by the palate. The alveolar process of upper jaw is separated from the upper lip and cheek by the appearance of labiogingival furrow.

7. DEVELOPMENT OF THE TONGUE Appears in the embryo at about 4 weeks Appears as two lateral lingual swellings and a median swelling, the tuberculum impar (origin: 1st pharyngeal arch) A 2nd median swelling, the copula of His or hypobranchial eminence, is formed by 2nd, 3rd and 4th pharyngeal arches. The caudal part related to the 4th arch forms the epiglottis.

8. The Anterior two-thirds of the tongue: Derived from the mandibular arch. Formed by the fusion of tuberculum impar and the two lingual swellings. Innervation: Lingual branch of Mandibular branch of trigeminal nerve. Chorda tympani branch of the Facial nerve.

9. The posterior one-third of the tongue: Formed from the cranial part of the hypobranchial eminence (copula) The 3rd arch mesoderm grows over the 2nd arch and fuses with the mesoderm of the 1st arch. Innervation: Glossopharyngeal nerve and vagus nerve.

10. The posterior-most part of the tongue Derived from the 4th arch. Innervation: Superior laryngeal nerve (nerve of 4th arch) The tongue muscles (except palatoglossus) are derived from myoblasts originating in occipital somites . Tongue musculature is innervated by the hypoglossal nerve The body of the tongue is separated from the posterior third by a V-shaped groove, the terminal sulcus.

11. TASTE BUDS: Formed in relation to the terminal branches of the innervating nerve fibres. MUCOSA: Anterior part contains numerous cone shaped papilla. Mucosa is covered by keratinised filiform papillae. Fungiform papillae are found scattered between filiform papillae and contain taste buds. Foliate papillae are found on the lateral parts of the tongue. Taste buds are found in its non keratinised regions. Circumvalate papillae develop from cranial part of hypobrancial eminence and migrate to anterior aspect of sulcus terminalis.

12. DEVELOPMENTAL DISTURBANCES OF THE TONGUE Macroglossia: Enlarged tongue. May cause lateral displacement of the teeth, open bite. Microglossia: Underdevelopment of the tongue. Leads to underdeveloped jaws. Aglossia: Very rarely the tongue may be absent. Ankyloglossia: The apical part of the tongue may be anchored to the floor of the mouth by an overlapped frenulum. Ankyloglossia superior: The tongue may be adherent to the palate. Glossoschissis: Bifid tongue or cleft tongue.

13. PRE NATAL DEVELOPMENT OF MAXILLA Around 4th week of intrauterine life t Prominent bulge appears on ventral aspect of embryo (Developing Brain) Shallow depression below the bulge - STOMODEUM 1st Pharyngeal Arch (MANDIBULAR ARCH) FRONTONASAL PROCESS 5 Pharyngeal arches form Mesoderm covering the developing forebrain grows downward and overlaps upper part of stomodeum Forms Nasomaxillary process Part of mandibular arch

14. At this stage, the primitive mouth or the stomodeum is overlapped from : above by the frontal process, below by the mandibular process and on either sides by the maxillary processes. Formation of the nasal pits divides the frontonasal process into two parts: The Medial Nasal Process and The Lateral Nasal Process

15. POST NATAL DEVELOPMENT OF MAXILLA The post natal growth of maxilla occurs by the following mechanisms: Displacement Growth at sutures Surface remodelling

16. DISPLACEMENT: It is the movement of the whole bone as a unit and it can be of two types: Primary displacement: occurs by growth of maxillary tuberosity in a posterior direction. 2. Secondary displacement:- - occurs in downward and forward direction due to growth of cranial base

17. GROWTH AT SUTURES: The maxilla is connected to the cranium and the cranial base by a number of sutures. These include: Fronto-nasal suture Frontomaxillary suture Zygomatico-maxillary suture Zygomatico-temporal suture Pterygopalatine suture

18. SURFACE REMODELLING: Remodelling occurs by bone deposition and resorption to bring about Increase in size Change in shape Change in functional relationship Change in proportion

19. PRE NATAL DEVELOPMENT OF THE MANDIBLE By 4th week of IUL, the pharyngeal arches are laid down on the lateral and ventral aspects of the cranialmost part of the foregut that lies in close approximation with the stomodeum. The mandibular arch forms the lateral wall of the stomodeum. Its gives off a bud from its dorsal end, maxillary process. Its grows ventro-medially, cranial to the main part of the arch, which is called the mandibular process. By 5th week of IUL, the mandibular processes of both sides grow towards each other and fuse in the midline, form the lower border of stomodeum i.e, the lower lip and lower jaw.

20. Secondary cartilages like the condyle, the coronoid and the symphyseal cartilages influence furthur growth of the mandible.

21. POST NATAL GROWTH OF MANDIBLE Of all the facial bones, the mandible undergoes the largest amount of growth post-natally and also exhibits the largest variability in morphology.

22. RAMUS: The ramus moves progressively posterior by a combination of deposition and resorption. Resorption occurs on the anterior part of the ramus, while bone deposition occurs on the posterior region. This results in a 'drift' of the ramus in a posterior direction.

23. BODY OF THE MANDIBLE Body of the mandible lengthens as the ramus exhibits bone deposition on the posterior aspect and resorption on the anterior aspect.

24. ANGLE OF THE MANDIBLE On the lingual side of the angle ofhe mandible, resorption takes places on the postero-inferior aspect while deposition occurs on the antero-superior aspect. On the buccal sid, resorption occurs on the antero-superior aspect while deposition takes place in the postero-inferior part. This results in flaring of the angle of the mandible as the age advances.

25. THE ALVEOLAR PROCESS As the teeth erupt the alveolar process grow in size by bone deposition at the margins. The alveolar process adds to the height and thickness of the body of the mandible. In the absence of teeth, the alveolar bone fails to develop and it resorbs in the event of tooth extraction.

26. THE CONDYLE The mandibular condyle has been recognised as an important growth site. The head of the condyle is covered by the condylar cartilage. The presence of the condylar cartilage is an adaptation to withstand the compression that occurs at the joint.

27. THE CORONOID PROCESS Its growth follows the enlarging “V” principle. Deposition occurs on the lingual surfaces of the left and right coronoid processes, when viewd from the posterior aspect of the longitudinal section. Viewing from the occlusal aspect, the deposition on the lingual surfaces of the coronoid processes brings about a posterior growth movement in the “V” pattern

28. DEVELOPMENT OF TEMPOROMANDIBULAR JOINT TMJ development takes place mostly between the 7th and 20th week of intrauterine life . A particularly sensitive period is morphogenesis between the 7th and 11th week .

29. There are three stages in TMJ development: B lastemic stage (7th-8th week; development of the condyles, articular fossa, articular disk and capsule), C avitation (9th-11th week; beginning of lower joint space development and condylar chondrogenesis), and M aturation stage (after the 12th week)

30. From the 8th until the 16th week of development, the primordial cartilages function as the primary temporomandibular or malleoincudal joint; auditory ossicles develop from the latter. This joint can perform only simple rotation or buccal movements, which appear in the 8th week of development. In the 9th week, chondrogenesis begins from the mesenchyme cells, laterally from Meckel’s cartilage, in the middle of the condylar blastema. In the 10th week, the condylar head and the entire conical condyle are apically surrounded by the lower jaw body, which is ossified intramembraneously.

31. Enchondral ossification of the condylar cartilage in the anterior part begins in the 17th week and after the 20th week the cartilaginous form of the condyle is present only on the surface. The existence of temporal bone is visible from the 8th and 9th week. In the 10th week, there is medial thickening of the disk with mildly pronounced concave contours. The articular fossa spreads cranially from the condyle in anterior direction and from the 12th week it has a concave shape and is in its permanent position between the temporal bone and condyle.

32. The mesenchymal development of the articular capsule starts in the 8th week and stretches from the squamous part of the temporal bone towards the articular disk and the condyle. In the 11th week, the capsule is positioned between the zygomatic arch of the temporal bone and the condyle and it is attached to the outer portion of the articular disk . Lower articular space starts developing earlier but slower than the upper one, in the 9th week, and follows the condylar base shape. The upper articular space starts forming in the 11th week between the zygomatic process of the temporal bone and the articular disk. The articular spaces are disproportionate until the 26th week. The secondary TMJ is fully developed after the 14th week of intrauterine growth, anteriorly from the otic capsule, and after the 16th week it assumes the primary joint function.

33. Postnatal development of the temporomandibular joint At birth, the articular surfaces of both the mandibular condyle an d temporal bones are covered with fibrous connective tissue. Extensive remodeling occurs during postnatal life. Changes involving increases in the posterior slope of the articular tubercle and in the depth of the mandibular fossa are reported up to the 4th decade of life. Postnatal remodeling occurs in the bone adjacent to the joint in harmony with the growth of the condyle and temporal fossa. Neuromuscular changes and occlusal status are also reflected in discrete bony and functional changes in the joint region.

34. DEVELOPMENTAL ANAMOLIES OF THE JAWS AGNATHIA: Defined as the complete or partial absence of one or both jaws. A very rare condition. Autosomal recessive condition. Occurs due to failure of migration of the neural crest cells into the maxillary prominence in the 4th - 5th week of gestation.

35. 2. MICROGNATHIA: It is a condition in which the jaw is undersized. It can also result due to abnormal positioning of the jaws (Apparent Micrognathia) True micrognathia can be either congenital or acquired. Congenital heart disease and Pierre Robin Sequence are commonly associated congenital abnormalities. Trauma, infection and ankylosis are some of the acquired causes.

36. 3. MACROGNATHIA: It is a condition where the jaws are abnormally large. Causes: Pituitary gigantism, Paget's disease of bone, Acromegaly. Mandibular protrusion or prognathism is a common occurance and a prominent chin is seen.

37. 4. FACIAL HEMIHYPERTROPHY: Also called as Friedreich's disease, Hemihyperplasia. Represents the hyperplasia of the tissue rather than its hypertrophy. It can be genetic in nature or be caused due to vascular abnormalities or due to endocrine dysfunction.

38. 5. FACIAL HEMIATROPY: Also called as Parry Romberg Syndrome or Progressive Facial Hemiatrophy. It is a rare disorder, characterised by slowly progressive wasting of the soft tissues of half of the face. The wasting is associated with the skin, cartilage, connective tissue, muscle and bone. Causes: Localised scleroderma, malfunction of sympathetic nervous system, loss of adipose tissue or even a familial tendency. Coup de sabre: Some patients may show sharp line of demarcation resembling large linear scar between normal and abnormal tissue.

39. 6. TREACHER COLLINS SYNDROME (MANDIBULOFACIAL DYSOSTOSIS) Rare syndrome characterized by bilaterally symmetrical abnormalities derived from the 1st and 2nd branchial arches. An autosomal dominantly inherited disorder. Arises from aberrations in the development of the facial structure during histodifferentiation morphogenesis between approximately the 20th day and 12th week of intrauterine life. These aberrations result from the destruction of neural crest cells before they migrate to form the facial processes.

40. DEVELOPMENT OF FACIAL MUSCLES During the 5th and 6th weeks, myoblasts within the mandibular arch begin proliferating and become oriented to the sites of origin and insertion of the muscles they form. By 7th week, the mandibular muscle mass enlarges and beguns migration and differentiation into the 4 muscles of mastication : the masseter, temporal, medial and lateral pterygoid muscles. The muscles within the hyoid arch and in the occipital myotomes undergo proliferation and migrate anteriorly towards the floor of the mouth to form muscles of the tongue. Muscles of the 3rd and 4th arc h form the pharyngeal muscles : stylopharyngeus, cricothyroid, levator palatini and constrictor muscles of pharynx.

41. By 10th prenatal week, the mandibular arch muscles become well organised bilaterally. The masseter and medial pterygoid muscle forms the vertical sling in the developing coronoid process. The lateral pterygoid muscle fibres, which also arise from the infratemporal fossa, extend horizontally to the necks of condyles and insert in the articular discs. The pharyngeal constrictor muscles differentiate and enclose the pharynx. The face changes shape considerably as it grows, and all its muscles develop to meet the increasing fuctional demand.

42. DEVELOPMENT OF SALIVARY GLANDS There are 3 pairs of major salivary glands: the parotid glands, the submandibular glands, and the sublingual glands. The development of salivary glands is a result of a highly orchestrated complex interaction between 2 distinct tissues, the oral epithelium and the underlying mesenchyme.

43. The major salivary glands begin development during the 6th to 8th week. The parotid develops in the lateral aspect of the stomodeum while the submandibular and sublingual glands develops in the floor of the stomodeum. Each gland develops from a bud of oral epithelium into the underlying mesenchyme.

44. The epithelial buds differentiate into solid cords of cells which later form lumen and get converted into ducts. Minor salivary glands develop during the 3rd month of prenatal life and they remain as separate acini scattered in the connective tissue under the oral mucosa. O blique view of a developing salivary gland acinus and ductal system

45. DEVELOPMENTAL DISTURBANCES OF SALIVARY GLANDS. 1. ABERRANCY It is defined as a situation in which salivary gland tissue develops at a site where it is not normally found. Also called as Ectopic salivary gland. Most frequntly reported in the cervical region, near the parotid gland or body of the mandible. Salivary tissue in neck lymph nodes are often mistaken for metastatic disease. They may become site for development of retention cyst or neoplasm.

46. 2. APLASIA AND HYPOPLASIA Aplasia is defined as the congenital absence of salivary glands. Any one of the gland or group of glands is missing either unilaterally or bilaterally. Patient complains of xerostomia and lack of saliva results in rampant dental caries. The oral mucosa appears dry and there is cracking of lips. Proper oral hygiene should be maintained in order to decrease dental caries and preserve teeth.

47. 3 . HYPERPLASIA OF SALIVARY GLAND It is the increase in size of the salivary gland. It is more common in minor salivary glanof the palate. These appear as small swellings of varying size in the hard palate or at the junction of hard and soft palate. They are usually asymptomatic and can be caused due to hormonal and metabolic disorders. Management is often by surgical excision.

48. 4. ATRESIA It is a congenital occlusion or absence of one or two major salivary gland ducts. The submandibular gland is most commonly affected. The newborn infant presents, within 2 or 3 days of life, with submandibular swelling on the affected site due to retention. It produces a relatively severe xerostomia.

49. THE TUBARIAL GLANDS