Orofacial Development PDF

Oral Histology Orofacial Development OROFACIAL DEVELOPMENT The development of the human embryo starts after fertilization by the cell division of zygote. The cell...

Oral Histology Orofacial Development OROFACIAL DEVELOPMENT The development of the human embryo starts after fertilization by the cell division of zygote. The cell division in the 1st week after fertilization will form aggregation of cell known as “Morula”. In the 2nd week the morula will change in sphere like structure with one layer of cells on the surface and few cells inside, which known as blastocyst. The cell inside known as embryonic cells / embryoblast. The embryoblast will organize in 2 layers: epiblast & hypoblast, which known as bilaminar embryo. Near the end of the 2nd week of prenatal development the epiblast surface will undergo invagination from one side (future tail end) and with time it will proceed to another side of the epiblast (head/cephalic). The latter is known as neural fold (primitive streak), which will form the neural tube. The neural tube will develop in CNS. At the same time of formation of neural fold, the epiblast cells divide to form new layer of cells between the epiblast & the hypoblast. The embryo now has 3 layers –trilaminar embryo- which consists of ectoderm (epiblast), mesoderm (new middle layer), and endoderm (hypoblast). During the 3rd week the neural fold sides- 1/19/2024 12  Oral Histology Orofacial Development neural crest, will give cells which migrate into the developing mesoderm. These cells known as neural crest cells and they have very essential role in development of human embryo. Near the end of the 3rd week the three embryonic layers will fold laterally and form curved tube like structure with two different ends. One bulged end is the cephalic end or head and the second end is the caudal end or tail. The cephalic end on the ventral side of embryo has processes, these processes are: head fold at the end of the embryo directed inferiorly and four processes in each side inferior to head fold known as branchial processes / arches (pharyngeal arches). 1/19/2024 13  Oral Histology Orofacial Development The neural crest cells locate between the mesoderm cells in both sides of the neural tube, and have active genes known as homeobox genes, which control the surrounding mesoderm that forms the skull bones, tooth tissues and some other tissues. The neural crest cells and surrounding mesodermal cells are called ectomesenchyme. These cells migrate anteriorly to the facial side with the growth of cranial nerves. DEVELOPMENT OF FACIAL STRUCTURES The face and its related tissues begin to form during the fourth week of prenatal development, within the embryonic period, which will be completed later in the twelfth week, within the fetal period. All three embryonic layers are involved in facial development. This development includes the formation of the primitive mouth, lower, mid, and upper face. Facial development depends on the five facial prominences surround the embryo’s primitive mouth: the single frontonasal process (head fold), the two mandibular processes (1st branchial arch), and paired maxillary processes (lateral & superior growths from 1st branchial arch). The facial processes are demarcated by grooves, which normally become flattened out by proliferative and migratory activity of the underlying mesenchyme, or abnormally leave clefts. Stomodeum and Oral Cavity Formation: The stomodeum initially appears before the fourth week, as a shallow depression in the embryonic surface ectoderm. In the future, the stomodeum will give the oral and nasal cavities. The stomodeum, at this time, is limited in depth by the oropharyngeal membrane, which consists of ectoderm overlying endoderm. At the beginning of the fourth week of prenatal development, the oropharyngeal 1/19/2024 14  Oral Histology Orofacial Development membrane disintegrates and the stomodeum becomes primitive mouth (oral pit). The primitive mouth gradually will increase in depth. The oral cavity will be lined by oral epithelium (ectodermal origin), which with underlying tissues will give rise to the teeth and associated tissues. Mandibular Arch and Lower Face Formation: During the fourth week, two bulges of tissue appear inferior to the primitive mouth, the two mandibular processes. These processes then fuse at the midline to form the mandibular arch. Then, it extends as a band of tissue inferior to the stomodeum and between the developing brain and heart. The embryonic mandible arch initially appears undeveloped. In the future, during the fetal period, it gives rise to the lower face, including the lower lip, the mandible, and other associated tissues. During the growth of the mandible arch, the primary cartilage (Meckel’s cartilage) forms within each side of the arch. Most of this cartilage later disappears as the bony mandible forms external to it, and only the posterior portion of it participates in the formation of the middle ear bones. The mandibular bone starts to develop from the 7th week by intramembranous ossification. The mandibular arch also forms the tongue body between the 4th & the 8th week. During the fifth and sixth weeks, primitive muscle cells begin to differentiate from the mesoderm of the mandibular arch. By the seventh week, the mandibular muscle mass has enlarged & will begin to differentiate into the four muscles of mastication: the masseter, medial and lateral pterygoids, and temporalis muscles. Maxillary Process and Midface Formation: The maxillary process forms (5th week of prenatal development) from increased growth of the mandibular arch, as an adjacent swelling, which grows superiorly and then anteriorly on each side of the primitive mouth. The maxillary process will form, in the future, the sides of the upper lip, the cheeks, the secondary palate, and the lateral posterior portion of the maxilla, including zygomatic bones and portions of the temporal bones. The maxillary processes also will give rise the maxillary canines and posterior teeth and associated tissues. The maxillary processes on each side of the developing face partially fuse with the mandibular arch to form the commissures of the mouth. 1/19/2024 15  Oral Histology Orofacial Development Frontonasal Process and Upper Face Formation: The frontonasal process is a bulge of tissue in the upper facial area, and it is the cranial boundary of the stomodeum. This process also forms during the fourth week, and in the future it gives rise to the upper face & central portion of the mid face, which includes the forehead, bridge of the nose, central portion of the upper lip, central portion of maxilla including incisor teeth, primary palate, and nasal septum. 1) Placode Development: Placodes are rounded areas of specialized, thickened ectoderm found at the location of developing special sense organs on the surface of the embryo. i. Lens placodes are found on each side of the frontonasal process, during the fifth week, and later they migrate medially from their lateral positions, because of the growth of the brain, and form the future sensory epithelium of eyes and related tissues. ii. Nasal placodes form in the anterior portion of the frontonasal process, just superior to the primitive mouth, during the fourth week and later, these two buttonlike structures develop into olfactory cells. iii. Otic placodes are more laterally and posteriorly placed and form into pits that create the future internal ear and related tissues. The external and middle ear form from 1st & 2nd branchial arches. The mandibular & hyoid arches cartilage forms the middle ear bone and the mandibular arch with hyoid arch are responsible for the formation of external ear. 1/19/2024 16  Oral Histology Orofacial Development 2) Nose Development: The tissue around the nasal placodes undergoes growth, during the fourth week, thus starting the nose development. The placodes then become submerged, forming a depression in the center of each one - the nasal pits, which later develop into the nasal cavities. The middle portion of the tissue growing around the nasal placodes appears as two crescent-shaped swellings located between the nasal pits, medial nasal processes. In the future, these processes will fuse externally to form the middle portion of the nose and center portion of the upper lip including the philtrum region. These processes also fuse internally and grow inferiorly inside the primitive mouth, forming the intermaxillary segment, which is involved in the formation of the anterior portion of maxilla with associated tissues, maxillary incisor teeth, primary palate, and nasal septum. On the external portion of the nasal pits are two lateral nasal processes. These processes, in the future, form the nasal alae. Fusion of lateral nasal, maxillary, and medial nasal processes forms the nares. 3) Upper Lip Formation: Upper Lip starts to form in the 5th week when the medial nasal processes grow inferiorly, and then during 6th week will start to fuse with lateral maxillary process in each side. The maxillary processes contribute to the sides of the upper lip and the medial nasal processes contribute to the middle of the upper lip. The philtrum is formed as a result of the fusion between two medial nasal processes. 1/19/2024 17  Oral Histology Orofacial Development DEVELOPMENT OF ORAL STRUCTURES The oral structures develop during the fourth week to the twelfth week of prenatal development, spanning the later embryonic and early fetal periods. Palatal Development: The palate is formed from two separate structures: the primary palate and the secondary palate. The palate begins formation in the fifth week, within the embryonic period, and is completed during the twelfth week, within the fetal period. 1. The Primary Palate Formation: The intermaxillary segment, which develops during the fifth week, gives rise to the primary palate, a triangular mass. The primary palate, later, will form the premaxillary portion of the maxilla, the anterior one- third of the final palate. This small portion is anterior to the incisive foramen and will contain the maxillary incisor teeth. The formation of the primary palate completes the first stage of palate development. 2. The Secondary Palate Formation: The bilateral maxillary processes, during the sixth week, give rise to two palatal shelves. These shelves grow inferiorly and deep on the inside of the stomodeum in a vertical direction, along both sides of the developing tongue. After growing in a vertical direction, flip in a superior direction within a few hours. 1/19/2024 18  Oral Histology Orofacial Development Thus, the shelves move into a horizontal position, now superior to the developing tongue. Next, the two palatal shelves elongate and move medially toward each other, fusing to form the secondary palate. This palate will form the posterior two-thirds of the hard palate, which will contain the maxillary canines and posterior teeth. It also gives rise to the soft palate and its uvula. 3. Completion of Palate: The secondary palate meets the posterior portion of the primary palate, and fuse together. During the twelfth week, the three processes are completely fused, forming the final Palate (hard and soft). The palatine processes also fuse with the overlying nasal septum in the midline, which causes a complete separation of the nasal cavities from the oral cavity posterior to the oropharynx. Ossification takes place in the anterior hard palate and is completed by the time palatal fusion, while in the posteriorly placed soft palate, mesenchyme migrates into the area to form palatal muscles. Tongue Development: The tongue develops during the fourth to eighth weeks from independent swellings located internally on the floor of the primitive pharynx, formed by the first four branchial arches and the mesoderm of the occipital somites. Three separate swelling mark the early development of the tongue. These are two lateral lingual swellings and the midline tuberculum impar. These structures, covered by ectoderm, develop from the mandibular arch and will form the anterior two-thirds of the tongue. The lateral parts rapidly enlarge and merge with each other, 1/19/2024 19  Oral Histology Orofacial Development overgrowing the central tubercle. Gradually, the three parts of the anterior portion of the tongue merge to form a unified structure. Immediately posterior to these three swellings, a fourth swelling, the copula in the middle of the 3rd arch, will become evident, which receives an influx of mesenchyme from the second, third, and part of the fourth branchial arches. A small contribution is also made by the remainder of the fourth arch to form a small prominence, the epiglottic swelling, just posterior to the copula. These two endodermally covered structures are referred to as the hypobranchial eminence and they will form the posterior one-third of the tongue. The fusion of the hypobranchial eminence with the anterior two-thirds of the tongue is demarcated in the adult by the sulcus terminalis, a “V-shaped” groove, whose apex points in a posterior direction. The depression in the apex of the sulcus terminalis is known as the foramen cecum, which is indicative of the beginning of the thyroglossal duct, the origin of the thyroid gland. Posterior to the sulcus terminalis the base of the tongue forms the lingual tonsil on its dorsal surface. In later stages of development, several types of papillae differentiate on the dorsal mucosa of the tongue’s body, and the lymphatic tonsil differentiates on the surface of the tongue’s base. The intrinsic muscles of the tongue are believed to originate from the mesoderm of the occipital somites, not from branchial arches. Cells from these somites migrate into the tongue, taking their motor never supply, the hypoglossal nerve (XII) with them. Therefore, the tongue is innervated by the following five cranial nerves: V, VII, IX, X, and XII. 1/19/2024 20  Oral Histology Orofacial Development Nasal Cavities and Septum Development: The nasal cavities form at the same time as the palate (5th to 12th weeks). The future nasal septum that separates the nasal cavities is also developing when the palate is forming. The nasal septum forms from the fused internally medial nasal processes “intermaxillary segment”. These tissues grow inferiorly and deep to the medial nasal processes and superior to the developing oral cavity. During the ninth week, the vertical nasal septum starts fusion with the horizontally oriented final palate, and completes it by the twelfth week. In the end, the paired nasal cavity and the single oral cavity in the fetus become completely separate. 1/19/2024 21