Oral Histology and Embryology Timeline Project

Week 1

The first period, the preimplantation period of prenatal development, takes place during the first week after conception. At the beginning of the first week, a woman’s ovum is penetrated by and united with a man’s sperm during fertilization. During preimplantation meiosis occurs, and this union of the ovum and sperm subsequently forms a fertilized egg or zygote. During fertilization, the final stages of meiosis occur in the ovum. The zygote then undergoes mitosis or individual cell division or cleavage. After initial cleavage, the solid ball of cells is known as a morula. Because of the ongoing process of mitosis and secretion of fluid by the cells within the morula, the zygote becomes a vesicle known as a blastocyst (blastula). By the end of the first week, the blastocyst stops traveling and undergoes implantation and thus becomes embedded in the prepared endometrium, the innermost lining of the uterus on its back wall. Then, the trophoblast layer later gives rise to important prenatal support tissue. Then, the embryoblast layer later gives rise to the embryo during the prenatal period that follows the embryonic period.

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Week

1

Week 2

During the second week of prenatal development, within the embryonic period, the implanted blastocyst grows by increased proliferation of the embryonic cells, with differentiation also occurring resulting in changes in cellular morphogenesis; every ridge, bump, and recess now indicates these increased levels of cellular differentiation.

This increased number of embryonic cells creates the embryonic cell layers (or germ layers) within the blastocyst. A bilaminar embryonic disc is eventually developed from the blastocyst and appears as a three-dimensional but flattened, essentially circular plate of bilayered cells. The placenta is also forming during this time and the umbilical cord is connecting to the baby. The placenta joins the pregnant female and developing embryo which develops from the interactions of the trophoblast layer and endometrial tissue.

Week

2

2nd to the End of the 8th

Week

The second period of prenatal development is the embryonic period. This begins in the 2nd week to the end of the 8th week. Certain physiologic processes or spatial and temporal events of patterning occur which in considered key to the further development of the fetus. The physiological processes during this time include, induction, proliferation, differentiation, morphogenesis, and maturation. These processes cause the structure of the implanted blastocyst to become an embryo with further development.

2nd-End

of 8th

Week

Week 3

At the beginning of the third week of pregnancy the primitive streak forms within the bilaminar disc. The primitive streak causes the disc to have bilateral symmetry or to have a right and left sides that are mirror images of each other. Due to the continued growth and differentiation of the embryo's cells during this week a new embryonic layer is formed. Now instead of resembling as a bilaminar disc, the embryo resembles a trilaminar disc with three layers. These layers are ectoderm (which was the old epiblast layer), endoderm (which was the old hypoblast layer), and a new layer in between called mesoderm. Because the triaminar embryonic disc has undergone so much growth during the past 3 weeks, certain structures of the disc become apparent. The disc during this time now has a cephilic end or head end and a caudal end. At the caudal end the cloacal memebrane forms, which is the location of the future anus or terminal end of the digestive tract. In addition, during the third week, another specialized group of cells, the neural crest cells (NCCs), develop from mesenchyme (a primitive connective tissue).

Week

3

Week 3-8

This is the embryonic period and major organs and systems have begun development. During this period, the embryo is most susceptible to teratogens.

During these weeks, hematopoiesis begins around the yolk sac at week 3 and is taken over by the liver, thymus, spleen, and final and lifetime production is done in bone marrow. During week 7, the embryo begins sexual differentiation.

Week

3-8

Week 4

During week 4 of prenatal development within the embryonic period, the trilaminar disc undergoes anterior (or cephilic) and lateral embryonic folding, which places forming tissue types into their proper positions for further embryonic development and also producing a tubular embryo. This folding is the result form extensive profileration of the ectoderm and differentiation of basic tissue and occurs mostly at the cephalic end (where the brain will form). The cephalic tissue then grows beyond the oropharyngeal membrane to overhang the developing hear. The folding also occurs at the caudal end and at the sides of the embryo simultaneously. Due to the folding, the positions of the embryonic cell layers take a more recognizable placement for the further development of the embryo. After the folding of the disc, the endoderm lies inside the ectoderm and fills in the areas between these two layers. This movement forms one long and hollow tube lined by ectoderm. This tube is the future digestive tract and is separated into three regions midgut, foregut, and hindgut. The anterior part of this tube is the foregut, which forms the primitive pharynx or primitive throat and includes a part of the primitive yolk sac as it becomes enclosed with folding. The midgut and hindgut form the rest of the mature pharynx as well as the remainder of the digestive tract. Additionally, during the fourth week, the face and neck begin to develop with the primitive eyes, ear, nose, oral cavity, and jaw areas. At the beginning of the fourth week, the primitive mouth has become the stomodeum which initially appeared as a shallow depression in the embryonic surface ectoderm at the cephalic end before the fourth week. At the end of fourth week there are no more membranes (The membrane also separates the stomodeum from the primitive pharynx).

Week

4

Facial development begins in the 4th week and ends in the 12th week. Before the rupture of the oropharyngeal membrane, the anterior lobe of the hypophysis or pituitary gland forms. Its development begins as a small evagination of the stomodeal ectoderm in the roof of the primitive mouth, just in front of the oropharyngeal membrane. The pit formed by this evagination is known as Rathke’s pouch. The cells of this pouch develop into the anterior lobe of the pituitary gland. At the beginning of the fourth week, the primitive mouth has become the stomodeum. At this time, the stomodeum is limited in depth by the oropharyngeal membrane. The membrane also separates the stomodeum from the primitive pharynx. The first event in the development of the face during the latter part of the fourth week of prenatal development is disintegration of the oropharyngeal membrane. With this disintegration of the membrane, the primitive mouth is increased in depth and enlarges in width across the surface of the midface. After formation of the stomodeum but still within the fourth week, two bulges of tissue appear inferior to the primitive mouth: the two mandibular processes. These paired mandibular processes then fuse at the midline to form the Mandibular arch the developmental form of the future lower dental arch, the mandible. After fusion, the mandibular arch then extends as a band of tissue .During the growth of the mandibular arch, meckles cartilage forms within each side of the arch. In future development, the developing mandibular arch directly gives rise to the lower face, including the lower lip and mandibular teeth. During the fourth week the frontonasal process also forms as a bulge of tissue but at the most cephalic end of the embryo. The facial area of the embryo also has two lens placodes, which are initially located fishlike on each side of the frontonasal process. On the outer part of the nasal pits are two other crescent shaped swellings, the lateral nasal processes. An adjacent swelling also forms from increased growth of the mandibular arch on each side of the stomodeum which is the maxillary proces. Primitve pharaynx formation also occurs during the fourth week. During this the endoderm of the pharynx lines the internal parts of the branchial (or pharyngeal) arches and passes into balloon-like areas of the pharyngeal pouches. However, this same endoderm does not come to line the oral cavity proper or nasal cavity. Instead, the oral cavity proper and nasal cavity are both lined by ectoderm as a result of embryonic folding. During the fourth week of prenatal development, stacked bilateral swellings of tissue appear inferior to the stomodeum and include the mandibular arch. These are the pharyngeal or branchial arches. These pharyngeal or branchial arches consist of six pairs of U-shaped bars of which the central core consists of mesenchyme derived from mesoderm invaded by NCCs, now referred to as ectomesenchyme. It is important to note that the

Week 4-8

The tongue develops during the fourth to eighth weeks of prenatal development.

It develops from independent swellings located internally on the floor of the primitive pharynx that were formed by the first four pharyngeal or branchial arches. Specifically, the body of the tongue develops from the first pharyngeal or branchial arch, and the base of the tongue originates later from the second, third, and fourth pharyngeal or branchial arches. The tongue structures involved during this time are the body: tuberculum, impar and lateral lingual swellings appear. The base: of the tongue also appears to be: Copula overgrowing the second pharyngeal or brachial arches.

Week

4-8

Week 5

During the fifth week of prenatal development and still within the embryonic period, the intermaxillary segment forms. The intermaxillary segment gives rise to the primary palate. At this time, the primary palate also serves as a partial separation between the developing oral cavity proper and the nasal cavity. During this time, the primary and succedaneous teeth also begin developing early in embryonic development around week 5. The process of tooth development begins around week 5.

Week

5

Week 5-6

During the fifth to sixth week, primitive muscle cells from the mesoderm in the mandibular arch begin to differentiate. These primitive muscle cells become oriented to the site of origin and insertion of the masticatory muscles that they will form. The formation of the palate starts during the fifth week of prenatal development within the embryonic period from two separate embryonic structures: primary palate and secondary palate. The primary palate forms from the Intermaxillary segment from fused nasal processes.

Week

5-6

Week 5-8

The crown, enamel, and dentin primarly develop during embryonic week 5 to 8.

Week

5-8

Week 5-12

The nasal cavity forms in the same time frame as the palate, from the fifth to twelfth week of prenatal development. It will later serve as part of the respiratory system. The future nasal septum of the nasal cavity is also developing when the palate is forming. The tissue types that form the nasal septum will grow inferiorly and deep to the medial nasal processes and superior to the stomodeum.

Week

5-12

Week 6

Embryology/prenatal development

During the sixth week of prenatal development, within the embryonic period, the bilateral maxillary processes give rise to two palatal shelves (or lateral palatine processes). During secondary palate formation, these shelves grow inferiorly and deep on the inside of the stomodeum in a vertical direction, along both sides of the developing tongue. The tongue is forming on the floor of the primitive pharynx at this time, and as it grows, it initially fills the common nasal and oral cavity. The shelves move into a horizontal position, which is now superior to the developing tongue. Next, the two palatal shelves elongate and move medially toward each other, meeting to join, and then fusing to form the secondary palate. The secondary palate will give rise to the posterior two-thirds of the hard palate, which contains certain anterior maxillary teeth (canines) and posterior teeth, all located posterior to the incisive foramen. It also gives rise to the soft palate and its uvula. During the 6th week, the upper lip is formed when each maxillary process fuses with each medial nasal process on both sides of the stomodeum, which are brought into proximity by proliferation of the NCC-derived mesenchyme.

Tooth development/oral tissues:

In the beginning of week 6 the stomodeum is also lined by ectoderm

The outer layer of ectoderm gives rise to oral epithelium. Ectoderm is Initially two horseshoe shaped bands of tissue at stomodeal surface. Ectomesenchyme also form at the same time as the ectoderm in week 6

Salivary gland development:

Of the three pairs of major salivary glands, the parotids are the first to develop, at 6 weeks of gestation. The epithelial buds of these glands are located on the inner part of the cheek, near the labial commissures of the primitive mouth. These buds grow posteriorly toward the otic placodes of the ears and branch to form solid cords with rounded ends near the developing facial nerve. Submandibular glands develop near the end of the sixth week. They develop bilaterally from epithelial buds in the sulcus surrounding the sublingual folds on the floor of the primitive mouth. Solid cords branch from the buds and grow posteriorly, lateral to the developing tongue

Week

6

Week 6-7

Tooth development:

The Initiation stage of tooth development happens 6-7 weeks in utero (the initiation stage is Interaction between physiologic process of induction/active interaction between embryologic tissue types). The dental lamina connects the developing tooth bud to the epithelial layer of the mouth for a significant time. Odontogenesis also occurs between 6 to 7 weeks of prenatal development.

Pulp:

During odontogenesis when pre-dentin forms around the papilla, the innermost tissue is considered pulp.

Week

6-7

Week 6-8

Salivary gland development:

Between the sixth and eight weeks of prenatal development, the three major salivary glands begin as epithelial proliferations or buds from the ectoderm lining of the primitive mouth. The rounded terminal ends of these epithelial buds grow into the underlying mesenchyme, producing the secretory cells of the glandular acini and the ductal system. The parts of the glands that contain supporting connective tissue, such as the outer capsule and inner septa, are produced from mesenchyme, which is influenced by neural crest cells. It is important to note that interaction between the developing components of the epithelium, mesenchyme, nerves, and blood vessels is necessary for complete development of the salivary glands.

Week

6-8

Week 6-12

Embryology/prenatal development:

During the 6th to 12th week, the secondary palate forms from fused palatal shelves from the maxillary process. The median palatine raphe within the mucosa lining and the associated deeper median palatine suture on the adult maxillary bone indicate the line of fusion of the palatal shelves.

Week

6-12

Week 7

Embryology/prenatal development

During the formation of the mandibular arch and lower face formation, by the seventh week, the mandibular muscle mass has enlarged. The cells have also begun to migrate into the areas where they will begin to differentiate into the four muscles of mastication as discussed. Muscle cell migration occurs before bone formation in the facial area. The fusion of the maxillary and medial nasal processes to form the upper lip is completed during the end of the seventh week of prenatal development as well. When the grooves between the processes are obliterated. The maxillary processes on each side of the developing face partially fuse with the mandibular arch on each side to create each labial commissure, with the mandibular arch already forming the lower lip. During apparatus formation, reichert cartilage, mostly disappears during development; however, parts of it are responsible for a middle ear bone, a process of the temporal bone, and parts of the hyoid bone. During the seventh week, the muscle cells from the mesoderm of the hyoid arches have begun to differentiate. These muscle cells then begin to migrate over the mandibular muscle masses.

Tooth development/oral tissues:

During the later part of the seventh week, oral epithelium grows deeper into the ectomesenchyme and produces a layer called the dental lamina. Dental Lamina also begins its formation at the midline in both arches and moves posteriorly.

Week

7

Week 8

Embryology/prenatal development

During the eighth week, the tongue completes formation during this week by the merging of anterior swellings of the body and copula of the base. By the end of the eighth week, the tongue has completed the fusion of these swellings. The tongue then contracts and moves anteriorly and inferiorly to avoid becoming an obstacle to the developing palatal shelves. The lingual papillae, small elevated structures of specialized mucosa on the dorsal surface, appear toward the end of the eighth week too.

Tooth Development:

The crown, enamel, and dentin of the teeth – succedaneous, develops during embryonic week 8 to after birth (5 years). The Bud Stage of tooth development also occurs during week 8 (8 weeks in utero). This is where the extensive proliferation of the dental lamina into the buds occur. Three-dimensional oval masses penetrate into the surrounding ectomesenchyme. At the end of the proliferating process involving the dental lamina of the primary dentition, both arches will have 10 buds each.

Each of these buds will eventually develop into a tooth germ. Only proliferation occurs during the bud stage.

Salivary system:

Sublingual glands appear in the 8th week of prenatal development, later than the other two major salivary glands. They develop from epithelial buds in the sulcus surrounding the sublingual folds on the floor of the mouth, lateral to the developing submandibular gland. These buds branch and form into cords that canalize to form the sublingual ducts associated with the gland. The rounded terminal ends of the cords form the acini.

Week

8

Week 8-12

The chance of a cleft palate occurs during the 8th week to the 12th week.

Week

8-12

Week 9

Embryology/prenatal development:

The fetal period begins during the 9th week. The fetal period encompasses the beginning of the ninth week or third month continuing to the ninth month, with the maturation of existing structures occurring as the embryo is enlarging to become a fetus. This is when the organs really start to grow. This process involves not only the physiologic process of maturation of the individual tissue types and organs but also further proliferation, differentiation, and morphogenesis (at this point we have an actual baby).

Tooth development:

The cap stage of tooth development also happens during this time-9 weeks in utero. This is where the process of proliferation continues and the enamel organ has taken the shape of a cap. Unequal growth of different parts of the tooth bud also occurs, leading to a three-dimensional cap shape over the ectomesenchyme. The cap will first be seen in primary incisor tooth germs. At the end of the cap stage; the tooth germ has formed, which is comprised of enamel organ, dental papilla, and dental sac

Dentin:

Dentinogenesis happens during the cap stage of tooth development which occurs during the 9th week. The origin comes from the dental papilla. The mesenchyme surrounding the enamel organ proliferates due to the influence of the proliferating epithelium of the enamel organ. The mesenchyme then condenses to form the dental papilla.

PDL and TMJ:

Similar to the alveolar process the PDL develops from the dental sac of the tooth germ and this is a mesodermally derived tissue.

Week

9

Week 9-12

Salivary system:

Minor Salivary Glands during develop during the 9th to 12th weeks of prenatal development. Much like the major salivary glands, the minor salivary glands arise from both the ectoderm and endoderm associated with the primitive mouth. They then remain after development as small, isolated acini and ducts within the oral mucosa or submucosa lining the mouth.

Week

9-12

Week 10

Embryology/prenatal development:

By the tenth week, the mandibular muscle masses have become well organized bilaterally into the four muscles of mastication. Nerve branches from the trigeminal nerve are incorporated early in these muscle masses. The muscle cells of the masseter and medial pterygoid muscles have formed a vertical sling that inserts into the site that will form the angle of the mandible. The temporalis muscle cells have differentiated in the temporal fossa and are inserting into the developing coronoid process. The lateral pterygoid muscle cells, which arise from the infratemporal fossa, extend horizontally into the mandibular condyle and disc of the temporomandibular joint. The filiform lingual papillae also develop during the early fetal period, which comprises the tenth to eleventh weeks. Around week 10 to 12, the sex of the fetus also becomes recognizable.

Tooth development/oral tissues:

At 10 weeks the cap stage of tooth development of primary teeth occurs and initiation for permanent dentition begins. Primordium for initially forming permanent teeth appears as an extension of the dental lamina into the ectomesenchyme. They will be lingual to the developing primary germs. Differentiation continues and the outer enamel epithelium, inner enamel epithelium, stellate reticulum (star shaped), and stratum intermedium begin to form. These continue to differentiate and complete in the bell stage.

Salivary system:

At approximately 10 weeks of prenatal development, the solid cords with rounded terminal ends near the developing facial nerve are canalized and form ducts, with the largest becoming the parotid duct for the parotid gland. The rounded terminal ends of the cords form the acini of the glands.

Week

10

Week 11-13

Week

11-13

Salivary system:

During weeks 11 to 10 the taste buds involved in taste sensation and that are associated with certain lingual papillae (circumvallate, foliate, and fungiform) develop during the eleventh to thirteenth weeks by inductive interaction between the epithelial cells of the tongue and invading nerve cells from the chorda tympani of the facial nerve or seventh cranial nerve as well as the glossopharyngeal nerve or ninth cranial nerve.

PDL and TMJ:

The TMJ develops in the 11th and 12th week of prenatal development, during the proliferation of the associated ligaments, muscles, and bones of the joint, as well as the joint spaces and joint disc.

Week 12

Embryology/prenatal development:

During week 12 facial development ends. The palate is also completed later during the twelfth week within the fetal period. The fusion of all three processes also occurs during week twelve, forming the final palate. Now, the mature oral cavity becomes completely separated from the nasal cavity, which has begun to undergo development of its nasal septum.

Tooth development:

The bell stage of tooth development also happens during this time-12 weeks in utero. During the bell stage, continuation of the ongoing process of proliferation, differentiation, and morphogenesis happens. Differentiation occurs to the furthest extent and four different types of cells are now found within the enamel organ, outer enamel epithelium, inner enamel epithelium (Lines the inside of the enamel organ and is a single layer of cuboidal cells and will differentiate by phases into ameloblasts), stratum intermedium, and stellate reticulum (SR). The stellate reticulum is between inner and outer enamel epithelium, they are star-shaped epithelial cells, and they protect the developing tooth-nourishment by allowing vascular fluids to move between cells. Stratum intermedium is composed of 1-3 layers of squamous cells, only found where enamel is being formed, and lies next to the ameloblasts.

Dentin:

Histodifferentiation occurs during the bell stage. This is where inner enamel epithelium becomes taller (these tall cells are the preameloblasts), peripheral cells adjacent to preameloblasts become columnar and are referred to as odontoblasts, and odontoblasts moves away from the preameloblast toward the center of the dental papilla and towards the pulp. Once the dentin matrix has formed, the preameloblasts differentiate into ameloblasts. The enamel matrix is formed opposite of the dentin.

Salivary system:

The cords of the submandibular gland later branch further and then become canalized to form the ductal part. The submandibular gland acini develop from the cords’ rounded terminal ends at 12 weeks.

Week

12

Week 16

Salivary system:

Secretory activity via the submandibular ducts begins at 16 weeks. Growth of the submandibular gland continues after birth with the formation of more acini. Lateral to both sides of the tongue, a linear groove develops and closes over to form the submandibular duct.

Week

16

Continues to grow & more acini

Week 18

Salivary system:

Secretion by the parotid glands via the parotid duct begins at approximately 18 weeks of gestation. Again, the supporting connective tissue of the gland develops from the surrounding mesenchyme.

Week

18

Week 24-25

Week

24-25

Salivary system:

The contractile myoepithelial cells, which are important in the secretion of saliva from each acinus, arise from neural crest cells and thus are ectodermal in origin. They surround the developing acini as well as parts of the ductal system and become active between the 24th and the 25th week of prenatal development.

During Tooth Eruption

The PDL, alveolar bone, and junctional epithelium form during tooth eruption. Development of the PDL begins during tooth eruption. PDL Fibroblasts secrete collagen fibers that become embedded in cementum and extend outwards (principal fibers). This process begins at the CEJ and continues apically as the root grows. The other end of collagen fibers is embedded within the alveolar bone, later, activated by tooth occlusion. This results in greater mobility of tooth roots during eruption. PDL collagen fiber bundles run in different directions, which can be classified as alveolar crest, horizontal, oblique, (peri) apical, interradicular (on multirooted teeth), and trans-septal. Dentin also continues to form during tooth eruption . Dentin is first laid down as an uncalcified matrix called predentin . Some dentin deposition occurs throughout life. Mantle dentin is the first predentin that forms and matures in the tooth. Thsi contains von korff's fibers. Mantle dentin is less calcified because of the thick Korff’s fibers.

During

Tooth

Eruption

Alveolar bone under microscope

After Tooth Eruption

After

Tooth

Eruption

Tooth development:

The apposition stage of tooth development happens up to 1-2 years after tooth eruption; this is where enamel, dentin, and cementum are secreted in layers. They are initially secreted by a matrix and the final process of induction continues to occur between the ectodermal tissues of the enamel organ. The dentino-enamel junction is then formed when the basement membrane completes. During this phase, the dental lamina is beginning to form the permanent teeth. The final stage of tooth development is maturation, where matrices of the hard tissue fully mineralize (this also occurs after tooth eruption). Root formation also occurs after tooth eruption. This is when the formation of the dentin and enamel begins at the incisal or cuspal areas and moves cervically. The tooth then splits into either one, two, or three roots. Hertwig’s Epithelial Root Sheath (HERS) is responsible for root formation. Herwig's form the outline of the root dentin before cementum formation begins. As HERS grows downward, it influences the peripheral cells adjacent to it to differentiate into odontoblasts. The dentin begins to form next to HERS and the root sheath begins to break up. The cells of the dental sac then begin to differentiate into cementoblasts and begin to form cementum.

PDL and TMJ:

Cells of the dental follicle other than cementoblasts differentiate into fibroblasts that secrete collagen. When the formation of cementum starts, these collagen fibers secreted by fibroblasts within the dental follicle, orientate themselves into bundles perpendicular to the surface of the root and form the principal fibers of the periodontal ligament. One end of these fibers become embedded in the developing cementum, and the other end in the alveolar bone and these are known as Sharpey's fibers In addition, the epithelial rests of Malassez (ERM) are present within the PDL. These PDL groups of epithelial cells are stranded in mature PDL after the disintegration of Hertwig epithelial root sheath during the formation of the root.