Issuu on Google+

Human Growth & Development ď Ž

Pre-natal Development

ď Ž

Post-natal Development


Leader in continuing dental education

[JX Huxley] ‘Self-multiplication of living substance’.


‘Increase in size, change in proportion & progressive complexity’.


‘Entire series of sequential anatomic & physiological changes taking place from the beginning of prenatal life to senility’.


‘Quantitative aspect of biologic development per unit of time’.


‘Change in any morphological parameter which is measurable’.

ď Ž

Thorough background in craniofacial growth & development is necessary for every dentist. Even for those who never work with children, it is difficult to comprehend conditions observed in adults without understanding the developmental processes that produced these problems.

[William R. Proffit]

“Embryology is the study of formation & development of the embryo from the movement of its inception up to the time when it is born an infant”.  Kaspar Friedrich (Described modern Embryology)

  

During first 2months we call the developing individual An Embryo From third month until birth Fetus From birth to 1yr of age An Infant A young child who is just beginning to walk Toddler Period shows onset of puberty. Period mark the secondary sex characters [12-15y(G) 1316y(B)] Teenager Developing from child to an adult (13-19y) Adolescent Body attained full growth & maturity of an organism (18-21y) Adult

ď Ž

As an Orthodontics we are interested in understanding how the face changes from its embryonic form through childhood, adolescence & adulthood.

ď Ž

To understand how & where growth occurs, how much growth is remaining, in which direction & when growth will express itself, what roles the genetic & environmental factors play in influencing facial growth & in turn how we can influence these factors with our Treatment, to achieved the optimum results in the potential of each individual person. Answer of all these questions will come when we know the human growth & development.

ď Ž

ď Ž

Although our understanding of gross Anatomy & Embryology are unlikely to change in future. Advances in clinical treatment are predicated on the bases of solid foundation of anatomic knowledge.

  

Prenatal Development

Period Of Ovum Period Of Embryo Period Of Fetus

Hippocrates—’Father of Medicine’ & ‘Founder of Anatomy’

Period of Ovum  From the fertilization to the end of 14th day.  Cleavage of the ovum.  Ovum is only 1.5mm in length. o

Period of Embryo  14th day to 56th day.  Formation of Embryonic Disc  Formation of Pharyngeal arches  Formation of nasal pits  Development of Cranial structures  Development of pre-oral region {Palate; Tongue; Maxilla; Mandible} 


Period of Fetus

56th day to 270 days/3rd to 9th months (till birth). B/w 8th to 12th weeks of IUL Fetus triples in length from 20mm to 60mm. By 3rd month Face assumes a more human appearance. From 12th to 36th week of IUL Head increases in length from approximately 18mm to 120mm; in width from 12mm to 74mm; in height from 20mm to 100mm

o    

Cranium to face ratio During Embryonic period—40:1 During 4th month of IUL—5:1 At Birth—8:1 At Adulthood—2:1

Aristotle—’Father of Ancient Anatomy’ He forecasted idea of ‘Genetic & Hereditary’

Teratogens— ‘Growing embryo if exposed to certain chemical/physical agents during pregnancy which causes structural developmental abnormalities’.

 First 2weeks  Not sensitive. {High rate of lethality}  3—8weeks  Greatest sensitivity Period for each organ.  9—38weeks Decreasing sensitivity. ‘Functional maturation Period’  3—8weeks ‘Period of Embryogenesis’.  8—38weeks ‘Period of Foetogenesis’.

ď Ž

Since the baby's face develops so early in the pregnancy, even when these factors are minimized through proper prenatal care, the damage may have already occurred to the child before the mother was even aware that she was pregnant.

Fertilization 

At the moment when one spermatozoon meets & fuses with one ovum. The development of new individual begins. This process of fusion is called Fertilization. Fused Ovum & Spermatozoon form Zygote. Fertilization occurs in the ampulla of the uterine tube.

Robert Hooke— identified & name the ‘cell’.

Cell Division ď ą

ď ą

When after cell division, the daughter cell must have chromosomes identical in number to those in the mother cell. This type of cell division is called Mitosis. When after division, the chromosome of the daughter cell becomes non-identical in number to those in the mother cell. This type of cell division is called Meiosis.

Two-Cell-Stage of Embryo ď Ž

During Fertilization Chromosomes of the ovum (23-Female chromosomes) fused with chromosomes of the spermatozoon (23-Male Chromosomes) so that the fertilized ovum now has the 46 chromosomes in all. Each of the 46 chromosomes then splits into two. Meanwhile, a spindle is formed, & one chromosome of each pair moves to each end of the spindle leading to the formation of two daughter cells.

ď ą

These two cells undergo series of divisions. This process of subdivision of ovum into smaller cells is called Cleavage.

zygote {after 30hrs after fertilization}: Repeated mitotic divisions Two blastomeres Four blastomeres Eight blastomeres ď ą

Formation of Morula As cleavage proceeds the ovum comes to have 12 to 32 blastomeres cells. The developing human is now called Morula. (It resembles a mulberry or black berry) 

Gregory Johann Mendel —Published experiments on plant hybridization. ‘Father of Genetics’

Formation of Blastocyst

When morula enters uterus (4th days after fertilization), fluid filled space appears inside the morula. ď ą As the fluid increases in the blastocystic cavity ,it separates the blastomeres into two parts: Trophoblast {outer cell layer} Embryoblast {inner cell mass} At this stage of development the conceptus is called a blastocyst ď Ž

TROPHOBLAST  Inner layer of cytotrophoblast  Outer mass of syncytiotrophoblast

Formation of Embryonic Disc 

Embryobalsts form the embryonic Spherical disc which forms three germ layers.

1] Endoderm (Hypoblast-Cuboidal cells)—Inside (1st germ layer to be formed) 2] Ectoderm (Epiblast-Columnar cell)—Outside (2nd germ layer) 3] Mesoderm –In the middle

ď Ž

Endoderm spread & lined the blastocystic cavity with the flattened cells, now this cavity is called Primary Yolk Sac.

Extra-embryonic Mesoderm Cells of the trophoblasts give origin to the mass of cells— Extra-embryonic mesoderm -which lie b/w trophoblast & primary yolk sac.  Extra-embryonic Somatic Mesoderm — Line the Cytotrophoblast & covers the amnion.  Extra-embryonic Visceral Mesoderm — Covers the yolk sac. 

Prochordal Plate 

Prochordal plate cells develops as a localized thickening of endodermal cells & indicate the site of buccopharyngeal membrane separating the oral cavity from Oropharynx (Future cranial region of embryo & Future site of mouth) At one area cubical cells of the endoderm become columnar— Prochordal plate. It determines the central axis of the embryo & divide the embryo into right & left halves & also distinguish its head & tail ends.

Primitive Streak ď ą

ď ą

Near the tail end of the disc, begin to proliferate & form an epiblastic elevation that bulges into the amniotic cavity— Primitive Streak. The primitive streak is at first a rounded swelling but with the elongation of the embryonic disc it becomes linear & lies at the central axis of the disc. Primitive pit surrounded by the elevated Primitive node is located at the cranial end of the primitive streak. Since the cells forming the primitive are leuripotent, their persistence may give rise to tumors containing all the three germ layers. Teratoma

Notochord ď ą

Notochord –It is the midline structure, which develops in the region extending from the cranial end of the primitive streak to the caudal end of the prochordal plate. (Notochord develops from the anterior extremity of primitive streak.)

Neural Tube ď ą

Ectoderm overlying the notochord undergoes changes that result in the formation of— Neural tube. Neural tube gives rise to brain & spinal cord.

Neural Plate Notochord functions as the primary inductor in the early embryo  The developing notochord induces the overlying embryonic ectoderm to thicken and form the “neural plate” [Primordium of central nervous system] 

Intra-embryonic Mesoderm

ď ą

Some cells proliferate in the region of the primitive streak, pass sideways pushing themselves b/w ectoderm & endoderm— embryonic

Gastrulation 

The process of formation of primitive streak & the intra-embryonic mesoderm by the streak is called ‘Gastrulation’. Intra-embryonic membrane spreads throughout the disc except in the region of prochordal plate, which is later form the Buccopharyngeal membrane. BPM is composed of ectoderm & endoderm.

Intra-embryonic mesoderm subdivided into three parts— 1] Paraxial mesoderm 2] Lateral mesoderm 3] Intermediate mesoderm

In 1638, 1st-human dissection was done in America.

[1] Paraxial Mesoderm On either side of the notochord. It is organized into the segments called “somitomeres”. These are arranged into somite. Each somite give rise to: Sclerotome –cartilage & bone of the axial & paraxial skeleton (remember neural crest cells give rise to cartilage & bone of the skull & face).  Myotome –segmental muscle component.  Dermatome—Segmental skin component.  Each myotome & dermatome has its own muscle & nerve component.

[2] Lateral Mesoderm Lateral side of the notochord. Differentiate into excretory unit of urinary system & Gonads.

[3] Inter-mediate Mesoderm Between Paraxial & Lateral mesoderms. Split into partial & visceral layers. ďƒ¨ In the head region, cranial to somites, somitomeres give origin to the mesenchyme. ďƒ¨ Striated muscle of the tongue is derived from the occipital myotome.

Stomodeum ď ą

Cranial most structure of the embryo enlarges & show two big bulging, cranial bulging develops future brain & a little below form pericardium. In-between them there is a depression called Stomodeum, the floor of which is formed by the BPM.

Foregut is bounded ventrally by the pericardium & dorsally by the developing brain. Cranially, it is at first separated from the stomodeum by BPM. When this membrane breaks down, the foregut open to the exterior through the stomodeum. Stomodeum is the future mouth.  Primordial of the craniofacial complex develops from Hensen’s node. 

Pharyngeal Arches 4th week after conception, neck is formed by the elongation of the region b/w the pericardium & stomodeum. This elongation is due to the appearance of a series of segmental mesodermal round tubular thickening in the wall of the cranial most of the foregut. These are called Pharyngeal/Branchial arches. These enlargement is bounded by cleft & grooves that helps defined each other.

Time table of the events Development of the embryo from fertilization up to the disc— ‘Pre-organogenesis period’.  2nd days after fertilization—Two-Cell-Stage embryo.  3rd days— Morulla developed.  4th day—Blastocyst formed.  8th day—Disc established (Bilaminar Disc stage)  14th day—Prochordal plate & Primitive streak formed.  16th day—Embryonic disc with three germ layers/ Gastrulation (Trilaminar Disc stage).

First Evidence of Formation 

Seen in late Somitic Period (4th-8th week IUL) During this period mesenchymal tissue derived from primitive streak, neural crest & occipital sclerotomes condense around the developing brain. Thus a capsule is formed around the brain called Ectomenix / Ectomeningeal capsule. Basal portion of this capsule gives rise to the future cranial base.

40 ď Ž

ď Ž

1) 2) 3) 4)


Day Onwards

Ectomeningeal capsule is slowly converted into cartilage (onset of cranial base formation). Conversion of mesenchymal cells into cartilage/ chondrification occurs in 4 regions. Parachordal Hypophyseal Nasal Otic

Parachordal ď Ž

Chondrification centers forming around the cranial end of the notochord are called Parachordal cartilages.

Hypophyseal 

Cranial to the termination of notochord, (at the level of the oro-pharyngeal membrane) hypophyseal pouch develops which give rise to anterior lobe of the pituitary gland. --On either side of the hypophyseal stem two hypophyseal / post sphenoid cartilages develop. These cartilages fuse together & form posterior part of the body of sphenoid. --Cranial to the pituitary gland, two presphenoid / trabecular cartilages develop which fuse together & form anterior part of the body of sphenoid. Anteriorly, the presphenoid cartilage forms a vertical cartilaginous plate called mesethmoid cartilage which gives rise to the perpendicular plate of ethmoid & cristagalli. --Lateral to the pituitary gland chondrification centers are

Nasal ď Ž

Initially during development, a capsule is seen around the nasal sense organ. This capsule chondrifies & forms the cartilages of the nostrils which fuse with the cartilages of the cranial base.

Otic ď Ž

A capsule is seen around the vestibulocochlear sense organs. This capsule chondrifies & later ossifies to give rise to the mastoid & petrous portion of the temporal bone. Otic cartilages also fuse with the cartilages of the cranial base.

ď Ž

ď Ž

Initially separate centres of cartilage formation in the cranial base, fuse together into a single irregular & greatly perforated cranial base. [Early development of the various nerve & vessels from & to the brain results in numerous perforations (foramina)]. The ossifying chondro-cranial meets the ossifying desmocranium (cranial vault) to form neurocranium.

Chondro-cranial Ossification ď Ž

ď Ž

Cranial base which is now in a cartilaginous form undergoes ossification. Bone of the cranial base undergo both endochondral as well as intramembranous ossifications.

“It is a process of formation of bone & it includes the proliferation of collagen & ground substance with subsequent deposition of calcium salts”. Calcification— “It is simply deposition of calcium slats without the presence of osteoblasts”. Ossification—

“Type of ossification in which mesenchyme is directly converted into bone”. Intra-cartilaginous Ossification— “Type of ossification in which at first mesenchyme is condensed & converted into cartilaginous models. These models are destroyed gradually & replacing in the bone”. Intra-membranous Ossification—

Centre of Ossification— “It is a particular point/points from which ossification starts”. It may be primary or secondary  Primary centre of Ossification— “It is that centre of ossification which appears first in a bone & usually before birth”.  Secondary centre of Ossification— “It is that centre of ossification which appears after the appearance of primary centre & usually after birth, from this centre small part of bone is formed”.

Law of Ossification 1)



When there are two secondary centers in a bone, the centre which appears first unites last. Duration of nutrient foramen is always away from the growing end of bone. When there are two growing ends of the bone. The end which grow longer & fuses later than the other end is considered the growing end of the bone.

Wolff’s Law 

“Bone formation & arrangement of trabeculae of cancelleous bone is directly proportional to stress & strain”. Stress & strain have influences in bone growth, compressive force helps resorption of bone whereas tensile force favors bone formation.

Occipital Bone Bone shows both endochondral & intra-membranous ossification. Seven ossification centres are seen, two intramembranous & five endochondral Supra- nuchal Squamous part ossifies intramembranously from one pair of ossification centres which appear in the 8th week of IUL. Infra- nuchal Squamous part ossifies endochondrally from two centres which appear at the 10th week of IUL. Basillar part ossifies endochondrally from a single median ossification centre appearing in the 11th week of IUL. This gives rise to the anterior portion of the occipital condyles & anterior boundary of foramen magnum. A pair of endochondral ossification centres appears in the 12th week forming the lateral boundary of ď Ž

Temporal Bone Ossifies both endochondrally & intramembranously from 11 centres. Squamous Part of the temporal bone ossifies from a single intra-membranous centre that appears in 8th week of IUL. Tympanic Ring ossifies from four intramembranous centres, appears in 12th week of IUL. Petrous part of temporal bone ossifies from four endochondral centers, appears in 5th month of IUL. Styloid Process ossifies from two endochondral centres. ď Ž

Ethmoid Bone Shows only endochondral ossification with three ossifies centres. One central located centrally that forms the median floor of the anterior cranial fossa. Two lateral centres in the nasal capsule. ď Ž

Sphenoid Bone Shows Both ossifications with 15 centres. Lesser Wing —Endochondral ossification, centre is seen in orbito -sphenoid cartilage Greater Wing & Lateral Pterygoid plate — Two intra-membranous ossification centres are seen in the alisphenoid cartilage. A part of the greater wing ossifies endochondrally. Medial Pterygoid plate —Ossifies endochondrally from a secondary cartilage in the hamular process. Anterior part of body of Sphenoid —Ossifies endochondrally from five centres (two paired & one in the midline). The centre of ossification is seen in pre-sphenoid cartilage. Posterior part of body of Sphenoid —Ossifies endochondrally from four centres. Centre of ossification is post-sphenoid cartilage. 

ď Ž

Cranial base (Chondro -cranium) is important junction b/w cranial vault & facial skeleton. Cranial base is relatively stable during growth compare to cranial vault & facial skeleton.

Flexure of the Cranial Base  During embryonic & early fetal period, cranial base flexed in the region b/w pituitary fossa & spheno -occipital junction.  Flexure of the cranial base is accompanied with flexure of developing brain stem, thus the spinal cord & foramen magnum which developed by flexed backwards, flexing & developing downwards. This flexure of the cranial base aids in increasing the neurocranial capacity.  At around 10th week of IUL, the flexion of the base is about 65° . This flattens out a bit at the time of birth.

Uneven Growth of ď Ž

Nature of Cranial Base

Anterior & posterior parts of the cranial base grow at different rates. Between 10th & 14th weeks of IUL, anterior cranial base increases in length & width by seven times, while during the same period the posterior cranial base increases only five fold.

4 ď Ž

ď Ž


Week IUL

A prominent bulge appears at the ventral aspect of the embryo. Below the bulge a shallow depression develops called Stomodeum (Primitive mouth). Floor of the stomodeum is formed by BPM which separates the stomodeum from the foregut. Around same time, branchial arches form in the region of future head & neck.

Maxilla develops from a centre of ossification in the mesenchyme of the maxillary process of the 1st mandibular arch. ď Ž 1st arch plays an important role in the development of naso -maxillary region. ď Ž

Fronto -nasal Process ď Ž

ď Ž

Mesoderm covering the developing forebrain proliferates as a downward projection that overlaps the upper part of stomodeum called Fronto -nasal process. Stomodeum is thus overlapped superiorly by the fronto -nasal process.

Mandibular Process ď Ž

Mandibular arches of both the sides form the lateral walls of the stomodeum. Mandibular arch gives of a bud from its dorsal end called maxillary process. maxillary process grows ventro-mediocranial to the main part of the mandibular arch which is now called mandibular process.

ď Ž

Thus at this stage stomodeum is overlapped from above by the frontal process, below by the mandibular process & on either side by the maxillary process.

32-days old Embryo

Nasal Pits Ectoderm overlying the fronto-nasal process shows bilateral localized thickenings above the stomodeum which soon sink & form the nasal pits. The formation of these nasal pits divides the fronto-nasal process into two parts. 1] Medial nasal process 2] Lateral nasal process. ď Ž

ď Ž

Two mandibular processes grow medially & fuse to form the lower lip & lower jaw.

ď Ž

No primary cartilage exists in the maxillary process , but the centre of ossification is associated closely with the cartilage of the nasal capsule. From this center, bone formation spreads posteriorly below the orbit towards the developing zygoma & anteriorly towards the future incisor region, ossification also spreads superiorly to form the frontal process.

Lateral Alveolar Plate ď Ž

As a result of this pattern of bone deposition a bony trough forms for the infraorbital nerve. From this trough a downward extension of bone forms the lateral alveolar plate for the maxillary tooth germs.

ď Ž

Ossification also spreads into the palatine process to form the hard palate.

Medial Alveolar Plate ď Ž

Medial alveolar plate develops from the junction of the palatal process & the main body of the forming maxilla. This plate together with its lateral counterpart, forms a trough of bone around a maxillary tooth germs.

Secondary Cartilage ď Ž

Secondary cartilage has some contributes into the development of maxilla. A zygomatic cartilage appears in the developing zygomatic process. At birth maxilla is relatively small because the maxillary sinus has not yet developed.

Palate is formed by Maxillary Process; Palatal Process; Fronto -nasal process. Fronto -nasal process gives rise to the premaxillary region (which carries the incisor teeth), while the palatal shelves form the rest of the palate.

Palatal Process ď Ž

From each maxillary process, a palate like shelves grows medially. This is called palatal process. As the palatal shelves grow medially, their union is prevented by the presence of tongue. Thus initially the developing palatal shelves grow vertically downwards towards the floor of the mouth. Sometime during the 7th week of IUL, transformation in the position of the palatal shelves occurs. They change from a vertical to a horizontal position.

Fusion of Two Palatal Processes ď Ž

Two palatal shelves, by 8 ½ week of IUL, are in close approximation with each other. Initially the two palatal shelves are covered by an epithelial lining. As they join, the epithelial cells degenerate. The connective tissue of the palatal shelves intermingle with each other resulting in their fusion.

ď Ž

Complete Palatal Fusion

Each palatal process fuses with posterior margin of the primitive palate (formed from the frontonasal process). Then two palatal processes fuse with each other in the midline. There fusion begin anteriorly & proceeds backwards. Medial edge of the palatal processes fuse with the free lower edge of the nasal septum, thus separating the two nasal cavities from each other & from the mouth.

Ossification of Palate  

Occurs from 8th week of IUL. This is an intra-membranous type of ossification. Mesoderm in the palate undergoes ossification to form the hard palate. Ossification does not extend into the posterior most portions, which remain as the soft palate. Palate ossifies from a single centre derived from the maxilla. The mid-palatal suture ossifies by 12-14years.

ď ‘ Oblique facial cleft results due toďƒ¨ Improper fusion b/w maxillary & lateral nasal process.



to 45


day IUL

Mandible is formed by Meckel’s cartilage of 1st branchial arch, which has close positional relationship to the developing mandible but makes no contribution (Guiding the growth of mandible ). ď Ž

6 ď Ž


Week of IUL

Meckel’s cartilage appears as a solid hyaline cartilaginous rod covered by fibro-cellular membrane.

ď Ž

Cartilage extends from a developing ear region (Otic Capsule) to the midline (Symphysis).

ď Ž

First structure to develop in the primordium of the lower jaw is the mandibular division of trigeminal nerve.

ď Ž

Trigeminal nerve has a close relationship to Meckel’s cartilage, beginning 2/3rd of the way along the length of the cartilage. At this point mandibular nerve divides into lingual & inferior alveolar branches, which run along medial & lateral aspects of the cartilage, respectively. Inferior alveolar nerve further divides into incisor & mental branches more anteriorly.

During 6 th Week of IUL ď Ž

On the lateral aspect of Meckel’s cartilage, a condensation of mesenchyme occurs in the angle formed by the division of the inferior alveolar nerve & its incisor & mental branches.

Neurotrophic Factors ď Ž

Produced by nerve induced osteogene sis in the ossification centers.

7 ď Ž


Week of IUL

Intramembranous ossification begins in this condensation. A single ossification centre for each half of the mandible arises in the region of mental foramen. From this centre of ossification, bone formation spreads rapidly anteriorly to the midline & posteriorly towards the point where the mandibular nerve divides into its lingual & inferior alveolar branches.

ď Ž

This spread of new bone formation occurs anteriorly along the lateral aspect of Meckel’s cartilage, forming a trough that consists of lateral & medial plates that units beneath the incisor nerve .

This trough of bone extends to the midline, where it comes into close approximation with similar trough of opposite side. The two separate centre of ossifications remain separated at the mandibular symphysis until shortly after birth. The trough is soon converted into a canal as bone forms over the nerve, joining the lateral & medial plates.

Inferior Alveolar Nerve Canal 

Backward extension of ossification along the lateral aspect of Meckel’s cartilage forms a gutter, later converted into a canal that contains the inferior alveolar nerve. This backward extension of ossification proceeds to the point where the mandibular nerve divides into the inferior alveolar & lingual nerves. From this bony canal, (extending from the division of mandibular nerve to the midline) medial & lateral alveolar plates of bone develop in relation to the forming tooth germs so that the tooth germ occupy the secondary trough of bone. This trough is partitioned, & thus the teeth come to occupy individual compartments, which finally are totally enclosed by growth of bone over the tooth germ. In this way the body of the mandible essentially is formed.

Ramus of the Mandible Develops by a rapid spread of ossification posteriorly into the mesenchyme of the 1st arch, turning away from Meckel’s cartilage. This point of divergence is marked by the lingula in the adult mandible (the point at which the inferior alveolar nerve enters the body of the mandible). ď Ž

ď Ž

Ossification stops at the site that will later become the mandibular lingula from where Meckel’s cartilage continues into the middle ear & develops into the auditory ossicles (Malleus & Incus).

Sphenomandibular Ligament ď Ž

ď Ž

From the sphenoid to the division of mandibular nerve into its alveolar & lingual branches, the cartilage is lost totally , but its fibro-cellular capsule persists as the sphenomandibular ligament. From the lingula forward to the division of the alveolar nerve into its incisor & mental branches, Meckel’s cartilage is resorbed completely.

10 ď Ž


Week of IUL

Rudimentary mandible is formed almost entirely by membranous ossification with (it must be stress) little direct involvement of Meckel’s cartilage. Forward from this point to the midline some evidence exits that cartilage might make a small contribution to the mandible by means of endochondral ossification???

Secondary Growth Cartilages Further growth of the mandible until birth is influenced strongly by the appearance of three secondary growth cartilages. [Endochondral Bone Formation] ď Ž

1] Condylar Cartilage Appears during 12th week of development & rapidly formed a carrot-shaped mass that occupies most of the developing ramus. This mass of cartilage is converted quickly to bone by endochondral ossification so that at 20weeks only a thin layer of cartilage is remains in the condylar head. This remnant of cartilage persists until the end of second decade of life, providing the mechanism for growth of mandible, in the same way as the epiphyseal cartilage does in the limbs.

2] Coronoid Cartilage Appears about 4th month of development, surrounding the anterior border & top of the coronoid process. Coronoid cartilage is a transient growth cartilage is believed to grow as a response to the developing temporalis muscle & disappears long before birth.

3] Symphyseal Cartilage

In the mental region, on either side of the symphysis, two cartilages, appear in the connective tissue b/w the two ends of Meckel’s cartilage but are entirely independent of it. They are obliterated within the 1st year after birth.

ď Ž

Thus the mandible is a membranous bone, developed in relation to the nerve of the 1st arch & almost entirely independent of Meckel’s cartilage.

Development of Lower Lip Mandibular processes of the two sides grow towards each other & fuse in the midline. They now form the lower margin of the stomodeum (remember mouth develops from stomodeum)— the two mandibular processes give rise to lower lip & lower jaw.

Development of Upper Lip ď Ž

ď Ž

Each maxillary process grow medially & fuses with the lateral nasal process & then medial nasal process. Medial & lateral processes also fuse each other. In this way the nasal pits (now called—External nares) are cut off from the stomodeum. Maxillary processes undergo considerable growth. At the same time the frontonasal process becomes much narrower from side to side, with the result that the two external nares come closer together. Stomodeum is now bounded above by the upper lip.

ď Ž

Mesodermal basis of lateral part of the upper lip is formed from the maxillary process. The overlying skin is derived from ectoderm covering this process. Mesodermal basis of medial part of the lip (called Philtrum) is formed from the frontonasal process.

Development of Nose ď Ž

It receives contribution from the frontonasal process & from the medial & lateral processes of the right & left sides. We have seen that the external nares are formed when the nasal pits are cut off from the stomodeum by the fusion of the maxillary process with the medial nasal process. We have also noted that the frontonasal process becomes progressively narrow & its deeper part ultimately form the nasal septum. Mesoderm becomes heaped up in the median plane to form the prominent of the nose. Simultaneously a groove appears b/w the region of the nose. Floor of the nasal cavity is formed byďƒ¨ Palatine process of maxilla & horizontal part of palatine bone.

Development of Cheek ď Ž

After formation of upper & lower lips, Stomodeum (which can now be called the mouth) is very broad. In its lateral part it is bounded above by the maxillary process & below by the mandibular process. These processes undergo progressive fusion with each other to form the cheeks.

ď Ž

We have seen that the during the formation of the upper lip maxillary process fuses with the lateral nasal process. This fusion not only occurs in the region of the lip but also from the stomodeum to the medial angle of the developing eye. For some time this line of fusion is marked by a groove called naso -optic furrow/ nasolacrimal sulcus. A strip of ectoderm becomes buried along this furrow & give rise to the nasolacrimal duct.

Development of Tongue Epithelium 

Anterior 2/3rd—From two lingual swelling & one tuberculum impar (from the 1st branchial arch)— Lingual Nerve & Chordatympani. Posterior 1/3rd— From the cranial half of hypobranchial eminence (From 3rd arch)— Glossopharyngeal Nerve. Posterior most part—(From 4th arch)—Vagus Nerve.

Muscles 

Develop from occipital myotomes—Glosso -pharyngeal Nerve.

Connective tissue

Develop from local mesenchyme.  1st papillae to appear on tongue — Circumvallate.  Intrinsic muscles of the tongue develop from— Occipital myotomes. 

Development of TMJ ďƒ¨ At 3mons of gestation, the secondary jaw joint, the TMJ begins to form. It starts with condensation of mesenchyme called the Temporal Blastema & Condylar Blastema. ďƒ¨ A cleft appears above the condylar blastema to form the inferior joint cavity & the condylar blastema differentiates into condylar cartilage. Another cleft occurs in the temporal blastoma to form the superior joint space. Simultaneously the articular disc is formed. The temporal blastoma gets ossified. TMJ is the last joint to be formed in the body. The complete development of this joint is over only by 12th year.

Development of Salivary Glands  Ectomesenchyme of 1st arch is essential for the complete development of salivary glands. 1st Parotid gland develops, followed by submandibular & sublingual glands.

 Primordial of the parotid & submandibular glands appear during the 6th week, whereas the primordium of the sublingual gland appears after 7th -8th weeks of fetal life.  The minor salivary glands begin their development during the 3rd month.

Human growth & development/ dental implant courses by Indian dental academy