Prenatal Growth of Maxilla

Prenatal Growth of Maxilla :

•  Prenatal period divided into three periods----------

•  first period of prenatal development =period of ovum .

•  (from time of fertilization to end of second weeks )

OVIDUCT:

-union of sperm and egg nuclei (zygote) -> first mitotic cell division (embryo) -> cell division continues -> formation of the morula .

UTERUS:

-morula forms a blastocyst or blastula (assymetrical ball of cells with a cavity) ->  implantation into the endometrium.

-formation of the blastula marks the beginning of morphogenesis - shaping of the embryo,migration of dividing cells to specific locations .

-blastula = blastocyst - hollow ball of cells.

-outer layer = trophoblast - forms extraembryonic tissues (e.g. placenta, yolk sac).

-inner mass of cells at one end - totipotent embryonic stem cells or embryoblasts.

Prenatal development :

§      Second period---(period of embryo)

§      From 2- 8 weeks of development

§      largely the differentiation of all major internal and external organs

§      very vulnerable stage

§      Third period -----(Period of fetus)

§      From 8-40 weeks

§      Largely a matter of growth and maturation

Development of the Upper Face :

•       day 24: development of the frontal prominence (covers the rapidly expanding forebrain)

–    beginnings of the mandibular and maxillary processes from the 1st branchial arch

–    well-defined boundaries of the stomatodeum results

•       day 26: well-formed maxillary and mandibular processes

•       day 27: appearance of the nasal placode and the odontogenic epithelium

•       day 28: localized thickenings develop  within the frontal prominence = olfactory placodes

Stomatodeum :

•       primitive stomatodeum forms a wide shallow depression in the face – limited in its depth by the buccopharyngeal membrane .

Branchial arches :

•       also called pharyngeal arches

•       fourth week: we can see four arches with fifth transitory branchial arch

•        first branchial arch (mandibular arch)

Development of the Upper Face :

•       From  (5-  8weeks)

•       the frontonasal prominence develops two sets of placodes (nasal and lens)

•       MAJOR EVENTS

–    development of medial and lateral nasal processes or swellings which encircle the nasal pits

–    fusion of the medial nasal processes at the midline = intermaxillary/premaxillary process or process

•       formation of the upper portion of the face is faster than the lower portion (finally cease to grow at puberty)

Development of the Upper Face :

–          the medial nasal processes of both sides + the frontonasal prominence give rise to the middle portion of the nose, upper lip, anterior portion of the maxilla and the primary palate

–          the medial nasal processes also fuse internally and form the premaxillary segment

–          the frontonasal prominence will also form part of the forehead

Upper lip formation :

•       From 6-7 weeks

•       fusion of the maxillary processes with each medial nasal process

•       this contributes to the  lateral sides of the upper lip – together with the medial nasal processes which contribute to the medial aspect of the upper lip

•       the maxillary processes also fuse with the lateral nasal processes – results in a nasolacrimal groove which extends from the medial corner of the eye to the nasal cavity

Development of the Palate (weeks 7 to 9) :

•       involves the formation of a primary palate, a secondary palate and fusion of their processes

•       Primary palate

–    forms from an internal swelling of the intermaxillary/premaxillary process (fusion of medial nasal processes)

•       Secondary palate

–    forms from the two lateral  palatine shelves or processes

–    develop as internal projections of the maxillary prominences

Primary palate :

•       fusion of the median nasal processes gives rise to the median palatine process – fuses to form the primary palate

Secondary Palate :

•       the common oronasal cavity is bounded anteriorly by the primary palate and occupied by the developing tongue

•       only after the development of the secondary palate can oral and nasal cavities by distinguished

•       three outgrowth appear in the oral cavity

–    nasal septum:

•       grows downward through the oral cavity

•       it encounters the primary and secondary palates

–    two palatine shelves

•       closure of the secondary palate is likely to involve the hardening of the palatine shelves – mechanism remains unknown + the withdrawal of the tongue

Palatine shelves :

•       Maxilla formation :

•       centers of ossification develop in the mesenchyme of the maxillary processes of the first branchial arch

•       spreads posteriorly below the orbit towards the developing zygoma and anteriorly toward the future incisor region and superiorly to form the frontal process

•       ossification also spreads into the palatine process to form the hard palate

•       at the union between the palatal process and the main body of the developing maxilla is the medial alveolar plate – together with the lateral plates – development of the maxillary teeth

•       a zygomatic or malar cartilage appears in the developing zygomatic processes and contributes to the development of the maxilla

References :

1- Essential of oral histology and embriohogy , James K.Avery .

2- Essential of oral biology , MAJI JOSE , First Edition 2008 .

3- Text book of orthodontics , second edition 2007 , Gurkeerat Singh, BDS , MDS ( ortho ) , M . Ortho ( RCS  London) , RCPS Glasgow.