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Development of oro - facial structures By jey Introduction • According to Todd – development is progress towards maturity • According to Moyer's – development refers to all the naturally occurring unidirectional changes in the life of an individual from its existence as a single cell to its elaboration as a multifunctional unit terminating in death. Introduction • • • • • • Gametes are located in the sex organs ‐ Gonads Male gonad – testis Female gonad – ovary Spermatogenesis – formation of sperm in the testis Oogenesis – formation of oocyte in the ovary Gametogenesis – formation of sperm & oocyte Introduction • Development begins with fertilization • Fertilization – is a process by which the male gamete the sperm, and the female gamete oocyte unite to form zygote. Chromosomes • • • • • • Human cell contain – 46 ch – diploid Autosomes – 44 Sex chromosomes or allsomes – 2 [x or y ] In man ‐ 44 + xy In women – 44 + xx Gametes contain – 23 ch ‐ haploid Introduction • Fusion of 2 cells with 46 ch is not possible • If happens new cell will have 92 ch • Thus require a cells with 23 ch [gametes], so that on fertilization cell with 46 ch will be reestablished • Process of formation of gametes is called ‐ meiosis Meiosis • Occurs in germ cells to generate male & female gametes • Consists of 2 divisions • At the beginning of meiosis duplication of DNA content takes place • Stages – 1 division • Prophase, metaphase, anaphase, telophase • Prophase • Leptotene – chromosomes become visible • Zygotene – chromosomes pair each other – synapsis Meiosis • Pachytene – chromosomes become distinct to form tetrad, crossing over of central chromatids occur – chiasma • Diplotene – chromosomes move each other & change of genetic material occurs • Metaphase1 – 46 chromosomes become attached to the spindle • Anaphase1 – differs from mitosis here there is no splitting of Centromere & one entire chromosome of each pair move to each pole • Other phases same as mitosis Meiosis Meiosis Meiosis I meiotic division Random distribution of ch along with crossing over Shuffling of genetic material Cells with distinctive genetic content Genetic shuffling in fertilization No 2 persons are identical Meiosis [clinical corelations] • • • • • Trisomy Monosomy Mosaicism Unbalanced trans locations Structural abnormalities Meiosis Gametogenesis Ovulation Prenatal life • Can be divided into • I phase – comprises about 4 weeks, involves cellular proliferation, migration & some differentiation • II phase – next 4 weeks, characterized by differentiation • III phase – end of 2nd phase to the term, maturation & growth occurs • I & II phase collectively called embryo • III phase is called fetus Formation of germ layers Fertilization Events after fertilization Cleavage Formation of blastocyst Events during 1st week Differentiation of ectoderm & endoderm Formation of extra embryonic mesoderm Formation of prochordal plate & primitive streak Formation of intra‐ embryonic mesoderm Formation of notochord Formation of notochord Formation of neural tube Differentiation of mesoderm Neural crest cells Fate of germ layers Folding of embryo • Increased length of the disc causes it to bulge upwards into the amniotic cavity • With further enlargement embryonic disc becomes folded on itself at the head and tail ends. Folding of embryo Effect of folds on positions of other structures Pharyngeal arches • After the establishment of head fold, foregut is bounded ventrally by the pericardium, dorsally by the developing brain. • Cranially it is separated from the stomatodeum by the buccopharyngeal membrane. When this later breaks down, fore gut opens to the exterior through the stomatodeum. Pharyngeal arches • At this stage the head is represented by the bulging caused by the developing brain and the pericardium occupying the future thorax. • These two are separated by the future mouth. this is apparent that the neck is not yet formed. • The neck is formed by the elongation of the region between the stomatodeum and the pericardium. • However this elongation is achieved mainly due to appearance of series of mesodermal thickenings in the wall of the cranial most part of the foregut.are called the pharyngeal or branchial arches. Pharyngeal arches‐coronal section structure Pharyngeal arches Derivatives Nerves & muscles of arches Fate of ectodermal clefts Fate of ectodermal clefts Fate of endodermal pouches Clinical correlations of pharyngeal arches • • • • Ectopic thymic tissue Ectopic parathyroid tissue Branchial cyst and fistula Disruption of neural crest cells results in – Trecher collin syndrome – Perrirobin syndrome Development of face Development of face Development of face Development of face Development of face Development of palate • Develop as primary & secondary palate • Primary palate from fronto nasal process • Secondary from maxillary process • In the 6th ‐ palatine shelves grow obliquely on each side of tongue • In the 7th week attain the horizontal position & fuse on the midline Development of palate Development of palate Anteriorly the shelves fuse with triangular primary palate • Incisive foramen is the midline landmark between primary & secondary palate • The medial edges of palatine shelves fuse with the free lower edges of nasal septum – separates the nasal cavity from oral cavity • Later stages under go intramembranus ossification to form hard palate, does not extend posterior most part to form soft palate • Development of palate Development of palate Clinical core relations • Cleft lip – non fusion of maxillary with medial nasal process • Cleft palate – non fusion of palatine process • Oblique facial cleft ‐ non fusion of maxillary with lateral nasal process Clinical co‐ relations Clinical corelations Development of tongue • Develops in relation to pharyngeal arches • Starts in the 4th week • Lingual swellings – proliferation of medial most parts of the mandibular arches • Tuberculum impar – partially separates lingual swellings • Thyroglossal duct – formed behind the tuberculam impar, from which thyroid gland develops subsequently this site marked by depression called foramen cecum • Hypobranchial eminence – another midline swelling in relation to 2nd, 3rd & 4th arches, shows cranial [copula] and caudal part Development of tongue Development of tongue Development of tongue Development of tongue • Anterior 2/3s is derived from – I arch • Posterior 1/3 is derived from – cranial part of eminence, here 2nd arch mesoderm gets buried, below the surface the 3rd arch mesoderm grows over it to fuse with 1st arch • Posterior most part derived from 4th arch Development of tongue • Anterior 2/3s supplied by mandibular nerve, Posterior 1/3 is supplied by glossopharyngeal nerve & posterior most part by superior laryngeal nerve • Musculature is derived from occipital myotomes – supplied by hypoglossal nerve • Epithelium is first made up of single layer, later becomes stratified squamous & papilla become evident Developmental anomalies of tongue • • • • • • • Macroglossia Microglossia Aglossia Bifid tongue Ankyloglossia Fissure tongue Lingual thyroid Molecular regulation of development • By Homeobox genes • First discovered in Drosophila in the year 1983 • Homeobox is conserved DNA motif of about 180 base pairs • Homeobox encode for protein Homeodomain • Homeodomain is DNA binding protein & regulate the transcription of other genes Molecular regulation of development • • • • These genes are arranged in clusters In drosophila – only one cluster & 8 genes In humans – 4 clusters & 39 genes They are called Hox genes present in 4 different ch Molecular regulation of development Molecular regulation of development • Homeobox genes act in concert with other regulatory molecules – growth factors & retinoic acids • Cells must express the receptors to have the effect of G F • G Fs are two types – autocrine , paracrine • Retinoic acids enter directly into the cell Molecular regulation of development Molecular regulation of development • As indicated previously much of the face is derived from – neural crest cells • Homeobox genes which regulate the facial development are carried to the pharyngeal arches by neural crest cells Summary 2days 3days 4days 8days 14days 16days Summary 17 days 22 days 21 days 4 & 5 week Summary 5,6 & 7 week By end of 9 weeks