Embryology - First Lecture - King Salman International University PDF

Faculty Of Dental Medicine Oral Biology Department Lecture: (Embryology I) Ass. Prof (Fatma Adel Saad)  By Dr. Fatma Adel Saad Associate professor of Oral Biology  Embryology It is the study of origin & development of an organism  Phases of intra-...

Faculty Of Dental Medicine Oral Biology Department Lecture: (Embryology I) Ass. Prof (Fatma Adel Saad)  By Dr. Fatma Adel Saad Associate professor of Oral Biology  Embryology It is the study of origin & development of an organism  Phases of intra-uterine life  I- Proliferation Period Formation of two germ layers Fertilization--------end of 2nd week IU II-The embryonic period 3rd week IU-------8th week IU  III-The fetal period 8th week IU----------birth 8th week Birth  I- Proliferation Period Formation of two germ layers Fertilization--------end of 2nd week IU Fertilization occurs at distal part of fallopian tube→ formation of zygote Zygote undergoes rapid proliferation → Blastocyst *Blastocyst (1st week IU) → gives *Bilaminar disc (2nd week IU)→Ectoderm & Endoderm germ layers will transform into → *Trilaminar disc (3rd week IU) → Ectoderm, Mesoderm & Endoderm germ layers Blastocyst at 1st w IU Trilaminar disc at 3rd w IU Amniotic Amniotic Yolk sac Yolk sac Bilaminar disc at 2nd w IU Embryo is formed of pear shaped bi-laminar disc (ectoderm and endoderm germ layers); From above → Amniotic cavity From below → Yolk sac Amniotic cavity II-The embryonic period 3rd week-------8th week (Intrauterine life)  II-The embryonic period 3rd week IU-------8th week IU *Trilaminar disc embryo is formed. *All organs & external features of embryo are developed. *This period → first trimester of pregnancy. *Any maternal illness and drug therapy cause congenital deformities. Now, the embryo (pear shaped bi-laminar disc) will transform into tri-laminar disc. Amniotic cavity *In third week IU, Embryonic bilaminar Yolk Sac Yolk Sac circular disc transform into Trilaminar disc; A mesodermal third layer is formed in between ectoderm & endoderm except for prochordal plate in head end.  Cardiac region Thus, mesoderm spread progressively between ectoderm and endoderm: *Along sides prochordal plate (in head region) *Anterior to prochordal plate forming cardiac region *Now, the embryo is a pear shaped disc formed of 3 germ layers: Ectoderm: Skin & its appendages, nails, oral mucous membrane, teeth enamel, peripheral and central nervous system Mesoderm: Cardiovascular system, blood vessels, lymphatic system, connective tissues, muscles, cartilage, bone, dental tissues except enamel. Endoderm: epithelial lining of respiratory and alimentary tract, secretory cells of liver & pancreas  III-The fetal period 8th week IU----------birth 8th week Birth Rapid growth, maturation & increase in the overall size of fetus Underlying mesoderm induce the overlying surface ectoderm to form neural plate, groove & finally Neural tube N.B: Neural plate, groove & tube are formed of neural ectoderm mesoderm mesoderm non - neural ectoderm ( neural ectoderm) At 4th week IU; Neural tube with their neural crest cells separate from overlying ectoderm (non neural surface ectoderm) → Neural tube its self enlarges giving brain in head region & spinal cord caudally Neural plate  non - neural ectoderm ( neural ectoderm) Neural crest cells → separate from the crest of the neural fold to migrate to head & neck region Neural crest cells → migrate to the mesenchyme of the head & neck region→ giving ectomesenchymal structures such as bone, cartilage, muscles , connective tissues and ectomesenchymal dental tissues. Neural tube /rostral During 4th week IU: Lateral folding The trilaminar disc embryo enlarges & folds in two axes : 1- Lateral folding Thus, the folded embryo is now encapsulated by Ectoderm. Antero-posterior folding 2- Antero-posteriorly → at both cranial & caudal ends. -Thus, the folded embryo is now encapsulated by Yolk Sac Ectoderm. -Organization of the embryonic structures in their places During the 4th week of intrauterine life Midbrain forebrain *Cranial end of the embryo shows three rounded prominences of the brain (forebrain, midbrain and hindbrain). *There is a deep groove called primitive Foregut oral cavity or stomodeum below the brain prominences & above developing stomodeum heart. Developing The stomodeum is separated from the heart foregut by the bi-layered buccopharyngeal membrane, which is formed of ectoderm (stomodeum side) & endoderm (pharynx side). It is originated from prochordal plate. 3 weeks IUL. embryo Brain prominences At end of 4th week IU : Buccopharyngeal membrane soon breakdown → stomodeum communicates with foregut why????. (It is formed of only ecto & endo , so no mesoderm for nutrition) Rathke's pouch: it is an upward ectodermal invagination from the roof of the stomodeum at end of 4th W IU. It will give rise to the anterior lobe of the pituitary glands Rathke’s Pouch Stomodeum Gut *1st week intra-uterine→ Blastocyst *2nd week intra-uterine→ bilaminar disc *3rd week intra-uterine→ trilaminar disc *4th week intra-uterine → -Formation of neural tube -Folding of embryo -Rupture of bucco-pharyngeal membrane -Development of Rathke’s pouch -Branchial arches During the 4th week IU life, the embryo is 3 mm length in average.  The branchial (pharyngeal) arches contribute extensively to the formation of the face & neck At the 4th week IU, Branchial arches: are six bilateral mesodermal thickenings on the ventral aspect of the embryo -The first four arches are clearly seen on ventro-lateral aspect of the embryo while the 5th & 6th arches are rudimentary. N.B: 5th arch disappears soon after its development -The arches expand in the lateral wall of : *Stomodeum → first arch. *Pharynx → remaining arches -For first mandibular & second hyoid branchial arches, the arches extend ventrally to merge with its opposite counterpart at the midline. -The remaining four arches do not meet at midline due to presence of a median swelling called copula of His or hypobranchial eminence arise from the pharynx floor Structure of branchial arches: Remember: *Stomodeum is lined by ectoderm. *Pharynx is lined by endoderm. Each branchial arch is a mesodermal condensation covered from outer surface by ectoderm. For each arch, It is covered or lined from (inside) inner surface → by endoderm (Pharynx side) except 1st arch is lined by ectoderm from inside. For each arch, -Outer surface → is covered by ectoderm. - Inner surface → is lined by endoderm (Pharynx side) N.B: 5th arch disappear soon except 1st arch after its development But, only first mandibular arch is covered from outside & inside by ectoderm WHY???? *As 1st branchial arch develops at the level & just lateral to the stomodium  T.S in Branchial Arches Ectoderm Mesoderm Endoderm -In between ectoderm and endoderm, there is mesoderm which gives rise to striated muscles. & Within the mesoderm there are: -Central hyaline cartilagenous bar surrounded by fibrous tissue. -Motor & sensory nerves, artery and vein. -Mesoderm of 1st arch gives rise to muscles of mastication -Mesoderm of 2nd arch gives rise to muscles of facial expression - Cartilagneous bar of 1st arch → called Meckel’s cartilage. - Cartilagenous bar of 2nd arch → called Reichert cartilage  Each branchial arch is supplied by double sensory innervation Post-trematic :The nerve of the arch itself (supply cranial side of each arch). Pre-trematic: Each arch also receives a branch from the nerve of the succeeding arch (supply caudal side of arch).. Each branchial arch is supplied by double sensory innervation The Branchial Arches His  First branchial Arch (pharyngeal Arch) Called mandibular arch 1st Mandibular branchial arch -It is the first & largest pair of branchial arches. -Bilateral 1st arches meet & merge at midline & ventrally. -It is covered by ectoderm from outside and inside Max.P Max.P -It has central cartilage called 1st Mand arch Meckel’s cartilage. -It is supplied by *Mandibular nerve (post-trematic) & chorda tempani of facial nerve (pre-trematic) * Maxillary & external carotid arteries  Maxillary process Mandibular process Mandibular arch divides into : 1- Maxillary process (dorso-laterally) 2- Mandibular process *The central cartilaginous bar of 1st mandibular arch called (Meckel’s cartilage) Function: Meckel’s cartilage mould the shape of the mandible.  Meckel’s cartilage mould the shape of the mandible and its remnants will give. Sphenomandibular ligament  Remnants of Meckel’s cartilagee: Incus and malleus Spheno-mandibular ligament Mental ossicles Lingula of the mandible Derivatives of first branchial arch Derivatives of the first arch: 1-The mandible and lower lip 2- Maxilla, lateral parts of upper lip & cheek. 3-All the teeth 4-All salivary glands 5-All muscles of mastication 6-Anterior 2/3 of tongue Note: muscles of facial expression arise from the second arch. Branchial clefts(grooves) & pharyngeal pouches T.S in Branchial Arches *Branchial arches are separated externally by shallow depression → branchial cleft (groove) → lined by ectoderm *Branchial arches are separated internally on pharyngeal side by deep depression → pharyngeal pouches → lined by endoderm  Branchial arch Branchial cleft Pharyngeal pouch The branchial arches contribute extensively to the formation of the face & neck Both first branchial cleft & pharyngeal pouch due to the deepening of the 1st cleft → give rise to → External auditory meatus, tympanic membrane, tympanic antrum and Eustachian tube Derivatives of Branchial clefts & pharyngeal pouches 1st cleft & External auditory meatus, tympanic membrane, 1st pouch tympanic antrum and Eustachian tube 2nd pouch Tonsillar fossa & epithelial covering of palatine tonsils 3rd pouch Inferior parathyroid and thymus gland 4th pouch Superior parathyroid gland  Foramen ceacum Thymus gland