1 formation.pdf

Formation of Three Germ Layers Dr Sandeep Gupta Assistant Professor Learning Objectives To understand formation of germ layers To understand development of the human face and oral cavity Overview Early facial development of all vertebrate embryos is similar Many events occur-c...

Formation of Three Germ Layers Dr Sandeep Gupta Assistant Professor Learning Objectives To understand formation of germ layers To understand development of the human face and oral cavity Overview Early facial development of all vertebrate embryos is similar Many events occur-cell migrations, interactions, differential growth, and differentiation Lead to formation of maturing structures progressively Sometimes abnormal developmental alterations may give rise to some common human malformations. Formation of germ layers After fertilization of the ovum, a series of cell divisions gives rise to an egg cell mass known as the morula in mammals major portion of the egg cell mass forms the extraembryonic membranes and other supportive structures, such as the placenta  The inner cell mass embryoblast separates into two layers, the epiblast and hypoblast  Only the epiblast forms the embryo The hypoblast and other cells forming supporting tissues, such as the placenta. Process of formation of 3 germ layer is called as GASTRULATION. Formation of Primitive Streak. Primitive Node.  Epiblast cells divide and move through the primitive streak and replace the hypoblast cells forming endoderm. The cells remaining between epiblast and endoderm forms the middle germ layer, the mesoderm. Cells remaining in the epiblast form the ectoderm, completing formation of the three germ layers. Notocord formation Depression appears in the center of Primitive Knot called as Blastopore. Prenotocordal cells pass cranially in the midline between the ectoderm and endoderm till it reaches the Prochordal plate These cell form a solid cord called as Notochordal process  Importance of Notochord – Position later occupied by the Vertebral column. Part of it persist in the region of each vertebral disc as NUCLEUS PULPOSIS. Provides scaffold for formation of Neural tube.  The Neural Tube formation The Ectoderm overlying Notochord in the midline undergoes Slipper shaped thickening forming Neural Plate in front of Primitive Pit. Formation of Neural Folds. Neural tube formation Neural Crest Cells The neural crest cells are multipotent cells. They give rise to variety of cells like Odontoblasts Melanocytes Ganglia Suprarenal Medulla Parafollicular Cells Of Thyroid Gland Connective tissue, and blood vessels of head and neck region  The enamel organ develops from the ectoderm. The ectomesenchyme consists of neural crest cells and mesodermal cells. The migration of sufficient number of neural crest cells is essential for the normal growth of head region. Development of Pharyngeal Arches The gradual appearance of pharyngeal (branchial) arches contributes to the development of the face and neck. In each arch a skeletal element, artery, muscles supplied by the nerve of that arch is formed. Ectodermal clefts and endodermal pouches thus formed between the arches give rise to various structures. Derivatives of first Pharyngeal arch Also called as mandibular arch Muscles Muscles of mastication Mylohyoid Anterior belly of digastric, Tensor veli Palatini & tensor tympani Cartilages Meckel’s cartilage- Symphysis region of mandible Malleus, Incus , anterior ligament of malleus Sphenomandibular ligament  Nerve Mandibular nerve supplies muscles of mastication Chorda tympani nerve Maxillary artery Derivatives of second Pharyngeal arch Also called as hyoid arch Muscles Muscles of facial expression Posterior belly of digastric Stylohyoid Stapedius  Cartilages Reichert’s cartilage: stapes, stylohyoid ligament, Lesser cornu and upper half of body of hyoid bone Styloid process Facial nerve Stapedial artery Derivatives of Third Pharyngeal arch Muscle: Stylopharyngeus Muscle Cartilage: Greater horn and lower part of body of hyoid bone Blood supply: Common carotid artery and its terminal branches Nerve supply: Glossopharyngeal nerve Derivatives of 4th and 6th Pharyngeal arch Muscle: Crico-thyroid Levator palatine Constrictor of Pharynx Intrinsic muscle of larynx: Cartilage: Laryngeal cartilages Blood supply: Common carotid artery and its terminal branches Nerve supply: (branches of Vagus nerve) Superior Laryngeal: 4th arch Recurrent laryngeal: 6th arch Pharyngeal Pouches and Clefts Thank You