Lecture 5 Three germ layers from placental mammal development: • ectoderm: formed from epiblast cells that stay on surface. • mesoderm: formed between epiblast and endoderm. • endoderm: formed from cells that replace hypoblast • However all layers are derived form epiblast cells! Organogenesis: process of tissues becoming organs. What do eat of the germ layers form? • Ectoderm: skin, and cns; • Mesoderm: skeleton, muscles, circulatory system, connectie tissue; • Endoderm: digestive and respiratory systems Notochord: forms from chordamesoderm, a swelling at one end of the primitive streak. It defines the body axis. neurulation: most of the ectoderm becomes epidermal skinhowever some of the ectoderm is induced through induction to form the DHNC. induction the process by which the notochord signals the overlying ectoderm to form the DHNC which eventually forms the brain and spinal cord. neurulation in Amphioxus: chordamesoderm induces overlying ectoderm to form the DHNC. Neurulation in vertebrates: in vertebrates the notochord forms from chordamesoderm and then induces the overlying ectoderm to form a neural plate. Neurulation in lampreys and teleosts: the neural plate forms a solid neural keel then the neural keel cavitates to form the DHNC. In sharks, basal osteichthyan fishes, and tetrapods: teh neural plate forms raised neural folds, the neural folds rise, creating a groove, then the folds meet to form DHNC. Where is the neural plate found in the human developing embryo? It is found adjacent to the hypoblast. neural crest: neural crest cells originate from ectodermal cells next to the neural tube-- What do neural crest cells eventually form? neural crest cells migrate to help form many structures: teeth, nerves, pigment cells, endocrine glands, membrane bones, and parts of sense organs. Mesoderm Differentiation: • The mesoderm cavitates and splits into three parts (from medial to lateral) • Somites: series of mesoderm blocks form from mesoderm next to notochord. • -Intermediate mesoderm: continuous strip of tissue just lateral to the somites. • lateral plate mesoderm: most lateral part of the mesoderm. • The cavity forms the coelom which forms the adult body cavities.