Lecture notes for Friday, February 8     Ectoderm subdivides by folding, to form 3 subdivisions: underlined below Neural Tube Ectoderm: Which itself subdivides to form the following: Brain Spinal cord Motor nerves (one segmental motor nerve per somite). Preganglionic Autonomic Nerves Neural Retina Pigmented Retina Failure of the neural tube to close results in the birth defect spina bifida.   Neural Crest Ectoderm:Which differentiates into many diverse cell types... Sensory nerves, dorsal root ganglia (one per somite) Postganglionic autonomic nerves Melanocytes, and other mesenchymal pigment cells Schwann Cells (but not oligodendrocytes) Facial Skeleton (cell types that would be mesodermal in any other part of the body!)   Somatic Ectoderm: most of which becomes epidermis Some parts in the head become placodes: A pair of olfactory placodes become nerves of the nose. A pair of lens placodes become the lenses of the eyes. A pair of otic placodes become the inner ear (semi-circular canals, cochlea, etc.). In fish and amphibians, the lateral line system develops from placodes. The inner ear uses neuromast cells to detect sound, gravity & water flow. The lateral line system also uses neuromast cells to detect flow.   Section through chick embryo pharyngeal region    Olfactory nerve and placodes in a living Xenopus tadpole           Neuromast cells, right above here Semi-circular canals in the same living tadpole   Otoliths in the same tadpole These granules of calcium carbonate are embedded in a gel, and detect which way is down, by pressure on neuromast cells.       Eyes Histological section through embryonic chick eye   lens   Lens of a mammal eye: Each cell is extremely long, extending from the posterior side of the lens, almost all the way to the front   Eye cup with lens         pigment cells in a tadpole   Stomodeum and Digestive Tract The most anterior end of the digestive tract is formed by an infolding of the ectoderm called the stomodeum. The mouth: mouse tongue seen with polarized light   higher magnification showing orientation of muscles in perpendicular directions   Teeth The outer layer of teeth (Ameloblasts --> Enamel) tooth rudiments   dentin and enamel section of a molar The dark area is empty space, where the enamel had been but broke off during preparation of the slide. The orange area is the dentin.   pharyngeal pouches: First Pharyngeal Pouch --> Eustachian Tubes In case you were wondering, the Eustachian tube is named for Bartolomeo Eustachia, a sixteenth-century Italian anatomist 16th century Italian anatomist. This question came up in the lecture, but will not be asked on any exam. Second Pharyngeal Pouch --> Tonsils Third Pharyngeal Pouch --> Thymus and Parathyroid Gland Fourth Pharyngeal Pouch --> Thymus and Parathyroid Gland A question that might show up on an exam: what would happen if the third and fourth pharyngeal pouches did not develop properly? Palatal shelves (which are only in mammals, not birds, not reptiles, not amphibia or fish) separate the oral cavity from the nasal cavity, very much like a pair of doors closing. (When the palatal shelves don't fuse, the result is a birth defect called "Cleft Palate") One of the 3 pairs of salivary glands and the anterior pituitary gland are stomodeal ectoderm. The rest of the digestive tract develops from endoderm, which will be covered in a later lecture.