Embryology - Biology 104, Spring 2006 - Albert Harris and Corey Johnson   TWENTYFIRST LECTURE - March 8, 2006, by Corey Johnson Head and Neck development II The brain develops from 3 segments into 5. Hindbrain (Rhombencephalon) - divides to form structures in the brainstem: Myelencephalon - medulla Metencephalon - cerebellum, pons Midbrain (Mesencephalon) - processing center for visual and auditory reflexes Forebrain (Prosencephalon): Diencephalon - forms the thalamus, hypothalamus, posterior pituitary, pineal gland, retinas Telencephalon - cerebral hemispheres, olfactory lobes, hippocampus The diencephalon is particularly interesting. Four evaginations give rise to neural structures in the adult. From the lateral walls of the diencephalon, 2 evaginations bud toward the ectoderm. These will give rise to the retinas. In the floor (ventral region) of the diencephalon, a structure known as the hypophysis forms. This evagination comes into contact with the ectoderm lining the mouth, the stomadeum. In response to this inductive influence, the ectoderm of the mouth evaginates toward the hypophysis forming a pocket in the roof of oral cavity known as Rathke's pouch. The two structures join to form the posterior (neural) and anterior (glandular, from the ectoderm) pituitary glands. Lastly, in the roof (dorsal region) of the diencephalon, an evagination forms the pineal gland. In many lizards, the ectoderm is induced to form a third eye. The xenopus tadpole also has a pineal eye, but not the adult. parietal eye on an iguanid Anterior-posterior patterning in the head. Not only does the dorsal lip of the blastopore have the ability to induce a second embryonic axis, but different regions of the organizer can induce anterior (head) structures or trunk structures. Spemann noted that the organizer from early gastrulae induced more anterior structures than the organizer from latter gastrulae. The very first cells that will form axial mesoderm form the prechordal plate. These cells are then followed by the chordamesoderm that will form the notochord. In some vertebrates the prechordal plate may be more like endoderm than mesoderm... some have split the difference and call it mesendoderm or endomesoderm. Really. The prechordal plate in vertebrates induces the head to form and initiates A-P patterning. In mammals, the prechordal plate is not sufficient to induce the most anterior of structures, but instead cooperates with another signaling center in the hypoblast, called the anterior visceral endoderm. How strange that head induction would involve an extraembryonic structure! Two genes that are expressed in the inducing tissues are Lim1 and Otx2. If either one is missing, the head anterior to rhombomere 2 does not form properly. Genes are expresses in boundaries that appear to segment the mid and forebrain, but morphological boundaries are not immediately obvious. The isthmus, which divides the midbrain from hindbrain, seems to be an organizing center that sends signals to tissues surrounding it instructing their fates. Palate development occurs late in embryonic life, when the lateral walls of the bones making up the face grow towards the center of the head. They ordinarily fuse. This involves the apical surfaces of epithelium making contact, degeneration of the epithelium, and finally fusion of the mesenchymal tissues. When this goes wrong, cleft palate may result. The fusion of these structures often occurs in conjunction with defects in the face such as cleft lip. This may reflect a deficiency in the mechanism of fusion, or a deficiency in the growth of the mesenchymal structures. Summary of tissues forming the head and neck: 1) Neural Crest: pigment, cartilage and bone of jaw, face, cranium, larynx. Other connective tissues including dermis 2) Mesoderm: Somitomeres (head mesoderm) : muscles of the face and jaw, extraocular muscles, blood vessels Somites (form from the neck to the tail, but not in the head): Base of skull. Neck muscles, dermis, and bone. 3) Ectoderm: epidermis, stomadeum, anterior pituitary, and other structures we'll discuss later 4) Endoderm: pharynx (but not oral cavity), tongue, and a number of glands thyroid, parathyroid, etc. Note: the pharyngeal arches are composed of the above 4 cell types. Can you draw a cross section of an arch, labeling its parts? 5) Neural ectoderm: brain regions, pineal gland, posterior pituitary, retinas

 

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