Placodal

The origin of the nervous tissue which comprises the sensory epithelium of the vomeronasal organ is in the anterior neural crest, from which the anterior neurogenic placode appears at the rostral tip (Fig. 4.1). 

Fig. 4.1 Differentiation of olfactory placode (O). Rostral CNS in vertebrates Fb = forebrain (from Graham and Begbie, 2000).

Fig. 4.2 Differentiation from olfactory placode of main and VN primordia (a) to (b), embryonic stages in reptiles (Squamates and Crocodylia). (a), Early invagination of placode (a ), late separation of primordia for AOS/MOS and (b), agenesis of presumptive VN(JO) cells in crocodile (NPT, nasal pit) (from Parsons, 1970).

Fig. 4.3 Morphogenesis during metamorphosis in Amphibia diagramatic sequence of MOS/AOS formation in African Clawed Toad (Xenopus). OP, olfactory placode PC, principal cavity and MC, mid-cavity (from Meyer and Jadhao, 1996).

The direct link between the nose and the reproductive system has been called a non-cognitive bridge (Stoddart, 1990). A part of VN projection is non-olfactory and reaches the limbic system via a direct and monosynaptic relay (Larriva-Sahd et al., 1993 and 1994 Matsumoto et al., 1994). In addition to the production of the peripheral olfactory cells (Sec. 4.1 above), the placode also gives rise to a group of important... 

Daikoku S., Koide I., Chikamoriaoyama M. and Shimomura Y. (1993). Migration of LHRH neurons derived from the olfactory placode in rats. Arch Histol Cytol 56, 353-370. 

Graham A. and Begbie J. (2000). Organisation of neurogenic placodes. Trends Neurosci 23, 313-316. 

Key S. and Wray S. (2000). Two olfactory placode derived galanin subpopulations luteinizing hormone-releasing hormone neurones and vomeronasal cells. J Neuroendocrinal 12, 535—545. 

Murakami S. and Arai Y. (1992). Origin of LHRH-neurons in Newts effect of olfactory placode ablation. Cell liss Res 269, 21-27. 

Wray S Grant P. and Gainer H. (1989). Evidence that cells expressing LHRH-mRNA in the mouse are derived from progenitor cells in the olfactory placode. Proc Natl Acad Sci 86, 8132-8136. 

The 20-26 somite rodent embryos should present a number of structures contributing to the face, including optic and otic placodes and some early formation of the nasal prominence. 

The facial region is morphologically assessed for developmental integrity of the nasal prominence, optic and otic placodes, as well as the relative spatial distance between the placodes as it pertains to the formation of the mesencephalic flexure (Fig. 13). 

Fig. 13. Facial morphology. Normal embryo with facial structures highlighted nasal prominence (np, oval circle), optic placode (op, arroWi and otic placode (otp, arroWi, and mesencephalic flexure light dotted ared). The mesencephalic flexure is surrounded by a central facial region filled with mesenchymal cell populations darkrectangle aredi.

Subtle changes that could be attributed to delayed development, such as poorly defined optic or otic placode borders (Fig. 14b, arrow) or mesencephalic flexure appearing less full in the area (Fig. 14b, circle), are assigned a score of 4. 

A score of 3 is assigned when the nasal prominence is small, or in cases when there are small or misshapen optic or otic placodes (Fig. 14c, arrows). A score of 3 is also assigned when the mesencephalic flexure is irregularly shaped or it is small/narrow. Frequently, facial hypoplasia (reduced expansion/distance between the optic and otic placodes) accompanies this finding as the flexure abnormalities can indicate alterations in craniofacial mesenchyme populations. 

Otic placode—Mild hypoplasia (borders not well defined) Otic placode—Small Otic placode—Irregular shape Otic placode—Poor definition Otic placode—Not evident... 

Optic placode—Mild hypoplasia Optic vesicle—Small Optic vesicle—Irregular shape Optic vesicle—Poor definition Optic vesicle—Not evident... 

Because elements that will later form the CNS appear earlier than most other systems in the developmental process, disturbances of neurulation may result in severe abnormalities of the brain and spinal cord. Other ectodermal thickenings, the otic placode and lens placode, develop into the inner ear and lens, respectively. A series of mesodermal blocks called somites form around the neural tube, and these somites give rise to the axial skeleton. The intraembryonic coelom appears in the lateral mesoderm and later divides into the pericardial, pleural and peritoneal cavities. The intermediate mesoderm forms the nephrogenic cord, which later becomes the kidneys. 

The sample for analysis is mixed with a spatula and quartered. The quarter reserved for analysis is rolled with a wooden rolling pin and then passed through a sieve with 2-mm round holes. Two samples of each soil are weighed out at the same time, a 10-g sample for analysis and a 5-g sample for moisture determination. The latter is dried at 105 deg C for 16 hrs, cooled in a desiccator, and weighed. The 10-g sample is placod in a 250-cu cm pyrex electrolytic beaker. 

ALUMEN - is known to all, and signifies Mercury, because it dissolves. It is the best of all crystals. Its species are various, of which some are called technically Jamenum, Roccum, Scissum, Rotundum, Zacharinum, Debelgamo, Genoese Nitre, Alum from the mines, Fusible Alum, Scaly Alum, Liquid Alum, Preserved Alum, Common Alum, Alum Placodes, Burnt Alum, Sodden Alum, Rock-Alum, and Native Alum. 

Miyasaka N, Knaut H, Yoshihara Y (2007) Cxcll2/Cxcr4 chemokine signaling is required for placode assembly and sensory axon pathfinding in the zebrafish olfactory system. Development... 

Baker CV, Bronner-Fraser M (2001) Vertebrate cranial placodes I. Embryonic induction. Dev Biol 232 1-61... 

A technique called microelectroporation was used locally to introduce a transgene into chick embryo and express the gene in spatial and temporal manner (84, 85). By the microelectroporation technique, DNA molecules were efficiently introduced into the optic vesicle (84), sensory placodes, surface ectoderm (86), neuroepithelium of the CNS, and into the somites and limb mesenchyme (87). 

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