Taenia tecta

The indusium griseum (IG) or dorsal hippocampal continuation receives input from but does not project to the olfactory bulb. It is a thin layer of cortex which runs parasagit-tally just dorsal to the corpus callosum. IG has been the subject of debate as to whether it is more related to the hippocampus or olfactory bulb (cf. Wyss and Sripanidkulchai, 1983 Adamek et al. 1984 for further discussion). It now seems clear that IG receives direct inputs to its tiny molecular layer from the olfactory bulb (Wyss and Sripanidkulchai, 1983 Adamek et al. 1984 De Olmos et al. 1978). This input is mainly to the rostral IG with fewer fibers running more caudally. The molecular layer of IG also receives input from the lateral and medial entorhinal cortex (Luskin and Price, 1983b). Since the entorhinal area receives direct olfactory bulb inputs and, in turn, projects to the dentate gyrus of the hippocampus it has been suggested that IG is a phylogenetically old part of the hippocampus that receives direct olfactory information as opposed to most of the hippocampus that receives only indirect olfactory input via the entorhinal area (Adamek et al. 1984). 

The anterior hippocampal continuation (AHC) has been described in detail elsewhere (Wyss and Sripanidkulchai, 1983 Adamek et al. 1984). It is located just ventral to the rostrum of the corpus callosum and dorsal to the taenia tecta. The AHC has also been called ttj (Switzer et al. 1985) and has been described in detail in the rat (Wyss and Sripanidkulchai, 1983). The inputs to AHC are similar to IG as are its efferent connections with the outstanding difference that IG does not project to the olfactory bulb while there is a modest projection from AHC to the olfactory bulb (Wyss and Sripanidkulchai, 1983 Scheibel and Scheibel, 1978 De Olmos et al. 1978). Other major efferent projections of the IG and AHC are to the mammillary bodies and anterior thalamic nuclei. 

The taenia tecta proper (tt2) projects strongly to the olfactory bulb (De Olmos et al. 1978 Shipley and Adamek, 1984). Haberly and Price (1978b) divided the taenia tecta into dorsal and ventral subdivisions. The ventral subdivision (Fig. 16) has reciprocal connections with the MOB and with parts of the olfactory cortex. The neurons of this cortical 

Structure are relatively tightly packed and are located dorsal to the anterior olfactory nuclei on the medial wall of the hemisphere anterior to the rostrum of the corpus callosum. 

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Major connections of the main olfactory system. Axons of MOB mitral/tufted cells (circles in the EPL and MCL, respectively) project as the LOT to synapse in a number of structures collectively referred to as primary olfactory cortex (POC). Centrifugal inputs to MOB include feedback projections from POC as well as inputs from subcortical forebrain and brainstem neuromodulatory cell groups. Abbreviations AON, anterior olfactory nucleus DP, dorsal peduncular cortex Ent, entorhinal cortex IG-AHC, indusium griseum-anterior hippocampal continuation LC, locus coeruleus NDB, nucleus of the diagonal band PeCo, periamygdaloid cortex PC, piriform cortex RN, raphe nuclei (dorsal and median raphe) TT, taenia tecta Tu, olfactory tubercle... 

MOB projects to several structures of the ipsilateral hemisphere, including the superficial plexiform layer of the anterior olfactory nucleus, piriform, periamygdaloid and lateral entorhinal cortices, taenia tecta, the anterior hippocampal continuation, in-dusium griseum and the olfactory tubercle (Figs. 14, 18, 19). Collectively, the regions directly innervated by the output of the MOB have been referred to as primary olfactory cortex (De Olmos et al. 1978). Most of these projections have been reported in several species (for review, cf. Shipley and Adamek, 1984). A recent study has also shown a... 

Abbreviations ACo = anterior cortical amygdaloid nucleus AOB = accessory olfactory bulb Me = medial amygdaloid nucleus AON = anterior olfactory nucleus (m = medial division) PCo = posterior cortical amygdaloid nucleus BST = bed nucleus of the stria terminalis DHR = dorsal hippocampal rudiment DPC = dorsal peduncular cortex DR = dorsal raphe nucleus Ent = entorhinal cortex LC = locus coeruleus LPO = lateral preoptic area MOB = main olfactory bulb MnR = median raphe BAOT = nucleus of the accessory olfactory tract NLOT = nucleus of the lateral olfactory tract DB = nucleus of the diagonal band PeCo = periamygdaloid cortex Pir = piriform cortex Tu = olfactory tubercle TT = taenia tecta... 

Luskin and Price (1983b) described additional outputs of the AON including projections to ventral taenia tecta, piriform cortex and olfactory tubercle, endopiriform cortex, ventral agranular insular cortex and nucleus accumbens (from ventroposterior portion of AON). According to Luskin and Price, there are very few projections outside olfactory cortex from AON, although Price et al. (1991) have confirmed a strong projection from AON to the lateral hypothalamus. 

Taenia tecta AON-medial, ventral, posterior, dorsal olfactory tubercle piriform cortex, layer II, layer III, lat. entorhinal... 

ISH-taenia tecta, AON (few), olf. tubercle, piriform (layer II, some III) cells associated with medial olf. tract, olf. tubercle all olfactory regions... 

AON-complementary to calbindin-IR neurons in AON mainly in rostral part of ventral, lat. and dorsal subdivision, numerous in taenia tecta Piriform - mainly in layer III... 

The hippocampus is the main site of localization of NGF, BDNF and NT-3 in the adult CNS. However, in situ hybridization shows specific, mainly non overlapping, patterns of expression for each one. NT-3 mRNA is found in the CA2 and the most medial CAl regions of the pyramidal cell layer, and in granule cells of the dentate gyrus. NT-3 mRNA was also present in the induseum griseum and taenia tecta, both developmentally derived from the hippocampus (Lauterbom et al.,... 

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