drawn from the cat’s and macaque’s WGA-HRP labelled serial sections for cat, macaque and human (herein illustrated sequen- tially) using Nissl stained serial sections. Three-dimensional images were drawn freehand according to these frontal, dorsal and lateral views. Results Structure of the Cat’s Red Nucleus Olivary injection sites (Fig. 1). In case 1, where there were four, 0.2-ml injections performed and the tissue processed 44 hrs later, the injection site was found to be centered into the cat’s olivary complex (see Fig. 1A). All olivary subnuclei were heavily stained throughout the rostro-caudal direction with additional partial involvement of the surrounding reticular formation. In case 2, where there were two, 0.2-ml injections performed and the tissue processed 43 hrs later, the injection site was found also to be centered into the cat’s olivary complex (see Fig. 1B), however this injection was dorsally shifted compared with case 1. Therefore this tracer strongly stained the dorsal accessory olive (DAO), the dorsal lamella (dl) of the principal olive (PO) and the caudal part of the medial accessory olive (MAO), and extended as well to the medullary reticular formation, although the lateral bend of PO and the ventral-most part of rostral MAO were free from staining. Distribution of labeled cells in the mesodiencephalic area (Figs. 2 and 3). The heavily labelled ND (sections B–L) and NB New Model of Human Red Nucleus (sections C–P) were found ipsilaterally in the ventral central gray, while the labelled dmpNr (sections C–M) and vlpNr (sections F–N) were located ipsilaterally in the reticular formation. All sections in this paragraph refer to Fig. 2. At the rostral level of the ND and NB (sections C–E), these nuclei fuse with each other and at this level the border between these structures is unclear. A narrowing of the nuclear zones is observed medially (sections F–J). The separation between ND and NB occurs at caudal levels (sections K and L). One of the causes of the separation between ND and NB might be the increase that occurred in number of descending fibers emerging from these nuclei and the consequent constriction of ND. The distal tip of ND did not extend to the level of the oculomotor nucleus (section M). The distal tip of NB exits dorsally to the rostral part of the oculomotor nucleus (sections N–P). The fibers of the fasciculus retroflexus (FR) penetrate the lateral side of the rostral tip of the ND (sections C and D), pass through the lateral side of the rostral ND and NB (sections C–E), between the rostral interstitial nucleus of MLF and Nint (sections D–F) and penetrate between dmpNr and vlpNr (sections F–H). The lateral part of the vlpNr overlaps with the rostral part of the mNr (sections H–N). In case 2, the pattern of the distribution of labeled cells was fundamentally similar to case 1. However, due to the fact that the number of labeled neurons of the NB were prominently reduced compared with case 1, the border of the NB was more readily definable along its rostrocaudal axis (see Fig. 3). Figure 3. Distribution of WGA-HRP-labelled cells in mesodiencephalic structures (case 2). Successive serial sections are rostrally arranged, i.e., section A is the most rostral. dmpNr–dorsomedial part of parvicellular red nucleus, EW–Edinger-Westphal nucleus, FR–fasciculus retroflexus, NB– nucleus accessorius medialis of Bechterew, ND–nucleus of Darkschewitsch, Nint–interstitial nucleus of Cajal, III–oculomotor nucleus. Scale bar = 200 mm in I (also applies to A–H). doi:10.1371/journal.pone.0006623.g003 PLoS ONE | www.plosone.org 5 August 2009 | Volume 4 | Issue 8 | e6623