Fig 2. Ultrastructural description of SON magnocellular neurons and their subcellular elements in the somatic zone.

Light (lMN) and dark (dMN) magnocellular neurones are morphologically distinct. The nuclei of lMN (A) and dMN (A') are different in shape, respectively rounded and indented. However, both contain dense fibillary condensed (dfc) and granular chromatin (gc). The cytoplasm of the IMN is less loaded (asterisks in A) than that of the dMN. (B-B') Both lMN and dMN showed high activity as reflected by the presence of a well developed machinery of neurosecretion and recycling. Rough endoplasmic reticulum (RER), Golgi apparatus (G), mitochondrion (M), polyvesicular body (pv), lysosomes (ly) and dense core secretory granules (arrowheads; 195.71 nm ± 4.02 and 157.5nm ±3.65) are the main membranous structures found at the subcellular level of MNs. The rough endoplasmic reticulum (RER) was abundant and was present in different degree of dilation (B,C). The rectangle in panel (B) identifies several phagosomes (polyvesicular bodies) with vesicles of heterogeneous size. The Golgi apparatus is scattered as discontinuous (B) or well developed continuous (C). (D) Cluster of mitochondria (M) observed close to plasma membrane. (E) Lysosomes (ly) and autophagosomes (white asterisks), internalizing cytoplasmic membranous structures (black asterisks). (F) Polyvesicular bodies containing heterogeneous vesicles.