The Suprachiasmatic Nucleus and the Circadian Timing System - Progress in Molecular Biology and Translational Science - page 10

retrochiasmatic area. The compact nature of the nucleus suggests relatively
less neuropil than adjacent nuclei, and there are no apparent SCN subdivi-
sions in Nissl-stained material. The general shape of the SCN in most species
is that of a triaxial ellipsoid extending from the rostral optic chiasm to the
caudal chiasm. A detailed quantitative analysis of the rat SCN was reported
by G¨ ldner 70,71 Each SCN has dorsoventral, mediolateral, and rostrocaudal
dimensions not exceeding 360, 450, and 750 m m, respectively, with an
astroglia to neuron ratio of approximately 1:3. The total neuron number
is about 8000 72 to 12,000. 6,71 SCN volume is about 0.064 mm 3 . 71,72
Although SCN subdivisions are not evident in Nissl-stained material, they
are with other methods; “the present study finds striking subpopulations of
cells within the SCN ... Although other minor subpopulations exist, two
predominant ones are designated the dorsomedial group of cells and the ven-
trolateral group of cells.” 72 In an extensive Golgi analysis and morphometry
on thick, plastic-embedded sections of the rat SCN, van den Pol 72 reported
that neurons in the dorsal and medial part of the nucleus had an area and
mean diameter of 84 4 and 7.8 0.9 m m, respectively, whereas those of
the ventrolateral SCN were 102 6 and 9.6 1.5 m m. Neurons in the ven-
trolateral region also had more nuclear invaginations and more complex
dendritic ramifications than those in the dorsomedial region. This division
of the SCN also is supported by developmental studies showing that
neurons of the ventrolateral region show final cell division and migration
prior to those of the dorsomedial division. 73-75
By the early 1980s, there were data from immunohistochemical studies
supporting this view of SCN organization (Refs. 76,77,85 ) . As available
information increased the nomenclature for the subdivisions was based on
their position in the nucleus (e.g., “ventrolateral” and “dorsomedial” for
the rat) became difficult to apply as it is not uniform among mammals, and
in 1996, I proposed a nomenclature for the divisions not based on relative
position of the subdivisions. In this, the subdivision lying above the optic
chiasm and predominantly populated by vasoactive intestinal polypeptide
(VIP þ ) and gastrin-releasing peptide (GRP þ ) immunoreactive neurons is
designated “core” and the subdivision lying above it containing arginine
vasopressin (AVP þ ) immunoreactive neurons is designated “shell”. 78 In
most species, core is largely surrounded by shell. This nomenclature has been
used quite extensively but has been criticized as being excessively simplistic
(Ref. 79 ) . In the next section, a detailed account of SCN organization and
summary of the argument that the SCN inmammals has twomajor functional
subdivisions will be presented.