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Light and electron microscopic analysis of two divisions of the suprachiasmatic nucleus in the young and aged rat.

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THE ANATOMICAL RECORD 237:71-88 (1993)
Light and Electron Microscopic Analysis of Two Divisions of the
Suprachiasmatic Nucleus in the Young and Aged Rat
WILLIAM H. WOODS, ERVIN W. POWELL, ANNETTE ANDREWS, AND
CHARLES W. FORD, JR.
Department of Biology, Philander Smith College and Department of Anatomy,
University of Arkansas for Medical Sciences, Little Rock, Arkansas
ABSTRACT
The suprachiasmatic nucleus (SCN) is a principal controller of mammalian circadian rhythms. However, in spite of documented disturbance of biological rhythms in old animals, few significant age-related
changes have been observed in this nucleus. This study examined agerelated differences in SCN volume, neuronal number, density, and ultrastructural features in the entire rat SCN and in its two divisions, the denser
ventromedial (compacta)and less dense dorsolateral (dissipata). Light and
electron microscopic morphometric techniques were utilized in weanlings
(21-28 days), young adults (3-6 mo), and aged ( 3 M 6 mo) animals.
The total SCN volume, as well as volumes of the compacta region, were
significantly greater in young adult and aged rats than in weanlings. Thus,
as the rat ages the SCN increases in total size. However, the dissipata region
appears to decrease in volume while the compacta increases. Even though
the total number of SCN neurons was quite constant in the three age groups,
the number of neurons in the dissipata region was decreased significantly
in the young adult and aged groups as compared to the weanling.
Neurons in the compacta region were usually spindled-shaped with two
dendritic processes, while oval to spheroidal cells with 3-4 processes predominated in the dissipata. Nuclei of SCN cells were often invaginated. In
weanlings, more SCN neuronal nuclei had invaginated nuclei in the dissipata region (66%) compared to the compacta (37%). In the two older age
groups of rats, a higher percentage of invaginated neuronal nuclei were
found in both regions. However, more were still found in the dissipata (90%)
compared to the compacta (72%),even though the number of these cells in
the compacta doubled. Thus, there was a large increase in the number of
invaginated nuclei, as well as the number of invaginations, in the young
adult rats compared to the weanling group, and this increase persisted in
aged rats.
SCN neurons usually had nuclei surrounded by a thin perimeter of cytoplasm containing sparse mitochondria and granular endoplasmic reticulum, multiple Golgi regions, and a moderate number of free ribosomes. In
weanlings, mitochondria contained dense cristae and the granular endoplasmic reticulum was relatively prominent. Degenerative ultrastructural
changes which included mitochondria1 enlargementlvacuolation, Golgi
vacuolation, lysosome, and lipofuscin development occurred in less than
10%of young adult SCN cells, and were more frequently found in the dissipata. In aged, rats 30%of the neurons showed degenerative changes in the
dissipata compared with 18%in the compacta. Degenerative changes appeared highly correlated with the degree of membrane folding. Ultrastructural degenerative cell changes and light microscopic morphometric observations are discussed in relation to loss of circadian rhythms with
advancing age. o 1993 WiIey-Liss, Inc.
Key words: Rat, Aging, Light morphometric, Degeneration, Ultrastructural
0 1993 WILEY-LISS. INC
Received April 20, 1992; accepted April 15, 1993.
Address reprint requests to Dr. William H. Woods, Department of
Biology, Philander Smith College, 812 W. 13th, Little Rock, AR
72202.
72
W.H. WOODS ET AL.
Since the early reports by Moore and Eichler (1972)
and Stephan and Zucker (1972) which showed a circadian rhythm control exerted by the SCN over adrenocorticosterone, drinking, and locomotion activity
rhythms, many studies have followed which tend t o
support this concept. Two subsequent reviews (Raisman and Brown-Grant, 1977; Rusak and Zucker, 1979)
documented evidence for the SCN involvement in other
major circadian rhythms including the estrous rhythm,
pineal N-acetyltransferase activity, sleep, water and
food intake, and others. More recent studies in the rat
have confirmed and extended the concept of SCN influence on biological rhythms related to drinking
(Drucker-Colin et al., 1984), insulin and glucose homeostasis (Yamamoto et al., 1984), sleep (Tobler et al.,
1983; Eastman et al., 1984),temperature (Powell et al.,
1979; Eastman et al., 19841, mitotic activity (Powell et
al., 1980) and neuro-transmitter receptors (Kafia et
al., 1981, 1985). Additional evidence in support of the
central influence of the SCN on circadian periodicity
has been provided by transplantation experiments in
which fetal SCN tissue introduced into the third ventricle of SCN-lesioned, arrhythmic animals restored
the lost drinking behavior in the rat (Drucker-Colin et
al., 1984; Ralph et al., 1990) and activity rhythm in
hamsters (Lehman et al., 1987; Martin et al., 1990).
In view of the numerous studies which support the
SCN as the principal controller of mammalian circadian rhythm and the fact that many major rhythmic
activities are fragmented in advancing age in humans
and laboratory animals (see Van Goo1 and Mirmiran,
1986; Miles and Dement, 1980; Ingram et al., 1982, for
reviews), several studies have examined cell loss
and/or degenerative changes in the SCN of aged animals. Peng et al. (1980) showed that running activity
and food intake decreased with age in the rat, but were
unable to show a significant loss in total SCN neurons.
Although this has been confirmed for the total cell
number, a decrease in a specific small population of
cells, i.e., vasopressin (VP) cells, within the SCN along
with an increase in nuclear volume and diameter and a
decrease in cell density occurred in the rat with age
(Roozendaalet al., 1987).A similar decrease in VP cells
in the SCN of aged human subjects and a “marked”
decrease in the volume of the nucleus has also been
observed (Fliers and Swaab, 1986).
Ogata (1986) has demonstrated a number of age-related changes in the dorsomedial part of the rat SCN
which he correlated with age-related loss in circadian
rhythmicity. These included a decrease in the number
of neurons and mitochondria and an increase in the
number of lysosomes, lipofuscin, and dark degenerating cells. The purpose of the present investigation was
to determine if age related differences in nuclear volume, neuronal number, density, and ultrastructure occur during aging in the dense medial and the less dense
lateral divisions of the rat SCN.
imals were housed under the standard laboratory conditions of a 12/12 light-dark cycle with food and water
ad libitum. Litters from breeding pairs were maintained under these conditions until they were weanlings (21-28 days) or young adult age (3-6 mo). Retired breeders were kept until old age (30-36 mo).
Light Microscopic and Morphometric Procedures
Seventeen animals were used for light microscopic
procedures: the first group (n = 8) were 21-28-day-old
weanlings which weighed 65 to 75 gm; a second group
(n = 3) were young adults (3-6 mo of age) that weighed
250 to 350 g, and the third group (n=3) were aged
animals (30-36 mo) weighing 300-400 g. The animals
were sacrificed under deep nembutal anesthesia (50
mgkg of body weight) during the morning hours of
1O:OO to 12:OO and their brains were flushed by intracardial perfusion with 0.9% saline and fixed immediately by perfusion with 10% formalin in a normal saline solution. The brains were then removed and
immersed in 10% formalin containing 20% sucrose for
3-7 days. The brains were frozen and 30 pm sections
were cut through the SCN. The sections were serially
mounted, stained with cresyl violet, and coverslipped
for study. Drawings of the SCN were made from these
slides.
SCN neurons were counted utilizing a calibrated ocular grid a t a magnification of 250 x . The method used
for determining the total number of neurons, nuclear
volume, and cell density has been previously reported
(Horikawa et al., 1988). Except for the fact that separate determinations were made for the ventromedial
(dense or compacta) and the dorsolateral (less-dense or
dissipata) parts of the SCN (Fig. l),the method was
essentially identical. We prefer to call these two regions compacta and dissipata, respectively. These regional designations are similar to those used for other
CNS areas, e.g., compacta and reticulata of the substantia nigra. Counts of neurons were made by taking
6 samples from each of the compacta and dissipata regions of the serially sectioned SCN; each sample was
from a different region of a given section and had a
volume of 0.000014 mm3. Counts from these six samples were averaged, and the average was divided by the
volume of a sample to obtain an average density. Only
neurons sectioned through a nucleolus were recorded to
avoid counting a neuron more than once. The volume of
each SCN (or its compacta and dissipata regions) were
determined by first tracing the scaled (16 x ) image outline of the respective areas onto drawing paper as described by Konigsmark et al. (1969). All drawings were
made by the same investigator who utilized the most
prominent appearing cell free zone in establishing
nuclear boundaries. Then, the area was determined by
using a calibrated digitizing tablet (Sumagraphics)
with a built-in microprocessor. The volume of a section
of a nucleus was determined by multiplying the area by
the
thickness. The volumes of all the sections of the two
MATERIALS AND METHODS
regions were added to obtain their total respective volThis study utilized light microscopical and ultra- umes. The formula used to calculate the total number
structural techniques, both of which involved quanti- of neurons, then, is
tative procedures. A combined total of 27 SpragueN = Dc x Vc + Dd x Vd
Dawley rats of both sexes, purchased from the Charles
Rivers Laboratories as retired breeders (8-10 mo) and where N = total number of neurons; Dc = neuronal
breeding pairs (3 mo), were used in this study. All an- density (average number of neurons per mm3 in the
EM ANALYSIS OF RAT SCN
73
using the same procedure employed for nuclear area
measurements. An average of 142 cells per animal
were traced from the compacta and dissipata regions of
the rostral, middle and caudal SCN.
The Kruskal-Wallis non-parametric test was used to
determine significance between the measured parameters in the three groups of rats. If significance existed,
the groups were paired (weanlings vs. young adult;
weanlings vs. old adult; and young adult vs. old adult)
then, the Mann-Whitney U-test was used to determine
which particular groups were significantly different
from each other.
Ultrastructural and Morphometric Procedures
Fig. 1. Coronal, cresyl violet stained 30 pm sections through the
SCN a t rostral (A), middle (B), and caudal (C) levels of the nucleus.
The dense ventromedial compacta (C) and less dense dorsolateral dissipata (D) regions are evident at each level. V, third ventricle; OC,
optic chiasm. x 250.
compacta region); Dd = neuronal density in dissipata
region; Vc = volume of compacta; and Vd = volume of
dissipata.
In addition, area determinations for 428 individual
SCN neurons from 3 young adult rats were made by
Electron microscopic (EM) procedures and analysis
were carried out on 10 Sprague-Dawley rats which belonged to 3 age groups; 21-28 days (n=3), 3-6 mo
(n = 3), and 30-36 mo (n = 4), and weighed 70-80,200300, 300-500 g, respectively.
All animals were sacrificed as described above. After
the saline flush, they were perfused with a mixture of
1.5% glutaraldehyde and 2% paraformaldehyde solution for 5 min followed by a 4% glutaraldehyde solution
in 0.1 M phosphate buffer (pH 7.3) for 15 min. Brains of
the animals were removed rapidly and immersed in 4%
glutaraldehyde after which they were blocked into an
area containing the hypothalamus. The latter was subdivided into medial and lateral hypothalamic nuclear
groups and then into dorsal and ventral tiers consisting
of strips 1-1.5 mm in depth, 1-1.5 mm in width, and 4
mm in length under a dissecting microscope with the
use of an atlas of the rat brain (Konig and Klippel,
1963). Major structural land marks (anterior commissure, 3rd ventricle, optic chiasm, optic tracts, mammillary bodies, fornix, etc.) were used as guides during the
dissecting procedures.
Brain strips containing the SCN were rinsed in 0.2 M
phosphate buffer, post-fixed in 1%osmium tetroxide in
0.1 M phosphate buffer, dehydrated, embedded in epon
in flat molds and allowed to harden. The blocks were
coronally sectioned in thick and thin sections on a
Nova I1 ultratome. Thin sections (60 nm) were taken
following each 1 p,m thick section through the rostrocaudal extent of the SCN. Thick sections were stained
with toluidine blue and used for light microscope confirmation of location within the SCN. Thin sections
were stained with uranyl acetate and lead citrate and
studied with a Philips 300 electron microscope. More
than a thousand (1,030) electron micrographs were
made from the SCN. Precise localization of the cells
dorsally and ventrally in the two halves of the nucleus
was confirmed from thick sections. Routinely, the grid
section which included the greatest number of neurons
under low magnification was photographed. Selected
cell(s) from the group were then photographed at
higher magnifications. Cells were analyzed for degenerative changes in the SCN, as defined under cytological age changes in the results section of this report.
The ratio of degenerative cells to “normal” ones was
compared between the compacta and dissipata regions
of the SCN within a given age group as well as between
the 3 different age groups. Based upon criteria detailed
below (see Results) the invagination of the nuclear envelope is considered a degenerative change in SCN
neurons. Therefore, the percentage of degenerative
74
W.H. WOODS ET AL.
TABLE 1. Comparison of volume, cell number and density in weanling, young and aged rats*
~
Category
Compacta volume ( ~ m ~ )
Dissipata volume (km3)
Total SCN volume (pm3)
Cell number for
compacta region
Cell number for
dissipata region
Total SCN cell number
Compacta region
density
p~m-~)
Dissipata region
~ m - ~ )
density
Total SCN density
(1OP urnp3)
Weanling
f S.E.M.
n = 6
29,224,000 t 2,491,5781p2
8,365,000 ? 1,179,927182
37,542,332 t 2,083,3551.2
Young adult
& S.E.M.
n = 3
41,319,000 f 614,5001
4,230,667 2 335,909l
45,549,668 f 758,638l
Aged adults
2 S.E.M.
n = 3
43,683,000 218,011'
4,638,000 f 476,323'
48,321,000 279,086'
*
*
11,134 ? 860
11,834 2 987
11,373 f 3,013
1,219 & 285ls'
12,353 ? 960
403 f 15l
12,237 999
334 2 100'
11,706 3,112
*
386
?
144
* 20
93
329
?
15
269
14l,'
288
2
*
211
264
*4
2
18
2
71'
70
* 17
244
65
*Significant difference is indicated by and (P<0.025).The SCN compacta region volume (P<0.025)
and total SCN volume (P<0.025)means
of the young and aged animals were significantly greater than the means for weanlings. The dissipata region volume (P<0.025)
and cell No.
(P<0.025) and the compacta region density (FY0.025)for weanlings were significantly higher than those for the young and aged rats.
cells with this characteristic was also determined in
the young adult and aged rats. More than 1,400 cells
(an average of more than 140 cells per animal) were
used in this analysis.
The Kruskal-Wallis non-parametric test was used to
determine if the occurrence of nuclear membrane invaginations in SCN neurons was significantly different
between the three groups of rats and between the compacts and dissipata regions of the SCN. The MannWhitney U-test was applied to see if significance was
found when the groups were paired (weanling vs.
young adult; weanling vs. old adult; young adult vs. old
adult).
RESULTS
Light Microscopic Morphometrics
The results of this study relating to total cell counts,
nuclear volume, and neuronal density of the SCN are
summarized in Table 1. Mean determinations for these
parameters were essentially the same in the SCN on
the right and left sides of the brain. Therefore, the
figures in the table represent only those for the right
SCN. The mean number of SCN compacta cells was
11,134, 11,834, and 11,373 in weanlings, young adults
and old rats, respectively. On the other hand, the dissipata region contained 1,132, 403, and 334 cells in
these respective groups. The total number of cells for
the SCN (compacta and dissipata regions) was slightly
less in the aged (11,706) as compared with weanlings
and young adult rats (12,353 and 12,237, respectively).
The difference in total cell number for the SCN between the three groups was not significant. However,
the difference in the number of cells in the dissipata
region of the nucleus was significant (P <0.025) in the
weanling as compared with the young and old adult
groups, but not between the young adult and old rats.
Likewise, the mean density of cells in both parts of the
SCN was reduced in young adult (288 cells x lop6
~ m - and
~ ) old animals (264 cells x
pmP3) as
contrasted with the cell density of the weanlings (386
cells x
~ m - ~but
) , the reduction was significant
only for the compacta region.
The mean total volume for the combined compacta
and dissipata parts of the SCN was calculated to be
0.038 mm3 in the weanlings as compared to 0.046 mm3
in young adults and 0.048 mm3 in the old rats. The
mean compacta region volume (P <0.025) and total
volume (P <0.05) of the young adult and old animals
were significantly greater than the same means for the
weanling group. However, the mean dissipata region
volume (P ~ 0 . 0 2 5 of
) the weanling was significantly
greater than those for the young adult and old animals.
General morphology of
SCN cells
General morphological features of SCN cells observed from the compacta and dissipata regions of the
nucleus (Fig. 1) as determined from light microscopic
and low magnification of thin sections are briefly described here. The cells which dominated the compacta
region of the SCN as determined from light and electron microscopy were small to moderate in size, averaging between 50 and 60 pm2 and had a mean average
of 56.6 4.0 pm2. The large prominent nuclei of SCN
neurons were usually surrounded by a small amount of
cytoplasm, which was most abundant at cell poles or
the site of origin of their processes. Characteristically,
one primary process emanated from each pole of these
spindle shaped cells (Fig. 2A) and less frequently, one
process was observed. Although these cells were dispersed throughout the nucleus, they appeared more
prevalent and concentrated in the ventral and medial
compacta region of the nucleus.
Larger oval to spheroid-shaped cells with up to 4
processes and oval to spheroid nuclei were more prevalent in the entire lateral division of the SCN but were
especially obvious in the dissipata region of the nucleus
(Fig. 2B). These were usually between 60 and 80 pm2
and had a mean average of 75.3 2.5 pm2. The cytoplasm and organelles were more abundant in these
cells, thus accounting for their larger size. As a rule,
the perikarya of medially located cells were closely
packed and oriented in dorso-ventral rows, while those
laterally had a dorsolateral orientation. It should be
emphasized that in the extreme dorsomedial part of the
*
*
EM ANALYSIS OF RAT SCN
Fig. 2. A The cell in this electron micrograph was taken from the
dissipata of a 5-mo-old rat. Note the spindle shape, the initial extension of two processes from the soma (arrows) and the more abundant
cytoplasm at opposite poles of the cell. B A ventrolateral dissipata
SCN cell of a 5-mo-old rat. The nucleus of this cell is surrounded by a
75
larger area of cytoplasm with more abundant organelles than the
smaller medial cells. The site of origin of three processes is shown by
arrowheads. A number of lysosomes (arrow) can be seen at this low
magnification.
76
W.H. WOODS ET AL.
smooth nuclear membrane. Cells with smooth nuclear
membranes were located throughout the SCN, but constituted the main type found in the compacta region
(Fig. 5). As Table 2 shows, the nuclear membrane of
neurons in the dissipata region of the SCN were usually mildly invaginated (58.7%) but often smooth
(34%),while extensively invaginated ones were uncommon (7.3%). In the SCN compacta the percentage for
neurons with mildly invaginated (35.6%) and smooth
Ultrastructural features
(63.2%)nuclear membranes was nearly the reverse of
The most distinctive appearing organelles within the the dissipata. Only 1.2% of the cells in the compacta
perikarya of SCN cells were mitochondria, the Golgi region showed an extensively invaginated nuclear
apparatus, and the granular endoplasmic reticulum. membrane (Table 2). Except for a few minor differMitochondria were moderate in number and usually ences, the ultrastructural features of major organelles
more heavily concentrated on one side of the cell. They in most SCN neurons of both the compacta and dissicontained dense transversely and sometimes horizon- pata regions (Figs. 5, 6) had essentially the same gentally oriented cristaes, and varied in shape from round eral morphology as described under general ultrastrucor oval to an elongated sausage form. The latter was tural features. The cristae of mitochondria were
often constricted andlor bent in an arc (Figs. 3,4) such distinct and relatively dense but the organelles were
that the two ends would occasionally meet and fuse rarely severely constricted, vacuolated or had unusual
forming “dumbbell” circular or other configurations configurations. The Golgi apparatus tended toward the
such as C or S shapes especially in adult and aged rats flattened or disk-shaped type. Granular endoplasmic
(Fig. 3,4). Golgi bodies were multiple and usually well reticulum, although not abundant, appeared more
developed. They were composed mainly of flattened prominently present and free ribosomes appeared to
disk-shaped vesicles along with a small spherical type have a higher density than in older rats. Lysosomes
of vesicle. In many instances the flattened vesicles and dense bodies were prominent in some cells but mawere in a circular or semicircular pattern and appeared ture lipofuscin granules were not observed. The cytoas a nearly concentrically laminated structure. The plasm was generally free of obvious vacuolations, exgranular endoplasmic reticulum was sparse to moder- cept for the flattened ones along the inner surface of
ate in amount and located peripherally or at the poles the cell membrane. Nematosomes were occasionally
of the cell from which primary dendritic processes observed as were nuclear vacuoles and inclusion bodarose (Fig. 3). Free ribosomes were rather evenly dis- ies.
tributed in the perikaryon, but were denser in the narrow rim of cytoplasm of spindle-shaped cells than in the Young adult animals
more abundant cytoplasm of the larger oval to spheroid
Several ultrastructural differences were noted that
or pear-shaped ones (Figs. 4, 11). This, in all probabil- distinguished many young adult rat SCN neurons from
ity, accounts for dark and pale (light) cells reported by those in the SCN of weanlings. The most obvious
others (Suburo and Pellegrino de Iraldi, 1969; Ogata, change was an increased incidence of nuclear mem1986). The cytoplasmic organelles and inclusions brane invagination (Table 2). There was a generalized
which appeared frequently and showed a distinct age increase in the percentage of compacta cells having
and regional correlation within the SCN were condens- invaginated nuclear membranes in the SCN, from
ing vacuoles (vesicles), cytoplasmic (matrix) vacuoles, 36.8% in weanlings to 74.0% in young adults. In addilysosomes and lipofuscin granules. Cytoplasmic nema- tion, there was a greater percentage of extensively intosomes, nuclear vacuoles, and inclusions were seen vaginated cells (41.1%)in the dissipata region in cononly infrequently and hence showed no distinct age or trast to the compacta region (9.8%) (see Figs. 7, 8). In
addition to nuclear membrane invagination, some disregional correlation within the nucleus.
sipata SCN cells (<lo%)showed degenerative changes
Age-related cytological changes in the SCN
including a rather striking increase in vacuolation, lyThe most obvious and distinctive age-related cytolog- sosome and lipofuscin development (Fig. 8). Vacuolaical changes in SCN neurons were nuclear membrane tion occurred within the cytoplasmic matrix, in the
invagination, organelle and cytoplasmic vacuolation, Golgi apparatus, and to a lesser extent in mitochonlysosomal aggregation and the development of mature dria. It was also rather obvious that free ribosomes
lipofuscin granules or L3 pigment granules (Sekhon decreased in density and multivesicular bodies were
and Maxwell, 1974). These features were used, collec- more frequent in the vacuolated cells of young adult
tively, to define degenerative neurons. On the basis of SCN cells.
nuclear membrane invagination, cells were categorized
as smooth (no invaginations), mildly invaginated (1-2 Aged animals
The most obvious change in 30-36-month-old rats
invaginations), and extensively invaginated (3 or more
invaginations) (Fig. 4). Age-related structural changes was a significant increase in the number of structurwere also observed in axons and dendrites, however, ally deteriorating cells. These characteristically contained 4 or more mature lipofuscin granules andlor agthese will not be addressed in the present study.
gregates of lysosomes combined with cytoplasmic
Weanlings
vacuolation, mitochondria1 alterations (enlargement,
The round to oval nucleus of the SCN neurons of vacuolation, and granulation), Golgi vacuolation and
21-28-day-old rats was surrounded by a predominantly occasionally dilated or vacuolated granular endoplas-
SCN, and to a greater extent in its dissipata region, the
cells were not as densely distributed as in the ventromedial part of the nucleus (Fig. 1). Another important
morphological feature of SCN cells is the characteristic
invagination of their nuclei. As will be explained below, this feature varied tremendously depending on the
age of the animal and the region of the SCN in which
the cell was found.
EM ANALYSIS OF RAT SCN
77
Fig. 3. A low power electron micrograph of a cell from the ventrolateral SCN of a 5-mo-old rat which contained a single large dendrite
(arrowhead). Note that the area of abundant cytoplasm which contains numerous variously shaped mitochondria (M)and other or-
ganelles is more abundant at the site of origin of the process than
other nuclear perimeters. The nuclear membrane is extensively invaginated with three relatively deep folds (arrows).
mic reticulum. In extreme cases, lipofuscin granules
crowded the cytoplasm, and the mitochondria appeared
swollen and ruptured. The assumption of varied morphological configurations by mitochondria appeared to
be accentuated in aged animals. Both light and dark
cells displayed such features and were dispersed
throughout the SCN but were more highly concen-
trated in the dissipata region (30%)of the SCN than in
its compacta region (18%),as is apparent in low power
micrographs of the 2 regions (Figs. 9 , l O ) . However, the
percentage of nuclear membrane invaginations in
the dissipata SCN neurons of old rats were essentially
the same as in young adults (Table 2). Also, the number of invaginations in the nucleus was highly corre-
78
W.H. WOODS ET AL.
Fig. 4. A higher magnification of the cell in Figure 3 which shows several forms of mitochondria1
constriction and elongation. An “S,” “dumbbell,” and a “C-shaped form are shown at the lower left
corner and middle of the cell, respectively (arrows). Free ribosomes (R) are rather evenly distributed in
the cell cytoplasm.
79
EM ANALYSIS O F RAT SCN
TABLE 2. Comparison of nuclear invaginations in the compacta and dissipata parts of the SCN in weanling,
young adult and old rats*
Compacta
E
S
M
Dissipata
E
S
22.8 8.6
25.5 t 3.4'
*
48.0 t 12.8
58.4 t 3.0
12.8 2.8
16.1 t 1.0'
*
7.0 2 2.3
11.2 t 3.1'
32.0 % 5.7
51.0 t 1.5
24.0 t 5.6
37.8 t 3.4'
20.3 t 0.9'
25.9 2.6l
*
50.3 t 6.0
64.2 t 1.8
7.7 t 0.9'
9.8* 0.3'
5.0 2.6l
7.5 t 3.6l
33.7 2.9
51.4 & 4.3
27.0 t 5.1'
41.1 t 7.1'
46.3 t 6.9l
63.2 t 1.5'
25.7 t 2.0
35.6 & 1.8'
1.0 t 0.6
1.2 t 0.8
22.3 t 2.4:
34.0 t 2.1
39.7 7.3
58.7 2 5.0'
4.3 t 1.3
7.3 t 3.3
M
Aged (n = 4)
Mean number of cells t S.E.M.
Mean percentages t S.E.M.
Young adult (n = 3)
Mean number of cells
Mean percentages t S.E.M.
Weanling(n = 3)
Mean number of cells t S.E.M.
Mean Dercentanes t S.E.M.
*
*
*S = smooth (no invagination); M = mild (1-2 invaginations); and E = extensive (3 or more invaginations). Significant differences were found for
compacta versus dissipata, e.g., weanling compacta smooth mean is compared to the weanling dissipata smooth mean. and indicatePs0.025.
lated with cytological degeneration, whether it was
found in the dissipata or the compacta SCN. On the
average, 56% of the cells in the dissipata with extensively invaginated nuclear membranes exhibited obvious degenerative changes (organelle vacuolation, lysosomes, and lipofuscin granule formation) while only
17% with mildly invaginated nuclear membranes
showed such changes in the aged rats. Less than 5%of
the SCN neurons with smooth nuclear membranes
showed characteristic deterioration regardless of location within the nucleus. We estimate that about twice
as many cells with degenerative changes occurred in
the middle and caudal SCN than in its rostra1 part. It
may be noted that not only did the number of invaginations increase with age but the depth of the invaginations also appeared to increase, often spanning the
entire depth of the nucleus (Fig. 11).
DISCUSSION
The results of light microscopic morphometric analysis showed essentially no difference in total SCN cell
number between weanling and young adult animals
(12,353 vs. 12,904). There was a slight decrease between these two groups and aged animals (11,7061,but
the decrease was not significant. The relative consistency of SCN cell numbers in all ages of rats is in
agreement with findings of Peng et al. (1980) and
Roozendaal et al. (1987). The total SCN cell count approximates the mean cell counts of 10,823 found by
Van den Pol (1980) and 11,650 (males and females combined) found by Guldner (1983). The total SCN volume
of 0.041 mm3 in young animals and 0.046 mm3 in aged
animals is less than the 0.050 mm3 reported by Guldner (1976), the 0.068 mm3 observed by Van den Pol
(1980) and the 0.064 mm3 observed by Guldner (19831,
but essentially the same as that found b? Roozendaal et
al. (1987) of 0.039 mm3 and 0.045mm in young (7-8
mo) and old (32-33 mo) rats. The discrepancy in these
figures could be explained in part by the difficulty one
experiences in determining the dorsolateral boundary
of the SCN between the compacta and dissipata regions
(Van den Pol, 1980). In view of the sparseness of cells in
this region, a larger area would not significantly
change the total SCN cell number.
Two important findings in the present study were a
significant decrease in cell number in the dissipata re-
gion (P <0.025) and a decrease in density in the compacts region (P <0.025) in young adults and old animals as compared to weanlings (Table 2). These
findings will be discussed below in light of our ultrastructural observations.
Cells of the SCN have spindle and oval shaped
perikarya, from which 1 to 4 processes originate, as
determined from thin and Nissl stained sections. The
cells contain multiple Golgi bodies, a small amount of
mitochondria and granular endoplasmic reticulum and
a large spheroidal or oval nucleus. The nucleus contained multiple nucleoli and a varying number and
depth of nuclear membrane invaginations. The most
frequently observed neurons were small to moderate
averaging 57 pm2 in size, spindle-shaped and possessed
one to two primary processes. Their nuclei were spindle
to ovoid-shaped and had none to several membrane invaginations. These seemed to be most highly concentrated in the ventral and medial parts of the compacta
region of SCN. Larger oval-shaped (averaging 75 bm2)
cells with more abundant cytoplasm, spheroidal nuclei
and 2-4 processes were observed in the dissipata and,
to a lesser extent, in the compacta region of the nucleus. In general, similar cellular features were described in young rats by Suburo and Pellegrino de
Iraldi (1969)and briefly outlined by Rusak and Zucker
(1979). In a detailed study of the SCN utilizing several
light microscopic techniques (including Golgi impregnation) and electron microscopy, Van den Pol (1980)
extended the list to five cell types. In addition to the
characteristic perikarya, morphological features and
processes of SCN cells described in the present study,
he also described spiny and "curly" bipolar neurons
(Van den Pol, 1980) based on dendritic trees of Golgi
impregnated materials. In the absence of Golgi stained
material, a complete correlation of cell types on this
basis cannot be made. However, the three remaining
cell types described were monopolar, simple bipolar
and multipolar types. In all probability, these cell types
observed correspond to those with 1, 2, and 3-4 processes seen in thick and thin sections in this study.
The presence of four or more mature lipofuscin granules together with organelle and cytoplasmic vacuolation along with increased nuclear membrane invaginations were major criteria used to define degenerating
SCN neurons. Except for nuclear membrane invagina-
80
W.H. WOODS ET AL.
Fig. 5. An electron micrograph of neurons in the compacta region of
the middle rostrocaudal extent of the SCN of a 28-day-old rat. The
neurons are generally spheroidal to oval in shape and have granular
endoplasmic reticulum (ger) with a characteristic polar or peripheral
distribution. Mitochondria (M) are typically oval, short cylinders or
round with no vacuoles and only occasional constrictions. The small
Golgi (GI region contains spherical and flattened vesicles often taking
a circular form. The nuclear membrane was almost always smooth
within this region of weanling rats.
tion, these criteria have been traditionally used to describe aged or degenerating features within neuronal
somas including those of the hypothalamus (Hasan et
al., 1974; Glees et al., 1975; Ogata, 1986).The degree of
nuclear membrane invagination (number and depth) of
SCN neurons appeared to show a high correlation with
the other morphological correlations of aging in these
cells. Therefore, we utilized the presence of an extensively invaginated nuclear membrane within SCN cell
somas as an additional indicator of cell deterioration.
EM ANALYSIS O F RAT SCN
81
Fig. 6. An electron micrograph of neurons taken from the same
animal and rostrocaudal level as in Figure 5, but these neurons were
taken from the dissipata region of the SCN. Note the three slight
invaginations (arrows) in the cell with a complete nucleus and the
single fold in the nucleus at bottom right of the picture. The morphology of mitochondria (M), Golgi (G),granular endoplasmic reticulum
(per) of these cells are similar to that described for the compacta cells
in Figure 5 except that they are more numerous.
We have observed this phenomenon in other hypothalamic neurons but not to the same degree (unpublished
observations). Matsumoto et al. (1982) also noted this
phenomenon in the arcuate nucleus of aged rats, and
Glees et al. (1975) noted i t in the hypothalamus of aged
monkeys. An increased frequency of invaginated nuclei
in the dissipata region of the SCN was observed by Van
den Pol (1980). In general, these authors explained increased invagination of hypothalamic neuronal nuclei
as a response to a n increased cell volume and also to a
decrease in the distance between nuclear membrane
and nucleolus. Since many of the nuclear membrane
invaginations in this report did not appear to be asso-
ciated with nucleoli, this morphological change is probably a response to some other change in the environment of the aging SCN.
Although no significant total SCN cell loss was found
in this study, cells showing structural features of degeneration occurred throughout the SCN, especially in
the lateral part of the nucleus including its ventral
half. Whether or not the development of these altered
morphological features within SCN neurons is SUEcient to indicate severe physiological deficits within
them is debatable. However, changes in the regular
circadian pattern of glucose utilization have been
shown to occur in middle aged ovariectomized (18-21
82
W.H. WOODS ET AL.
Fig. 7.An electron micrograph of neurons in the compacta region
with their characteristic spindle-shape and close soma1 apposition are
shown from middle SCN of a 8-mo-old rat. The cytoplasm is abundant
at one end of the spindle to oval-shaped nucleus. Mitochondria (MI,
some of which are swollen and vacuolated, are oval to short cylinders
with mainly transverse cristae. The Golgi (G) body primarily consists
of flattened and spherical vesicles. Granular endoplasmic reticulum
(per) is sparse and peripheral or polar in distribution. The nuclear
membrane is generally smooth or mildly invaginated (1or 2 invaginations).
E M ANALYSIS OF RAT SCN
Fig. 8. An electron micrograph of neurons in the dissipata region of
the SCN from the same animal as in Figure 7. Neurons of this region
are usually oval to spheroid and have a larger volume and lighter
density of cytoplasm than those of the denser compacta part of the
nucleus. Although only about one-half of the nuclei of two of these
83
three larger cells is shown, each membrane was extensively invaginated (3-4 invaginations). Note the presence of vacuoles (V),several
lysosomes (Ly), and isolated strands of endoplasmic reticulum (ger)
and small Golgi bodies ( G )in these cells.
84
W.H. WOODS ET AL.
Fig. 9. The four neurons shown in this electron micrograph were
taken from the compacta region of the SCN of a 36-mo-old animal.
The cells retain their usual spindle shape and are surrounded by a
thin rim of dense cytoplasm. No severe signs of degeneration except
occasional lipofuscin (Lf) granules or Lysosomes (Ly) are evident in
these medial cells. No more than two invaginations (mildly invaginated condition) occur in either of the cells, the Golgi ( G ) contain
normal, flattened and small spherical vesicles and mitochondria (M)
are the regular spherical or cylindrical (tubular) forms.
mo) as compared to young (3-4 mo) rats by Wise et al.
(1987,1988). These authors associate their results with
a decline in oxidative metabolism which in turn contributes to physiological deficits. The increased prevalence of altered SCN neuronal morphology, especially
vacuolated, swollen and ruptured mitochondria, tends
to support their concept.
Another point that merits consideration is whether
the loss of a small population of dissipata cells with a
decline in functional efficiency of many of the remaining cells in the lateral division of the SCN could be
associated with documented losses of circadian
rhythms in laboratory animals (Miles and Dement,
1980; Ingram et al., 1982) and humans (Van Goo1 and
Mirmiran, 1986). The SCN is complex in its organization and contains a significant number of immunocy-
E M ANALYSIS OF RAT SCN
Fig. 10. This electron micrograph was taken from the dissipata region of the SCN of the same animal
in Figure 9. Cells at the top and right of the figure show degenerative changes. Note small Golgi bodies
(G),vacuolated mitochondria (M), cytoplasmic vacuolation (V) and lipofuscin (Lf) granules. Although the
complete nuclei of three cells are not shown, all were either mildly or extensively invaginated.
85
86
W.H. WOODS ET AL.
Fig. 1 1 . A high power electron micrograph of an extensively invaginated nuclear membrane within a
SCN neuron of the dissipata of a 36-mo-old rat. The membrane has six folds (arrows), one of which is
relatively deep. Obvious lipofuscin granules (Lf), free ribosome (R),and a distinct Golgi apparatus (G)
are present in the cytoplasm.
tochemically identified cells which are involved in a n
unusual degree of local circuitry (Van den Pol, 1980;
Moore, 1983; Moore and Card, 1985; Van den Pol and
Gorcs, 1986; Guy et al., 1987). Due to this extremely
varied cell population, it is impossible to state with
certainty, the immunocytochemical identity of the
SCN neurons which we have shown to be more sensitive to the aging process. However, ultrastructural
analysis of the present study revealed that in the aged
rat pronounced degenerative changes occurred not only
87
EM ANALYSIS OF RAT SCN
in the dissipata, but also in many cells of the ventrolateral compacta division of the SCN. In view of the
fact that EM tissue samples from different age groups
were frequently processed simultaneously and degenerative changes were prevalent only in aged animals, it
is reasonable to conclude that such changes are age
related. As a rule, the cells which displayed degenerative changes were larger than those of the ventromedial part of the nucleus, spheroidal in shape and contained large extensively invaginated nuclei. These
general morphological features are essentially identical to those described for vasoactive intestinal peptide
(VIP) containing neurons which have been shown to be
highly concentrated in the ventrolateral SCN (Card et
al., 1981; Maegawa, 1987; Chee et al., 1988). Vasoactive intestinal peptide neurons along with immunoreactive vasopressin, somatostatin and glutamic acid decarboxylase make up 50% or more of SCN neurons
(Card and Moore, 1984) but only the VIP neurons predominate in the ventrolateral region of the nucleus
(Card and Moore, 1982; Maegawa, 1987). The heavy
concentration of VIP cells laterally in the SCN has lead
to their being designated as the most likely positioned
neurons to receive and integrate visual afferent projections (Card and Moore, 1982). Further, VIP cells of the
SCN have been shown by Maegawa (1987)to be among
the first to appear, developmentally, and along with
vasopressin cells show a day-night rhythm of appearance. Hence, he indicated that VIP cells may function
as a spontaneous oscillator displaying a light-dark
rhythm.
Finally, a few comments should be made as noted
earlier in this discussion relative to the high degree of
degenerative ultrastructural alterations and cell loss
observed in the dissipata region of the aging rat SCN.
The significant decrease in cell number in this region
may be indicated by the predominance of a diffusely
dispersed single neuronal cell type localized in this
area following loss of the more susceptible cell type. As
pointed out above, morphological features displayed by
the majority of cells in this region are identical to
those described for VIP cells by others. Specific
cell-type loss has been reported for vasopressin
(Roozendaal et al., 1987) and VIP cells (Chee et al.,
1988). The latter study suggested “selective cell death”
to explain their findings of a decreased vasopressin
and VIP cell loss in the SCN of aged rats while the
total cell number remained stable. Because the SCN
dissipata is populated by fewer cell types than the
compacta, a single cell type, e.g., VIP cells, that
undergoes early degenerative changes would result in
a significantly decreased cell number in this region
and would be more readily detected than a decrease for
the total SCN. Additional immunocytochemical studies a t the ultrastructural level and, possibly, electrophysiological studies of specific regional cell types in
aged animals must be undertaken in order to obtain a
definitive correlation between cell loss in the aging
SCN and the loss or alteration of normal circadian
rhythms. A move in this direction is indicated by the
recent report of Miller and Fuller (1992), who
identified a subpopulation of isoperiodic firing of
rat SCN neurons in vivo. These authors also suggested that most, if not all, of these neurons were VIP
cells.
ACKNOWLEDGMENTS
The authors would like to express their extreme
gratitude to Mrs. Patricia Marks for her help with statistical data, Ms. Janice Gray for her clerical help, and
Mrs. Evelyn Payne-Allen for her technical assistance.
We are equally grateful for the care and time devoted
to reading the manuscript by Dr. Robert Skinner, Dept.
of Anatomy, University of Arkansas for Medical Sciences. This research was supported by National Institute of Health grant S14GM02716.
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