Anatomical and Histological Study of the Excretory System in the Boscs Fringe-Toed Lizard (Acanthodactylus boskianus)
The lizard, Acanthodactylus boskianus has recently been reported from the west of Iran. The purpose of this study was to examine the morphology and histological structure of excretory system in this species. To perform this task, a number of 12 lizards (male and female) were collected from their natural habitat and transferred to the laboratory where their kidneys and excretory ducts were dissected out. The dissected samples were then processed to be prepared for histological examinations. Microscopical observations showed that in this taxon there is no evident boundary between the cortical and medullar regions of kidneys. It also demonstrated that each kidney consists of very few nephrons which do not show any loop of Henle. Each nephron is composed of a glomerulus surrounded by a doubled-walled Bowmans capsule, the other segments are proximal tubule, distal tubule, connecting tubule and collecting duct. The epithelial tissues lining the lumen of these segments were simple cubiodal cells, cubiodal cells, columnar and tall columnar cells, respectively. A ureter is located on the ventral surface of each kidney. The ureters open into the urodaeal cavity of the cloaca with a common duct or tube. In general, this lizard as other reptiles live in such areas display few glumeruli in the kidneys and highly likely produce the urine that is hyposmotic with blood.
Received: November 11, 2012;
Accepted: March 13, 2013;
Published: May 03, 2013
The morphology and physiology of reptilian kidneys has previously been investigated
by some researchers (Roberts and Schmidt-Nielsen, 1966;
Davis and Schmidt-Nielsen, 1967). These investigations
have revealed that the morphology of the tubular cells is similar to what seen
in avian kidneys but shows considerable differences from that of mammalian and
amphibian kidneys. Like other vertebrates, the functional units of kidneys are
called nephrons (Bradshaw and Bradshaw, 2002). Unlike
the human and other mammals whose kidneys consist of approximately two million
nephrons, reptiles have a relatively few nephrons. In reptiles, each nephron
is comprised of several segments including, renal corpuscule (Bowmans
capsule plus glomerulus), the proximal tubule, distal tubule which connects
to the collecting duct (Jacobson, 2007; Davis
et al., 1976). However, there are variations and differences in the
structure and functioning of various reptilian kidneys that adapt them to the
environment in which the animals live. These include variations in the number,
size, length and structural complexity of nephrons. To illustrate, it has been
reported that some reptiles have no glumeruli at all. Moreover, there is a report
that snakes and lizards have many aglomerular nephrons. The glumeruli of snakes
and lizards are small and few in number whereas, those of aquatic turtles are
much larger and more abundant (Edward, 1998; Mcnab,
2002). The boscs fringe-toed lizard, Acanthodactylus boskianus
has recently been collected and reported from west of Iran (Rastegar-Pouyani,
2000). This taxon is usually found in semi-arid areas and live on the sandy
hills covered by various species of Astragalus spp. Considering its harsh habitat,
these species were choosen to determine whether there is any specific morphological
and histological characters which are not found in related species found in
more humid regions.
MATERIALS AND METHODS
Specimens of A. boskianus were collected from the vicinity of Harsin
(34°17' N, 47°242'E), a town in Kermanshah province, West of Iran, in
spring and summer 2010. A sample of 12 females and males was examined in this
research. Snout-vent and tail lengths (to the nearest 0.5 mm) were measured
for each lizard. The largest specimen used for this research had a 60.5 mm snout-vent
length and 115 mm tail length. Specimens were then euthanized with ether within
2 days of collection. The gender of each animal was determined according to
morphological features and testified after dissection. The specimens were dissected
and the excretory system (kidney, ureter and cloaca) was removed from each individual.
The excretory system was immediately fixed in 4% formalin saline fixative. Tissues
subsequently were dehydrated in a graded series of ethanol, cleared in xylene
and embedded in paraffin. About 7 micron sections were prepared by a rotary
microtome and the sections were then stained with routine Haematoxylin-Eosin
protocol. The sections were finally observed under a Nikon microscope equipped
with a camera for photography.
Observations and results: Morphologically, the excretory organs consist
of two kidneys, two ureters and a cloaca, all of which contribute to the final
composition of the urine. The two kidneys seem flatten and the symmetrical organs
are located on each side of the cloaca. They lie dorsal to the peritoneum against
the body wall. They present a convex laterodorsal surface and a more flattened
medioventral surface containing the renal vessels. The firm, light-reddish kidneys
are covered by a connective tissue capsule. Each kidney consists of seven to
eight lobules. A ureter is located on the ventral surface of each kidney. Ureters
are opened into the ureodeum of the cloaca (Fig. 1).
In terms of histology, the transverse sections of the kidney showed that it
is surrounded by a very thin capsule, composed of reticular fibers and smooth
muscles (Fig. 2). There is no distinct border between the
cortex and medulla (Fig. 1). Few of nephrons and urinary tubules
all were connected by connective tissues (Fig. 2). Our examinations
revealed that in this taxon glomeruli consist of a simpler system of capillary
loops, with connective tissues, surrounded by a doubled-layered Bowmans
capsule including an outer (parietal) and an inner (visceral) layer. The results
also clarified that glomeruli have two poles, urinary and vascular (Fig.
3a). Four types of tubular segments were found in each nephron. The first
proximal tubule is lined by a layer of cuboidal cells. These cells display a
dense cytoplasm and moderately dense nuclei located near to the cell base. The
cells in the proximal tubules contain microvilli (Fig. 3b).
The next tubule is the distal tubule and lined by cuboidal cells which are recognized
by their rare cytoplasm and dense nuclei which are centrally or basally located
(Fig. 3c). The distal tubule is followed by connecting tubule
which is lined by low columner cells. There are two types of cells in the connecting
tubule. The pink cells, believed to be mucosa cells, are distributed between
the non mucous cells (cells having a fairly dense cytoplasm and dense nuclei
which are basally located).The mucous cells are taller than the proximal or
distal tubular cells and contain basally located nuclei.
||Photograph displaying the position of kidneys and other organ
in the lower region of the body cavity, 1: Kidney, 2: Cloaca, 3: Rectum
and 4: Testis x2
||Transverse section of the kidney showing the organ is surrounded
by thin capsule and tubular segments are observed in stroma. x100, H and
In many instances, the apical regions of the cytoplasm bulge into the lumen
as though in an active process of excreting their contents into the tubular
lumen. The non mucous cells are about the same size as mucous cells (Fig.
The collecting duct shows a larger diameter than the previous segment. The
tall columnar cells in the collecting duct represent the largest cell type found
in the entire renal tubule. The nuclei of these cells are basally located and
are surrounded by a moderately dense, thin fringe of cytoplasm. The apical two-thirds
of the cells, however, are slightly stained and contain vacuoles and granular
materials. In most cases, the plasma membranes between the cells are clearly
defined (Fig. 3d).
The ureters have been descended to lie dorsally. They are embedded dorsolaterally
in an urodaeum and lined with a simple columnar epithelium (Fig.
4a). The ureters are very close to each other, between which there is a
common duct. At the end, the common duct joins with the urodaeal cavity. Each
of the ureters are lined with a stratified epithelium (transitional, mostly
cuboidal), involves several layers in thickness (Fig. 4b).
||Micrographs showing cross sections of different kidneys
tubular segments. (a) Histology of glomeruli and Bowmans capsule revealing
(1) Parietal and (2) Visceral layers, (3) Urinary pole and (4) vascular
pole, (b) A cross section of proximal tubules with its cuboidal cells. (c)
Distal tubules are lined by a layer of cuboidal cells (1), while two types
of cells line the connecting tubule, non mucous cells (2a) and the mucous
cells (2b) and (d) A histological section through a collecting duct. The
tall columnar cells in the collecting duct represent the largest cell type
in the entire renal tubule. x400. H and E
||Micrographs taken from ureters and the common duct. (a) Photograph
revealing ureters (Ur) are embedded dorsolaterally in the (Uc) urodaeum
cavity and (b) Section showing the common duct between ureters is opening
to the urodeum cavity (Uc) (Colored arrow denotes merging point). Glands
(G). x400, H and E
It is thought that the number of glumeruli in kidney is related to the habitat
of the animal, since lower glumerular filtration rate is associated with a smaller
output of urine. The lizard A. boskianus inhabits a semi-desert environment
in the west of Iran and need to conserve water due to a limit water supply.
The results presented here demonstrated that in agreement with other reports
performed on lizards inhabiting arid environment, A. boskianus also displays
a small number of glumuli in the kidney. Even though the real number of glumeuli
was not counted here, but according to the observations made on serial sections
it was possible to make a rough estimation. It was estimated that this number
is around 1500 for each kidney. In the lizard, Sceloporus cyanogenys
inhabiting an arid environment in the south-western part of the United State
a number of 2000 glumeluli has been counted in two kidneys (Davis
et al., 1976). It has also been reported that in the lizard Uromastyx
microlopis collected from Iraq, neighboring to Iran and not very far from
where A. boskianus was collected, kidneys display a small number of glomeuli
(Khalef and Ata, 2010). According to Oshea
and Bradshaw (1993) this number is 400 to 6000 per kidney in the lizard,
Ctenophorus ornatus. There are also other studies that support the belief
about existence of a low number of renal glumeli in lizards (Gabri
and Butler, 1984). Moreover, the present study showed that in comparison
with other vertebrate the cortical and medullary regions of kidney are not separated
by a distinct border. It was also revealed that there were very few non-Henle
nephrons in the kidney while all the urinary tubules were connected by connective
tissues. In this respect it is in agreement with observations previously made
by others (Mcnab, 2002; Edward, 1998).
The results elucidated that the kidneys glomeruli of this species is composed
of a simpler system of capillary loops with mesangium and this testify the results
provided by other workers (Mcnab, 2002; Gabri
and Butler, 1984). The observation that glomeruli are surrounded by doubled-walled
Bowmans capsule is in agreement with Jacobson, 2007.
Moreover, the finding that four types of segments are observed in nephron tubules
have been also reported in different reptiles (Davis et
al., 1976; Jacobson, 2007). Here, it was reported
that in A. boskianus, the ureters form a common duct which fuses with
the dorsal surface of the urodaeal cavity. However, in contrast to our finding,
there are reports that in other species (Cnemidophorus, Gerrhonotus and
Elgaria multicarinata) the ureters are joined to the oviduct (Blackburn,
1998; Fox, 1977). In most lizards and snakes, the
ureters open into the urodaeum (Blackburn, 1998; Kuchel
and Franklin, 2000), yet in the families of Polychrotidae, Tropiduridae,
Xantusiise, Teiidae, Anguidae, Phrynosomatidae, Scincidae and Liolemidae, it
has also been found in the proctodaeum (Sanchez-Martinez
et al., 2007). It has been documented that ureters show a simple
columnar epithelium and in some species they are composed of a few layers of
stratified columnar epithelium (Sanchez-Martinez et
The present study showed that, unlike desert mammals which have abundant long
and complicated nephrons in their kidneys, arid-dwelling lizards display few
small, short and simple nephrons which seems to produce the hyposmotic urine
compared to the blood. This is possible because of their lower metabolic rate
compared with mammals. The general morphology and histology of nephron in this
taxon resembles of the nephrons reported from other species. However, because
of the lack of references it was not possible to compare the kidney and nephron
parameters in this species with those lizards living in more humid regions.
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