The in alloxan induced DM which may serve

The histological
structure of the salivary glands is linked to the physiological function of the
gland. An alteration in the structure of the gland will manifest itself as a
dysfunction of the gland with serious pathological consequences. Change in structure
of the salivary glands can be attributed to the hyperglycemia associated with
DM. The cause of change cannot be assumed to be a single mechanism but rather a
complex interaction of multiple factors which result in the observed changes.
This study focuses on the actual histological changes that occur within the
parotid gland and not the mechanism by which they occur since they are
multifactorial.

 

Numerous studies
on the submandibular, sublingual and lingual salivary glands have been
performed and changes in histological structure in DM have been reported. The
greatest of these changes occur in the parenchyma of the gland whereby acinar
atrophy and death have been observed. The blood vessels within the glands have
shown significant changes in luminal diameter and wall thickness and stiffness.
In addition to this, the ductal system in the gland, responsible for essential
modification of saliva, also undergoes changes in their epithelium and luminal
diameter which then results in a significant alteration in the composition of
saliva. All these factors must remain unaltered to ensure the normal
functioning of the gland.

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The parotid gland is the largest
salivary gland and secretes the largest amount of saliva compared to the
submandibular and the sublingual glands. The saliva not only aids in
mastication but contains protective constituents that prevent against oral
infections. Diabetes has been shown to affect the parotid gland by reducing the
salivary flow rate resulting in xerostomia. (S. Conner et al, 1970). Most
studies done on the parotid have studied the changes in the salivary flow rate
and changes in the ion concentrations that occur in poorly controlled DM and
therefore are mostly physiological. However, there is no adequate anatomical
description of the actual histological changes that occur in the acini, blood
vessels and excretory ducts of the parotid gland in alloxan induced DM which
may serve to explain the changes in salivary flow rate and alterations in
composition that have been previously observed. Therefore, this study aims to
explore these areas in depth to characterize these changes as they occur over a
period of time.

 

LITERATURE REVIEW

 

Diabetes mellitus is a disease with a
significant impact on salivary glands’ histoarchitecture which leads to a
decline in the secretory capabilities of the glands. (Antonio D. Mata et al,
2004). The changes that take place within the structure of the gland cannot be
attributed to a single mechanism but are a combination of a network of factors
that work in tandem to bring about the changes. With the secretory capabilities
compromised, saliva production reduces, leading to dry mouth also known as
xerostomia and other numerous complications. 
Therefore, understanding the changes that take place in the histomorphology
of the parotid gland (albeit they are multifactorial) is crucial since the
parotid is the principal and most essential salivary gland, indispensable to
oral function.

 

 NORMAL STRUCTURAL
ANATOMY OF THE PAROTID GLAND

 

The parotid gland is the largest of the paired salivary
glands in the human and weigh about 30 to 40 grams. It is located bilaterally
in the preauricular region along the posterior border of the mandible. The
facial nerve runs through the gland, separating it into a superficial and a
deeper lobe. The superficial lobe is lateral to the facial nerve and overlies
the lateral surface of the buccinator muscle. The deep lobe is medial to the
facial nerve and is found between the mastoid process and the ramus of the
mandible. (F. Christopher Holsinger and
Dana T. Bui, 2007)

The parotid gland is confined to a space that is bounded
superiorly by the zygomatic arch, inferiorly by the sternocleidomastoid muscle,
posteriorly by the mastoid and anteriorly by the buccinator muscle. The deep
lobe lies within the Para-pharyngeal space. (Grant J, 1972). In some cases, an
accessory parotid gland is present over the masseter muscle. (Frommer J., 1977)

The deep fascia of the neck continues superiorly over the parotid
gland to form the parotid fascia that then splits into a superficial and a deep
layer. The superficial layer is the thicker of the two and extends from the
masseter and sternocleidomastoid muscles to the zygomatic arch while the deep
layer extends to the stylomandibular ligament. (Orabi AA et al,2002).

All glands are derived from epithelial cells and consist of
parenchyma (the secretory unit) and the stroma (connective tissue). Salivary
glands are exocrine glands that secrete their products into ducts. This is
because they secrete their saliva from blind-ended secretory units called the
acinus. There are 3 main types: Serous, mucinous and seromucous. The parotid
gland has exclusively serous acini which are roughly spherical and produce a
watery protein secretion that is minimally glycosylated or non-glycosylated
from secretory granules. The individual acinar cells are pyramidal with basally
located nuclei. Flat myoepithelial cells envelop the acini by their filamentous
processes to aid in forced secretion thus allowing rapid salivary secretion.
These myoepithelial cells have also been found around intercalated ducts but
are spindle in shape and vary in size (Junquera L et al, 2003)

The acini secrete saliva that is isotonic however, the saliva that
enters the oral cavity is hypotonic which suggests that there is some degree of
ductal modification. The striated duct cells are the principal cells
responsible for reabsorption of sodium chloride which render the saliva
hypotonic (Thaysen JH et al, 1958). The acinar cells and ductal cells also
secrete bicarbonate which buffers any acidic pH in the oral cavity thus
protecting the enamel from demineralization. (Qin L et al, 2012)

Other important constituents of saliva secreted by the parotid
gland include statherin, carbonic anhydrase, secretory IgA and IgG, albumin,
lysozyme, lactoferrin, interleukin 8 and defensin. These components play
significant roles in preventing caries formation and act as immune modulatory
substances. (Gordon B. Proctor, 2016)

The numerous essential roles that saliva plays in normal
functioning of the oral cavity cannot afford to be underestimated. However, one
of the most common diseases attributed alteration of gland structure and
decline of saliva production and salivary flow rate leading to oral pathologies
is diabetes mellitus.

DIABETES MELLITUS

 

Diabetes mellitus (DM) is one of the
most prevalent diseases that affects individuals of all age groups without
discrimination of age or gender. It is a metabolic disorder of multiple
etiologies characterized by chronic hyperglycemia with the impairment of
carbohydrate, fat and protein metabolism resulting from defects in insulin
secretion, insulin action, or both (carlos Antonio, Olinda tarzia, 2010).
Glucose levels in the human body are regulated very closely within a range.
This range set out by the World Health Organization (WHO) is as follows:
Fasting plasma glucose concentration of between 5.5 to 7.0mmol/L (126 mg/dl)
while the range for the whole blood is between 3.2 to 6.7mmol/L (120mg/dl). Any
value above this range may be considered diabetic (K.G.M.M Alberti, P.Z Zimmet
for WHO consultation, 1998). The disease can be categorized into two main
groups: Diabetes mellitus type 1 and type 2. Diabetes type 1 indicates the
autoimmune processes of beta-cell destruction that may ultimately lead to DM in
which insulin is required to prevent the development of ketoacidosis, coma and
death. Type 2 diabetes indicates disorders of insulin action and insulin
secretion, either of which may be the predominant feature. DM type 1 is the
most common disease of childhood with 1 in every     400 – 600 children being diagnosed with it
and it constitutes 5% of all diabetic cases worldwide (Romesh Khardori, 2017)

 

The long-term consequences of DM
include polyuria, retinopathy, neuropathy, nephropathy, Charcot joints,
features of autonomic dysfunction, cardiovascular diseases, peripheral vascular
and cerebrovascular diseases (carlos Antonio, Olinda tarzia, 2010). Besides
damaging the kidneys, eyes, nerves, blood vessels, and heart, chronic
hyperglycemia can also be associated with histological changes of the salivary
glands that leads to physiologic alterations in the quantity and quality of
saliva production from the glands. The reduction in salivary flow leads to
taste alterations, burning mouth syndrome, greater tendency to buccal
infections, delayed healing process, decays coated tongue and halitosis (Carlos
Antonio Negrato, Olinda Tarzia, 2010)

 

PARENCHYMAL CHANGES OF SALIVARY GLANDS IN DIABETES
MELLITUS

 

The parenchyma of salivary glands
refers to the secretory unit of the gland. This includes: the acini,
intercalated ducts and the myoepithelial cells that surround the acini. The
parotid gland is the only salivary gland that has a parenchyma consisting of
exclusively serous acini while other glands contain either mucinous or
seromucous acini (Nagao T et al, 2012).

 

The parotid gland is affected in diabetes
in an adverse way, leading to decreased saliva output and bilateral
sialadenosis. Hypertrophy of the acini have been documented in diabetic rats,
however the details on other aspects such as cellular density changes, blood
vessel wall changes and ductal changes still remain vague and imprecise.

 

A study done by C. O Reuterving et all
in 1987 on rat submandibular glands showed that the salivary gland weight was
significantly reduced in diabetic rats and only the diabetic rats had lipid
inclusions within the acini while none of this was observed in non-diabetic
rats. The acinar cell size was significantly increased in long term diabetic
rats compared to short term diabetic rats. The conclusion from this study
stated that there is a relationship between the duration of the diabetes and
the extent of changes observed in the salivary glands. However, no such
information is available for the parotid gland.

 

A separate study on the submandibular
gland has described and numerous striking changes that were seen to occur: significant
acinar alterations and epithelial degeneration was observed; dilation
of many ducts within the gland were seen, with an eosin-stained material and
lime material observed in most of the cases. In the stroma, a round-cell
infiltration was seen, as well as a fibrous proliferation around ducts and
blood vessels; collagen fibers entrapped in the acini in a localized fashion,
thus showing the destruction of parenchymal architecture (Jun Masuno et al 1984).
The autonomic nerves to the glands that supply the salivary glands also undergo
a process of degeneration in hyperglycemia that causes the acini within the
glands to die. After death, they are replaced with connective tissue. (C. O Reuterving et all, 1987)

 

The
sublingual glands have been shown to exhibit changes in diabetes as well. Light
microscopy showed vacuole-like structures with various sizes in the cells of
the serous Demilune cells. In addition to this, there were granule-like
structures within the cells that appeared to be lipid droplets. This suggests
that the sublingual mucous cells become dysfunctional during the development of
diabetes (Masaki Kamata et al, 2007)

 

Perivascular
foci of lymphatic infiltration and glandular disorganization of normal salivary
gland structure was observed in the lingual salivary glands. Diabetes causes
structural changes that increase the susceptibility to oral tissue inflammation,
infection and caries owing to the reduction in salivary flow and decrease in
salivary protein (Ahmed Elayat, 2000)

 

Keeping
in mind that the salivary glands do not have the same secretory cells and
differ in the composition of secretions, different salivary glands have shown
different responses to diabetes. Therefore, it is of importance to characterize
the changes that would possibly occur in the parotid gland considering its
importance in the functioning of the oral cavity and hence its impact on systemic
health.

 

The
parotid gland bears a striking to resemblance to the structure of the pancreas
since both these glands are serous and are exocrine in part. Therefore, any
changes that occur in diabetes to the exocrine portion of the pancreas may also
be extrapolated to the parotid gland. The pancreas shows a 33% reduction in
volume in diabetic rats compared with normal controls which was statistically
significant (Matteo Piciucchi et al,2015).  Histomorphological studies also described
acinar fibrosis and atrophy including diminution of pancreatic size, fatty
infiltration and loss of acinar cells in diabetes mellitus (Philip D. Hardt et
al, 2002)

 

BLOOD VESSEL CHANGES
IN DIABETES MELLITUS

 

Diabetic
angiopathy is one of the most significant processes that occurs in diabetes and
leads to serious conditions such as retinopathy and glomerulonephritis. The DM
manifests as endothelial cell dysfunction, structural changes of large and
small arteries, deficits in tissue perfusion and hypoxia. The structural
changes that occur are due to altered lumen to wall ratio since DM causes a
characteristic thickening of the of the tunica intima and media along with
increased stiffening (Gaia Spinetti et al, 2008). If these changes take place
in the eye and the kidney then it is also possible that they do occur in
parotid gland as well and is worth investigating.

 

In the
submandibular gland, a sclerotic alteration was seen in arteries and also in
arterioles and capillaries, making the latter appear numerous. Also,
alterations indicating a certain diabetic vascular lesion were also observed.
(Jun Masuno et al 1984). A separate study performed on the blood vessels in the
submandibular gland found that the greatest histological differences were found
in the endothelium of small arteries and consisted of significant proliferation
of swollen endothelial cells, obliterating the lumen, marked fibrosis and
hyalinization (Arthur R. Colwell 1960).

 

Similarly,
another study done on the submandibular gland showed that the lumen of the
blood vessels was changing as the DM progressed over time indicating pronounced
diabetic microangiopathy. The endothelial cells showed signs of damage by day 42
and by day 56 the morphometry of the endothelial cells was significantly
altered and the lumen of the capillaries was far smaller compared to the
controls (Kotyk Taras et al, 2014)

The
lingual salivary glands showed degenerative changes of the blood vessels in
form of hyalinized thickenings of their walls (Ahmed Elayat, 2000). Increased
capillary density in streptozotocin induced diabetic rat submandibular glands
was observed and positively correlated to the edema seen in the gland. This
lead to significant dysfunction of the salivary gland (Anderson et al, 1992)

 

The
increased capillary density is due to the fact that the diabetes causes hypoxia
in tissues by reduction in luminal size and increased wall thickness of the
blood vessels. Therefore, the tissues release intrinsic angiogenic factors
which are responsive to changes in the oxygen tension and so are elevated in
DM. The support for this theory comes from a study performed on the
submandibular gland which showed that chronic hypoxia causes an increase in the
capillary density (Scott and Gradwell, 1989).

 

DUCTAL
CHANGES IN DIABETES MELLITUS

 

The
ducts of salivary gland begin at the acinus. The acini secrete into the
intercalated ducts (intralobular ducts) which link the acinar lumen with the
larger components of the duct system within the salivary gland. Intercalated
ducts drain into the secretory or striated ducts (interlobular ducts). The
intercalated ducts are composed of a simple squamous and in some cases, low
cuboidal epithelium, while the secretory ducts are of a simple columnar
epithelium. The striated ducts are the principal ducts that are responsible for
ionic exchange from the saliva. The secretory ducts drain into excretory ducts
which vary in histological appearance from simple columnar to stratified
squamous epithelium. Stenson’s duct is the main excretory duct of the parotid
gland. (Francesca Testa riva et al, 1995)

 

The Stensen’s duct emerges from the gland and runs forward
along the lateral side of the masseter
muscle. In this course, the duct is surrounded
by the buccal fat pad. (Ruiz Liard et al, 2005) It takes a steep turn at the border of the masseter
and passes through the buccinator
muscle, opening into the vestibule of the mouth, the region of the mouth between the cheek and the gums, at the parotid papilla, which lies across the second
superior molar tooth. (Bath-Balogh and Fehrenbach, 2011)

 

It is a well-established fact that diabetes causes a
change in the composition and volume of saliva produced by the salivary glands (Antonio
D. Mata et al 2004). As mentioned previously, the ducts cause modification in
salivary composition, this suggests that there may be alterations with the
ductal epithelium which result in a deficiency of ion exchange. There is little
literature to this effect regarding the parotid gland. However, the
submandibular glands and the lingual glands have been observed to show ductal
changes in DM.

 

A study done by M.M Moneteiro in 2016 showed that the
morphology of the submandibular ducts was affected in the hyperglycemic state
with a reduction of the convoluted duct area which was observed by morphometry.
In addition to this, a separate study on the submandibular glands showed that
there was a clear reduction in size and number if the granular ducts as early
as three weeks after the initial induction of diabetes (Leigh C. Anderson et al
1994). This study also found that the most striking feature at light
microscopic level involved the granular ducts of the gland and their
alterations in DM.

 

The lingual salivary gland’s striated ducts also showed
hypotrophic changes in response to the DM. Periductal foci of lymphocytic
infiltration was observed which can explain the dysfunction associated with the
gland (Ahmed Elayat, 2000).

 

The
literature available regarding the ductal changes in the submandibular and
lingual glands is abundant, however, changes in the parotid gland ducal system
still, to this day remain scarcely documented. Literature on the main excretory
duct of the parotid gland (Stenson’s duct) is even more so scarce. Therefore,
this study is set to explore these vague areas and describe the changes which
occur within the main excretory duct of the parotid gland.

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