Identify the basic histology of the pancreas and differentiate
endocrine and exocrine both histologically and functionally.
INTRODUCTION
The salivary glands are involved in the secretion of enzymes that
aid in digestion as well the moistening of food. The pancreas
secretes numerous enzymes which aid in digestion. It also plays
the major role in glucose regulation.
Salivary glands
Salivary glands are classified according to their function, and
location as well as the predominance of either serous or mucous
acini. The major salivary glands are the parotid, submandibular
(submaxillary), and the sublingual. The minor salivary glands are
the buccal, palatine, labial, and the lingual.
Three types of salivary secretory units are present in these
glands. The serous ones contain amylase. The mucous ones secrete
sialomucins, and mixed units contain both serous and mucous
components.
- Serous acini consist of pear-shaped groups of epithelial cells
surrounded by a distinct basement membrane. The epithelial cells
have a dense cytoplasm and a basal nucleus. The acini have a
central lumen into which secretions flow from the epithelial
cells.
- Mucous acini are larger than serous acini. The secretory cells
have an abundant cytoplasm filled with clear mucous. The
epithelial cells are pyramidal with a flattened basal nuclei.
- Mixed acini can be characterized by a crescent-shaped
formation of serous cells capping a mucous acini (serous demilune)
or be a mixed pattern of serous and mucous acini. The former
pattern is seen typically in the submandibular gland.
The gland structure consists of a connective tissue stroma with
blood vessels, nerves,lymphatics and ducts. The connective tissue
septae divide the glands into lobules. The acini make up the
parenchymal component.
The duct system transports the saliva from the gland to the oral
cavity and modifies its concentrations of electrolytes.
- It is composed of different segments. The first two segments
the intercalated and the striated ducts are intralobular. They
are secretory ducts. The intercalated duct is the first segment
and is lined with squamous or low cuboidal epithelium. It has an
irregular layer of myoepithelial cells. The striated ducts have a
simple columnar epithelial lining. It has characteristic
striations on the basal side caused by membrane invaginations and
mitochondria. This structure is involved in the transport of water
and electrolytes.
- The interlobular ducts are located in the connective tissue of
the septae. They are lined with pseudostratified columnar
epithelium along with sparse goblet cells. As the diameter
enlarges, the epithelium becomes a stratified columnar and as the
duct enters the oral cavity it is lined distally with
nonkeratinized stratified squamous epithelium.
The major salivary glands are paired structures. The parotid
gland is of the serous type. The submandibular and sublingual
glands are of the mixed type. The submandibular gland has a
predominance of the serous acini and the sublingual gland has a
predominance of the mucous acini.
Myoepithelial cells (basket cells) are flat and have long
cytoplasmic processed that aid in contraction. They lie between
the epithelial cells and the basal lamina of acini.
Gland Acini Excretory duct
Parotid serous Stenson's duct
Submandibular mixed Wharton's duct
(serous predominate)
Sublingual mixed empty into floor
(Mucous) of mouth
THE PANCREAS
In the adult the average pancreas is about 12-15 cm in length and
weighs 60 to 140 g. Histologically, the pancreas has two separate
components, exocrine and endocrine glands. The exocrine portion
makes up approximately 80% of the organ, and consists of the
numerous acini aggregated into lobules that can be seen grossly.
The acini are separated by scant connective tissue.
General Histology
The pancreas has a poorly defined capsule. It is covered by thin
areolar connective tissue capsule. This capsule gives rise to the
connective tissue septae that divide the pancreas into lobules.
Blood vessels, nerves, lymphatics and ducts transverse through the
septae. Each lobule is supplied by a principal small artery.
Acinar cells constitute the majority of the organ. The cells form
rounded or elongated acini usually at the ends of the intercalated
ducts.
- Normal acinar cells are large, pyramidal shaped cells with a
single nucleus.The nucleus lies close to the base of the cell
which rests on the basal laminae. The nucleus is round with
clumped chromatin present.
- The apical portion of the cell is filled with eosinophilic
zymogen granules. The basal portion is strongly basophilic
because the cytoplasm is filled with RER. The cells secrete
directly into acinar lumen through the apical surface. No
myoepithelial cells are present.
The ducts of the exocrine pancreas
- The intercalated ducts are formed by low cuboidal epithelium.
The nucleus is ovoid with inconspicuous nucleoli.
- The intralobular ducts vary in diameter. Lined by simple
cuboidal epithelium. A single rounded nucleus appears to fill
each cell.
- Interlobular ducts are larger and are lined by simple columnar
epithelium. They are invested by a layer of collagenous tissue.
- Two major ducts are the ducts of Santorini and Wirsung. They
have tall columnar epithelium with basal nuclei.
Merocrine secretion of proenzymes by the acinar cells is regulated
by secretin, cholecystokinin, and nerve stimulation from the
vagus.
The endocrine pancreas constitutes 1-2% of the adult pancreas.
The endocrine cells form scattered aggregates that form the Islets
of Langerhans. Each islet is a lightly stained, rounded group. A
small amount of connective tissue accompanies the large
capillaries that run through each islet. The islet cells are not
uniformly distributed in the pancreas.
There are three major cell types in the endocrine pancreas:
cell type product relative amount
alpha (A) glucagon 15-20%
beta (B) insulin 60-70%
delta (d) somatostatin 5-10%