Accessory Organs of the - PowerPoint Presentation

Slide1

Accessory Organs

of the

Digestive System

Dr. Buthaina

Ibrahem

Al-

ezzi

Zahraa

Ahmed

The accessory organs of the digestive system are located outside of the digestive tube. Excretory glands

from the salivary glands open into the oral cavity. The

liver, gallbladder,

and

pancreas

are also

accessory organs

of the digestive tract that deliver their secretory products to the small

intestine by

excretory ducts.

The

common bile duct

from the liver and the

main pancreatic

duct

from the

pancreas join in the duodenal loop to form a single duct common to both organs. This

duct then

penetrates the duodenal wall and enters the lumen of the small intestine. The

gallbladder joins

the common bile duct via the cystic duct. Thus,

bile

from the gallbladder and

digestive enzymes

from the pancreas enter the duodenum via a common duct.

LIVER : GENERAL FEATURES :

Liver is

situated

mainly in the

right hypochondrium

, below the right dome of diaphragm in the

abdomen. It

is irrigated by two types of blood vessels, namely, portal vein (70%) and hepatic artery (30

%). Liver performs

the following exocrine and endocrine functions and is involved in:

–

Synthesis

and secretion of bile (exocrine function)

for fat digestion

,

–

Excretion

of bilirubin into the bile, which is a toxic chemical formed

in

the body

during degradation of worn

out erythrocytes

by

the

phagocytic cells

(Kupffer’s

cells) of liver,

–

Synthesis

and secretion of plasma proteins like albumin,

prothrombin

and fibrinogen

(endocrine function),

–

Storage

of glucose as glycogen,

–

D

etoxification

of various drugs and harmful substances like alcohol,

–

H

emopoiesis

in

fetuses

,

–

Clearing

the blood of cellular debris and particulate material by

the

phagocytic

function of the Kupffer’s cells.

Liver Lobule :

A classical liver lobule is hexagonal (polygonal) in shape and forms the structural unit of

liver. It

has a vein at the

center, called the

central vein.

human

liver lobules are not completely demarcated by connective tissue

septa.

So, it is very

difficult to

precisely identify the limit of the lobule.

The

portal tract contains connective tissue derived from Glisson’s capsule, containing three structures, namely, a

portal venule

, hepatic arteriole and a small hepatic ductule. As these three structures are always found in the portal tract,

the portal

tracts are often referred to as portal triad

.

The main structural and functional components of the liver are the hepatocytes, which are arranged in one cell thick

plates radiating

from the central vein towards the periphery of the

lobule. The

irregular spaces between the hepatic plates are occupied by liver

sinusoids

which are lined by discontinuous

fenestrated endothelial cells. Some

of the endothelial cells are

modified

to become phagocytic cells called

Kupffer’s cells

which phagocytose worn

out RBCs

. These cells form a part of the mononuclear phagocytic system

.

Portal Triad

PANCREAS : GENERAL

F

EATURES :

Pancreas

is an

exocrine

digestive gland as well as an

endocrine

gland.

It extends from the concavity of the duodenum on the right to the spleen on the left in the posterior abdominal

wall retroperitoneally.

1- Exocrine

Pancreas

:

The

exocrine part of pancreas is formed by

serous acini

arranged into many

lobules, the

lobules are separated by interlobular

septa

of connective tissue which carry neurovascular structures and ducts.

Each

serous acinus is made of pyramidal serous cells surrounding a small lumen. These cells are darkly stained at the

base and

lightly at the apex and contain zymogen granules

.

There are no myoepithelial cells. Instead, there are

myofibroblast

like cells called

pancreatic stellate cells

found encircling the base of the acinus in the periacinar connective tissue

.

Some

of the acini exhibit pale staining

centroacinar cells

within the

lumen, they

are cuboidal in shape and represent

the intra-acinar

part of the intercalated duct, which instead of arising from the periphery of the acinus, has invaginated

into the

acinus and starts from inside it

.

Intercalated Duct

2-Endocrine Pancreas :The endocrine part of pancreas is formed by

islets of Langerhans

, which appear as pale staining spherical bodies

among the

serous

acini, they

are more in the tail region of

pancreas. There

are about 1 to 2 million islets in the

pancreas. They

are made of branching cords of endocrine cells of the following types, supported by reticular

fibres and a rich

network of

capillaries, :

1. Alpha (

α

)

cells :

Form 20% of the total

population they are

large cells with eosinophilic

granules and found

mainly at the periphery of

islet, they

Secrete

glucagon

,

that increases glucose level in the blood.

2. Beta (

β

)

cells :

Form 70% of the

population they are

small cells with basophilic

granules found

mostly in the

center, they

Secrete

insulin

which decrease glucose level in the blood.

3. Delta (

δ

)

cells :

Form 5% of the

population,

Secrete

somatostatin

,

which inhibits secretion of GH, glucagon and insulin. They also decrease pancreatic

exocrine secretion

and bile secretion.

4. F cells/PP

Cells :

Secrete

pancreatic polypeptides

which inhibit pancreatic secretion.

GALL BLADDER :

Gall

bladder has from inner to outer, the following coats:

Mucosa :

-

It

includes the lining epithelium of simple columnar variety and the supporting

lamina

propria

rich in elastic

fibres and

blood

vessels.

- As

this epithelium is involved in absorption of water, it is provided with microvilli

which

give

a brush border

appearance to

the epithelium under light

microscope.

Mucosa

is thrown into small folds when the bladder is

empty.

- Muscularis

mucosa and submucosa are absent. (Muscularis mucosa and

Muscularis

externa

fuse.)

2. Fibromuscular layer :

This layer is composed of circularly arranged smooth muscle

fibres

intermixed with connective tissue.

3. Serosa/Adventitia :

The fundus and lower surface of body of gall bladder is covered by peritoneum (serosa), whereas the upper

surface is

attached to the fossa for gall bladder by means

of connective

tissue (adventitia). So depending on the area

selected for

sectioning, the outer coat is made of either serosa or adventitia.

It is

characterized

by

(i) presence of mucosal folds lined

by

simple

tall columnar epithelium

;

(ii) presence of

fibromuscular

layer;

(iii) absence of muscularis mucosa

and submucosa

.

References :1- Textbook of Histology A Practical guide - Gunasegaran, J.P. 2nd ed 20102- DiFiore’s Atlas of Histology with Functional Correlations, 11

th

ed 2008

Thanks for your attention