Objectives:
By the time you have finished this worksheet and any other additional reading you should be able to:
Background Reading
Wheater's Functional Histology (5th ed. 2006) Ch. 15 & Ch. 17 pp 342 - 343. Also Dr Hunter's and Dr Whittle's lecture notes
INTRODUCTION
This practical is concerned with the glands associated with the digestive tract i.e. the liver, gall bladder and pancreas (salivary glands have been considered previously).
1) LIVER
The liver is unusual in having a double blood supply. It receives blood both from the hepatic portal vein and the hepatic artery. Where has this blood come from and how does its composition differ in the two vessels?
Which vein drains the liver?
Briefly list some of the functions of the liver:
The hepatocytes of the liver are arranged in structures known as lobules. In some species, e.g.the pig the lobules are very clearly defined. You should be able to identify a typical liver lobule, the connective tissue around the lobule, hepatocytes, central vein and portal tracts. Label these structures in Figure 3.1
Figure 3.1 A Typical Liver Lobule
What are the three types of vessels normally present in portal tracts?
The liver has very little connective tissue, which explains its softness and susceptibility to tearing in abdominal trauma. However, a fine network of connective tissue fibres supports the hepatocytes and sinusoid lining cells. What type of fibres are these?
Fig 15.7 in Wheater's Functional Histology shows the arrangement of the hepatocytes in a lobule. Note that hepatocytes form branching plates of cells, often only one cell thick, between a system of sinusoids that connect the portal tracts to the central vein. In which direction does the blood move in these sinusoids?
What is the advantage of this structural arrangement? (Think about the blood supply to the hepatocytes)
The sinusoids are lined by two types of cell - phagocytic cells called Kupffer cells which are though to play a role in removing worn out blood cells from the circulation and sinusoid lining cells which are similar to the endothelial cells that line other blood vessels. What is the space of Disse and with what other space is it continuous?
Make sure that you understand the relationship between hepatocytes, space of Disse, sinusoid lining cells and sinusoids. Label these structures in Figure 3.2
Figure 3.2. The Relationship between Hepatocytes, the Space of Disse and Sinusoids
Hepatocytes synthesise and secrete bile into a system of tiny bile canaliculi which are present between adjacent hepatocytes. These canaliculi do not have a duct-like structure but consist merely of localised enlargements of the intercellular space between adjacent cells. What evidence is there that bile secretion is an active process?
2) GALL BLADDER
The gall bladder is a muscular sac - what are its functions?
Observe the organisation of the components of its walls and identify the mucosa, submucosa, muscle layer and adventitia.
What type of epithelium lines the gall bladder and in what way is it specialised for its function?
How does the gall bladder empty and how is this stimulated?
3) PANCREAS
The pancreas has both exocrine and endocrine functions. Its exocrine portion consists of closely packed serous acini similar to those of the digestive glands and its endocrine portion consists of isolated islands of lighter staining cells called islets of Langerhans.
In Figure 3.3 label the lobules, connective tissue septa, ducts and islets of Langerhans.
Figure 10.3 The Pancreas
A) Exocrine Pancreas
Look at the portion of the exocrine pancreas shown in Wheater's Functional Histology, Fig 15.15 and draw or describe the structure of a typical acinus.
Which organelles are particularly noticeable in these cells? Why is this?
What, briefly, do these acini synthesise and secrete and how do these secretions leave the acini?
What stimulates this secretion?
B) Endocrine Pancreas
DG42 shown an islet of Langerhans, a clump of secretory cells supported by reticulin fibres.How do they differ in appearance from exocrine cells?
In H&E-stained sections the cells in the islets are indistinguishable from each other but in fact a number of different types of secretory cell types are present. What are the different types called and what does each secrete?
How do these secretions leave the islets and which components of the islets facilitate this?
What causes diabetes mellitus and what, briefly, are its symptoms?
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