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Histology, microscopy, anatomy and disease
Histology, microscopy, anatomy and disease

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2.1 Introduction to different tissues

The rest of this week focuses on the task of identifying different types of tissue using evidence provided by the virtual microscope. Specifically, this involves working through Activity 3, looking at Slides 6–11 from the ‘Week 2’ collection.

Activity 3

Open the virtual microscope in a new browser window or tab and find the ‘Week 2’ collection. Slides 6–11 have been chosen to introduce a range of different tissues, so that you can recognise them again later on in the course. Use the instructions below, and the associated legends to navigate around the sections.

Lung (Slides 6 and 7)

Compare the normal lung (Slide 6) and the ‘inflated’ lung (Slide 7). You should be able to see that the alveoli are much larger on the inflated lung, which was injected to inflate it, before processing.

Both sections are from normal tissue. However, this pair of slides illustrates that the way in which a tissue is prepared can affect its appearance.

Blood vessels (Slide 8)

Arteries and veins often run together as pairs, although they appear quite different. The vein has a much thinner wall and, therefore, has collapsed and is less prominent than the artery in this slide.

The section also includes a number of smaller blood vessels. Arteries branch into arterioles and then into capillaries, the smallest blood vessels. Capillaries drain into venules, which feed into the veins.

Liver (Slide 9)

The liver has a complex organisation with an ‘open’ circulation. The circulation is described in this way because the blood within the organ is not confined within blood vessels.

Instead, the liver is supplied by the arterial blood (hepatic) artery and receives blood from the gut (hepatic portal vein). Blood flows through the liver along sinusoids, rather than capillaries.

Pancreas (Slide 10)

The pancreas contains two distinct types of tissue.

  • The exocrine pancreas produces digestive enzymes.
  • The Islets of Langerhans produce a number of hormones, including insulin, which control aspects of metabolism.

Notice that the Islets can most easily be identified using the lowest magnification objective.

Thymus (Slide 11)

The thymus is a lymphoid organ, in which a type of leukocyte known as T lymphocytes (or ‘T cells’) develop. (You have already encountered lymphocytes in the blood smears.) After their initial development in the thymus, T cells distribute to other lymphoid organs, including lymph nodes, the spleen and Peyer’s patches. The tissues where lymphocytes initially develop, thymus and bone marrow, are referred to as primary lymphoid organs, and the other organs, such as lymph nodes, are secondary lymphoid organs. T cells are involved in the recognition of infected cells and they also control and coordinate many other elements of immune responses.

T cell development occurs during the early years of life, and the thymus becomes gradually smaller with age. Notice the distinct areas within the tissue. T cell development starts in the cortex (outer layer) and the cells mature as they progress to the medulla (centre) of the organ.