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Thyroid Gland

The epithelium of this endocrine gland is arranged in fluid-filled spheres called follicles, which look like large circles with pink centers ( HA3 f). The pink material is the precursor for thyroid hormone. The size of the follicles varies with plane of section. The follicular epithelium is primarily simple cuboidal, but some (upper left) is simple squamous. As you will learn in Unit 3, the morphology (shape, appearance) of an endocrine epithelium varies with the synthetic activity of the cells. Which would you predict is less active, squamous or cuboidal cells?

The spaces among the follicles are blood vessels. You probably expected to see blood in the vessels, but they are empty because of how the tissue was fixed. There are two general techniques used to fix tissue for light microscopy:

•  immersion fixation , where the tissue is removed from the animal and placed into a vial of fixative

•  perfusion fixation , where saline is pumped through the vasculature to flush out the blood, then fixative is pumped through to fix the tissues

So, depending on how the tissue was fixed, there may or may not be blood in the vessels.

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