Thyroid & Parathyroid Glands

AtlasWheater’s, Thyroid & Parathyroid Glands
TextRoss and Pawlina, Thyroid & Parathyroid Glands

OBJECTIVES:

  1. Explain how structures seen in the thyroid gland, at both the light and electron microscope levels, are involved in the production of thyroglobulin, its storage, and its subsequent breakdown to yield thyroid hormones.
  2. Recognize the parathyroid gland in histological section, and within the gland identify the chief cells (source of parathyroid hormone) and oxyphil cells.

I. THYROID GLAND

Slide 217 Thyroid gland, parathyroid H&E Webscope Imagescope orientation
Slide 218-norm Thyroid gland rat Masson Webscope Imagescope
Slide 218-hypo Thyroid gland rat hypoactive Masson Webscope Imagescope
Slide 218-1  Thyroid gland rat hyperactive Masson  Webscope   ImageScope

Examine slide 217 at low magnification, then at higher magnifications. Note that the thyroid gland is made up of functional units called follicles, which in three dimensions are approximately spherical, their walls being composed of a simple cuboidal epithelium, surrounding a lumen that contains colloid. Note that the follicles vary in size and that the height of the follicular epithelial cells may also vary. The colloid is composed primarily of thyroglobulin, a glycoprotein synthesized by the follicular epithelium. Under stimulation from pituitary TSH, the thyroid cells break down the thyroglobulin to release thyroid hormones (T­3 and T4), which pass into nearby capillaries.

Occasional parafollicular cells (C-cells), source of the hormone calcitonin, are also present between the follicles and in the follicular epithelium #217   Webscope   ImageScope . However, they are difficult to distinguish in routine histological slides of human thyroid, and you are NOT expected to recognize them based on light microscopy alone (but you should know that they are the source of calcitonin which is packaged into secretory granules that makes these cells readily identifiable by electron microscopy).

There are three versions of slide 218 that show a rodent thyroid at three different levels of functional activity: (1) normal [example] , (2) hypoactivity due to hypophysectomy [example] , and (3) hyperactivity #218-1   Webscope   ImageScope due to treatment with the drug thiouracil. Compare the tissue shown in each slide --the variation is not overwhelming since the experiments were performed conservatively, but you should be able to see some differences in epithelial cell height and in the size of the follicular lumens. After hypophysectomy there is no stimulation by TSH, so the follicular epithelial cells become reduced in height and the colloid in the lumen is abundant, since it is not being resorbed to make thyroid hormones (in these hypoactive slides, C-cells #218-1   Webscope   ImageScope are more obvious as these cells are fully functional and not dependent on TSH). In contrast, in the hyperactive follicles of thiouracil-treated animals the epithelium is columnar, and the follicular lumen is much reduced in size. The reason for this hyperactivity is that thiouracil blocks the oxidation of iodide, with the result that functional thyroid hormones can no longer be produced. The lack of thyroid hormones in the blood stream leads to stimulation of the pituitary to produce large quantities of TSH, causing the thyroid follicular cells to hypertrophy and resorb colloid very actively from the lumen, reducing its size. The frantic effort of these cells is futile, however, since the oxidized iodine necessary to make functional thyroid hormones is unavailable. INDO267

II. PARATHYROID GLAND

Slide 217 Thyroid gland, parathyroid H&E Webscope Imagescope orientation
Slide 220 Parathyroid H&E Webscope Imagescope
Slide 221 Parathyroid H&E Webscope Imagescope

Sections of parathyroid gland can be seen on slides 217, 220, and 221. In slide 217, parathyroid tissue will be found on one side of the much larger mass of thyroid tissue. To find the parathyroid tissue on slides 217, scan around the periphery of the thyroid tissue at low magnification. (Note: slide 217 in a few of the glass slide sets lacks parathyroid.)

The parenchyma of the gland slide 217 Webscope Imagescope is made up of two identifiable cell types: the predominant chief (or principal) cells (source of parathyroid hormone) and occasional oxyphil cells. Observe the arrangement of chief cells in the parathyroid as seen on slides 217, 220, and 221. The chief cells are arranged as interconnecting cords or clusters, with blood vessels and connective tissue forming the partitions between the cell cords. The capillaries in slide 221 may be more easily seen because erythrocytes have been retained within the lumens. The individual chief cells, seen well in slide 220, have relatively little cytoplasm, which may be almost unstained or lightly basophilic. The lightly stained cells are thought to be quiescent while the more basophilic cells are believed to be more actively involved in the synthesis and secretion of parathyroid hormone. INDO268

In either slide 217, 220, or 221 try to find oxyphil cells. Oxyphil cells are much less numerous than chief cells, and can be differentiated from them by the following criteria: (1) larger than chief cells, with more extensive, eosinophilic cytoplasm, (2) nuclei smaller and darker staining, (3) usually occur in isolated groups. Not every specimen in the glass slide sets contain readily identifiable oxyphil cells, but they can be found on each of the virtual slides:  #217   Webscope   ImageScope , #220  Webscope Imagescope , and #221 Webscope Imagescope

 

Electron Micrographs