8.2 Thyroid Gland

The thyroid gland (image 8.2a) is located anterior to the upper part of the trachea near the larynx. The thyroid gland is divided into the right and left lobes that are joined by a thin band called the isthmus. Image 8.2b is a photomicrograph of thyroid tissue at about 300x, showing hollow balls of cells called thyroid follicles (labeled "a" in image 8.2b). All of the follicles are filled with a protein substance, the colloid that contains a stored form of thyroid hormones. The arrow in image 8.2b pinpoints the follicular cells that enclose the colloid. Thyroid hormones generally function to maintain a proper metabolic rate, contribute to growth, and help mineralize the skeleton. In image 8.2c the large, solid areas are the colloid filled follicles. The spots are the nuclei of the follicle cells.

8.3 Parathyroid Glands

The parathyroid glands are four small glands on the dorsal surface of the thyroid, which are drawn as red, bean-shaped ovals on the diagram in image 8.3a. Image 8.3b, shows a region dominated by principal (chief) cells (labeled "a" in image 8.3b), one of the two major cell types which secrete parathormone.

8.4 Adrenal Glands

Adrenal glands (suprarenal glands) are paired, each located superior to a kidney. Each adrenal gland is structurally and functionally differentiated into two regions: the outer adrenal cortex making up the bulk of the gland and the inner adrenal medulla (as shown in image 8.4a). The two regions are also visible in the light micrograph in image 8.4b. The adrenal cortex has three sublayers which secrete mineral corticoids, glucocorticoids, and sex hormones; while the inner adrenal medulla secretes adrenalin and noradrenalin during alarm reactions. Only the adrenal medulla is innervated.

Image 8.4c shows a sectioned adrenal gland. The outermost layer of the cortex is the zona glomerulosa (labeled "g" in image 8.4c); the middle layer of the cortex is the zona fasciculata (labeled "f" in image 8.4c); and the deepest layer of the cortex is the zona reticularis (labeled "r" in image 8.4c) The medulla can also be seen (labeled "m" in image 8.4c). Note the differences in texture among the different layers. Image 8.4d shows a higher magnification of the adrenal cortical stratification. The layer labeled "g" in image 8.4d is the primary source of mineralcorticoids; the layer labeled "f"in image 8.4d mainly secretes glucocorticoids; and the layer labeled "r" in image 8.4d secretes miniscule amounts of gonadocorticoids. Image 8.4e shows an array of hormone-producing cells called chromaffin cells, which surround bright red sinuses containing blood.

8.5 Pituitary Gland

The pituitary gland is a bean-sized organ that lies in the sella turcica of the sphenoid bone and is attached to the base of the hypothalamus by a stalk-like infundibulum. The pituitary is divided structurally and functionally into a large anterior lobe and a smaller posterior lobe (right and left, respectively, on the diagram in image 8.5a). This image also shows the the projection of nerve fibers from some neuro- secretory cells in the hypothalamus down into the posterior pituitary. Image 8.5b is a sectioned skull with the dark, ovoid pituitary marked with an arrow. The pituitary is a source of many hormones, many of which exert a controlling ("trophic") effect upon the secretion of other endocrine glands. This has given the pituitary the name as "the master gland" in the human body.

Image 8.5c shows a transverse section of the pituitary (which is also called "hypophysis"). The anterior lobe (labeled "a" in image 8.5c) makes up 75% of the pituitary by weight and the posterior lobe (labeled "b" in image 8.5c) is actually a neuron fiber extension of the hypothalamus. Image 8.5d is a magnified view of the anterior pituitary. This shows the variety of cell types that compose the pituitary, each of which secretes a specific hormone.

8.6 Pancreas and Islets of Langerhans

The Islets of Langerhans are isolated clumps of endocrine cells surrounded by exocrine tissue in the pancreas. The large, yellow-colored, fish-shaped object, centrally positioned on the diagram in image 8.6a is the pancreas, and the solid portion represents the exocrine gland of the pancreas. The patches with black dots, which are exposed on a cut section of the pancreas, represent Islets of Langerhans, the endocrine portion. The exocrine portion secretes pancreatic juice into the duodenum to help with the digestion of food; the Islets secrete glucagon and insulin, two hormones that antagonistically help control the blood sugar level. Image 8.6b is a photomicrograph showing an Islet (labeled "a" in image 8.6b), among the darker and much more abundant exocrine cells. Image 8.6c shows an Islet at higher magnification. A lighter, ovoid Islet mass dominates the screen and extends off the left margin of the field. The Islet is surrounded by exocrine cells.

8.7 Thymus Gland

Often a bi-lobed lymphatic organ, the thymus gland is located in the upper portion of the mediastinum, posterior to the sternum and between the lungs, as seen on the diagram in image 8.7a. The thymus reaches maximum size during childhood and then it quickly atrophies, but it retains a low level of function in the adult. In image 8.7b are lobules of thymus tissue with the cortex (labled "a" in image 8.7b) and medulla (labeled "b" in image 8.7b) of a thymic lobule identified.

Image 8.7c shows a close-up of a thymic lobule with the characteristic thymic (Hassall's) corpuscles (labeled "a" in image 8.7c). Image 8.7d shows a closeup of Hassall's corpuscle which is composed of concentric layers of epithelial cells. There is no known function for these corpuscles, but many think that they are centers of cellular death within the thymus.

8.8 Pineal Gland

The pineal gland (image 8.8a) is composed of supportive neuralgia cells and secretory cells. Although many peptides and amines (including serotonin, norepinephrine, and histamine) have been isolated from this minute gland, only melatonin is known with certainty to be a major secretory product. Melatonin concentrations in the blood correlate to diurnal sleep cycles with the peak levels at night and the lowest levels at noon. The pineal gland (arrow in image 8.8b) lies just superior to the colliculi of the midbrain.