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| The thyroid gland is located anterior to the upper part of the trachea, near the larynx and is divided into the right and left lobes that are joined by a thin band called the isthmus. |
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| A photomicrograph of thyroid tissue at about 300x, showing hollow balls of cells called thyroid follicles. Label "a" marks a follicle---all of the follicles are filled with a protein substance, the colloid that contains a stored form of thyroid hormones. The arrow 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. |
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| The large, solid pink areas are the colloid filled follicles. The purple spots are the nuclei of the follicle cells. |
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| A patient with a hypertrophied thyroid, a goiter. An exophthalmic or thyrotoxic goiter is caused by an over secretion of thyroxine, but a simple goiter is caused by an iodine dietary deficiency. Iodine is an essential component of thyroid hormones (excluding calcitonin). |
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| A patient with exophthalmic goiter, hyperthyroidism, or Grave's disease. The bulging eyes are a side effect of fat and water retention by fatty tissues in the eye orbit. |
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| 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. |
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| "a" marks a region dominated by principal (chief) cells, one of the two major cell types which secrete parathormone. |
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| 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 (yellow), and the inner adrenal medulla (the dark red area). |
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| The two regions are also visible in the light micrograph. 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. |
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| A sectioned adrenal gland. Label "g" is the outermost layer of the cortex, the zona glomerulosa, while "f" marks the middle layer of the cortex, the zona fasciculata. And finally, "r" is the deepest layer of the cortex, the zona reticularis. Label "m" marks the medulla. Note the differences in texture among the different layers. |
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| A higher magnification of the adrenal cortical stratification. Layer "g" is the primary source of mineralcorticoids. Layer "f" mainly secretes glucocorticoids. Layer "r" secretes miniscule amounts of gonadocorticoids. |
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| Array of hormone-producing cells called chromaffin cells, which surround bright red sinuses containing blood. |
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| 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). |
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| 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. |
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| A transverse section of the pituitary (which is also called "hypophysis"). Label "a" marks the anterior lobe which makes up 75% of the pituitary by weight. Label "b" is the posterior lobe which is actually a neuron fiber extension of the hypothalamus. |
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| Magnified view of the anterior pituitary. This shows the variety of cell types that compose the pituitary, each of which secretes a specific hormone. |
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| 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 is the pancreas, and the yellow portion represents the exocrine gland of the pancreas. The blue 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. |
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| Photomicrograph showing an Islet ("a"), among the darker and much more abundant exocrine cells. |
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| 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. |
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| 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. The thymus reaches maximum size during childhood and then it quickly atrophies, but it retains a low level of function in the adult. |
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| Lobules of thymus tissue. Label "a" marks the cortex, and "b" is the lighter stained medulla of a thymic lobule. |
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| Close-up of a thymic lobule, with "a" marking the characteristic thymic (Hassall's) corpuscles. |
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| 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. |
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| The pineal gland (green) 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 lies just superior to the colliculi of the midbrain (blue). |
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| The pineal gland (arrow) 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 lies just superior to the colliculi of the midbrain. |
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