Learning Objectives
Understand how organs of the endocrine system exert their effect via two types of hormones
Describe the Mitchcroscopic structure and vascular supply of the anterior and posterior parts of the pituitary gland, and name the hormones synthesized by each cell type
Explane how the hypothalamus promotes or inhibits reslease of hormones from the anterior and posterior parts of the pituitary gland
Describe the Mitchcroscopic structure of the cortical layers of the adrenal gland and compare the hormones released from each
Describe the Mitchcroscopic structure of the thyroid gland, parathyroid glands, and pineal gland
No slide
We have previously discussed exocrine glands. How do endocrine glands differ from exocrine glands?
Exocrine glands secrete their products via a duct, so generally cells in these glands will show polarity (a top and a bottom) and will be arranged into secretory units such as tubules and acini.
Endocrine glands secrete their products directly into the blood stream. Endocrine cells can be single, arranged into plates or cords within an organ, or comprise an entire organ. Because they secrete their products into the blood stream, they are highly vascular and frequently found with abundant capillaries that are usually fenestrated (sometimes they can be sinusoids).
SLU Slide 28: Pituitary
Iowa Virtual Slidebox: Pituitary(147)
What are the two components of the pituitary? How do they differ from one another at low magnification?
The pituitary has an adenohypophysis (anterior pituitary) that is epithelial/glandular in nature, and a neurohypophysis (posterior pituitary). The anterior pituitary stains more darkly because of its glandular tissue and tightly packed parenchyma, while the posterior stains more lightly because of its nervous tissue.
What are the three parts of the posterior pituitary? What kinds of cells are in the neurohypophysis?
Pars nervosa, infundibular stalk, and median eminence. The neurohypophysis consists of nervous tissue. The glial cells in the pituitary are called pituicytes.
Locate a Herring Body. What is its significance?
Herring bodies are expanded axon terminals with an accumulation of neurotransmitters (granules)such as oxytocin and ADH (aka vasopressin).
What are chromophobes? What distinguishes them from chromophils?
Chromophobes are support and precursor cells to the chromophils. They do not yet possess the secretory granules present in chromophils, so do not stain particularly strongly with either acidic or basic dyes.
Chromophils can be divided into acidophils and basophils. Which hormones are secreted by acidophilic cells in the pituitary? By basophilic cells? Identify these cells in your slide. (Hint- Do you know any mnemonic devices for these?)
Acidophils synthesize and release proteins such as growth hormone (somatotrophs) and prolactin (mammotrophs). You can’t distinguish the different types of acidophils from one another.
Basophils synthesize and secrete luteinizing hormone and follicle stimulating hormone (gonadotrophs); thyroid stimulating hormone (thyrotrophs), and adrenal cortical stimulating hormone (ACTH). You can’t distinguish the types of basophils from one another.
EM Atlas: Observe the different types of capillaries in the pituitary/hypophysis. How does their structure relate to the function of the pituitary as an endocrine organ?
Endocrine glands secrete their products into the bloodstream, so abundant capillaries with larger fenestra are necessary to receive the products and distribute them throughout the body.
No Slide: Pituitary adenomas frequently involve the proliferation of somatotrophs. What changes would you expect to find in a child with such an adenoma, and how would they differ from the changes that you would see if that same person developed their adenoma as an adult instead of during their childhood?
A pituitary adenoma that presents during childhood would be associated with excessive growth (pituitary gigantism).
Pituitary adenomas presenting during adulthood also produces more bone growth, but instead of affecting bones at the growth plate, will result in thickened bones (acromegaly).
SLU Slide 29: Thyroid Gland
Iowa Virtual Slidebox: Thyroid (49)
Begin by locating the outer fibrous capsule of the thyroid. Identify several of the numerous blood vessels present in the capsule. Note how the capsule extends through fine collagenous septa to divide the gland into lobules.
Observe thyroid follicles and both low and high power. They consist of a simple epithelium surrounding a lumen filled with an eosinophilic substance. What is the name of the homogenously stained substance in the center, and of what is it composed? What are the cells at the periphery?
This substance is called colloid, and is mostly iodinated thyroglobulin. The cells at the periphery are follicular cells.
Generally, describe the general steps for thyroid hormone synthesis, including where each takes place:
Follicular cells synthesize thyroglobulin into the follicular lumen. They also take up iodine and transport it to the follicular lumen. This iodine then reacts with the thyroglobulin to produce T3 and T4 (within the follicular lumen).
Follicular cells respond to TSH by releasing T3 and T4 into capillaries.
T3 and T4 act on a variety of tissues to increase basal metabolic rate.
Relative activity level of the thyroid gland can be assessed based upon the appearance of epithelial (i.e., follicular) cells and colloid under light microscopy. Draw or describe a thyroid follicle in each level of activity (hypothyroid, normal activity, and hyperthyroid) that indicates the changes in these two features.
Your hypothyroid (inactive thyroid gland) state should include follicular cells that are squamous.
Your normal thyroid gland should include cells with a cuboidal epithelium.
A hyperactive thyroid gland should have tall columnar cells.
Locate some parafollicular cells. What do they secrete, and where are they found?
Parafollicular cells are cells that are located in the intrafollicular connective tissue (in between follicles). They secrete calcitonin, which gives their cells a pale cytoplasm (they are sometimes called Clear cells). These cells are responsive to blood calcium levels.
SLU Slide 30: Parathyroid Glands
Iowa Virtual Slidebox: Parathyroid (149) and Thyroid-Parathyroid (148)
Begin at low magnification and identify the two main cell types. Which one makes up the majority of the gland? How can you distinguish between them?
Chief/Principal cells and oxyphils are the two types of cells in the parathyroid glands, with chief/principal cells predominating.
Chief cells are small and basophilic, and arranged in clusters. Their nuclei are centrally located. These are the cells primarily responsible for secreting PTH.
Oxyphils are large eosinophilic cells that can appear on their own or in small clusters. They are not known to have a secretory role.
How does the parathyroid gland regulate calcium levels in the blood?
Activity by the parathyroid gland is regulated by serum calcium levels. In response to low calcium, PTH is released, acting on osteoblasts, which in turn activate osteoclasts.
(PTH can also reduce absorption of calcium and phosphate ions from the kidney, and increase its absorption in the intestine.)
Optional: Iowa Virtual Slidebox: Parathyroid (351)
Note that in older individuals, fat cells may be come abundant.
SLU Slide 31: Adrenal gland
Iowa Virtual Slidebox: Adrenal (294); Adrenal (272)
Begin at low magnification and identify the outer cortex and an inner medulla. Which one makes up the majority of the gland? How can you distinguish between them?
The cortex occupies the greatest area on your slide. The cortex is just deep to the connective tissue capsule and is made up of three regions discussed below. The cortex is made of continuous cords of cells separated by sinusoids.
The medulla is deep to the cortex, is located centrally and is made of clumps of large, round, usually lightly staining cells. Compared to the staining of the medulla the cortex is often more eosinophilic than the cells of the medulla which are often more basophilic.
How can you distinguish the three regions of the cortex?
I typically start by identifying the zona fasciculata, because I think it's nthe easiest layer to identify. It is a thick zone of cells arranged in straigt cords as it is a thick zone of cells arranged in straight cords, one or two cells thick, which run at right angles to the surface of the gland. Cells in the zona fasciculata are lightly stained and have a frothy or foamy appearance due to the extraction of lipid droplets from the cell cytoplasm during tissue processing.
Next, I locate the zona glomerulosa, which is superficial to the zona fasciculata and consists of cells arranged in rounded or arched clusters. These cells typically stain darker and are smaller than those in the ZF.
The zona reticularis is deep to the zona fasciculata, and the cells are arranged in cords which form an irregular anastomosing network.
What steroid hormones are secreted by each of the three layers of the cortex?
Mineralocorticoids are secreted by the zona glomerulosa.
Glucocorticoids (cortisol and corticosterone) are secreted by the zona fasciculata.
Precursor androgens (dehydroepiandrosterone and androstenedione) are secreted by the zona reticularis.
What steroid hormones are secreted by the medulla?
Epinephrine and norepinephrine are secreted by the chromaffin cells of the adrenal medulla.
EM Atlas: What is the most extensive cytoplasmic organelle found in cells in the zona fasciculata? What does the presence of this explain about the function of these cells?
Cells of the zona fasciculata contain extensive smooth endoplasmic reticulum. This is because these cells produce steroid hormones.
EM Atlas: How can you distinguish the epinephrine and norepinephrine cells of the adrenal medulla in electron micrographs?
Epinephrine secreting cells have smaller vesicles with light granules while norepinephrine secreting cells contain larger vesicles that are more darkly staining.
SLU Slide 61: Pineal Gland
Iowa Virtual Slidebox: Pineal Gland 151
Start by observing the connective tissue that surrounds this organ, and extends inward to divide the pineal gland into lobules with cords and clumps of cells. These cells are Pinealocytes and glial cells.
Pinealocytes have large, slightly indented or lobed nuclei. Glial cells have smaller, more elongated, and more darkly staining nuclei.
Which hormone(s) is/are the primary secretion of pinealocytes?
Melatonin (at night) and serotonin (during the day).
Identify corpora arenacea. Compare the shape and morphology of these bodies between the two slides here to appreciate the variation that exists with regard to their formation.