HISTOLOGY FULL-TEXT

William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA

Chapter 26 HORMONES

A GENERAL CHARACTER

l These are potent chemical substances travelling via the bloodstream from one cell to another. They work in conjunction with the nervous system to control the homeostasis of the body, and to anticipate future events such as birth, lactation, fighting or fleeing. Powerpoint
2 The hormone reaches many cells but, except for hormones affecting growth and some general metabolic processes, only certain target or end-organ cells respond. The response is often a start or increase in a cell's activity, e.g., contraction, release of a secretion, growth by proliferation (hyperplasia) or by an increase in size (hypertrophy). However, a hormone may sometimes inhibit a cell's activity, e.g., calcitonin inhibits osteoclasts' resorption of bone.
3 The hormone may stimulate its target cell either by binding with a membrane receptor in the plasmalemma that starts a signal transduction sequence, say, to alter the level of a second, internal, messenger, within the cell, e.g., cyclic AMP or GMP; or by penetrating the cell membrane and binding with a cytoplasmic receptor. Once inside the cell the bound hormone itself, the second messenger, or downstream effectors such as Ca2+, can trigger the release of secretion, an increase in nucleus-controlled synthesis, a contraction, or some other useful task.
4 In its usual concentrations, a hormone's action is called physiological. Pharmacological effects are seen when abnormally high quantities are injected.
Pathological effects are observed when: too little or too much hormone is present; the target organ is insensitive to the hormone; or the hormone molecule is defective.

B CHEMISTRY OF HORMONES

l Hormones differ chemically and may be classified thus: 2 Hormones may be bound to proteins for storage, e.g., to thyroglobulin in thyroid colloid; or for transport, down a neurosecretory cell's axon, or in the blood.
3 The small amounts of hormone normally circulating in the blood, or present in a tissue, can be measured by two principal methods:

C CYTOLOGY OF HORMONE SECRETION

l Hormones are formed in:
.. (a) pure endocrine glands, e.g., thyroid;
.. (b) mixed exocrine and endocrine glands, e.g., pancreas and testis;
.. (c) some of the cells in organs having other functions, e.g., placenta, kidney, GI tract.
2 The hormone is a product synthesized and released by glandular cells, mostly epithelial, but some are modified neurons or muscle cells.
3 Cells have organelles associated with synthesis, e.g., granular or smooth ER, Golgi complex, and may store the hormone or prohormone as membrane-bound inclusion granules. Lysosomes may be used to destroy excess hormone. Actin filaments are used to discharge the granules by exocytosis. The chemical nature of the hormone is reflected in the cytology, e.g., steroid cells store the lipid precursor, but not the hormone, and have much smooth ER.
4 The secretory granules may stain selectively because of the chemical nature of the hormone, e.g., glycoprotein with the PAS reaction, and have a distinctive size, shape and density in EM.
5 The stored hormone can be demonstrated in its cell by immunostaining, using an antibody that binds specifically with that hormone, coupled with a visually demonstrable tag, e.g., a fluorescent compound (for LM) or a peroxidase (for EM and LM).
Catecholamines can be seen with fluorescence microscopy after treatment with an aldehyde. The mRNA for the hormone or its precursor, or for enzymes necessary to its synthesis, can be seen by using in situ hybridization.
6 The synthesis of a hormone can be followed within the cell, moving progressively between organelles and inclusions, by sequential radioautographic following of a radioactively labelled precursor, e.g., an amino acid, or 125I.
7 The stimulus for the release of hormone, and the synthesis of more hormone, may be:
.. nervous by synaptic action, e.g., adrenal medulla;
.. another hormone, e.g., TSH for thyroid follicular cells; or
.. the blood level of a non-hormonal chemical, e.g., Ca2+ for parathyroid chief cells.
8 To facilitate the blood-endocrine cell interactions, cords and small clusters of endocrine cells are supported by reticular fibres, close to numerous wide capillaries (sinusoidal capillaries), lined by fenestrated but non-phagocytic endothelial cells.

D QUESTIONS TO CHARACTERISE A HORMONE

l For each hormone - questions for the physiological state:
  1. Its chemical nature?
  2. What cell, in what part, of which organ, forms it?
  3. What cell specializations are associated with its synthesis, storage and release?
  4. What are other morphological details of the cell? e.g., size.
  5. What controls hormone release?
  6. Is the pathway for the control of release simple and direct, or complex and indirect?
  7. What are the target tissues?
  8. What actions does the hormone have on those tissues?
  9. Do some effects appear more important than others?
  10. Do other hormones affect the same tissue? If so, how do the hormones interact?
  11. Do the endocrine cells and the effects of the hormone on the target change with time? e.g., the timescales of menstruation, gestation, childhood growth.
  12. Are the hormone's effects the same in the two genders? e.g., the role of prolactin in the male?
  13. What pre-secretion proteolytic processsing is required to release active hormone? For example, the corticotrophs of the anterior pituitary gland employ a serine endoprotease to release ACTH and small amounts of beta-endorphin from a 30 kDa peptide precursor - Pro-OpioMelanoCortin (POMC). In the intermediate lobe, extra convertase enzymes increase the output of endorphin and process ACTH to alpha-Melanocyte Stimulating Hormone (MSH).
2 For each hormone - questions for abnormal states, pathological and induced:
  1. What are the effects of its loss?
  2. Is it essential for life?
  3. What happens when there is an excess from a tumour, or by experiment?
  4. To find out the pathways and mechanisms controlling hormone release: What are the effects on the endocrine cells of:

William A Beresford, Anatomy Department, School of Medicine, West Virginia University, Morgantown, WV 26506-9128, USA - - e-mail: -- wberesfo@wvu.edu -- wberesfo@hotmail.com -- beresfo@wvnvm.wvnet.edu -- fax: 304-293-8159