Chapter 14 Discussion questions and answers

14.1 Why are the pituitary gland and the hypothalamus referred to as the ‘master organs’ of the endocrine system?

Answer

  • The pituitary and hypothalamus are often referred to as the ‘master organs’ of the endocrine system because the hormones under their influence regulate the physiological activities of many other endocrine glands and tissues.

14.2 Discuss two examples where immunohistochemistry is useful in identifying and localizing individual cell types within endocrine glands.

Answers

  • Antibodies are currently available for growth hormone (GH), thyrotrophic hormone (TSH), adrenocorticotrophic hormone (ACTH), luteinizing hormone (LH) and prolactin (PRL). This enables the clarification of the normal distribution of the cells within the pars distalis secreting these hormones, correlates structure and function, and can be useful in the classification of pituitary tumours.
  • There are antibodies currently available to thyroglobulin and calcitonin.
  • In some clinical situations, the use immunocytochemistry (ICC) for thyroglobulin and PTH can be helpful in categorising parathyroid lesions encroaching on the thyroid.
  • Specific histochemical staining confirms that epinephrine and norepinephrine are synthesized and secreted by two different cell types.
  • Using a combination of ICC and transmission electron microscopy (TEM), three major and three minor islet cell types can be identified, each associated with a specific peptide hormone and each located in a specific site within the islet.

14.3 How do hormones act and how is their production by endocrine cells controlled?

Answers

  • There are three types of hormonal action mechanisms: endocrine, where the hormone is released into the bloodstream and is transported to the target cells: paracrine, where the hormone is released from an endocrine cell into connective tissue spaces and diffuses to hormone-specific receptors on adjacent cells, and autocrine, where the target cell is the cell that has produced the hormone.
  • Two groups of hormone receptor can be identified. Cell surface receptors bind with protein hormone molecules that are too large to enter the cell through the cell membrane, which initiates the production of small intracellular second-messenger molecules activating a cascade of reactions to produce hormone-specific responses in the target cell. Intracellular receptors are localized within the target cell, usually inside the nucleus; these are used by steroid hormones which are able to penetrate both plasma and nuclear membranes and bind to the DNA. This leads to mRNA transcription and the production of new proteins within the cell that result in hormone-specific responses in the stimulated cell.
  • One process by which substances cross the plasma membrane to enter and leave cells is known as vesicular transport. The integrity of the plasma membrane is maintained, and molecules can also be transported between different compartments within the cell. Substances enter the cell by endocytosis, and leave the cell by exocytosis (see Chapter 1). The secretory vesicles of most secretory cells, as well as the synaptic vesicles of neurons, release their secretory products by exocytosis.
  • The production of hormones by endocrine cells is often controlled by feedback mechanisms from the target cells or organ. The action of the hormone (the stimulus) has an effect on the hormone-secreting cell (the original stimulus). Negative feedback occurs when the response to the stimulus reduces the original stimulus; positive feedback (less common) occurs when the response increases the original stimulus.

14.4 List the endocrine glands.

Answers

  • The major endocrine glands (or ductless glands) are the pituitary (also known as the hypophysis), hypothalamus, thyroid, parathyroids, adrenals, pancreatic islets of Langerhans and the pineal gland. The placenta, only present during pregnancy, secretes several hormones; and organs such as the kidney, heart, thymus, ovaries and testes, and intestine have individual or small groups of cells with endocrine functions.

14.5 Discuss the four categories of endocrine disease.

Answers

  • Endocrine diseases can be broadly classified into four categories:

Hormone overproduction: This is most commonly caused by an increase in the number of cells producing a specific hormone, due to abnormal antibodies mimicking the action of a hormone (eg TSH in Graves’ disease is overproduced, resulting in an increase in thyroid cells), or due to a genetic abnormality affecting the regulation of hormone synthesis and release, or gene mutations resulting in the proliferation of mutant hormone-producing cells.

Hormone underproduction: This can be due to the destruction of an endocrine gland by a disease process or an autoimmune disease where antibodies are produced that target and destroy hormone-producing cells. Underproduction can also result from genetic abnormalities that cause abnormal development of endocrine glands, abnormal hormone synthesis, abnormal regulation of hormone secretion, or an iatrogenic cause where endocrine tissue is surgically removed (eg removal of parathyroids at thyroidectomy)

Altered tissue responses to hormones: as a result of genetic mutations of the hormone receptors.

Tumours of endocrine glands: rather than causing overproduction of hormones, tumours instead can compress nearby organs or destroy other organs by metastasis.

  • Hormones are used to treat endocrine disease, either administered orally in the case of thyroid and steroid hormones, or by injection for protein hormones such as insulin.
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