Page 807 - Textbook of Pathology, 6th Edition

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Chapter 27 The Endocrine System
Chapter 27

ENDOCRINES: THE BASIC CONCEPT Group I: Those interacting with cell-surface membrane
receptors:
The development, structure and functions of human body 1. Amino acid derivatives: thyroid hormone, catecholamines.
are governed and maintained by 2 mutually interlinked 2. Small neuropeptides: gonadotropin-releasing hormone
systems—the endocrine system and the nervous system (Chapter (GnRH), thyrotropin-releasing hormone (TRH), somatostatin,
30); a third system combining features of both these systems vasopressin.
is appropriately called neuroendocrine system.
Group II: Those interacting with intracellular nuclear
receptors:
NEUROENDOCRINE SYSTEM
3. Large proteins: insulin, luteinising hormone (LH), para-
This system forms a link between the endocrine and nervous thormone hormone.
systems. The cells of this system elaborate polypeptide 4. Steroid hormones: cortisol, estrogen.
hormones; owing to these biochemical properties, it has also 5. Vitamin derivatives: retinol (vitamin A) and vitamin D.
been called as APUD cell system (acronym for Amine The synthesis of these hormones and their precursors takes
Precursor Uptake and Decarboxylation properties). place through a prescribed genetic pathway that involves:
However, though having common biochemical properties, transcription → mRNA → protein synthesis → post- CHAPTER 27
the cells of this system are widely distributed in the body in translational protein processing → intracellular sorting/
different anatomic areas and hence is currently called membrane integration → secretion.
dispersed neuroendocrine system. Cells comprising this system Major functions of hormones are as under:
are as under: i) Growth and differentiation of cells: by pituitary hormones,
1. Neuroendocrine cells are present in the gastric and intestinal thyroid, parathyroid, steroid hormones.
mucosa and elaborate peptide hormones. ii) Maintenance of homeostasis: thyroid (by regulating BMR),
2. Neuroganglia cells lie in the ganglia cells in the sympathetic parathormone, mineralocorticoids, vasopressin, insulin.
chain and elaborate amines. iii) Reproduction: sexual development and activity, pregnancy,
3. Adrenal medulla elaborates epinephrine and norepinephrine. foetal development, menopause etc.
4. Parafollicular C cells of the thyroid secrete calcitonin. A basic feature of all endocrine glands is the existence of The Endocrine System
5. Islets of Langerhans in the pancreas (included in both both negative and positive feedback control system that
endocrine and neuroendocrine systems) secrete insulin. stimulates or regulates hormone production in a way that levels
6. Isolated cells in the left atrium of the heart secrete atrial remain within the normal range (abbreviated as S or R
natriuretic (salt-losing) peptide hormone. respectively with the corresponding hormone e.g. TSH-TRH
In addition to above, other non-endocrine secretions pathway, GnRH-LH/FSH pathway etc). This system is
include neurotransmitter substances such as acetylcholine and commonly termed hypothalamic-pituitary hormone axis for
dopamine released from neural synapses, and erythropoietin different hormones schematically illustrated in Fig. 27.1. The
and vitamin D elaborated from the kidney. stimulatory or regulatory action by endocrine hormonal
3
secretions may follow paracrine or autocrine pathways:
Paracrine regulation means that the stimulatory/
THE ENDOCRINE SYSTEM
regulatory factors are released by one type of cells but act on
Anatomically, the endocrine system consists of 6 distinct another adjacent cell of the system.
organs: pituitary, adrenals, thyroid, parathyroids, gonads, Autocrine regulation refers to action of the factor on the
and pancreatic islets; the last one is included in neuro- same cell that produced it.
endocrine system also). Understanding the patholgy of these With this brief overview of principles of physiology of
endocrine organs requires the knowledge of overall frame- hormones, we now turn to the study of diseases of the
work of hormone secretions, their actions and broad endocrine organs. In general, pathologic processes affecting
principles of feedback mechanisms. endocrine glands with resultant hormonal abnormalities may
Broadly speaking, human hormones are divided into 5 occur from following processes:
major classes which are further grouped under two headings Hyperfunction: This results from excess of hormone secreting
depending upon their site of interactions on the target cell tissues e.g. hyperplasia, tumours (adenoma, carcinoma),
receptors (whether cell membrane or nuclear receptor): ectopic hormone production, excessive stimulation from