What are D cells that secrete somatostatins?

D Cells: The Somatostatin-Secreting Cells of the Pancreatic Islets

D cells are specialized endocrine cells located in the pancreatic islets and gastrointestinal tract that secrete the hormone somatostatin, which acts as a paracrine inhibitor regulating the secretion of other hormones including insulin, glucagon, and pancreatic polypeptide.

Characteristics of D Cells

• Location and Distribution:

  • Comprise approximately 5% of pancreatic islet cells 1
  • Present in both pancreatic islets and throughout the gastrointestinal tract 2
  • In the pancreas, they are predominantly located at the periphery of islets
  • In the GI tract, they are found in the mucosa as enteroendocrine cells

• Morphology:

  • Complex cellular structure with dendrite-like processes that extend to contact multiple neighboring cells 1
  • Majority (84%) are located in close proximity to capillaries 3
  • Tend to form networks with other D cells (67% are arranged in contiguity to other D cells) 3
  • Immunoreactive material (somatostatin) accumulates in cell parts facing capillaries 3

Physiological Function

• Somatostatin Production:

  • D cells synthesize and secrete two main forms of somatostatin:
    • Somatostatin-14 (predominant in gastric and pancreatic D cells)
    • Somatostatin-28 (predominant in intestinal mucosal cells) 4

• Regulation of Secretion:

  • D cells contain ATP-sensitive potassium channels that:
    • Open at low glucose levels
    • Close when glucose is elevated, leading to membrane depolarization and increased somatostatin secretion 1
  • Factors released by neighboring α-cells and β-cells amplify glucose-induced somatostatin secretion

• Physiological Actions:

  • Acts locally within islets as a paracrine inhibitor of:
    • Insulin secretion from β-cells
    • Glucagon secretion from α-cells
    • Its own secretion (autocrine inhibition) 1
  • Inhibits gastrointestinal secretions and delays absorption of glucose and amino acids 2
  • Functions as a "shock absorber" in regulating endocrine and exocrine functions 2

Clinical Significance

• Pathological Conditions:

  • Somatostatinomas: Rare neuroendocrine tumors arising from D cells 5

    • Represent approximately 1% of pancreatic neuroendocrine tumors 6
    • Associated with high risk of metastasis (80-90%) 6
    • Can cause diabetes mellitus and/or diarrhea/steatorrhea 6

  • Altered somatostatin secretion in diabetes:

    • Perturbed in animal models of diabetes mellitus
    • May explain loss of appropriate hypoglycemia-induced glucagon secretion 1
    • Somatostatin receptor antagonists may help restore glucagon response to hypoglycemia

• Diagnostic Considerations:

  • Somatostatinomas can be diagnosed through:
    • Measurement of plasma somatostatin levels
    • Imaging studies including somatostatin receptor scintigraphy 6
    • Histological examination showing distinctive D cell morphology 5

Therapeutic Implications

• Somatostatin antagonists or agents that suppress somatostatin secretion have been proposed as adjuncts to insulin therapy in diabetes 1
• Understanding D cell physiology is important for developing targeted therapies for diabetes and neuroendocrine tumors

D cells represent a critical regulatory component of the pancreatic islets, with their somatostatin secretion serving as an important local modulator of glucose homeostasis through the inhibition of both insulin and glucagon secretion.