Types of Secretion / Signaling Mechanisms

1. Eccrine Secretion

• Mechanism: The secretory cell remains intact. Products are released by exocytosis (vesicle fusion with the cell membrane) without any loss of cytoplasm.
• Cell fate: Cell survives and continues to function.
• Classic example: Eccrine sweat glands — the most numerous sweat glands in humans, distributed all over the body surface. They secrete a watery, hypotonic sweat primarily for thermoregulation.
• Other examples: Salivary glands, pancreatic acini, goblet cells (mucus).
• Also called: Merocrine secretion.

2. Apocrine Secretion

• Mechanism: The secretory product accumulates at the apical portion of the cell, which then pinches off and is released, taking a portion of the cytoplasm and cell membrane with it.
• Cell fate: Cell loses part of its cytoplasm but survives and regenerates.
• Classic examples:
  • Apocrine sweat glands (axilla, groin, areola) — produce a thicker, odorless secretion that becomes pungent when metabolized by skin bacteria.
  • Mammary glands (lipid component of milk).
• Key distinction from eccrine: Partial sacrifice of the cell apex.

3. Holocrine Secretion

• Mechanism: The entire cell disintegrates and becomes the secretion. Cells accumulate their product, die, and are released in full.
• Cell fate: Cell dies and is shed as the secretion.
• Classic example: Sebaceous glands — produce sebum (an oily mixture) that lubricates and waterproofs hair and skin. The entire sebocyte ruptures to release sebum.
• Key distinction: Most "destructive" mode — the product IS the dead cell.

4. Paracrine Secretion (Signaling)

• Mechanism: A cell releases signaling molecules that act locally on adjacent or nearby cells in the same tissue. The signal diffuses through the interstitium over short distances (does NOT enter systemic circulation).
• Cell fate: Cell survives.
• Classic examples:
  • Pancreatic δ cells release somatostatin, which inhibits nearby β cells (suppressing insulin) and α cells (suppressing glucagon).
  • Histamine release from mast cells acting on adjacent blood vessels.
  • Growth factors (EGF, FGF, PDGF) in tissue repair.
• Distinction from endocrine: Endocrine hormones travel through the bloodstream to distant targets; paracrine signals act locally.

Harrison's Principles of Internal Medicine, 21st Ed., p. 10537: "Paracrine regulation refers to factors released by one cell that act on an adjacent cell in the same tissue."

Summary Comparison Table

Quick Memory Tip

• Holocrine = whole cell dies
• Apocrine = apex of cell is lost
• Eccrine (merocrine) = exocytosis, cell intact
• Paracrine = acts on parallel/adjacent cells (local signaling)