Secretion

from latin: secernere - to deposit
Synonym: Secretio
German: Secretion

1 Definition

Secretion denotes the delivery of essential substances (e.g. hormones or digestion enzymes) by specialised cells, mainly glandular cells. Also the secreted substances are referred to as secretion.

2 Kinds of secretion

Depending from kind and localisation, different kinds of secretion may be distinguished:

• Endocrine secretion (internal secretion, incretion): delivery of the relevant substances into blood plasma. The conceptually similar forms of autocrine or paracrine secretion denote delivery into communicating compartments like lymph or cerebrospinal fluid. Predominantly, these kinds of secretion apply to hormones or controlling mediators.
• Exocrine secretion: delivery of the relevant substances to internal or external surfaces (directly or via glandular ducts). Examples are saliva, pancreatic secretion and sudor. Here again, different kinds of delivery are distinguished:
  • Eccrine secretion: secretion without visible loss of cytoplasm.
  • Merocrine secretion: secretion with minimal loss of cytoplasm
  • Apocrine secretion: secretion by delivery of parts of the cell, including the apical part of the cell membrane
  • Holocrine secretion: secretion by decomposition of the total cell

Usually, also the excretion of metabolic end-products is referred to as secretion.

3 Pathology

The secretion from glands may be pathologically alterated in form of:

• increased secretion: Hypersecretion
• decreased secretion: Hyposecretion