Apud System: Characteristics, Structure, Functions

The APUD system is a group of secretory cells independent of the specialized glands of the endocrine system . Its name derives from the initials in English “( A) mine and ( P) recursor ( U) ptake ( D) ecarboxylation .

This system is also known as the “Diffuse Endocrine System”. It was defined for the first time in 1966 by Professor AGE Pearse (1916 -2003), who observed that the cells belonging to this system had the ability to absorb and decarboxylate precursors of amine compounds.

Pearse suggested that the cells of this APUD system belonged to a group of primitive secretory cells and predecessors to the specialized cells that structure the endocrine glands.

The cellular representatives of this system share many characteristics with the typical structure of secretory cells; they contain large numbers of free polyribosomes and ribosomes , large mitochondria with high activity, and secretory granules close to the cell membrane.

At present, all cells that have been cataloged as part of the APUD system have common histochemical and ultrastructural characteristics; it has even been observed that they have the same embryological origin.

Many doctors classify the cells of the APUD system as one-third of the nervous system , since it has been observed that they control homeostasis between the neurotransmitters of the autonomic nervous system and the tissues sensitive to them.


A distinctive feature that scientists use to identify cells of the diffuse endocrine system is the presence of the compounds 5-hydroxytryptamine and catecholamine, which are detected by irradiating florescences after incubation with formaldehyde.

In vitro studies carried out with the cells of the APUD system have shown that they have a high affinity for the uptake of precursor amines of the synthesis of hormones such as L-dopa and 5-hydroxytryptophan.

All the cells of this system contain the enzyme DOPA-decarboxylase. This enzyme is responsible for decarboxylating the compounds or amino acids that cells use as substrates to produce the amines or peptides they excrete.

Thus, as its name indicates, the APUD system is characterized by its ability to capture and decarboxylate amine hormonal precursors, which is achieved by its cells mainly thanks to the presence of the enzyme DOPA-decarboxylase.

Furthermore, electron microscopy has shown that all the cells of the APUD system have a diminished rough endoplasmic reticulum and a highly developed smooth endoplasmic reticulum, with a prominent tubular or vesicular shape.

Structure and distribution

The APUD system is present in all tissues of mammalian animals. However, it is predominantly in the pancreas and in tissues where hormone receptors are found.

It is estimated that up to one million cells belonging to the APUD system can be found in the pancreas and that these represent between 1 and 3% of the total content of pancreatic cells.

Originally, AGE Pearse identified only 8 types of APUD cells: chromaffin cells in the adrenal medulla, intestinal enterochromaffin cells, mast cells, melanotropic and corticotropic cells of the pituitary , β cells of the pancreas, and C cells of the thyroid. .

Subsequent research has established that many other cell types exist, including epithelial cells in the thymus cortex, Leydig cells in the testes, and endocrine cells in the prostate and heart.

Structurally speaking, all the cells that make up the diffuse endocrine system share the following structural characteristics:

– Large nuclei.

– Triangular, piriform or oval shape.

– Large amount of granules in the cytosol, rich in the precursor chemical species of the hormones they secrete.

– They are found isolated or grouped in cell clusters, corpuscles or islets.


The cellular representatives of the APUD system contain dopamine , norepinephrine, epinephrine, and serotonin within them. In addition, they can synthesize these hormones from exogenous amine compounds.

The main function of this system, according to specialists in endocrinology, is the secretion of peptides with hormonal functions or peptides with functions in neurotransmission.

Some authors classify the cells of this system as “para-neurons” and this is due to their capacity to produce polypeptides and proteins that serve as neurotransmitters in target cells of the pancreas, gonads or respiratory cavities.

In many publications, the APUD system is classified as a “diffuse neuroendocrine system”, since it has even been found to interact with cells of the hypothalamus, ganglia, with the autonomic peripheral nerves, with the pituitary, the epiphysis and certain chemoreceptors of the placenta.

The classification of a “diffuse neuroendocrine system” also corresponds to the peptide secretion capacity both within neurons and somatic cells, as well as in the intracellular space and towards the cellular exterior in animal tissues.

However, in the scientific world there are some detractors who disapprove of the classification of this system as a neuroendocrine system, since no evidence of nerve endings has been found in the cytosol of cells, although these are always found in the vicinity of the cells. .


Gastritis, whether acute or chronic, is a pathology associated with the malfunction of the cells of the APUD system, since, in some cases, the cells of the gastric mucosa are “overlapped” or covered by an excessive proliferation of APUD cells. .

Many of the studies of diseases related to the APUD system focus on the origin of tumors that are identified as “apudomas” and that cause a very complex symptomatology due to all the endocrine processes in which these cells are involved.

Apudomas can inhibit or increase the hormonal secretions of ganglia and glands, triggering the hypo- or hyper-functioning of the organs in which these tumors are found.

It was thanks to the study of the pathologies involved in the APUD system that the true endocrine scope of this diffuse endocrine system has been understood in greater depth.

However, in humans the function of the system and the diseases related to the defects inherent in it are not yet known with certainty.


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  7. Zhou, Y., Xie, B., Duan, Y., Su, W., Yi, X., Liu, W.,… & Xiao, D. (2016). Case Report Primary gastric malignant melanoma might originate from amine precursor uptake and decarboxylation cells. Int J Clin Exp Pathol, 9 (12), 13003-13009.

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