section 30.6

Organization of the Endocrine System

723

Androgens

Estrogens

FIGURE 30-12

Organization of the endocrine system. The endocrine system can be classified into three levels. At level I are those

endocrine tissues that are, or were embryologically derived from, nervous tissue; these include the hypothalamus, the

adrenal medulla, the thyroid C-cell, and several of the gastrointestinal mucosal cells. Hormones produced by these

tissues resemble neurotransmitters to the extent that their effects are rapid in onset and short term. At level II are those

endocrine tissues that are directly or indirectly influenced by the nervous system; these include the anterior pituitary,

the pancreatic islets, and the parathyroids. Hormones produced by these tissues, which are polypeptides, proteins, or

glycoproteins, exert both short- and long-term effects. At level III are the adrenal cortex, the thyroid, and gonads; these

endocrine tissues exhibit strong dependence on the anterior pituitary. Hormones produced by these tissues are small,

hydrophobic molecules that ultimately affect gene expression in the target cells; their effects are thus slow in onset but

long term. Although the level III tissues are not directly regulated by the central nervous system (CNS), they are, by

virtue of their control by the anterior pituitary, indirectly regulated by the hypothalamus. Neural regulation by the CNS

is critical; —, neural regulation by the CNS exists but is not critical;------ , little or no regulation by CNS;

— >

,

hormonal regulation.

to regulate the level of substances that profoundly affect

nerve function (glucose, calcium, sodium, potassium).

The organization of the endocrine system can best be

described in relation to the central nervous system. Three

levels of endocrine tissues can be distinguished on the

basis of their association with the central nervous system

(Figure 30-12). The first level consists of those that are (or

were) derived from nerve cells; these include the hypotha-

lamus, adrenal medulla, thyroid C-cell, and gastrointesti-

nal enterochromaffin cells. The hypothalamus and adrenal

medulla still retain their neural connections and can there-

fore be regarded as endocrine extensions of the nervous

system. The C-cell and the gut cells, however, are APUD

cells and lack neural connections. These four tissues pro-

duce hormonal peptides or amines having, like neurotrans-

mitters, rapid-onset, short-term effects.

The second level of endocrine tissues are the ante-

rior pituitary, pancreatic islets, and parathyroids, which

show varying degrees of dependence on neural regula-

tion. Because it is highly dependent on the hypothala-

mus for control of hormone synthesis and release, the

anterior pituitary is highly dependent on the nervous sys-

tem. On the other hand, the pancreatic islets and parathy-

roids synthesize and release their hormones in the ab-

sence of neural signals; however, the rate of hormone

release can be influenced by autonomic nerve stimula-

tion. These three tissues produce polypeptide, protein, or

glycoprotein hormones having both short- and long-term

effects, often involving the regulation of cellular protein

synthesis.

The third level of endocrine tissues includes the

adrenal cortex, thyroid follicles, and gonads, all of which