324
UNIT 3
Organ Systems
Figure 17-11B
Clinical Correlation: Pheochromocytoma
Figure 17-12A
Adrenal Medulla
Figure 17-12B
Cells of the Adrenal Medulla
Pineal Gland
Figure 17-13A
Overview of the Pineal Gland
Figure 17-13B
Pinealocytes and Brain Sand of the Pineal Gland
Figure 17-13C
Clinical Correlation: Pineoblastoma
Endocrine Pancreas
Figure 17-14A,B
Islets of Langerhans, Endocrine Pancreas
Figure 17-15A,B
Pancreatic Islet Cells, Islets of Langerhans
Figure 17-16
Clinical Correlation: Type 2 Diabetes Mellitus
Synopsis 17-1
Pathological Terms for the Endocrine System
Table 17-1
Endocrine Organs
Introduction and Key Concepts
for the Endocrine System
The
endocrine system
is very closely associated with the nervous
system and is much like the
nervous system
in some ways. The
nervous system
sends messages related to sensation, thought,
and motor control using electrochemical signals (
action poten-
tials
) that are carried by neurons and axons. The
endocrine
system
sends messages to control and regulate the metabolic
activity of the body using chemical signals (
hormones
) that are
released by endocrine secretory cells and carried by the blood
circulatory system. The endocrine system includes (1)
endo-
crine glands
, such as the pituitary gland, thyroid and parathy-
roid glands, adrenal glands, and the pineal gland; (2)
clusters
of endocrine cells
located in the organs such as
islets of Langer-
hans
in the pancreas; and (3)
isolated endocrine cells
in certain
tissues, such as the
enteroendocrine cells
in the epithelium of
the
respiratory and digestive tracts (see Chapters 11, “Respira-
tory System,” and 15, “Digestive Tract”). The endocrine organs
that are discussed in this chapter include the pituitary gland,
parathyroid glands, adrenal glands, and the endocrine pancreas
(islets of Langerhans). Other endocrine organs, such as the tes-
tes and ovaries, are discussed in Chapters 18, “Male Reproduc-
tive System,” and 19, “Female Reproductive System.”
Endocrine secretions (hormones) are delivered through
the capillary network of the vascular system to the target
organs rather than through a series of ducts as in the exocrine
system. The timing of hormone release is controlled by the
hypothalamus
. The hypothalamus acts as a command center,
controlling the activity of the pituitary gland. The
pituitary
gland
functions as a master gland, releasing hormones to con-
trol other endocrine glands and organs. The organs or tissues
that are activated by released hormones are called
target organs
or
tissues
. The cells in the target organ/tissue have appropriate
receptors
, which are able to recognize and respond to speci±
c
hormones (Fig. 17-2).
The hormones can be divided into three classes based on
their structure:
1.
Steroid hormones
are lipid hormones that have the characteristic
ring structure of steroids (terpenoid lipids) and are formed from
cholesterol. Examples of these hormones include estrogen, tes-
tosterone, cortisone, and aldosterone.
2.
Peptide hormones
are composed of amino acids and are
usually produced by the partial hydrolysis of proteins. The
majority of hormones of this type are secreted by the pituitary
gland (e.g., adrenocorticotropic hormone [ACTH], thyroid-
stimulating hormone [TSH], follicle-stimulating hormone
[FSH], prolactin, and growth hormones) and parathyroid
glands (parathyroid hormone [PTH], or parathormone).
3.
Amine hormones
are derived from the amino acid
tyrosine
.
Examples include triiodothyronine (T
3
) and thyroxine (T
4
)
released by the thyroid and sympathomimetic hormones
(adrenaline/epinephrine and noradrenaline/norepinephrine)
secreted by the adrenal medulla.
Pituitary Gland
The
pituitary gland
is a neuroendocrine organ located inside the
skull and considered a part of the brain (Figs. 17-2 to 17-6B).
It consists of two divisions: the
adenohypophysis
(anterior
lobe) and the
neurohypophysis
(posterior lobe). The pituitary
gland produces various types of hormones that act on many
target organs, many of which also secrete hormones (Fig. 17-2).
Secretion of the pituitary gland is controlled and regulated by
releasing hormone
and
inhibitory hormone
secreted by the
hypothalamus or by nervous system signals from the hypotha-
lamic nuclei, including the paraventricular nuclei, the supraoptic
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