The anterior pituitary is crucial for normal growth, sexual maturation,
and endocrine function. The pituitary integrates, mediates, and
modulates the influence of the brain and the hypothalamus on the
endocrine system (Fig. 521-1). The hypothalamus
regulates anterior pituitary function by synthesizing and secreting
small peptides that either promote or inhibit pituitary hormone
secretion. These include growth-hormone releasing hormone (GHRH),
somatostatin, thyrotropin-releasing hormone (TRH), corticotrophin-releasing
hormone (CRH), and gonadotropin-releasing hormone (GnRH). Release
of these factors is controlled in part by central nervous system
neurotransmitters such as dopamine, norepinephrine, serotonin, and endogenous
opioids. These neuropeptides are carried through the hypothalamic-hypophyseal
portal circulation to anterior pituitary cells, where they bind
membranous receptors and elicit intracellular cascades of activity
that lead to release of the specific pituitary hormones. Pituitary
hormones can modulate their own secretion by affecting the secretion
of hypothalamic peptides or by acting directly on the anterior pituitary gland.
These phenomena are called short and ultra-short feedback
loops, respectively. The peripheral endocrine glands, whose secretion
of hormones is stimulated by pituitary trophic hormones, also exert
negative feedback on anterior pituitary function at the level of
the hypothalamus, the pituitary gland, or both. Through these exquisitely
sensitive and complex interactions, the anterior pituitary gland
regulates much of the endocrine milieu of the body.
Brain-hypothalamic-pituitary-target organ axis. This
schema depicts the complex regulatory interrelations among brain
neurotransmitters and peptides, hypothalamic releasing and inhibiting
factors, pituitary trophic hormones, and target organ hormones.
Arrows indicate the avenues of regulation. Signs (+ or –)
indicate whether the effect is stimulatory or inhibitory, respectively.
Negative feedback of target gland hormones is illustrated by the
arrows on the left. The short and ultra-short-loop negative feedback
mechanism is illustrated on the right by the inhibitory effects
of trophic hormones on secretions from the hypothalamus and anterior
The anterior pituitary gland is derived from invagination of
ectodermal cells in the roof of the primitive oral cavity (stomodeal
ectoderm), whereas the posterior pituitary gland (neurohypophysis)
is derived from neural cells in the floor of the third ventricle.
A group of homeobox genes encode multiple intracellular transcription
factors and extracellular signaling peptides that direct temporal
and anatomic development of the pituitary.1,2 The
developing pituitary and hypothalamus are in close proximity, and
their embryogenesis is codependent.
The pituitary is connected to the hypothalamus by a stalk and
is separated from the brain by a reflection of the dura mater (Fig. 521-2). The hypothalamic-releasing and
hypothalamic-inhibiting factors reach the anterior pituitary gland
through the hypothalamic-hypophyseal portal circulation. At its
source in the floor of the hypothalamus, this vascular system is
a dense network of arterioles and capillaries. In the pituitary
stalk, it is composed of portal veins that drain into sinusoidal
capillaries in the anterior pituitary gland. Arterial blood derived from
branches of the ...