Page 52 - 86 human physiology part-2

The Endocrine System

The posterior lobe is composed of neural tissue [neural ectoderm] and is derived from
hypothalamus. Its function is to store oxytocin and Antidieuretic hormone. When hypothalamic
neurons fire: hormones release into the capillaries of the lobe.

The posterior pituitary is, in effect, a projection of the hypothalamus. It does not produce its own
hormones, but only stores and releases the hormones oxytocin and antidiuretic hormone (ADH - also
known as vasopressin).

Anterior pituitary

Communication between the hypothalamus and the anterior pituitary occurs through chemicals
(releasing hormones and inhibiting hormones) that are produced by the hypothalamus and delivered to
the anterior pituitary through blood vessels. The releasing and inhibitating hormones are produced by
specialized neurons of the hypothalamus called neurosecretory cells. The hormones are released into a
capillary network or primary plexus, and transported through veins or hypophyseal portal veins, to a
second capillary network or secondary plexus that supplies the anterior pituitary. The hormones then
diffuse from the secondary plexus into the anterior pituitary, where they initiate the production of
specific hormones by the anterior pituitary. Many of the hormones produced by the anterior pituitary
are tropic hormones or tropins, which are hormones that stimulate other endocrine glands to secrete
their hormones

The anterior lobe is derived from oral ectoderm, composed of glandular epithelium it
communicates with the hypothalamus via a network of capillaries.

The anterior pituitary lobe receives releasing hormones from the hypothalamus via a portal vein
system known as the hypothalamic-hypophyseal portal system.

The anterior pituitary secretes:

• growth hormone
• prolactin
• follicle-stimulating hormone
• luteinizing hormone
• thyroid-stimulating hormone
• adrenocorticotropic hormone
• endorphins
• and other hormones

It does this in response to a variety of chemical signals from the hypothalamus, which travel to the
anterior lobe by way of a special capillary system from the hypothalamus, down the median eminence,
to the anterior lobe. These include:

• TRH (thyrotropin-releasing hormone)
• CRH (corticotropin-releasing hormone)
• DA (dopamine, "prolactin inhibiting factor"/PIF)
• GnRH (gonadotropin-releasing hormone)
• GHRH (growth hormone releasing hormone)

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