Well, let’s first of all
look at a hormone, its
chemical nature or structure. Hormones are the
secretory products of the endocrine cells.
Sometimes, other cells, not part of the endocrine
system, also secrete products that we referred
as being chemical messengers. Hormones can
be classed chemically into three types that
I’ll go through in a moment. But I just want
to make sure we know the difference between
endocrine secretion, paracrine secretion,
and autocrine secretion, and also exocrine
secretion. In a separate lecture on exocrine
glands, I explained that these glands are
derived from epithelial surfaces. The epithelium
grows down underneath the epithelial surface
within the connective tissue, the lamina propria
and there, these cells develop into secretory
cells, and they maintain their connection
to the surface through what becomes the duct
of the exocrine gland. In the case of endocrine
glands, the cells lose their connection to
the surface. The duct disappears and so these
secretory cells then release their products,
hormones, into the surrounding interstitium. And thereby,
the product is taken from the interstitium
into the blood stream. And so these hormones can
travel right throughout the body, and they
can affect target organs at some distance
from the original source of the hormone.
That’s a true endocrine gland, true endocrine secretion.
Sometimes cells can secrete hormones or products
that affect their own function. That’s called
autocrine cells. Being autocrine
meaning secretion, secreting to affect oneself.
Sometimes these cells can secrete hormones
that move along the interstitial fluid and
affect neighboring cells, that's referred to
as paracrine secretion. But in this lecture, I’m
going to refer mainly to only true endocrine
type secretion. Well, hormones are of three types.
One, they can be steroid hormones, or they
can be peptide protein or polypeptide hormones.
Some hormones can be derived from amino acids.
And here on this slide, I’ve listed the various
locations that these hormones and the organs
are secreting, and I’ll refer to this in
a later lecture. But the reason why I’m
showing you these three different classes is
because the chemical nature of these hormones
is different. And therefore, sometimes when you
see the locations of these cells in the various
endocrine glands of the body, you can stain
for these different sorts of hormonal products.
And these stains can often be used to distinguish
the various types of endocrine glands.
As we’ll see in this lecture on the pituitary
gland, we will see that various stains are
used because they differentiate the components
of the secretory cells. And again, as I said,
this will be repeated in another lecture on
the other endocrine glands. Well, when a hormone
is secreted, it can affect the cell at two
sites. One, it can bind to a receptor on the
surface of the target cell. In some other
cases, it moves into the cell and it reacts
with receptors within the cell itself and
the cell biologist and the endocrinologist
will describe these processes in other courses.
It’s important to understand a little of
about the regulation of the endocrine system. We
call it a feedback system, a feedback mechanism,
whereby, the stimulus then goes through the
endocrine cells, and the endocrine cells secrete
a hormone, and that hormone has an effect. And when
the effect then feeds back on the original
stimulus and diminishes that stimulus, we
call that negative feedback. And that’s the
most common feedback mechanism we have in the
endocrine system. Rarely though, sometimes,
we have a positive feedback system, which
is the opposite.