INTRODUCTION TO ENDOCRINE SYSTEM

1. Introduction to the Endocrine System
General Features and Definitions
Types of Hormones
Functions of the Endocrine System
Components of the Endocrine System
Chemical Structure of Hormones
Homeostasis
Control of Endocrine Activity
Regulation of Hormone Secretion
Transport and Distribution of Hormones
Mechanism of Hormone Action
Regulation of Hormone Receptors

2. General Features of the Endocrine System
1. Endocrine glands are ductless
2. Endocrine glands have a rich supply of blood.
3. Hormones, produced by the endocrine glands are
secreted into the bloodstream.
4. Hormones travel in the blood to target cells close
by or far away from point of secretion.
5. Hormones receptors are specific binding sites on
the target cell.

3. Important Definitions:
Endocrine System
• Endocrine--endo means within. This is a
system which controls body function through
hormones.
• Endocrine System is composed of a number of
glands.
• Glands are specialized tissues that produce a
hormone.

4. Endocrine Glands

5. Important Definitions
• What are hormones?
Hormones are organic chemical messengers produced and
secreted by endocrine cells into the bloodstream. Hormones
regulate, integrate and control a wide range of physiologic
functions.
Silverthorn, Human Physiology, 3rd
edition Figure 6-1&2

6. Important Definitions
• What are endocrine glands?
Endocrine glands are ductless glands comprised of
endocrine cells. This means that these glands do not
have ducts that lead to the outside of the body. For
example, sweat glands are NOT endocrine glands
(they are instead exocrine glands) because sweat
glands have ducts that lead to the outside surface of
your skin (that’s how the sweat gets out). The fact
that endocrine glands are ductless means that these
glands secrete hormones directly into the blood
stream (instead of to the outside of your body).

7. Important Definitions
• What are target cells?
Target cells refer to cells that contain specific receptors (binding
sites) for a particular hormone. Once a hormone binds to
receptors on a target cell, a series of cellular events unfold
that eventually impact gene expression and protein synthesis.
Silverthorn, Human Physiology, 3rd
edition Figure 6-1&2

8. Important Definitions
• What are hormone receptors?
Hormone receptors are binding sites on the target cell (either on
the surface or in the cytoplasm or nucleus of the target cell)
that are activated only when specific hormones bind to them.
If a hormone does not/cannot bind to it’s receptor, then no
physiologic effect results.
See next slide for a picture of a hormone bound to its receptor

9. Growth hormone regulates cell growth by
binding to growth hormone receptors on target
cells.

10. Types of Hormones
Steroid Hormones
• These are all derived
from cholesterol.
• Examples: testosterone,
estrogen, progesterone,
mineralicoids,
glucocorticoids.
• Steroids can cross the
plasma membrane!
Protein Hormones
• These are made of
amino acids.
• Examples: Insulin,
hypothalmus-signaling
hormones.
• Protein hormones
cannot cross the plasma
membrane!

11. Chemical Structure of Hormones
• Two general classes of hormones: water soluble
and lipid soluble.
• Water soluble (polar): proteins, glycoproteins,
polypeptides, amino acid derivatives.
• Lipid soluble (nonpolar): steroids, amino acid
derivatives, fatty acids.
• Different classes have different mechanisms of
action, different modes of transport through the
body, and differing stability in the circulation.

12. Examples of Water Soluble Hormones
• Proteins: growth hormone, prolactin, insulin
• Glycoproteins: follicle-stimulating hormone (FSH),
luteinizing hormone (LH) , thyroid-stimulating
hormone (TSH)
• Polypeptides: arginine vasopressin, oxytocin,
somatostatin
• Amino acid derivatives:epinephrine, melatonin

13. Examples of Lipid-Soluble Hormones
• Steroids: estrogen, progesterone, testosterone,
glucocorticoids, mineralocorticoids
• Amino acid derivatives: Thyroid hormones (T3, T4)
• Fatty acids: prostaglandins, thromboxanes

14. Classes of Hormones
• Steroids vs. Peptide Hormones
Hormones fall into 2 general classes based on
their molecular structure and synthesis.
• All steroid hormones are made initially from
the precursor (precursor = first step in
biosynthetic pathway) cholesterol.
See next slide for a diagram of the biosynthetic pathway of steroid hormones
from cholesterol.

16. Steroid Hormones
• Steroid hormones are produced by the gonads
and adrenal cortex.
• Thyroid hormones are not steroids, but will be
categorized with steroids for simplicity.
• Steroid hormones are made from cholesterol
in the smooth endoplasmic reticulum and
mitochondria of endocrine cells.

17. Steroid Hormones
• Steroid hormones cannot be stored in vesicles
in the endocrine cells that produce them. As
soon as steroid hormones are produced, they
diffuse out of the endocrine cell and enter the
bloodstream.
• Steroid hormones are lipid soluble and their
receptors are located inside their target cell.

19. Peptide Hormones
• Peptide hormones are comprised of chains on
amino acids.
• Like most proteins, peptide hormones are
synthesized on ribosomes of the (rough)
endoplasmic reticulum of endocrine cells.
• Peptide hormones can be stored in vesicles in
endocrine cells until they are needed at some
later point.

20. Peptide Hormones
• Peptide hormones do not readily pass through
cell membranes (lipid bilayers) and they are
referred to as water soluble.
• Receptors for peptide hormones are found on
the cell surface of their target cells.

21. Some General Actions of Hormones
• Hormones cause cells to change.
• Hormones can result in changes in gene
expression (DNA-RNA-Protein).
• Hormones can result in enzyme cascades
which control our metabolism.
• Hormones drive our reproductive systems.

22. ACTION OF A NONSTEROID
HORMONE

23. Some Specific Actions of Hormones
• Fetal development and differentiation
• Cell growth and cancer
• Metabolism
• Cardiovascular function
• Renal function
• Skeletal function
• Reproductive function
• Immune function
• Central nervous system function

24. Homeostasis
• Definition: the maintenance of a constant
environment (internal).
• Parameters regulated: Temperature, osmolarity,
pH, nutrient levels, hormone levels, etc.
• Homeostasis is critical for cell viability and proper
functioning.
• Loss of homeostasis results in disease/death.
• Homeostasis is maintained by feedback
mechanisms (primarily negative feedback).

27. Another Example: Regulation of LH Release in
the Male
• LH increases production of testosterone from the
testis.
• Testosterone feeds back upon the pituitary to inhibit
LH release.
testosterone
LH
(-)
pituitary
testis

28. Endocrine overview
• Hormones are released by glands.
• Hormones are released by feedback.
• Our body works to carefully regulate hormone
levels.
• Negative feedback usually controls hormone
secretion.

29. Principal functions of the endocrine
system
• Maintenance of the internal environment in the body
(maintaining the optimum biochemical
environment).
• Integration and regulation of growth and
development.
• Control, maintenance and instigation of sexual
reproduction, including gametogenesis, coitus,
fertilization, fetal growth and development and
nourishment of the newborn.

30. Response vs. distance traveled
Endocrine action: the hormone is distributed in blood and binds to
distant target cells.
Paracrine action: the hormone acts locally by diffusing from its source to
target cells in the neighborhood.
Autocrine action: the hormone acts on the same cell that produced it.

31. What determines the size of hormone effects?
1) The amount of hormone in the circulation
(reaching the target tissue)
- the more hormone, the greater the effect
2) The presence and number of receptors for that
hormone on the target tissue.
- no receptor, no response
- some receptors, some response
- many receptors, higher response

32. How do you regulate hormone levels?
• Hormones are generally not secreted at a constant
rate.
• Regulation of hormone levels involves:
- regulation of hormone production
- regulation of hormone secretion (often a
separate step)
- sometimes, regulation of hormone metabolism

33. Patterns of Hormone Secretion
• There are three basic patterns of secretion: pulsatile,
acute, and cyclic.
• Pulsatile: relatively constant level of hormone, over a
long period
• Acute: rapid increase in hormone level for a short
time in response to a stimulus
• Cyclic: hormone increases and decreases in a
constant pattern

34. Mechanism of Hormone Action: Receptors
• For hormones to act on a cell, that cell must have a
receptor for that hormone.
• Receptors bind the hormone, resulting in a biological
response.
• Receptors are found only in target tissues for that
hormone.
• Receptors are very specific (they only bind a specific
hormone, not all hormones)
• Receptors have high affinity for their hormone (bind
hormone at very low hormone concentration).