ENDOCRINOLOGYThe scientific study of the endocrine

glands and their hormones

Insulin-containing cells on direct immunofluorescence of islets of LangerhansTaken from images.MD

Outline Lecture 106/06/05

• General Features of the Endocrine System and Important definitions

• Evolution of the Endocrine System

• Classes of Hormones

• Actions of Hormones

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.

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

Important Definitions Cont.

• 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).

Important Definitions Cont.• 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

Important Definitions Cont.

• 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

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

Outline Lecture 106/06/05

• General Features of the Endocrine System and Important definitions

• Evolution of the Endocrine System

• Classes of Hormones

• Actions of Hormones

Evolution of the Endocrine System

• It is likely that the vertebrate endocrine system evolved from bacterial signaling systems that allowed bacteria to both perceive extra- and intracellular signals.

• Primitive endocrine signaling likely arose from cells releasing chemical signals to communicate with neighboring cells (paracrine signaling).

Silverthorn, Human Physiology, 3rd edition Figure 6-1&2

Outline Lecture 106/06/05

• General Features of the Endocrine System and Important definitions

• Evolution of the Endocrine System

• Classes of Hormones

• Actions of Hormones

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 hormonesfrom cholesterol.

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.

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.

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.

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.

Outline Lecture 106/06/05

• General Features of the Endocrine System and Important definitions

• Evolution of the Endocrine System

• Classes of Hormones

• Actions of Hormones

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