TABLE OF CONTENTS
KEY CONCEPTS IN THIS LECTURE 1.The endocrine system consists of ductless glands which secrete hormones into the blood to act at a distant target organ. The "classic" endocrine glands are the anterior pituitary, pancreas, parathyroid, thyroid, adrenal gland, and gonads, but other organs, such as the stomach and the small intestine, also secrete hormones. The anterior pituitary gland is especially important for two reasons: 1) trophic hormones (e.g., ACTH) regulate function of other endocrine glands; and 2) the anterior pituitary is in part regulated by neural centers in the hypothalamus (=the hypothalamic-hypophyseal axis) via secretion of releasing factors (e.g., CRF). 2. There are two general classes of hormone: 1) peptide and 2) steroid. Peptide hormones readily dissolve in water while steroids do not. This difference affects how the hormones are transported in the blood as well as their action at the target cell. Peptide hormones usually act at a specific receptor on the plasma membrane. The signal is then communicated to the cytoplasm by one of the signal transduction pathways (e.g., receptor-hormone, G protein, adenylyl cyclase, cAMP, kinase). Steroids readily pass through both the plasma and nuclear membranes. They bind with a specific Hormone Response Element in the nucleus to activate DNA transcription. Hormone titer in the blood is regulated directly (blood glucose effects on insulin release) or via the hypothalamic-hypophyseal axis (inhibition of FSH by Testosterone feedback). 3. In mammals, sex is determined by the presence or absence of the Y chromosome. The Y chromosome contains a gene that produces H-Y antigen, a protein which causes differentiation of the fetal testis. Mullerian Inhibiting Factor is secreted by the fetal testis causing atrophy of the female reproductive tract. Testosterone is also secreted which stimulates the differentiation of the internal male sex organs, enlargement of the brain's sexual dimorphic nucleus, secondary sexual characteristics, and sexual behavior. If one of these events is omitted, the default state of "femaleness" occurs at that step. In other vertebrates, sex determination can be influenced by changes in the physical environment (e.g., temperature), age (e.g., younger fish are male; older fish are female), social context (e.g., absence of a male causes a female to change to a male), etc. LECTURE OBJECTIVES 1. Compare and Contrast the general
functional characteristics of peptide and steroid hormones.
2. Discuss the genetic and physiological basis for sex determination in mammals and in other vertebrates LECTURE OUTLINE I. AN INTRODUCTION TO THE ENDOCRINE SYSTEM **
A. Endocrines -- ductless glands that secrete hormones
into the blood
1. General characteristics
a. often has a more general influence than nerve
stimulation
b. slower onsets with longer-term effects
c. often multiple targets are affected by single hormone
II. THE BASICS OF ENDOCRINE FUNCTION
A. Classic endocrine glands
1. Anterior Pituitary -- often called the "master gland"
because it regulates other endocrine glands (e.g.,
gonads via tropic hormones)
a. Prolactin is non-tropic hormone of the anterior pituitary
2. Pancreas -- metabolism of glucose, etc.
3. Parathroid -- Ca++ regulation
4. Thyroid -- metabolism, but also involved in some behaviors
5. Adrenal gland -- active during stress
6. Testes and Ovaries -- involved in reproduction and
reproductive behavior
B. General characteristics of hormones
1. Two general classes: 1) peptides (& amines) and 2) steroids
2. Characteristics of peptides and steroids
a. hydrophilic or lipophilic?
b. site of synthesis, method of release
c. site of hormone action
3. Hormone action in greater detail
a. What do all hormones have in common? (a receptor,
signal amplification, etc.)
b. Peptides act at the plasma membrane
1) Involves various membrane bound proteins and
a second messenger (Animation)
c. Steroids and thyroxine act on a receptor in the nucleus to
activate a hormone response (HRE) element on the
DNA. mRNA released followed by protein synthesis
(Animation)
4. Fate of circulating hormone
5. Regulation of hormone titer in the blood
a. Negative and Positive Feedback mechanisms
1) Examples: Testosterone feedback & Estrogen feedback
III. SEX DETERMINATION
A. Most organisms are diploid
1. A karyotype reveals this diploid condition
2. X and Y chromosomes determine sex (Female is XX
and Male is XY)
a. The Y is critical for "maleness"
1) Evidence from non-disjunction
2) Default (absence of the Y) is the female condition **
B. What does the Y provide?
1. SRY gene is only on the Y chromosome
a. Immature reproductive system is bisexual
2. SRY produces H-Y antigen which differentiates the
fetal testis
3. Muellerian Inhibiting Factor (MIF) causes atrophy of female
Muellerian duct
4. Testosterone (T) stimulates:
a. formation of internal male sex organs
b. enlargment of the sexual dimorphic nucleus in the
hypothalamus
c. male behavior and secondary sexual characteristics
5. Dihydrotestosterone (DHT) stimulates development of
male external genitalia
6. Summary of Sex Determination Events **
7. Spotted Hyena: A unique case of sexual differentiation
a. Female hyenas are dominant to males and are
very aggressive
b. Females have external genetalia resembling that
of the male
c. Fetal androgens derived from ovarian
4-androstenedione may be responsible for
masculinization.
C. Other influences on sex determination
1. Other forms of genetic sex determination
a. Birds=female is ZW and male is ZZ
2. Temperature dependent sex determination
a. Incubation temperature is important in some
ectotherms
b. High temperatures stimulate one sex; low the other.
3. Age of the animal may determine its sex
a. Young black porgies are male but many change to
female during the third breeding season
1) the gonad contains both ovarian and testicular
tissue
b. This process is hormonally controlled
c. Hormone manipulations can alter the timing of sex
reversal
4. Behavioral sex determination
a. Sex can change in many fish (female --> male)
b. Sociobehavioral factors influence sex change
c. Example: bluehead wrasse
d. Sex may be related to GnRH level: Male fish have
high GnRH **
Anterior and Posterior Pituitary function from the Hebrew University.
Hormone Action of peptide and steroid hormones with some animations. From Colorado State.
Sex Determination in Fish. A web page on the factors influencing sex determination in fish. Stress in fish is also discussed. Go to: |
