1. Adolescence and Puberty
MCB 135E
Lecture 29

2. ADOLESCENCE: Period extending
from onset of reproductive function to adulthood;
characterized by maturation of all functions
By adolescence, some functions are already established and continue to grow/develop until optimal function in adulthood
The reproductive function is NOT developed by adolescence and develops during adolescence
in males: persists throughout life
in females:ceases at menopause

3. Why is reproductive function so late to mature?
PUBERTY: Maturation of reproductive function is only one of many physiologic changes occurring at adolescence
Why is reproductive function so late to mature?
Immaturity is not at the level of the gonads or the anterior pituitary
BUT at the level of the BRAIN
(primarily limbic structures and hypothalamus)

4. Status of Gonadal Function in Children
Testis and ovary are differentiated as are the secondary male and female organs
Very low levels of sex hormones: testosterone (T) for males and estrogens (E) for females
Very low levels of GnRH (in hypothalamus) and FSH/LH (in anterior pituitary)
Therefore the negative feedback regulation does NOT seem to be operative at this age

6. Status of Gonadal Negative Feedback Regulation in Adults
Levels of sex hormones inhibit GnRH release from hypothalamus
Levels of GnRH reduce FSH/LH release from anterior pituitary
Levels of FSH/LH cease to stimulate testis or ovary to secrete T and E
Levels of sex hormones in blood stimulate GnRH release stimulating FSH/LH release
Levels of FSH/LH stimulate T and E secretion.
Thereby, normal sex hormone levels are established in blood and tissues.

7. FSH/LH (anterior pituitary)
During Childhood
Blood sex steroids
FSH/LH (anterior pituitary)
GnRH (hypothalamus)
During Puberty
Limbic system matures and, in preparation to maturation of sexual function, signals GnRH neurons to increase their responsiveness to changes in sex hormone levels.

8. CNS Status in Children
At birth and in childhood, brain/head are proportionately more developed than other organs
CNS matures progressively from birth to late childhood by:
Dendritic branching & number of synapses: better communication
Glial cell number: better metabolism, neurotransmission, myelination, faster and more efficient neuronal communication, specific stimulus: learning social interactions
Threshold to stimuli is decreased, therefore neurons are responsive to lower intensity stimuli
Of particular significance for the maturation of the reproductive function is the maturation of the limbic system

9. Limbic System: Major Functions
Cognition
Affect
Conation (urge to take action)
Feeding Behavior
Sexual Behavior
(urge to copulate, coordinate sequence of events in males and females that lead to pregnancy)
Fear and Rage
Motivation

10. Major Structures: Amygdala and Hippocampus, Septal nuclei
Some Limbic-Hypothalamic Pathways: Stria Terminalis, Amygdala-hypothalamic fibers

11. Limbic System

12. Why is Reproductive Function so Late to Mature?
Is the immaturity at the level of the hypothalamus? YES!
Is it at the level of the anterior pituitary?
No.
Is it at the level of the gonads?
Is it at the level of limbic-hypothalamic pathways? YES!

13. How do we know that the limbic-hypothalamic system is responsible for the immaturity of the reproductive system before puberty?
1. The immature gonads transplanted into an adult animal will mature immediately and demonstrate a normal reproductive function.
2. The pituitary, taken from a prepubertal animal and transplanted into an adult animal, will regulate a normal reproductive function.
3. Alterations/lesions/trauma of the hypothalamus and specific areas in the limbic system (amygdala, hippocampus) will prevent the prepubertal animal to develop a normal reproductive function

14. Hypothalamus as a “Gonadostat”
With adolescence, the threshold to low sex hormone levels decreases
thereby
inducing stimulation of GnRH neurons
&
initiating the cascade of hypothalamo-pituitary-gonadal hormones and their feedback regulation

15. Changes in the “Gonadostat” & Onset of Reproductive Function
Before puberty: secretion of GnRH and Gns is inhibited by the very low levels of gonadal steroids
At puberty: decreased sensitivity to negative feedback of gonadal hormones on hypothalamic GnRH and pituitary FSH/LH synthesis & release establishes normal reproductive function

16. A. Maturational changes evoke potentials in the amygdala of the developing rats.
A. Amygdala
B. Hypothalamus
B. Maturational changes evoke potentials in the hypothalamus of developing rats.
Time Table Maturational Events
10 days: eye opening 21 days: weaning
39 days: maturation of reproductive function Adult

17. Maturational Changes in the Brain of the Rat (sagittal section)
10 Days: eyes opening
21 Days: Weaning
39 Days: Maturation of Reproductive Function