Download Adolescence and Puberty

Adolescence
and
Puberty
MCB 135E
Lecture 29
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
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)
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
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.
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.
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
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
Major Structures: Amygdala and Hippocampus, Septal nuclei
Some Limbic-Hypothalamic Pathways: Stria Terminalis,
Amygdala-hypothalamic fibers
Limbic System
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?
No.
Is it at the level of limbic-hypothalamic pathways?
YES!
How do we know that the limbichypothalamic 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
Hypothalamus as a
“Gonadostat”
With adolescence, the threshold to low sex
hormone levels decreases
thereby
inducing stimulation of GnRH neurons
&
initiating the cascade of hypothalamopituitary-gonadal hormones and their
feedback regulation
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
A. Amygdala
A. Maturational
changes evoke
potentials in the
amygdala of the
developing rats.
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
Maturational Changes in the
Brain of the Rat (sagittal section)
10 Days: eyes opening
21 Days: Weaning
39 Days: Maturation of Reproductive Function