Adolescenceand

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

adolescencein males: persists throughout lifein 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 ChildrenTestis 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 ChildrenAt 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 communicationGlial 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 FunctionsCognition

AffectConation (urge to take action)

Feeding BehaviorSexual 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 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

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

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. Maturational changes evoke potentials in the amygdala of the developing rats.

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

A. Amygdala B. Hypothalamus

Maturational Changes in the Brain of the Rat (sagittal section)

10 Days: eyes opening

21 Days: Weaning

39 Days: Maturation of Reproductive Function