Puberty and Hormones

Hormones and Puberty

Overview of the Hypothalamus-Pituitary-Gonadal Axis (HPG Axis)

• The HPG axis is crucial for the regulation of reproductive hormones.

• It involves the interaction between the hypothalamus, pituitary gland, and gonads (ovaries/testes).

Regulation of the HPG Axis

1. Mechanisms of Regulation

   • Positive Feedback:

     • Involves hormones inducing hypothalamic neurons to secrete GnRH (Gonadotropin-releasing hormone).

     • High levels of GnRH lead to increased secretion of gonadotropins (LH and FSH).

   • Negative Feedback:

     • Hormones can inhibit hypothalamic neurons, resulting in very low levels of GnRH.

2. Impact During Puberty:

   • Ontogenetic shift from negative to positive feedback mechanisms takes place, allowing for reproductive function maturation.

Physiological Processes Controlled by the HPG Axis

• The HPG axis controls various reproductive physiological processes:

  • Regulation of menstrual cycles in females.

  • Control of spermatogenesis in males.

  • Ovulation triggers in females.

Simplified Example of HPG Axis Regulation in Females

• Positive Feedback:

  • Rise in Estradiol stimulates the surge center of the hypothalamus, increasing GnRH, leading to an LH surge and subsequent ovulation.

• Negative Feedback:

  • Progesterone inhibits GnRH and in turn decreases LH and FSH, which can lead to incomplete follicular development due to lower FSH.

• Neurons from the SURGE CENTER primarily affect the positive feedback mechanism while those from the TONIC CENTER influence negative feedback.

Phases of the HPG Axis Regulation in Females

• Midcycle: High levels of estradiol trigger ovulation through an LH surge.

• Luteal Phase: Progesterone elevation leads to reduced GnRH and gonadotropin levels.

Simplified Example of HPG Axis Regulation in Males

• Negative Feedback Example:

  • Increased testosterone leads to decreased GnRH, which reduces LH and FSH levels.

Puberty Criteria and Indicators

• Key Indicators of Puberty:

  • Age of first estrous and ovulation.

  • Age capable of supporting pregnancy without issues.

  • Behavioral traits' expression age.

  • Male indicators include age of first ejaculation and quality of ejaculate (spermatozoa presence).

• External Factors: External environmental and physiological factors can influence the timing of puberty.

Sex Differences in the Hypothalamus

• Tonic Center & Surge Center:

  • Testosterone affects the differentiation of these centers during embryogenesis, impacting reproductive capabilities.

  • Testosterone converts to estradiol, which defeminizes the hypothalamus, with the default condition being female.

Estradiol and the Defeminization Process

• Fetal estradiol does not defeminize the hypothalamus in females because:

  • Alpha-fetoprotein (⍺FP) binds estradiol, preventing it from crossing the blood-brain barrier.

  • ⍺FP does not bind testosterone, enabling its influence on male brain differentiation.

Hormonal Concentration Patterns

• Types of Hormonal Secretion Patterns:

  • Episodic: Rapid bursts of hormone secretion.

  • Basal: Low, consistent background levels.

  • Sustained: Steady hormone levels over time.

  • Pulsatile patterns organization into predictable rhythms.

Post-Puberty Hormonal Dynamics

• After puberty, hormone levels become more pulsatile, with a pronounced surge in estrogen generating a well-organized LH surge during the ovulation cycle.

Neural Activity in the HPG Axis

• The full neural activity of hypothalamic centers must be reached for effective regulation post-puberty:

  • Tonic and surge centers manage different patterns of GnRH secretion.

  • Tonic center maintains basal levels, while surge center handles pulsatile stimulation.

Male Feedback Mechanism During Puberty

• In males, feedback primarily involves simple negative feedback:

  • As puberty approaches, GnRH neurons become less sensitive to negative feedback from testosterone and estradiol, leading to increased LH and FSH secretion from the pituitary.

Metabolic Signals Affecting GnRH Neurons

• Metabolic signals can also influence GnRH neuron activity:

  • Various factors such as blood glucose, leptin, and fatty acids may modulate signaling pathways involving GnRH.

  • Interaction between kisspeptin and GnRH neurons may play a role in reproductive endocrine regulation.