Cell-cell Communication - The Endocrine System

Hormones

Overview

Hormones are chemical messengers synthesized and released by gland tissues in the body, which play crucial roles in regulating physiological functions, growth, and metabolism.

Gland Tissue

• Hormones are produced in specialized cells located in various glands throughout the body.

• These gland tissues synthesize or build specific hormones through gene expression and then release them into the extracellular fluid (ECF) or more commonly, into the bloodstream.

Target Tissue and Hormone Receptors

• Hormones exert their effects by binding to specific receptors located on target tissues.

• The target tissues are prepared to receive these hormones because they express the genes that synthesize the hormone receptors.

• Hormones can affect organs and tissues in several ways:

  • Inducer or Organizational Effects: These are permanent changes that lead to long-lasting adjustments in tissue or organ function.

  • Activational Effects: These are transient changes in physiological functioning resulting from the presence of the hormone; effects are not permanent and may cease when hormone levels drop.

Types of Hormone Secretion

Hormones can be secreted in various manners, classified into:

1. Autocrine: Hormones act on the same cells that produce them.

2. Paracrine: Hormones affect nearby but different cells.

3. Endocrine: Hormones are released into the blood and travel to distant target cells.

4. Neurohormones: Secreted by nerve cells and travel through the bloodstream to target cells, influencing various bodily functions.

5. Pheromones: Chemicals produced by one member of a species that can affect the behavior of others in the same species; their effects in humans are still under investigation.

Two Chemical Types of Hormones

1. Peptide Hormones: Comprised of amino acids, these hormones are water-soluble and do not easily pass through cell membranes. They typically exert their effects through second messenger systems inside target cells.

2. Steroid Hormones: These hormones are lipid-based, derived from cholesterol, and are fat-soluble. They can easily cross cell membranes and typically act on gene expression directly by binding to receptors inside cells.

The Four Biological Molecules

• Carbohydrates: Provide energy and structure.

• Proteins: Serve as enzymes, hormones, and structural elements; they can coexist with water.

• Lipids: Hydrophobic molecules, including steroids and fats; do not mix well with water.

• Nucleic Acids: DNA and RNA, essential for genetic information and protein synthesis.

Receptors for Hormones

• Cells need to have specific receptors to detect and respond to hormones effectively; without receptors, no physiological response will occur.

Focus on Sex Hormones

• Androgens:

  • Testosterone: Acts on target tissues through its receptors (Tr). It is critical for:

    • Spermatogenesis (sperm production).

    • Development of male secondary sexual characteristics and mature genitalia.

    • Increased sex drive.

• Estrogens:

  • Estradiol: The primary form found in females; affects:

    • Regulation of the menstrual cycle (in females).

    • Secondary female sexual characteristics (e.g., breast development).

    • Synthesized in ovarian follicles, placenta, brain, adrenal glands, and adipose tissue.

• Progesterones:

  • Progesterone: Important for:

    • Supporting pregnancy.

    • Growth and maintenance of the uterine endometrial lining.

    • Works in conjunction with estrogen.

• Cortisol: A glucocorticoid hormone that regulates metabolism and immune response; known to elevate in stress.

Health Implications of Hormonal Levels

• Hormonal imbalances or excessive hormone levels can lead to various health issues:

  • Cardiac problems, including heart disease.

  • Chronic headaches.

  • Diminished sperm production in males.

  • Liver dysfunction.

  • In females, visible signs like a deeper voice and increased facial hair due to excess androgen levels.

• Important Note: Excess or insufficient levels of progesterone can disrupt the synthesis of several other hormones e.g., cortisol, aldosterone, testosterone, DHT, estradiol, leading to systemic health issues.

Important Enzymes in Hormonal Regulation

• Aromatase: Converts testosterone (T) into estradiol (E), playing a key role in regulating estrogen levels in both males and females.

• 5-alpha-reductase: Converts testosterone (T) into dihydrotestosterone (DHT), critical for the development of male secondary sexual characteristics and external genitalia.

• P450scc: An enzyme that converts cholesterol into pregnenolone; this step is vital in steroid hormone biosynthesis.

Focus on Gonads and Gonadotropins

• The gonads (ovaries and testes) are central to the production of sex hormones and gametes.

• Gonadotropin Hormones:

  • Gonadotropin-releasing hormone (GnRH): Stimulates the release of Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH).

  • Luteinizing Hormone (LH): Triggers ovulation in females and stimulates testosterone synthesis in males.

  • Follicle Stimulating Hormone (FSH): Critical for follicular development in females and stimulates Sertoli cells for sperm production in males.

• Other hormones like Oxytocin and Prolactin also play roles in reproductive functions and lactation.