Endocrine Glands: Major Hormones and Their Effects

Overview of Endocrine Glands

The endocrine system consists of glands that secrete hormones directly into the bloodstream, regulating various physiological processes. Hormones act on specific target organs to elicit responses essential for growth, metabolism, reproduction, and homeostasis.

• Endocrine glands include the pituitary, thyroid, adrenal glands, gonads, and others.

• Hormones are chemical messengers that influence target organs and tissues.

• Regulation often involves feedback mechanisms to maintain balance.

Major Hormones, Target Organs, and Effects

The following table summarizes key hormones, their primary target organs, and their main physiological effects.

Key Terms

• Basal Metabolic Rate (BMR): The rate at which the body uses energy while at rest to maintain vital functions.

• Gonadotropins: Hormones (FSH and LH) that stimulate the gonads (testes and ovaries).

• Feedback Mechanism: A process in which the output of a system regulates itself, often through inhibition or stimulation.

Hypothalamic-Pituitary Axis

Structure and Function

The hypothalamic-pituitary axis is a central regulatory system for endocrine function. The hypothalamus produces releasing and inhibiting hormones that control the anterior pituitary, which in turn secretes hormones affecting various target organs.

• Hypothalamus: Located in the brain, it integrates neural and endocrine functions.

• Anterior Pituitary: Releases hormones such as TSH, ACTH, FSH, LH, prolactin, and growth hormone.

• Posterior Pituitary: Stores and releases ADH and oxytocin produced by the hypothalamus.

Hormonal Pathways

• TSH (Thyroid-Stimulating Hormone): Stimulates the thyroid gland to produce thyroid hormones.

• ACTH (Adrenocorticotropic Hormone): Stimulates the adrenal cortex to produce glucocorticoids.

• FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone): Stimulate the gonads (ovary and testis) for gamete production and sex hormone secretion.

• Prolactin: Stimulates milk production in the mammary glands.

• Growth Hormone: Stimulates growth in bone, muscle, and adipose tissue.

Gonads: Testes and Ovaries

Overview and Hormonal Secretion

The gonads are the primary reproductive organs, responsible for producing gametes and sex hormones. The testes secrete androgens, primarily testosterone, while the ovaries secrete estrogens and progesterone.

• Testes: Produce sperm and secrete testosterone, which regulates male secondary sexual characteristics and spermatogenesis.

• Ovaries: Produce ova and secrete estrogens (estradiol-17β, estrone, estriol) and progesterone, which regulate the menstrual cycle and female secondary sexual characteristics.

Key Hormones and Their Functions

• Testosterone: Stimulates development of male reproductive tissues, secondary sexual characteristics, and sperm production.

• Estrogens: Promote development of female reproductive tissues, secondary sexual characteristics, and regulate the menstrual cycle.

• Progesterone: Prepares the uterus for pregnancy and maintains gestation.

Chapter 20: Reproduction

Section 1: Male Reproductive System

1.1 Testis Structure

The testes are divided into two main compartments: seminiferous tubules and interstitial tissue.

• Seminiferous tubules: Site of spermatogenesis; contain Sertoli cells with FSH receptors.

• Interstitial tissue: Contains Leydig cells with LH receptors; responsible for testosterone production.

1.2 Seminiferous Tubules (Spermatogenesis)

Spermatogenesis is the process of sperm cell development within the seminiferous tubules.

• Sertoli cells: Support and nourish developing sperm; form the blood-testis barrier; have immunological privilege; phagocytose residual cytoplasm.

• FSH: Stimulates Sertoli cells to promote spermatogenesis.

• Testosterone: Required for the completion of spermatogenesis.

1.3 Interstitial Tissue: Leydig Cells

Leydig cells produce testosterone in response to LH stimulation.

• Testosterone: Promotes development of male reproductive organs, secondary sexual characteristics, and maintenance of spermatogenesis.

• Testosterone derivatives: DHT (via 5α-reductase) and estradiol (via aromatase) have specific roles in target tissues.

1.4 Interactions Between Compartments

Hormonal regulation coordinates spermatogenesis and testosterone production.

• FSH: Enhances spermatogenesis via Sertoli cells.

• LH: Stimulates Leydig cells to produce testosterone.

• Testosterone: Acts on seminiferous tubules and exerts negative feedback on LH secretion.

1.5 Control of Gonadotropin Secretion in Testes

• LH secretion: Controlled by negative feedback from rising testosterone levels.

• FSH secretion: Controlled by inhibin (from Sertoli cells) and testosterone.

• Testosterone derivatives: DHT and estradiol also participate in feedback regulation.

1.6 Male Accessory Sex Organs

• Include prostate, seminal vesicles, and other glands; growth and function maintained by androgens.

• Common disorders include benign prostatic hyperplasia (BPH).

Section 2: Female Reproductive System

2.1 Ovary and Ovarian Cycle

The ovaries contain follicles at various stages of development. The ovarian cycle involves maturation of oocytes and hormonal changes.

• Primary oocytes: Present at birth; only a fraction mature during reproductive years.

• Follicular phase: FSH stimulates growth of follicles; estradiol production increases.

• Ovulation: Occurs mid-cycle; mature Graafian follicle releases secondary oocyte.

• Corpus luteum: Forms from the ruptured follicle; secretes progesterone and estradiol.

2.2 Menstrual Cycle

The menstrual cycle is regulated by interactions between the hypothalamus, pituitary, and ovaries.

• FSH and LH: Secreted by anterior pituitary; stimulate follicle development and ovulation.

• Estradiol: Promotes endometrial growth; feedback regulates FSH and LH secretion.

• Progesterone: Prepares endometrium for implantation; maintains pregnancy if fertilization occurs.

• Phases: Follicular phase, ovulation, luteal phase, menstruation.

Summary Table: Phases of the Menstrual Cycle

Key Equations

• BMR (Basal Metabolic Rate):

• Negative Feedback Example:

Additional info:

• Estradiol and progesterone are essential for the regulation of the menstrual cycle and maintenance of pregnancy.

• Testosterone and its derivatives are crucial for male reproductive development and function.

• Feedback mechanisms involving hypothalamic and pituitary hormones ensure proper regulation of reproductive cycles.