Chapter 27 Part A: The Reproductive System – Overview and Hormonal Regulation

The Reproductive System: Overview

Introduction to the Reproductive System

The reproductive system is unique among organ systems in that it does not function continuously throughout life, but becomes active at puberty. Male and female reproductive organs differ in structure but share four fundamental tasks essential for sexual reproduction.

• Formation of Gametes: Specialized cells for sexual reproduction are called gametes.

  • Sperm – male gametes

  • Ova (eggs) – female gametes

• Bringing Gametes Together: Gametes from male and female are brought together through sexual intercourse (copulation).

• Fertilization: Genetic information from gametes is combined during fertilization.

  • Zygote – the first cell of a new individual, formed when sperm and egg fuse. All body cells arise from the zygote.

• Support of Fetal Development: The system supports the development of the fetus (gestation) and birth of the baby (parturition).

27.1 Male and Female Reproductive System: Shared Features

Gonads and Accessory Reproductive Organs

Male and female reproductive structures are homologous, meaning they share a common origin during embryonic development. For example, male testes and female ovaries, or male penis and female clitoris.

• Primary Sex Organs (Gonads):

  • Testes (male) and ovaries (female) produce two main products:

    1. Gametes: Sperm (male) and ova (female), formed by cell division called meiosis.

    2. Sex Hormones: Steroid hormones such as testosterone (males), estrogens and progesterone (females). These hormones are vital for the development and function of reproductive organs, sexual behavior, and sexual drives.

• Accessory Reproductive Organs: Include ducts, glands, and external genitalia that aid in the transport and support of gametes.

Reproductive Hormone Secretion

The Hypothalamic-Pituitary-Gonadal (HPG) Axis

The production of gametes and sex hormones is regulated by a sequence of hormonal events involving the hypothalamus, anterior pituitary gland, and gonads (testes or ovaries). This regulatory pathway is known as the hypothalamic-pituitary-gonadal (HPG) axis.

• Key Hormones:

  • Gonadotropin-releasing hormone (GnRH): Released from the hypothalamus, it reaches the anterior pituitary via the hypophyseal portal system.

  • Follicle-stimulating hormone (FSH) and Luteinizing hormone (LH): Gonadotropins released from the anterior pituitary, which stimulate the gonads.

  • Sex hormones: Testosterone (males), estrogens and progesterone (females), which act at target tissues and exert negative feedback on the hypothalamus and anterior pituitary.

  • Inhibin: Released from the gonads, inhibits FSH release from the anterior pituitary.

Negative Feedback Mechanisms

Sex hormones and inhibin regulate the HPG axis through negative feedback:

• Testosterone, estrogens, and progesterone inhibit GnRH, FSH, and LH secretion.

• Inhibin specifically inhibits FSH release.

Activation of the HPG Axis at Puberty

Puberty is the period when reproductive organs grow to adult size and become functional, marking the earliest time reproduction is possible.

• Before puberty, low levels of circulating sex hormones suppress GnRH secretion.

• As puberty approaches, the hypothalamus becomes less sensitive to inhibition by sex hormones and begins releasing GnRH in a pulsatile manner.

• FSH and LH levels rise, stimulating the gonads to produce more sex hormones.

• This leads to the adult pattern of hormone interaction and reproductive capability.

Summary Table: Key Hormones of the HPG Axis

Hormone	Source	Target	Main Function
GnRH	Hypothalamus	Anterior Pituitary	Stimulates release of FSH and LH
FSH	Anterior Pituitary	Gonads (Sertoli cells in males, ovarian follicles in females)	Stimulates gamete production
LH	Anterior Pituitary	Gonads (Leydig cells in males, ovarian theca cells in females)	Stimulates sex hormone production
Testosterone	Testes (Leydig cells)	Various tissues	Promotes male secondary sex characteristics, spermatogenesis
Estrogens/Progesterone	Ovaries	Various tissues	Promote female secondary sex characteristics, regulate menstrual cycle
Inhibin	Gonads	Anterior Pituitary	Inhibits FSH release

Relevant Equations

• General Negative Feedback Equation: $\text{Hormone Output} = \text{Baseline Secretion} - \text{Inhibitory Feedback}$

Example: Hormonal Regulation at Puberty

As puberty begins, the hypothalamus increases GnRH secretion, leading to increased FSH and LH, which stimulate the gonads to produce sex hormones. This results in the development of secondary sexual characteristics and reproductive capability.

Additional info: The HPG axis is fundamental for the regulation of reproductive function in both males and females, and disruptions can lead to disorders of puberty and fertility.