In-Depth Notes on Human Reproduction

Human Reproduction

Introduction to Human Reproduction

• Chapter 46 introduces the topic of human reproduction.

• Two main types of reproduction exist in the animal kingdom:

  • Sexual Reproduction: Involves fusion of male and female gametes to form a zygote.

  • Asexual Reproduction: Creation of offspring without egg and sperm fusion.

Differences Between Asexual and Sexual Reproduction

Mechanisms of Asexual Reproduction

1. Budding: New individuals arise from existing ones (e.g., hydra).

2. Binary Fission: Parent organism divides into two or more individuals of similar size (e.g., bacteria, amoeba).

3. Fragmentation: Pieces of the parent develop into new individuals (e.g., earthworms, starfish).

   • Example of Budding: In hydra, a portion buds out and grows into an adult.

   • Example of Binary Fission: In bacteria, the cell replicates its chromosomes and divides into two diploid cells.

   • Example of Fragmentation: Segmenting an earthworm can produce multiple adults from cut pieces.

Costs of Sexual Reproduction

• Sexual reproduction is considered evolutionary costly as females have half the number of daughters compared to asexual females (termed as a twofold cost).

• Despite this, most eukaryotic species reproduce sexually.

• Illustration of Cost: Starting with one female in asexual reproduction results in all female progeny, while sexual reproduction results in half female and half male.

Variations in Sexual Reproduction

• Hermaphroditism: Individuals have both male and female reproductive systems; can mate with themselves or another hermaphrodite (e.g., snails, worms).

• Sex Reversal: Some species can change sex ratios; for instance, certain oysters exhibit male-to-female or female-to-male changes.

Fertilization Mechanisms

• Fertilization involves mechanisms that unite sperm and eggs of the same species, classified into:

  • External Fertilization: Eggs are shed and fertilized in external environments (requires moist habitat). Examples include spawning in salmon where eggs and sperm are released into water together.

  • Internal Fertilization: Sperm is deposited in or near the female reproductive tract (leads to fewer gametes but higher zygote survival). This also involves mechanisms for embryo protection and parental care.

Male Reproductive System

Overview

• Key Structures: Testis, epididymis, vas deferens, prostate, penis, urethra.

• Temperature Regulation: Sperm production requires lower temperatures than the body’s core (testes are located in the scrotum).

Testes and Sperm Production

• Seminiferous Tubules: Coiled tubes within the testes where sperm is produced.

• Leydig Cells: Secrete testosterone and are found between the seminiferous tubules.

• Ejaculation Process: Sperm flows from the testes to the epididymis, then through the vas deferens, and exits the body via the urethra during ejaculation.

Penis Structure

• Composed of three cylinders of erectile tissue, which fill with blood during arousal, causing an erection.

• The head of the penis (glands) is sensitive, covered by foreskin.

Semen Composition

• Semen consists of sperm, fructose (energy for sperm), prostaglandins (trigger contractions), buffer (neutralizes urine acidity), and mucus (lubrication).

• Accessory glands (seminal vesicles, prostate gland, bulbo-urethral gland) contribute fluids to semen.

Female Reproductive System

Overview

• Comprises two parts:

  • Internal Structures: Pair of ovaries, oviducts (fallopian tubes), uterus, cervix.

  • External Structures: Clitoris, labia.

Ovaries and Oviducts

• Ovaries contain follicles that support partially developed eggs.

• Eggs travel via oviducts to the uterus, where they implant if fertilized.

Uterus and Vagina

• Endometrium: Lining of the uterus, rich in blood vessels, thickens for implantation.

• Cervix: Opens into the vagina, which is the canal for sperm entry and childbirth.

Clitoris and Associated Structures

• Clitoris has a sensitive gland and is covered by a small hood (clitoral hood).

• Rich in blood vessels and nerve endings, contributing to sexual arousal.

Mammary Glands

• Functionally important for reproduction; secrete milk during and post-pregnancy.

Gametogenesis

Sperm Formation (Spermatogenesis)

• Continuous process; millions of sperm produced daily.

• Takes about 7-10 weeks for sperm to mature, with an approximate cycle of 74 days.

• Meiosis leads from primordial germ cells to mature sperm.

Egg Formation (Oogenesis)

• Longer, cyclic process compared to spermatogenesis; immature eggs form during embryonic development.

• At puberty, one primary oocyte matures and is ovulated approximately every 28 days.

Hormonal Regulation of Reproduction

Male Regulation

• Hypothalamus releases Gonadotropin-Releasing Hormone (GnRH), stimulating anterior pituitary to release FSH and LH.

• FSH promotes Sertoli cells, aiding in spermatogenesis. LH stimulates Leydig cells for testosterone production.

• Inhibin is secreted by Sertoli cells to provide negative feedback to the anterior pituitary.

Female Regulation

• The ovarian cycle consists of follicular and luteal phases; the uterine cycle corresponds with menstruation and prepares the endometrium for possible implantation.

• Ye old menstrual cycle, divided into three phases: menstrual flow phase, proliferative phase, and secretory phase. If no implantation occurs, menstruation begins again.

Conclusion

• Menopause: Occurs after approximately 500 cycles, signifying a decline in reproductive hormones and cessation of menstruation, potentially allowing better care for offspring.

• Understanding these reproductive processes is crucial for studying biology, human development, and health issues related to reproduction.