Animal Reproduction

• Asexual reproduction


  Creation of offspring without the fusion of egg and sperm. One parent clones offspring.
• Sexual reproduction

  Creation of an offspring by fusion of a male gamete (sperm) and a female gamete (egg) to form a zygote.
• Zygote


  • Diploid cell that developed from the fusion of haploid gametes.
• Egg

  • Larger and nonmotile.
• Sperm


  • Smaller and motile.
• Asexual reproduction methods

  1. Budding

  2. Fragmentation

  3. Parthenogenesis
• Parthenogenesis is the development of a new individual from an unfertilized egg.
• Sexual females have half as many daughters as asexual females; this is the "twofold cost" of sexual reproduction.
• Advantages of sexual reproduction

  • Increase in variation in offspring
  • Increase in the rate of adaptation
  • Shuffling of genes and the elimination of harmful genes from a population
• Ovulation

  Release of mature eggs at the midpoint of a female cycle.
• Reproductive cycles

  Controlled by hormones and environmental cues.
• Hermaphroditism


  Each individual has both male and female reproductive systems.
• Hermaphroditism is an adaptation that is common to sessile (stationary) animals like barnacles, corals, clams, and burrowing animals.
• External fertilization


  Eggs shed by the female are fertilized by sperm in the external environment.
• Internal fertilization

  Sperm are deposited in or near the female reproductive tract, and fertilization occurs within the tract.
• Cloaca


  Common opening between the external environment and the digestive, excretory, and reproductive systems.
• Female reproductive system

  • Ovaries
  • Oviducts and uterus
  • Vagina and vulva
  • Mammary glands
• Male reproductive system

  • Testes
  • Ducts
  • Accessory glands
  • Penis
• Spermatogenesis

  1. Stem cells divide mitotically to form spermatogonia

  2. Spermatogonia generate spermatocytes through mitosis

  3. Each spermatocyte gives rise to four spermatids through meiosis

  4. Spermatids undergo differentiation into sperm
• Oogenesis

  1. Oogonia divide by mitosis to form cells that begin meiosis

  2. Meiosis stops at prophase I before birth, forming primary oocytes

  3. At puberty, follicle-stimulating hormone stimulates follicles to resume growth and development

  4. One follicle fully matures each month, with its primary oocyte completing meiosis I

  5. Meiosis II begins but stops at metaphase, releasing the secondary oocyte at ovulation
• Spermatogenesis and oogenesis both generate haploid gametes via meiotic divisions of a set of dedicated diploid cells, with support cells in the gonad playing an essential role.
• Primary oocytes


  About 1-2 million contained in the ovaries, of which about 500 fully mature between puberty and menopause
• Follicle-stimulating hormone (FSH) stimulation


  Periodically stimulates a small number of follicles to resume growth and development at puberty
• Follicle maturation


  Typically, only one follicle fully matures each month, with its primary oocyte completing meiosis I
• Meiosis II

  Begins but stops at metaphase, with the secondary oocyte released at ovulation when the follicle breaks open
• Sperm penetration of the oocyte

  Resumes meiosis II
• Corpus luteum


  The ruptured follicle left behind after ovulation, which secretes estradiol and progesterone
• Corpus luteum degeneration

  New follicles mature during the next cycle if the egg is not fertilized
• Differences between spermatogenesis and oogenesis

  • One egg forms from each cycle of meiosis in oogenesis, while four sperm form in spermatogenesis
  • Cytokinesis is unequal in oogenesis, with most cytoplasm going to the egg, while equal in spermatogenesis
  • Oogenesis ceases later in life, while spermatogenesis continues throughout adult life
  • Oogenesis has long interruptions, while spermatogenesis is continuous
  • Males produce hundreds of millions of sperm each day, while egg development takes years
• Gonadotropin-releasing hormone (GnRH)


  Secreted by the hypothalamus, directs the release of FSH and LH from the anterior pituitary
• Follicle-stimulating hormone (FSH) and Luteinizing hormone (LH)


  Regulate processes in the gonads and the production of sex hormones
• Sex hormones


  • Androgens
  • Estrogens
  • Progesterone
• Androgens, estrogens, and progesterone

  Found in both males and females, but at quite different concentrations
• Effects of sex hormones at puberty

  • In males, androgens cause voice deepening, facial/pubic hair, and muscle growth

  In females, estradiol stimulates breast and pubic hair development
• Functions of sex hormones

  • Development of primary sex characteristics during embryogenesis

  Development of secondary sex characteristics at puberty

  Regulation of sexual behavior and sex drive
• Sertoli cells

  Nourish developing sperm and are regulated by FSH
• Leydig cells


  Secrete testosterone and other androgens, regulated by LH
• Testosterone

  Regulates the production of GnRH, FSH, and LH through negative feedback
• Inhibin


  Hormone secreted by Sertoli cells that reduces FSH secretion from the anterior pituitary
• Menstrual cycle

  Changes in the uterus/uterine lining with blood vessels