Fertilization and embryonic development are crucial processes in animal reproduction. From internal fertilization in mammals to external fertilization in fish, these strategies ensure species survival. The journey from gamete formation to fetal development involves complex stages like cleavage, blastulation, and gastrulation.

Animals have evolved diverse reproductive adaptations to thrive in various environments. Mammals nurture offspring internally, while birds lay hard-shelled eggs. Amphibians and fish often use external fertilization, and invertebrates showcase a wide range of strategies to maximize reproductive success.

Fertilization and Embryonic Development

Internal vs external fertilization

• Internal fertilization
  • Sperm deposited into the female reproductive tract
  • Fertilization occurs inside the female's body requires copulation between male and female
  • Ensures higher survival rate of offspring by providing protection and nourishment (mammals, birds, reptiles)
  • Typically occurs in the oviduct, where sperm and egg meet
• External fertilization
  • Eggs and sperm released into the external environment
  • Fertilization occurs outside the bodies of both parents requires synchronization of gamete release
  • Higher number of offspring produced to compensate for lower survival rates due to exposure to predators and environmental factors (many fish, amphibians, marine invertebrates)

Gamete Formation and Fertilization

• Meiosis produces haploid gametes (sperm and egg)
• Fusion of haploid gametes results in a diploid zygote
• Zygote undergoes implantation in the uterus wall for further development

Stages of prenatal development

• Cleavage
  • Rapid mitotic cell divisions without growth produces a solid ball of cells called a morula
  • Increases the number of cells while maintaining the same overall size
• Blastulation
  • Formation of a fluid-filled cavity called the blastocoel embryo becomes a hollow ball of cells called a blastula
  • Marks the beginning of cell differentiation and the establishment of the inner cell mass
• Gastrulation
  • Rearrangement and migration of cells forms three primary germ layers: ectoderm, mesoderm, endoderm
  • Critical stage that establishes the basic body plan and sets the stage for organ development
• Organogenesis
  • Development of organs from the three primary germ layers
    1. Ectoderm gives rise to the nervous system and epidermis
    2. Mesoderm forms the skeleton, muscles, circulatory system
    3. Endoderm develops into the digestive system and associated organs
  • Involves complex cell signaling and tissue interactions to form functional organs
• Fetal development
  • Growth and maturation of the organs and tissues
  • Development of the placenta for nutrient and waste exchange between mother and fetus
  • Continued growth and preparation for birth marked by rapid increase in size and weight

Reproductive adaptations in animals

• Mammals
  • Internal fertilization and development provides protection and nourishment for the developing embryo
  • Mammary glands for nourishing offspring with milk ensures proper nutrition and immune support
  • Uterus for supporting embryonic and fetal development allows for prolonged gestation and live birth
• Birds
  • Internal fertilization ensures higher fertilization success and genetic diversity
  • Hard-shelled eggs with yolk for nourishment and protection during embryonic development
  • Parental care and incubation of eggs maintains optimal temperature and humidity for embryo survival
• Reptiles
  • Internal fertilization increases chances of successful fertilization
  • Amniotic eggs with shells for protection and preventing desiccation in terrestrial environments
  • Some species exhibit viviparity (live birth) as an adaptation to specific ecological niches (vipers, some skinks)
• Amphibians
  • External fertilization in most species allows for high fecundity and dispersal of offspring
  • Eggs laid in water or moist environments to prevent desiccation and provide oxygen for embryonic development
  • Larvae (tadpoles) undergo metamorphosis to develop into adults, an adaptation to exploit different ecological niches
• Fish
  • Most species have external fertilization, allowing for high fecundity and dispersal of offspring
  • Some species exhibit internal fertilization (sharks, guppies) as an adaptation for increased fertilization success
  • Eggs may be laid in nests or scattered in the water column, depending on the species and environmental conditions
• Invertebrates
  • Diverse reproductive strategies, including both internal and external fertilization, to suit various ecological niches
  • Some species have specialized structures for brooding or protecting eggs (egg cases in cephalopods, egg sacs in spiders)
  • Asexual reproduction is common in some groups (cnidarians, sponges), allowing for rapid colonization of new habitats