Notes on Chicken Embryonic Development

Chicken Embryonic Development

Fundamentals of Fertilization and Initial Development

• Conception: Embryonic development begins immediately after fertilization.
  • Sperm (male) fuses with ovum/egg (female).
  • Both contribute $50\%$ of genetic information, forming a single cell.
• Cell Division: This single cell undergoes rapid division:
  • $1 \to 2 \to 4 \to 8 \to 16 \to 32 \to 64 \to 128$, continuing until thousands of cells are formed.
• Pre-Laying Development: Cell division starts inside the hen's body (specifically, in the oviduct) before the egg is laid.
  • When a hen lays an egg, it already contains approximately $3,000$ cells that have begun developing.

Timeline of Embryonic Development

• Total Development Duration: Approximately $22$ days.
  • Internal Phase: $1$ day inside the hen's body (in the oviduct) right after fertilization.
  • External Phase (Incubation): $21$ days under favorable incubation conditions.
• Egg Laying Cycle: A hen typically takes around $24-26$ hours to lay an egg.

Embryonic Germ Layers and Organ Formation

• From a single cell, the embryo forms three primary germ layers during embryonic development:
  • Ectoderm: The outermost layer, which gives rise to:
    • Skin
    • Feathers
    • Beak
    • Claws
    • Eyes (specifically the lens and retina)
    • Nervous system
    • Mouth and band lining
  • Mesoderm: The middle layer, which develops into:
    • Muscles
    • Bones
    • Blood
    • Reproductive system
    • Excretory system
  • Endoderm: The innermost layer, which forms:
    • Digestive system
    • Respiratory system
    • Endocrine system
• All these organ systems are composed of millions of cells, stemming from the initial single cell that multiplied to form a multicellular organism.

Critical Role of Temperature in Incubation

• Delayed Development: Embryonic development should be delayed until the fertilized egg is placed in an incubator.
  • This is achieved by storing eggs at a temperature of $68^<br /> \circ$F or lower.
• Risks of Fluctuating Temperatures:
  • Any temperature above $68^<br /> \circ$F will cause the embryo to begin developing.
  • Fluctuations in temperature (start-stop-start development) lead to embryonic mortality.
  • Example: An egg left at $86^<br /> \circ$F for $3$ days without hen care will start developing, then stop, leading to embryo death.
• Egg Collection Practices:
  • Eggs are collected frequently to prevent exposure to fluctuating ambient temperatures.
  • Small farms: Daily collection.
  • Speaker's farm: Twice daily.
  • Large commercial farms: $4-6$ times daily.
• Optimal Incubation Temperature: Between $99.6^ \circ$F and $100.4^ \circ$F.
  • The speaker uses $99.6^<br /> \circ$F in their system.
• Hen's Body Temperature: A hen's body temperature ranges from $105^<br /> \circ$F to $107^<br /> \circ$F.
• Importance of Cool Rooms: To maximize hatchability, eggs should be immediately placed in a cool room after collection to slow down embryonic development until they are ready for controlled incubation.
• Incubation Systems: Discussed (but detailed comparison saved for later class):
  • Multi-stage incubation: Embryos of different ages in the same incubator.
  • Single-stage incubation: All embryos placed at the same time and hatched at the same time.

Extraembryonic Membranes and Their Functions

• Chicken embryos lack an anatomical connection to the hen (unlike mammalian fetuses). Therefore, all necessary nutrients (except oxygen) must be present within the egg itself.
• Key Extraembryonic Membranes:
  • Yolk Sac: Contains most of the nutrients. Features numerous blood vessels that break down nutrients and transport them to the embryo. Functions similarly to an umbilical cord.
  • Allantois: Serves multiple vital functions for the embryo:
    • Digestive system
    • Respiratory system
    • Excretory system
  • Amnion: The membrane that protects the embryo from physical impact.
  • Also mentioned: Vitelline and Chorion.
• Oxygen and Carbon Dioxide Exchange:
  • Oxygen is the only compound not found in the egg that the embryo requires directly from the environment.
  • The eggshell has approximately $20,000$ pores, allowing oxygen ($O<em>2$) to enter and carbon dioxide ($CO</em>2$) produced by the embryo to exit.

Fertility vs. Hatchability

• Fertility: Refers to the percentage of eggs that have been fertilized by sperm.
• Hatchability: Refers to the percentage of fertilized eggs that successfully develop and hatch by day $21$.
• Significance: Understanding the stages of embryonic development is crucial for diagnosing issues during incubation. Tables exist to correlate embryonic mortality at specific stages (e.g., first $5$ days, days $5-8$) with particular underlying problems in the incubation process.

Day-by-Day Stages of Embryonic Development

• Day 1: Appearance of whitish embryonic tissue (thousands of cells).
• Day 2: Visible tissue development and initial appearance of blood vessels.
• Day 3 (approx. $72$ hours): The heart becomes clearly visible, along with prominent blood vessels.
• Day 4: Eyes become pigmented; the embryo enters the