Egg Composition and Formation - Anatomy & Physiology

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Schematic Diagram of a Chicken Egg

^{©RVC 2008}

Size

• Varies greatly between species.
• Altricial species lay much smaller eggs than Precocial species.
  • Precocial: Chicks have natal down,hatch with their eyes open and can survive outside the nest within 1-2 days.
  • Altricial: Chicks are born blind and nakes,require long periods of feeding.

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Shell

The shell provides physical protection. It protects the embryo from microorganisms and regulates evaporation so that the embryo does not become dessicated. The shell is also a source of calcium carbonate, which is important for bone formation in the embryo.

Shell membranes

• Organised into an inner and outer layer which lie in close apposition.
• Part at one end to form the air cell, where the hatching chick draws its first breath.
• Meshwork of protein fibres (protein and glycoprotein) with minor constituents collagen, hydroxyproline and hydroxylysine.
• Prevent dessication and infection.

Testa

• Main thickness of the shell
• Matrix fibres and calcium carbonate
• Calcium carbonate layes contains pores to allow gas exchange.
• Pigmented by porphyrin and biliverdin

Cuticle

• Water repellent
• Barrier to infection

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Yolk

• Protein and lipids are synthesized in the liver and travel to the oocyte in the ovary, where they are made into yolk (vitellogenesis).
• Principle stored nutrients, provides main nutrient for the embryo.
• Produces water and energy, allowing survival in arid environments.
• Thick and viscous
• Contains maternal antibodies, primarily IgG.

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Albumin

• Less viscous than yolk
• Composed mainly of water and protein.
• Buffers the embryo from sudden changes in temperature.
• Serves as a shock-absorber to protect the embryo.
• Thin layer encloses the yolk membranes and suspends the yolk in the centre of the egg by twisted strands called Chalazae.

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Germinal Disc

• Small, circular, white spot on the surface of the yolk.
• Contains cytoplasm and the oocyte.

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Embryonic Membranes

Within the egg, three extraembryonic membranes support the life & growth of the embryo:

Amnion

• Surrounds only the embryo
• Inner layer of cells secretes amniotic fluid in which the embryo floats.
• Fluid keeps the embryo from drying out and provides protection.

Chorion

• Surrounds all embryonic structures and serves as a protective membrane (chorioallantoic membrane).
• Also important for:
  • Transpiration
  • Metabolism
  • Waste collection
  • Calcium transfer from the shell to the embryo.

Allantois (or allantoic sac)

• Grows larger as embryo grows
• Fuses with the chorion and is called the chorio-allantoic membrane.
• Works together with chorion to permit respiration (exchange of oxygen and carbon dioxide) and excretion.
• Important in storage of nitrogenous wastes (uric acid).

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