Model Answer
0 min readIntroduction
Fertilization in the chick, like in other avian species, is a complex internal process that initiates embryonic development. Unlike many mammals, avian fertilization is characterized by "physiological polyspermy," where multiple sperm penetrate the ovum, although only one ultimately fuses with the egg nucleus to form a viable zygote. This process occurs in the infundibulum of the hen's oviduct, shortly after ovulation. Understanding these stages is fundamental to avian reproductive biology and provides insights into the unique developmental strategies of birds. The chick embryo serves as a classic model for studying vertebrate development due to its accessibility and significant development prior to laying.
Stages of Fertilization in Chick
Fertilization in the chick occurs internally within the infundibulum of the oviduct. The process is characterized by several distinct stages, including sperm transport, sperm-egg interaction, entry of multiple sperm (physiological polyspermy), and finally, syngamy.1. Sperm Transport and Storage
Following copulation, sperm from the male are transferred to the female's cloaca (often through a "cloacal kiss") and then travel up the oviduct. The hen's oviduct has specialized sperm storage tubules (SSTs), particularly in the infundibulum and utero-vaginal junction, where sperm can be stored for extended periods, sometimes up to several weeks. This storage ensures a continuous supply of viable sperm for multiple ovulations.
2. Ovulation and Ovum Release
The ovum, consisting primarily of the large yolk (oocyte) surrounded by the vitelline membrane, is released from the ovary into the infundibulum of the oviduct. This release is crucial for initiating the fertilization process, as sperm are released from the SSTs in response to the ovum's presence.
3. Sperm-Egg Interaction and Penetration of the Perivitelline Membrane (PVM)
- Initial Contact: Sperm encounter the ovum in the infundibulum. The outer layer of the ovum, the perivitelline membrane (PVM), is the primary barrier.
- Acrosome Reaction: While the exact timing and nature of the acrosome reaction in birds differ from mammals, sperm undergo changes to enable penetration. Sperm possess enzymes that help in digesting glycoproteins of the PVM.
- PVM Penetration: Multiple sperm typically penetrate the PVM, creating small holes. This is a characteristic feature of avian fertilization, known as physiological polyspermy.
4. Entry of Sperm into the Germinal Disc (Blastodisc)
After penetrating the PVM, the sperm reach the germinal disc (also known as the blastodisc or germ disc), a small, circular, white spot on the surface of the yolk containing the female pronucleus. In chicks, anywhere from 20 to 60 sperm are typically found within the egg cytoplasm at fertilization, though this number can vary. This physiological polyspermy is essential for normal embryonic development in birds, unlike in most mammals where it is lethal.
5. Cortical Reaction and Prevention of Further Sperm Entry (Functional Monospermy)
Despite multiple sperm entering the ovum, only one sperm nucleus will ultimately fuse with the female pronucleus. The mechanism for this "functional monospermy" is not a typical polyspermy block at the egg surface as seen in mammals. Instead, avian eggs regulate the fate of supernumerary sperm nuclei. These extra sperm nuclei often move towards the periphery of the germinal disc and degenerate, or undergo mitosis without contributing to the zygote's genetic material.
6. Syngamy and Zygote Formation
The nucleus of one successful sperm fuses with the female pronucleus (amphimixis) to form the diploid zygote. This event signifies the completion of fertilization and initiates embryonic development. The fertilized blastodisc (now called the blastoderm) will then undergo cleavage as the egg travels down the oviduct, with layers of albumen, shell membranes, and the shell being added around it.
Diagram of Fertilization in Chick
[**Please imagine a diagram here illustrating the following:**]
Figure: Stages of Fertilization in a Chick Egg
The diagram would show a cross-section of the chick ovum (yolk) in the infundibulum.
- An outer layer representing the perivitelline membrane (PVM).
- The central yolk mass with the germinal disc (blastodisc) containing the female pronucleus.
- Multiple sperm cells approaching and penetrating the PVM.
- Several sperm cells entering the germinal disc.
- One sperm nucleus fusing with the female pronucleus.
- Supernumerary sperm nuclei visible at the periphery, in the process of degeneration.
- Labels would include: Ovum (Yolk), Germinal Disc (Blastodisc), Female Pronucleus, Perivitelline Membrane (PVM), Sperm, Fusing Sperm Nucleus, Supernumerary Sperm Nuclei.
The fertilized egg, now undergoing initial cleavage, continues its journey through the oviduct, where the albumen (egg white), shell membranes, and hard calcareous shell are sequentially added, taking approximately 20-24 hours before the egg is laid.
Conclusion
The fertilization process in chicks is a unique and highly coordinated series of events occurring within the hen's oviduct. Characterized by physiological polyspermy, it highlights a distinct evolutionary strategy compared to the strict monospermy observed in most mammals. The entry of multiple sperm, followed by the selective fusion of only one, ensures robust embryonic initiation, with supernumerary sperm playing a functional role in early embryo survival. This intricate mechanism underscores the adaptive diversity in reproductive biology and the complex interplay between male and female gametes to ensure the successful propagation of avian species.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.