Embryology Problem Based Questions (Langman - Q7)

ANATOMY AIIMS, GROSS ANATOMY, EMBRYOLOGY, NEUROANATOMY, MICROANATOMY, APPLIED/ CLINICAL ANATOMY

What are the three phases of fertilization, and what reactions occur once fusion of the sperm and oocyte membranes takes place?

Answer:

The phases of fertilization include

● Phase 1, penetration of the corona radiata

● Phase 2, penetration of the zona pellucida

● Phase 3, fusion of the oocyte and sperm cell membranes

After fusion of membranes-

Cortical and zona reaction

Resumption of second meiotic division

Metabolic activation of egg

During the fertilization process, there are several molecular and cellular events that occur in a specific sequence to ensure successful fusion and formation of the zygote. Here are some additional details about the different phases of fertilization:

Penetration and Activation:

After reaching the oocyte, the sperm binds to its receptors, which triggers intracellular signaling cascades leading to the release of calcium ions in the oocyte. The sudden influx of calcium ions into the oocyte cytoplasm is a critical step that activates the oocyte and initiates the cortical reaction.

The cortical reaction causes the release of cortical granules from the oocyte's cortex, which modifies the zona pellucida to make it impermeable to other sperm. The cortical granules also cause the oocyte's membrane to separate from the zona pellucida, preventing any further sperm from entering.

Fusion:

After the cortical reaction, the sperm undergoes the acrosome reaction, which involves the release of enzymes that digest the zona pellucida, enabling the sperm to penetrate the oocyte. Once the sperm penetrates the zona pellucida, it binds to the oocyte membrane, and the two membranes fuse. The fusion of the sperm and oocyte membranes allows the sperm to enter the oocyte's cytoplasm and initiate oocyte activation.

Activation of the Oocyte:

Oocyte activation involves a series of events that prepare the oocyte for fertilization and subsequent embryonic development. After the fusion of the sperm and oocyte membranes, the sperm nucleus enters the oocyte's cytoplasm and forms the male pronucleus. At the same time, the oocyte's meiotic spindle reorganizes, and the second meiotic division is completed, forming the female pronucleus.

The male and female pronuclei then migrate towards each other and fuse, forming the diploid zygote. This process is essential as it leads to the formation of a genetically distinct individual. The newly formed zygote undergoes rapid cell division, forming the morula, which then develops into a blastocyst that implants in the uterus.

References:

Wassarman, P. M. (2017). Fertilization. Current Biology, 27(17), R870-R875.

Ducibella, T., Fissore, R., & Heindryckx, B. (2019). The cortical reaction and development of activation competence in mammalian oocytes. Journal of Assisted Reproduction and Genetics, 36(2), 221-231.

Johnson, M. H., & Everitt, B. J. (2018). Essential reproduction (7th ed.). Hoboken, NJ: John Wiley & Sons. Chapter 4, Fertilization.