SEM02_Gametogenesis_and_Fertilization

In mammals, fertilisation takes place inside the fallopian tube, while in birds, only the left ovary and oviduct remain functional in adults.

In birds, oogenesis starts with a number of primary follicles, each containing a primary oocyte surrounded by a granulosa layer.

Follicular development in birds involves the accumulation of yolk without the formation of a fluid-filled follicle.

The oocyte increases in size and moves closer to the ovary surface as it accumulates yolk.

During ovulation, only the yolk is released from the ovary, while the rest of the egg components are added along the oviduct.

Fertilisation occurs at the beginning of the oviduct in birds, with multiple sperms typically binding and fusing with the oocyte, but only one reaching the female pronucleus and fusing with it.

In mammals, the zygote is the fertilised egg that results from the union of a sperm and an egg, and the single cell undergoes cleavage to become subdivided into smaller cells.

The zygote contains all the essential factors for development, but they exist in encoded genetic instructions within the chromosomes.

In birds, follicular development involves the production of oestrogen, which stimulates the liver to produce yolk material for the growing follicles.

The largest follicle produces high levels of progesterone, which induces the luteinising hormone surge and leads to ovulation.

The ovary in the breeding season resembles a bunch of grapes, while outside of the breeding season, it resembles a collection of millet seeds.

In mammals, fertilisation begins with sperm-zona pellucida binding and ends with the initiation of zygote cell division.

In birds, multiple sperms bind and fuse with the oocyte, but only one reaches the female pronucleus and fuses with it.

In mammals, the zygote undergoes a brief stage before cleavage, during which all the essential factors for development exist in encoded genetic instructions.

The oviduct reduces in size outside of the breeding season and the ovary resembles a collection of millet seeds.

The yolk in birds appears as a viscous fluid with suspended granules and globules as the oocyte increases in size.

The largest follicle in birds produces high levels of progesterone and induces ovulation.

The oocyte in birds is activated by the fusion of the sperm head with the oocyte plasma membrane and undergoes meiosis, giving rise to an ovum and a second polar body.

The male and female haploid nuclei (pronuclei) fuse in the zygote in birds, restoring the diploid condition in the zygote nucleus.

In birds, the oviduct adds the white (protein) and the shell to the egg as it passes along it.

In mammals, the zygote immediately cancels its membrane potential and undergoes the cortical reaction, releasing enzymes and preventing polyspermy.

In birds, multiple spermatozoa bind and fuse with the oocyte membrane, but only one reaches the female pronucleus and fuses with it, initiating amphimixis.

Both sperm and egg contribute DNA, but the egg provides most of the cytoplasm for the zygote.

Egg cytoplasm is rich in nutrients and organelles for embryo survival, including nucleic acids, proteins, and mRNAs.

Egg provides membranes that protect the embryo before implantation, produced by the egg itself or follicle cells.

Primary membrane of the zygote is equivalent to the cytoplasmic membrane.

In mammals, secondary membranes include the pellucid zone and corona radiata, and in birds, the vitelline membrane.

Pellucid zone is a specialized glycoprotein layer surrounding the oocyte and consists of four types of zona pellucida glycoproteins.

Corona radiata is a layer of cells adhering to the outer surface of the zona pellucida.

In birds, the secondary membrane is equivalent to the vitelline membrane, which plays a crucial role in species-specific fertilization.

Tertiary membranes are specific to egg-laying animals, including the white albumen and shell membrane.

White albumen is a protein-rich fluid that surrounds the yolk in the egg.

Shell membrane is a double membrane surrounding the albumen and shell, formed of keratin and protecting the egg.

Shell is the outer covering of eggs in land animals, made of calcium carbonate and porous for gas exchange.

Yolk is the nutritive material in the cytoplasm of the female gamete, providing nourishment for the embryo.

Depending on the quantity and distribution of yolk, eggs are classified as oligolecithal, mesolecithal, or polylecithal.

Yolk has influenced animal evolution by providing essential nutrients for embryonic development.