Biological Development and Fertilization: Key Concepts and Stages

What is the definition of development in a biological context?

All changes that occur during the entire life cycle of an organism, including embryonic development and stages after birth or hatching.

What are the three primary results of fertilization?

Restoration of the diploid (2n) number, determination of sex, and activation of the egg to stimulate developmental reactions.

What are the four major steps in the fertilization process?

Sperm dissolves protective layers, sperm binds to surface receptors, changes occur to the egg surface to prevent polyspermy, and nuclei fuse.

What are the three layers surrounding a mammalian egg, from inside to outside?

Plasma membrane, zona pellucida (glycoprotein matrix), and follicle cells.

What occurs during the acrosomal reaction in mammals?

Sperm pushes through follicle cells, binds to receptors in the zona pellucida, and the acrosome bursts to release enzymes that aid in penetrating the zona pellucida.

What triggers the calcium wave in an egg during fertilization?

Plasmogamy (the fusion of sperm and egg plasma membranes) triggers the release of Ca2+ from the smooth ER.

What is the function of the cortical reaction?

Cortical granules fuse with the plasma membrane and release enzymes that trigger the slow block to polyspermy by hardening the zona pellucida.

How does the slow block to polyspermy prevent multiple sperm from fertilizing an egg?

Enzymes catalyze changes in the zona pellucida, causing it to harden and altering sperm receptors so no additional sperm can bind.

What metabolic changes occur during egg activation?

Increased respiration, activation of maternal enzymes and proteins, and the initiation of protein synthesis using pre-existing mRNA.

At what stage of meiosis is a secondary oocyte arrested before fertilization?

Metaphase II.

What is karyogamy?

The process of nuclear fusion where microtubules guide the sperm nucleus to the egg nucleus to form a zygote.

What is the definition of a totipotent cell?

A cell that has the potential to give rise to all cell types in an organism.

When is fertilization considered complete?

When the zygote undergoes its first cell division.

What is the primary characteristic of the cleavage stage in development?

A series of rapid cell divisions without overall growth of the embryo.

How does yolk distribution affect cleavage patterns?

Yolk acts as food for the embryo; little yolk allows for faster, equal divisions, while high amounts of yolk lead to slower, unequal divisions.

What is a blastula?

A hollow embryo characterized by the presence of a fluid-filled cavity called the blastocoel.

What characterizes equal holoblastic cleavage?

Complete division of the zygote resulting in cells of equal size and a central blastocoel, seen in echinoderms and mammals.

What characterizes unequal holoblastic cleavage?

Complete but asymmetric cleavage due to uneven yolk distribution, which slows down cytokinesis, as seen in amphibians.

What is meroblastic cleavage?

A pattern of cleavage where only the regions of the egg without yolk undergo complete division, common in reptiles, birds, and insects.

Why is the zona pellucida important in mammalian fertilization?

It acts as a barrier to interspecific fertilization and contains receptors that ensure species-specific sperm binding.

What is the role of the blastocoel?

It is the central, fluid-filled cavity within a blastula that forms during the cleavage stage.