Embryonic Development: Stages, Fertilization, and Processes

Stages of Embryonic Development

A key feature that differentiates living things from non-living things is the ability to reproduce. Reproduction occurs through cell division. In more complex organisms, this includes sexual and asexual reproduction. The essential phenomenon is mitosis in plants and animals. Sexual reproduction involves meiosis, the process of gamete formation, or specialized cells for reproduction, with the participation of individuals of different sexes. Genetic and hormonal events, along with synchronization mechanisms, ensure the release of gametes for successful fertilization. This is followed by a series of events that characterize successive development and differentiation, transforming the egg or zygote into an adult individual.

Sexual Reproduction and Gamete Production

Individuals with specialized organs produce sex cells, or gametes. Most animals produce heterogametic gametes, meaning different types of gametes, each with a specific function.

• The Spermatozoon: The male gamete is produced by meiosis in the animal testicles.
• The Egg: The female sex cell is also produced by meiosis in the ovules. The human egg has a small diameter of 0.2 to 0.5 mm.

There is a direct relationship between egg size and the type of embryonic development. Larger eggs contain enough nutrients for near or total development of the embryo. In mammals, development relies on reserves that are very short, and the embryo soon begins to receive food through the mother's blood.

Fertilization

Fertilization is the union of the haploid nuclei of the sperm and egg to form the diploid nucleus of the zygote. This process is secured by:

• Chemical agents that stimulate sexual activity.
• Environmental stimuli, such as changes in season or day length.
• Mating, the union between male and female.

Fertilization can occur within the female body (internal fertilization) or through the encounter of gametes outside the female's body (external fertilization).

The Fertilization Process

The fertilization process can be studied in three stages:

1. The encounter between the egg and sperm. Eggs have a layer called the zona pellucida, surrounded by the corona radiata. An enzyme produced by the sperm acts on this area. Upon penetration, changes occur in the surface layer of the egg, preventing further sperm entry.
2. The activation of the egg.
3. The fusion of the egg nucleus and sperm, giving rise to the zygote.

Fertilization in Humans

In humans, fertilization occurs in the upper part of the fallopian tubes. About a week after fertilization, the morula reaches the uterus, where implantation occurs in the thick wall of the endometrium. The embryonic development process continues from there.

Processes of Development

The term 'development' is used with two different meanings:

• Individual Development: The processes involved in the transformation of a fertilized egg, or some other vestige of a parent organism, into a new adult organism.
• Phylogenetic Development: The gradual historical transformation of life forms, from the simplest to the current diversity of life on our planet. This is also called evolution.