Embryogenesis: Fertilization

• Embryogenesis is the process of embryo development.

Fertilization

• Embryonic life commences with fertilization, which involves the fusion of an oocyte and sperm.
• Sperm passes through the corona radiata and penetrates the zona pellucida via acrosome secretion.
• The sperm head attaches to the oocyte surface; their plasma membranes fuse at the contact point.
• Sperm's head and tail then enter the oocyte's cytoplasm.
• Oocyte reacts by changing the zona pellucida to prevent entry by more sperm.
• The second oocyte completes the second meiotic division, expelling the second polar body and becoming the female pronucleus.
• Sperm tail degenerates, and the head enlarges into the male pronucleus.
• Male and female pronuclei come together at the center, lose nuclear membranes, and maternal/paternal chromosomes intermingle in the first mitotic division of the zygote, ending fertilization.
• At ovulation, oocyte transported by fimbriae of the uterine tube sweeping the ovum into the infundibulum.
• Oocyte moves to the ampulla of the uterine tube with cilia beating and muscular contraction.
• Sperm swims through uterus to uterine tubes where fertilization occurs, usually in the ampulla (widest part).

Cleavage

• As the zygote passes down the uterine tube it undergoes rapid mitotic cell divisions known as cleavage.
• Division into two daughter cells called blastomeres initiates the second stage of development approximately 30 hours post-fertilization.
• Following divisions occur rapidly, creating a solid mass of 16 cells (morula) about 3 days later.
• Fluid enters the morula from the uterine cavity, occupying intercellular spaces; this fluid increase separates cells into two parts.
• These parts create the trophoblast (outer cell layer providing nutrition for the placenta) and the inner cell mass (embryoblast).
• The fluid-filled space fuses into a blastocyst cavity which converts the morula into a blastocyst.
• The Blastocyst lies free in uterine secretions for ~2 days, the zona pellucida degenerates disappearing between days 5-6, and the blastocyst attaches to endometrial epithelium.
• The endodermal layer, the first of three germ layers, forms during the first week of uterine life.

Embryogenesis begins with fertilization, where sperm and oocyte fuse. The sperm penetrates the oocyte, triggering reactions to prevent further sperm entry. Male and female pronuclei merge, leading to the first mitotic division of the zygote.