Chapter 12. Fertilization - Review of Medical Embryology Book

Definition: fertilization is the union of the male (sperm) and female (oocyte) gametes to form a zygote and marks the beginning of pregnancy. Embryonic life begins with fertilization. Fertilization process requires about 24 hours
Maturation of a follicle takes place in the ovary from an oocyte to a graafian follicle
Ovulation coincides with the first maturation division and with the elimination of the first polar body. The ovum is "captured" by the ampulla of the uterine tube whose fimbriae sweep over the ovary
Fertilization takes place in the distal third of the uterine tube. Spermatozoa arrive about 10hours after coitus. The ovum must be fertilized within 24 hours after ovulation

Dispersal of cells in vitro is the result of enzymatic action of tubal mucosa and semen. Sperm tail movements also help penetration of corona and zona pellucida

Sperm penetrates the zona pellucida: digests a path by action of enzymes released from its acrosome

Only 1 sperm enters the oocyte and fertilizes it, even though several may penetrate the zona pellucida
Two sperm may take part in fertilization during an abnormal process called dispermy resulting in a triploid embryo (69 chromosomes), but it nearly always aborts or dies shortly after birth
If 2 female pronuclei take part in fertilization, it is called polygyny

Sperm head attaches to surface of the oocyte, plasma membranes of oocyte and sperm fuse, and then break at contact point

Head and tail of sperm enter oocyte cytoplasm with sperm's plasma membrane being attached to oocyte's plasma membran Once inside the cytoplasm of the oocyte, the sperm tail degenerates

Zonal reaction: change in zona pellucida inhibits entry of more sperm, due to substance of oocyte cytoplasm
Secondary oocyte completes second meiotic division and its chromosomes (22 plus X) arrange themselves in a vesicular nucleus called the female pronucleus. The second polar body is extruded

Sperm head enlarges and forms the male pronucleus
The male and female pronuclei approach each other in the oocyte center, meet, and lose their nuclear membranes. They resolve their chromatin into a complete single haploid set of chromosomes which become organized on a spindle
After the maternal and paternal chromosomes intermingle, metaphase of the first cleavage mitosis takes place, and the normal chromosome number is reconstituted
Anaphase of the first cleavage mitosis then occurs
The first 2 blastomeres are next seen, following cell division, and they are surrounded by the zona pellucida