Describe Fertilisation
Sexual intercourse of a male and a female is normally the way in which a sperm and egg cells meet although artificial insemination can also be possible. Through this sexual intercourse, there is a penetration of the penis inside the vagina. Therefore, sperm is deposited in to vagina through ejaculation, which is a reflex via muscle contractions.
Normally, the vagina kills most sperm but a few hundred make it to the fallopian tubes via the cervix and uterus. The sperm that makes it there in time penetrates the egg and the egg then disallows any other sperm to enter. The other sperm die.
The 23 egg chromosomes and 23 sperm chromosomes form a full amount of chromosomes to create a human. This is initially referred to as a zygote. On day 1 and 2 of this process, the zygote begins to divide through mitosis, and a 4 cell embryo is formed. On day 5 the cells form something called a blastocyst which is what will eventually attach to the uterus wall. By day 10 it is implanted. This is when the process of precidualisation and decidualisation occurs.
After ovulation, the endometrium becomes more vascularised, and produces cells that coat the uterine cavity called decidual cells. If pregnancy takes place, the decidualisation process begins. Decidual cells fill with lipids and glycogen. The outer wall is termed the trophoblast and creates a placenta.
The inner area of the cell mass creates 3 or 4 blastocyst cells which form the yolk sac and amniotic cavity. The chorion develops from the trophoblast to create a placenta. The surface of the chorion is then coated with chorionic villi which attaches the placenta to chorion. The inner cells form 3 germ layers, the mesoderm, endoderm and ectoderm. These create individual parts of the foetus, such as the various organ systems.( Biology-online. org 2000)
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