Blastula, or blastocyst, is the term used to describe an early stage in the development of an embryo. At this stage, the embryo consists of a hollow ball of cells enclosing a cavity called the blastocele. A fluid-filled cavity forms between one side of the inner cell mass and the outer cell mass, and the zona pellucida, a jellylike layer of cells covering a newly released egg, begins to disintegrate. The cells of the blastula divide as it floats free in the uterus for one or two days.
About the fifth or sixth day of pregnancy, the blastula becomes attached to the internal surface of the uterus. The outer cells of the blastula, called the trophoblast, secrete an enzyme that breaks down the lining of the uterus. The trophoblast begins to divide rapidly, invading the uterine tissue. The process of attachment to the uterine wall is called implantation. By the 11th day of the pregnancy, the blastula is firmly implanted in the uterus. Embryonic stem cells can be grown in the laboratory from the blastula and made to differentiate into nerve, liver, muscle, blood, and other cells.
See also Coelom ; Reproduction, Human (The first days of pregnancy) ; Stem cell .
