Amnion With Fetus Inside
The specimen shows the amniotic cavity with the developing fetus inside. The amnion is a membrane that closely covers the embryo when first formed. It fills with the amniotic fluid which causes the amnion to expand and become the amniotic sac which serves to provide a protective environment for the developing embryo or fetus. The amnion, along with the chorion, the yolk sac and the allantois form a protective sac around the embryo.
The amniotic fluid allows the free movements of the fetus during the later stages of pregnancy, and also protects it by diminishing the risk of injury from outside. It contains less than two percent solids, consisting of urea and other extractives, inorganic salts, a small amount of protein, and frequently a trace of sugar.
The amnion is an extraembryonic membrane that surrounds a developing embryo. It acts as a protective sac along with three other extraembryonic membranes: the chorion, the yolk sac, and the allantois.
An embryo is the early stage of development of a multicellular organism. Embryonic development is the part of the life cycle that begins just after fertilization and continues through the formation of body structures, such as tissues and organs, i.e., from conception to eighth week of development. After the eighth week of development, it is called the fetus until birth.
The amniotic fluid is a protective liquid contained by the amniotic sac of a developing embryo or fetus. This fluid serves as a cushion for the growing fetus, but also serves to facilitate the exchange of nutrients, water, and biochemical products between mother and fetus. Excessive increase in the amount of amniotic fluid is called polyhydramnios and the abnormal decrease is called oligohydramnios.
The chorion is a double-layered membrane formed by the trophoblast and the extra-embryonic mesoderm, which eventually will give rise to the fetal part of the placenta.
The yolk sac is the first fetal membrane to be formed in all mammals and is the oldest of the extra-embryonic membranes. It is endodermal in origin and covered with extra-embryonic mesoderm. The yolk sac lies outside the embryo connected by a yolk stalk (vitelline duct, omphalomesenteric duct) to the mid gut.
The allantois is derived from splanchnopleure (endoderm and splanchnic mesoderm). It arises as a diverticulum of the hindgut and gradually fills the entire extraembryonic coelom in most species.