Prenatal development

Prenatal development is the process in which an embryo or fetus gestates during pregnancy, from fertilization until birth. After fertilization the embryogenesis starts. In humans, when embryogenesis finishes, by the end of the 10th week of gestation, the precursors of all the major organs of the body have been created.

Fertilization

 * Main Article: Fertilization

Fertilization is the union of an ovum and sperm cell. It comprises a sequence of events that begins with the contact of a sperm cell with an egg cell and ends with the fusion of their two pronuclei to form a new diploid cell, called a zygote.

Sperm are equipped with flagellum that allows them to travel to the ovum. In cases of external fertilization, both ovum and sperm and frequently released into water and fertilization is not possible in drought conditions for the specific reason that motility requires an aqueous condition. During internal fertilization in humans, sperm must travel through the cervix, uterus, and then up the uterine tube. Smooth muscle contractions in the uterine tubes as well as ciliary activity (waving of hair-like structures) of the tube’s lining both are important in the transport of sperm up, and of the ovum into and then down, the uterine tube.

Frequently tens or even hundreds of millions of sperm are ejaculated, but only one fertilizes an ovum. Those sperm that do come into the vicinity of the ovum must get through the material covering the ovum (the corona radiata and the zona pellucida) and finally contact and bind to the ovum’s membrane, by means of specialized structures in the head of the sperm cell. When a sperm does get into the ovum, then the ovum membrane changes so that other sperm cannot enter. Meanwhile, the sperm cell in the egg is also undergoing changes and its specialized structures fall away. Prior to nuclear union, the haploid nuclei of both the sperm and the egg are called male and female pronuclei. Both swell, as their densely packed DNA loosens up prior to replication, and they also migrate toward the center of the ovum. Then their nuclear membranes disintegrate and the paternally and maternally contributed chromosomes pair up, an event called syngamy. At this time, the chromosome number is restored, and a new complete genome comes into being. The result of syngamy is an entity with an individual genome. The fertilized egg is now called a zygote. It is at this point already entering the first stage of its first mitotic division, and beginning cleavage.

Embryogenesis

 * Main Article: Embryogenesis

Embryogenesis begins when a sperm fertilizes an egg and creates a single cell that has the potential to form an entire organism. In the first hours after fertilization, this cell divides into identical cells. Approximately 4 days after fertilization and after several cycles of cell division, these cells begin to specialize, forming a hollow sphere of cells, called a blastocyst. The blastocyst has an outer layer of cells, and inside this hollow sphere, there is a cluster of cells called the inner cell mass. The cells of the inner cell mass will go on to form virtually all of the tissues of the human body.

The blastocyst has an outer layer of cells, and inside this hollow sphere, there is a cluster of cells called the inner cell mass. The cells of the inner cell mass will go on to form virtually all of the tissues of the human body. Although the cells of the inner cell mass can form virtually every type of cell found in the human body, they cannot form an organism. Therefore, these cells are referred to as pluripotent, that is, they can give rise to many types of cells but not a whole organism. Pluripotent stem cells undergo further specialization into stem cells that are committed to give rise to cells that have a particular function. Examples include blood stem cells that give rise to red blood cells, white blood cells, and platelets, and skin stem cells that give rise to the various types of skin cells. These more specialized stem cells are called multipotent—capable of giving rise to several kinds of cells, tissues, or structures.

Pregnancy

 * Main Article: Pregnancy