Human embryogenesis

Human embryogenesis is the process by which the embryo forms and develops. It starts with the fertilization of the egg cell (ovum) by a sperm, a process that leads to the formation of a zygote. This zygote then undergoes rapid mitotic divisions, a process known as cleavage. Embryogenesis covers the first eight weeks of development; at the beginning of the ninth week, the embryo is termed a fetus.

Fertilization and the zygote

Fertilization is the fusion of the sperm and egg cell to form a zygote. This process occurs in the fallopian tube and results in the formation of a single cell entity with 46 chromosomes, half from each parent. The zygote begins to divide by mitosis, a process known as cleavage.

Cleavage and blastocyst formation

The zygote undergoes rapid cell divisions with no significant growth, producing a cluster of cells that is the same size as the original zygote. The cells divide into a solid ball of cells, a morula, and then into a hollow ball of cells, 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.

Gastrulation

Gastrulation is a significant stage in embryogenesis where the embryo transforms from a blastocyst with a single layer of cells, into a structure with multiple layers of cells. These layers are the ectoderm, mesoderm, and endoderm, which will eventually form all the tissues and organs of the body.

Organogenesis

Organogenesis is the formation of organs during human embryogenesis. It follows the stage of gastrulation and results in the formation of the major organs of the human body. This includes the heart, lungs, liver, kidneys, nervous system, and digestive system.

Neurulation

Neurulation refers to the folding process in vertebrate embryos, which includes the transformation of the neural plate into the neural tube. The neural tube will eventually form the brain and spinal cord.

See also

• Developmental biology
• Prenatal development
• Gestational age
• Fetus