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Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth. During organogenesis, the three germ layers formed from gastrulation (the ectoderm , endoderm , and mesoderm ) form the internal organs of the organism.
Human embryology is the study of this development during the first eight weeks after fertilization. The normal period of gestation (pregnancy) is about nine months or 36 weeks. The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus .
Human organs and organ systems develop in a process known as organogenesis. This begins in the third week of embryonic development, when the gastrula forms three distinct germ layers, the ectoderm, mesoderm and endoderm. The ectoderm will eventually develop into the outer layer of skin and nervous system.
Neurulation forms the nervous system, and organogenesis is the development of all the various tissues and organs of the body. A newly developing human is typically referred to as an embryo until the ninth week after conception, when it is then referred to as a fetus.
A germ layer is a primary layer of cells that forms during embryonic development. [1] The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans (animals that are sister taxa to the sponges) produce two or three primary germ layers.
Several signals mediate the organogenesis of the ectoderm such as: FGF, TGFβ, Wnt, and regulators from the hedgehog family. The specific timing and manner that the ectodermal organs form is dependent on the invagination of the epithelial cells. [9] FGF-9 is an important factor during the initiation of tooth germ development.
The human embryo forms five sets of endoderm-lined pharyngeal pouches. The third and fourth pouch are responsible for developing into the inferior and superior parathyroid glands, respectively. [ 8 ] The third pharyngeal pouch encounters the developing thyroid gland and they migrate down to the lower poles of the thyroid lobes.
The human embryonic heart displays cardiac activity approximately 21 days after fertilization, or five weeks after the last normal menstrual period (LMP), which is the date normally used to date pregnancy in the medical community.