Embryology is the study of the development of an organism from its earliest stages. The process begins with fertilisation, the process of male sperm fusing with the female ovum. This forms a single celled zygote, which then undergoes rapid growth and division. The zygote becomes a blastocyst, which contains the inner cell mass (ICM) and an outer cell mass (trophoblast).
The ICM undergoes a highly specialised transformation known as gastrulation, during which the bilaminar disk of the ICM is rearranged to form the trilaminar disk. This disk consists of ectoderm, mesoderm and endoderm, the three germ layers which are the first layers of the embryo. The ectoderm forms the skin, which is innervated by spinal nerves. This “single area of skin innervated by a single spinal nerve” is known as a dermatome. Likewise, “a group of muscles innervated by a single spinal nerve” is called a myotome.
The cardiovascular system is one of the early systems to take shape within the embryo. It begins as a primitive heart tube and subsequently folds laterally to form an input and output. The cephalocaudal folding of the heart tube places it in the thorax, and further folding of the heart results in the creation of the four chamber heart system. This consists of two atria and two ventricles. The looping of the heart tube causes the primitive atria to shift superiorly and posteriorly, while the primitive ventricles move inferiorly and anteriorly.
The development of the face, head and neck are closely interconnected and depend on special structures known as pharyngeal arches. These arches are derived from mesenchyme, in addition to the frontonasal prominence which plays an important role in facial formation. The reproductive and urinary system share a caudal endpoint known as the cloaca and both are derived from the intermediate mesoderm of the gonadal ridge. The respiratory system is formed from a diverticulum of the oesophagus and the primitive gut tube. The trachoesophageal septum separates the two, resulting in the formation of the lungs.
In summary, the process of embryonic development involves specialised transformations of the zygote, including gastrulation, which develops the embryo into its three germ layers. Subsequent development of the cardiovascular system, reproductive system, urinary system, respiratory system and facial, head and neck structures, occur in an intricate and delicate pattern. These processes lead to the formation of the adult organism.
