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Organ system development
Development of the cardiovascular system
Development of the ear
Development of the eye
Development of the face and palate
Pharyngeal arches, pouches, and clefts
Development of the digestive system and body cavities
Development of the gastrointestinal system
Development of the teeth
Development of the tongue
Development of the integumentary system
Development of the axial skeleton
Development of the limbs
Development of the muscular system
Development of the nervous system
Development of the renal system
Development of the reproductive system
Development of the respiratory system
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The respiratory system starts developing during week 4 of intrauterine life, and it begins when a lung bud sprouts out of the primitive digestive tract.
The lung bud is an outgrowth of the foregut portion of the digestive tract, and it turns into lower respiratory tract structures such as the larynx, trachea, and lungs.
Around week 4, the embryo has developed all three embryonic germ layers, and of the three, respiratory tract structures arise from the endoderm and mesoderm.
The larynx starts developing at the beginning of week 4, as nothing more than a slit between the fourth and the sixth pharyngeal arches.
The pharyngeal arches are paired, symmetrical embryonic structures that sprout from the foregut and arch towards the embryo’s front midline.
They consist of a mesoderm core and that mesoderm is made up mostly of mesenchyme—a soupy, fetal tissue that eventually turns into circulatory tissue, lymphatic tissue, and musculoskeletal tissue.
The arches are covered on the outside by the pharyngeal cleft, which is made of ectoderm, and lined on the inside with the pharyngeal pouch, which is made of endoderm.
In fish, these arches develop into gills, but in our case they serve as a foundation for many important structures around the head and neck.
The endoderm of the 4th and 6th pharyngeal arches forms the laryngeal epithelium and glands, and the mesoderm forms the laryngeal cartilages, while the arches themselves carry the laryngeal branches of the vagus nerve to these structures.
The respiratory system develops during week 4 of gestation, where it is initially with a primitive gut tube, which later becomes the gastrointestinal system. At around week 4, the respiratory diverticulum detaches as an outpocketing on the proximal part of the primitive gut tube.
A longitudinal ridge called the tracheoesophageal septum forms between the gut tube and the respiratory diverticulum, becoming two separate structures. The respiratory diverticulum later gives lung buds, from which the lungs and trachea develop. The larynx arises from the fourth and sixth pharyngeal arches around week 12.
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