Development of the respiratory system

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.

All in all, these two arches give us our ability to talk. Doesn’t beat breathing under water, but still pretty cool, right?

In week 5, a laryngeal orifice forms, which is a T-shaped opening that leads to the larynx.

The epithelium inside the larynx turns into laryngeal ventricles, which give rise to the vocal cords.

In week 6, the epiglottis forms, and in week 12 the laryngeal opening has its adult shape, as well as thyroid, cricoid, and arytenoid cartilages.

Now let’s switch gears and look at the trachea and lungs.

During week 4, two tracheoesophageal ridges form and grow towards one another, ultimately fusing into a septum that divides the foregut into two regions: a posterior esophagus and an anterior lung bud.

The lung bud has an endoderm core, which develops into the epithelial and glandular structures of the trachea and lungs, surrounded by visceral mesoderm, which develops into muscles, cartilages, and connective tissue.

Now, the lung bud basically just hangs off the foregut, and at its loose end it bifurcates into two bronchial buds.

These bronchial buds give rise to the lungs, which grow and develop inside the pleural cavities.

The bronchial buds develop into lungs in four stages.

The first stage is the pseudoglandular stage, and it lasts from week 5 to week 16.

During this time, the bronchial buds divide into a left and right main bronchus, and these divide again into three secondary bronchi for the three right lung lobes and two secondary bronchi for the two left lung lobes.

Secondary bronchi turn into ten tertiary bronchi on the right side and eight on the left—corresponding to the number of lung segments on either side.

Tertiary bronchi divide repeatedly until all the terminal bronchioles are formed.

So, now the lungs enter the second stage of development, called the canalicular stage.

This stage lasts from week 16 to week 26, and during this time the terminal bronchioles continue to divide and form the respiratory bronchioles.