Alright, we’re back at it but let’s keep going on embryology. Never meant to make an embryo blog but it sure seems to be shaping up that way! This week – the respiratory system!
The respiratory system can be broken into two parts, the lower and the upper. We’re going to start with the development of the lower respiratory, which consists of the larynx, trachea, bronchi and lungs. Our very first view of it is when the respiratory diverticulum is formed. We’re back in week 4 and we’re looking at the ventral wall of the primitive foregut. The end of the respiratory diverticulum gets bigger and forms the lung bud. Almost like how you might picture an anyuresim forming.
The lung bud then splits to make two bronchial buds which are going to start branching. They make the main, lobar, segmental and subsegmental bronchi. When it first starts out, the respiratory diverticulum is completely attached to the foregut, but that won’t last for long. The mesoderm starts forming folds called tracheoesophageal folds to split it off. In fact, those will even fuse to make the tracheoesophageal septum. That’s the generic part, let’s look at some specific structures.
The larynx is formed from the laryngeal orifice. Remember the when the respiratory diverticulum started ballooning off of the foregut? That opening is going to be the laryngeal orifice. Remember, the endoderm is going to spread down into here so that it can create the epithelium and associated glands. If you’re into the pharyngeal arches thing at this point, we’re also talking about 4 and 6 which become a tremendous amount of important things in the body.
Next, moving inferiorly (down) the lower respiratory system is the trachea. Again, epithelium and glands come from the endoderm. The rest is visceral mesoderm. Think about it, we don’t really control it, do we? Makes sense that it should come from there. As it is just south of the larynx in its final form, its embryological start is the same. So, we’re just working down that balloon. We’ll get another article up that talks just about tracheoesophageal fistulas and their abnormalities. High yield and easy stuff once it’s clear!
Okay, next on the list is the bronchi. These actually come from the lung buds dividing. First they make the bronchial buds, then in week 5 they enlarge to form the main bronchi. That further divides into the lobar bronchi which then divides into the segmental bronchi. Those segmental bronchi correspond to the various bronchopulmonary segments we have as adults (is that another article idea?!). Those segmental bronchi also subdivide into subsegmental bronchi. So, we start with the lung buds and they just keep dividing and dividing, expanding as they go to fill the primitive pleural cavity. In fact, they push the visceral mesoderm out and sort of into the somatic mesoderm. This causes the formation of the visceral pleura (from the visceral pleura) and the parietal pleura (from the somatic mesoderm) and creates the space known as the pleural cavity.
Now, starting with the largest bronchi, the lungs are going to start maturing in a sort of down and outward direction. It occurs across the whole area at once, but its important to remember that it starts at the top. So, that area will always be more advanced. There is a tremendous amount of development that happens pretty much from week 7 on. We’re going to absolutely nutshell it, list form, below.
Psuedoglandular period (weeks 7-16) – numerous endodermal tubules lined by simple columnar epithelium surrounded by mesoderm with some capillaries. Each ET branch ends in 15-25 terminal bronchioles.
Canalicular period (16-24) TBs branch into three or more respiratory bronchioles. Each ofthose have 3 to 6 alveolar ducts, now lined with simple cuboidal epithelium and rocking more capillaries.
Terminal sac period (24-birth) Terminal sacs come off the ADs, dilatine and expanding into the mesoderm around them. They get split by primary septae. The simple cuboidal epithelium differentiate into type I pneumocytes (which make the blood-air barrier) and type II pneumocytes (which make surfactant – the stuff that keeps them from sticking to themselves and collapsing) and we have a ton more capillaries.
After birth until the kid is around 8 years is called the alveolar period. Secondary septae form in the alveoli to cause the increasing lung size as we get older. Yeah, we actually get more alveoli. And yet, kids never seem to run out of breath first.