Respiratory System Development PDF

Development of the RESPIRATORY SYSTEM By Guta B. (Msc.)  Development of the RESPIRATORY SYSTEM develops as a ventral diverticulum’s of the foregut in front of the oesophagus as a respiratory diverticulum (lung bud)...

Development of the RESPIRATORY SYSTEM By Guta B. (Msc.)  Development of the RESPIRATORY SYSTEM develops as a ventral diverticulum’s of the foregut in front of the oesophagus as a respiratory diverticulum (lung bud) 1. Respiratory diverticulu 2. Foregut This diverticulum appears at first as a 3. Esophagotracheal sept esophagotracheal groove at around the middle of the fourth week (26-27 days) The 2 margins of the laryngo-tracheal groove fuse together forming a septum, esophagotracheal septum 1. Pharynx 2. Trachea 3. Esophagus 4. Lung buds 3 4  esophagotracheal septum, separate the lumen of the primitive pharynx into 2 parts:  Dorsal part ---- the pharynx & esophagus.  Ventral part ---- laryngo-tracheal tube ===Fusion of the laryngo-tracheal folds stops cranially leaving a communication between the laryngo-tracheal tube and the pharynx. ===This opening is called the laryngeal inlet 5 ===Since the outgrowth is from ventral wall of the foregut The cartilages, muscles, connective tissues, blood vessels and lymphatics of the respiratory tract develop from visceral or splanchnic mesoderm The epithelial and glandular anlage of the respiratory tract develop from endoderm ==As the groove closes, the diverticulum elongates caudally and divides by the process of dichotomy to form the endodermal lung (bronchlal) buds  That develop in to the right and left primary or principal bronchi of the corresponding lung buds. The part proximal to the first division becomes the larynx and trachea Development of the larynx The internal lining of the larynx originates from endoderm The cartilages and muscles originate from mesenchyme of the fourth and sixth pharyngeal arches The primitive laryngeal opening (aditus) is cranially bounded by the hypobrachial eminence, which later becomes the epiglottis Caudal to the opening on the ventral wall two arytenoid swellings appear and these bring about the narrowing of the aditus. Later the arytenoids swelling fuse laterally with the epiglottis forming the aryepiglottic fold. As a result of rapid proliferation of this mesenchyme, the laryngeal orifice changes in appearance from a sagittal slit to a T-shaped opening LARYNX… Laryngeal orifice and surrounding swellings at successive stages of development 8  When mesenchyme of the the fourth and sixth pharyngeal arches transforms into  The thyroid, cricoid, and arytenoid cartilages, the characteristic adult shape of the laryngeal orifice can be recognize The laryngeal epithelium also proliferates rapidly, resulting in a temporary occlusion of the lumen.  Subsequently, vacuolization and recanalization produce a pair of lateral recesses the laryngeal ventricles.  These recesses are bounded by folds of tissue that differentiate into the vocal cords The laryngeal cartilages start to develop in the second month from the visceral mesoderm but a plate of cartilage appears in the epiglottis later in the foetal life. 9 Development of the tracheo-bronchial tree and the lungs At the beginning, the trachea is a short but rapidly growing epithelial tube By the end of 7th wk cartilaginous rings appear in the trachea  The formation of these cartilaginous rings extends to the level of the lobar and segmental bronchi. The two lung buds at the caudal end of the diverticulum give rise to the tissue of the corresponding bronchial tree  These buds divide into daughter buds, which are three on the right side and two on the left side forming the corresponding 12  … Tracheo-bronchial tree As the result of the continuous dichotomic division the following parts of the tracheo-bronchial tree are formed. Trachea Primary (principal) bronchi → secondary (lobar) bronchi → tertiary (segmental /lobular) bronchi → terminal bronchi Bronchioles Terminal bronchioles → respiratory bronchioles alveolar ducts → alveolar sacs → alveoli The surrounding splanchnic mesoderm forms cartilaginous plates, smooth muscles,connective tissue & blood capillaries of a bronchopulmanary segment  Therefore inspite of the incomplete development of the alveoli, even until birth, a foetus can survive in vitro after the seventh month of pregnancy.  Invasion of the pleura: each lung grows laterally around the foregut and invaginates the part of the intraembryonic coelom, (i.e., the pericardio- peritoneal canal) which gives the pleural sac. The visceral pleura is derived from the splanchnic mesoderm, whereas the parietal pleura is derived from the somatic mesoderm.  Therefore, the visceral pleura is supplied by autonomic nerves and is insensitive  Whereas the parietal pleura is supplied by somatic nerves and is sensitive to pain. The space between the parietal and visceral pleura is the pleural cavity 16 17 stages of the lungs development There are four stages or periods in the development of the lungs. 1. Pseudo glandular period (5th -17th wk/till the 4th month) Only terminal bronchioles are formed. Histologically, the lung resembles an exocrine gland At the end of the 17th week all parts of the lung are formed but gas exchange is not yet possible  Therefore fetuses born during this period can not survive 2.Canalicular period (16th -25th wk/ 4-6 months) The terminal bronchioles divide into respiratory bronchioles and from each respiratory bronchioles 3-5 alveolar ducts develop  Survival is theoretically possible but in reality, a fetus born at this stage dies even with intensive care The ends of these alveolar ducts become saccular forming the terminal sacs//primitive alveoli, lined by cuboid epithelium. Some of the primitive alveoli are thin walled and their surrounding tissue is well vascularised,  There is a minimum possibility of respiration around the end of this period.  Fetuses born at the end of this period (22nd -25th wk) have a chance of survival under intensive care. But they die usually for their respirator system and other organs are not fully mature 3.Terminal period or period of alveolar formation (24th wk – birth)  Between the 24th -28th weeks the epithelial lining of the primitive alveolar sacs become so thin that the capillaries bulge into the sacs, During this time the epithelial lining becomes simple flattened (squamous) epithelium.  This simple squamous cells form the type I pneumocytes. Scattered between these cells there are large rounded secretory epithelial cells with numerous microvilli, the type II pneumocytes  Type II pneumocytes appear & secrete surfactant which is a phospholipid that lines the alveoli & help in their expansion after birth.  Surfactant production reaches adequate levels by the 7th month.  A fetus born by that time can survive with intensive care  Functional development of the lung A B A. The canalicular period lasts from the 16th to the 25th week. Note the cuboidal cells lining the respiratorybronchioli. B. The terminal sac period begins at the end of the sixth and beginning of the seventh prenatal month 4. Alveolar period  From late prenatal period - 8 years postnatal  At the beginning of this period, the alveolar ducts end in the very thin walled primitive alveoli which are separated from each other by a loose connective tissue.  The number of alveoli increases (95% develop after birth).  Before birth this immature alveoli appear as small outpocketings of the wall of the alveolar duct.  Mature (thin walled) alveoli don’t develop except after birth.  Now, the alveolo-capillary membrane is thin enough to permit sufficient gaseous exchange.  Until the age of three years the lungs continue to increase in size  Most alveoli develop in the last six weeks of intrauterine life and in the first few years after birth. Lung tissue in a newborn 23  Thereafter, both the size and number of the alveoli gradually increases until the age of 8 years. Due to the respiratory movement during intrauterine life there will be aspiration of fluid into the lungs, Therefore at birth the lungs are almost half filled with fluid. The sources of this fluid are the amniotic fluid, tracheal glands and the lungs themselves. After birth this fluid in the lungs is removed: 1.Through the mouth and nose 2.By the pulmonary capillaries 3.By the lymphatic vessels of the lungs