SEM_09_10_Pharynx, pharyngeal arches and respiratory system_PARTE2.docx

Larynx The larynx (voice box) and the trachea (windpipe) originate as an evagination of the endoderm along the caudal pharyngeal floor, called laryngotracheal groove. A tracheoesophageal septum grows from the lateral walls of the laryngotracheal groove and fuses along the midline. The septum create...

Larynx The larynx (voice box) and the trachea (windpipe) originate as an evagination of the endoderm along the caudal pharyngeal floor, called laryngotracheal groove. A tracheoesophageal septum grows from the lateral walls of the laryngotracheal groove and fuses along the midline. The septum creates a laryngotracheal tube as a separate entity from the oesophagus. The larynx develops rostrally, where the lumen of the laryngotracheal groove keeps open and connected with the pharynx. https://sway.office.com/eYCkf1IQuJ2y9KDH#content=NmhYsiw6zEd3lE - The first indication of the differentiation of the respiratory system is the formation of the laryngo-tracheal groove on the floor of the foregut. This groove deepens and grows caudally until it becomes separated from the proper foregut by the formation of the tracheo-oesophageal septum. This septum separates the oesophagus (the dorsal portion of the foregut) from(trachea (the ventral part) which develops roughly parallel with the oesophagus. The trachea bifurcates at its caudal end to form two lung buds. The development of the specific features of the lung is related to the gradual branching of the bronchial tree. The wall of the larynx develops from bilateral laryngeal swellings or arytenoid swellings formed by the branchial mesoderm (from the fourth and sixth branchial arches) that surround the entrance to the laryngotracheal tube. This mesoderm differentiates into the muscles and cartilages (arytenoid, thyroid and cricoid) that constitute the laryngeal wall. The laryngeal cavity is lined by the endoderm which forms the laryngeal mucosa and the laryngeal folds (vocal and vestibular folds) that characterise the interior of the larynx. Cranial to the opening of the larynx into the pharynx, an additional swelling in the midline, called epiglottal swelling, gives rise to the epiglottic cartilage, which sticks out toward the pharynx to control the closing/opening of the larynx. https://sway.office.com/eYCkf1IQuJ2y9KDH#content=TPTIE5D5qPMpKA - The larynx (respiratory system) and the oesophagus (digestive system) separate at the level of the fourth pharyngeal arch where an epiglottal swelling forms the opening of the larynx which develops ventrally on the floor of the foregut. Dorsally, the pharynx is continued by the oesophagus. The arytenoid swelling forms the wall of the larynx. Trachea, bronchi and lungs The trachea is a tube with cartilaginous rings that connects the larynx to the lungs allowing the passage of air through. The tracheoesophageal fistula and oesophageal atresia are the two most common congenital anomalies derived from defects in the development of the tracheoesophageal septum. A fistula, from the Latin meaning ‘a pipe,’ is an abnormal connection running either between two tubes or between a tube and a surface. In tracheoesophageal fistula, it runs between the trachea and the oesophagus. This connection may or may not have a central cavity; if it does, then food within the oesophagus may pass into the trachea and then to the lungs or alternatively, the air in the trachea may cross into the oesophagus. Congenital tracheoesophageal fistula can arise due to a failed fusion of the tracheoesophageal ridges after the fourth week of embryological development. Atresia is a condition in which an orifice or passage in the body is closed, absent or abnormally narrow. Oesophageal atresia causes the oesophagus to end in a blind-ended pouch rather than connecting normally to the stomach. It is characterised anatomically by a congenital obstruction of the oesophagus with interruption of the continuity of the oesophageal wall. https://sway.office.com/eYCkf1IQuJ2y9KDH#content=1Bps5k0kP09gZD - Tracheoesophageal fistula and oesophageal atresia A tracheoesophageal fistula is when the trachea and the oesophagus are connected. In this condition, food or milk can get into the lungs when the patient swallows. This can cause pulmonary aspiration leading to breathing problems and even pneumonia. In oesophageal atresia, the upper oesophagus does not connect (atresia) to the lower oesophagus and stomach. Oesophageal atresia can be associated with oesophageal fistula. As the trachea grows into the thorax, its blind caudal end bifurcates into the left and right bronchial buds, the primordia of the lungs. As the lung buds elongate, the principal bronchi (primary bronchial branches for each lung), ramify to form the lobar bronchi (secondary bronchial branches for each pulmonary lobe). Further ramifications of the lobar bronchi give rise to the segmental bronchi (tertiary bronchial branches for each bronchopulmonary segment). The segmental bronchi continue to divide for various generations until they become the bronchioles, too narrow to be supported by cartilage. The terminal bronchioles mark the end of the conducting zone and the beginning of the respiratory zone. The respiratory bronchioles can be identified by the presence of some alveoli along their walls. The respiratory bronchiole splits into a number of alveolar ducts, which terminate in alveolar sacs. At the end of the foetal period and during the first stage of the postnatal life, the terminal sacs are partitioned by secondary septa to form the individual alveoli. Initially, as the lung buds grow into the thorax, the bronchial branches are solid cores of endodermal cells elongating in the surrounding mesoderm (they grow like exocrine gland). Eventually, they become hollow and the terminal bronchioles dilate in sac-like diverticula lined with a thin endodermal epithelium. The ciliated respiratory epithelium, rich in goblet cells and glands, is derived from the endodermal lining of the primitive gut. The visceral mesoderm that surrounds the lung buds is the origin of the blood vessels, cartilages, muscles and connective components that complete the wall of the respiratory system. - Bronchial Tree The trachea branches into the right and left primary bronchi at the carina. The carina is a raised structure that contains specialized nervous tissue that induces violent coughing if a foreign body such as food is present. A bronchial tree (or respiratory tree) is the collective term used for these multiple-branched bronchi. The main function of the bronchi, like other conducting zone structures, is to provide a passageway for air to move into and out of each lung. In addition, the mucous membrane traps debris and pathogens. Although the formation of the bronchial system begins early in gestation, embryonic period, the bronchial branches continue to form throughout the foetal period and in the postnatal period. The embryonic period. This period extends from the formation of the laryngotracheal groove to the formation of the segmental bronchi. In this period the developing lungs expand into the common pleuro-pericardial cavity and become surrounded by the visceral pleura. The fetal period is when the ramified bronchi are formed and the perliminary structures for gas exchange are establish. It can be subdivided in the following periods: The pseudoglandular period. This period is named according to the gland-like ramifications of the segmental bronchi that give rise to the small terminal bronchioles, which mark the end of the conducting zone; no respiratory bronchioles or alveoli are still present. The canalicular period. This period is characterised by the formation of the primordia of the respiratory zone: those portions of the lungs which are engaged in the gaseous exchange. The lungs become vascular, and the terminal bronchioles give rise to the respiratory bronchioles that can be identified by the presence of some alveolar sacs along their walls. Finally, the respiratory bronchioles divide into the smaller canalicules or alveolar ducts. Breathing is theoretically possible, but the lungs are too immature, and the premature foetuses are not likely to survive if they are born in this stage. The terminal sac or saccular stage. A large number of terminal alveolar ducts and terminal sacs bud off from the respiratory bronchioles. These terminal sacs correspond to the primitive alveoli. Initially, they are lined by the cuboidal epithelium which later differentiates into two types of cells: squamous cells (type I alveolar cells or membranous pneumocytes) and cuboid or cone-like cells (type II alveolar cells or granular pneumocytes). Type I alveolar cells are involved in the gaseous exchange and account for more than 90% of the surface of the alveoli. Type II alveolar cells are cuboid cells that secrete surfactant which creates a phospholipid layer covering the luminal surface of the alveoli. Surfactant reduces the surface tension and thereby prevent adhesion of alveolar walls during development. This surfactant also facilitates the expansion of the alveoli during inspiration (lung's compliance) and prevents their collapse during expiration (atelectasis). Despite the incomplete pulmonary development and the limited amount of surfactant being produced, human foetuses born towards the end of this stage may survive with intensive care. The alveolar period. In this period, which extends postnatally, alveoli continue to form through a septation process increasing the gas exchange surface area. - Pulmonary surfactant is a mixture of lipids and proteins which is secreted by the epithelial type II cells into the alveolar space. Its main function is to reduce the surface tension at the air/liquid interface in the lung in order to prevent the collapse or closure of a lung resulting in reduced or absent gas exchange (atelectasis) and to increase the lung's ability to stretch and expand (lung compliance, or pulmonary compliance). The deficiency of pulmonary surfactant is the principal cause of respiratory distress syndrome in premature individuals. - Phases of lung development During foetal development, the lungs are filled with fluid, secreted from the developing glands and supplemented by the aspirated amniotic fluid. The presence of the fluid is considered to be an important stimulus for the expansion of the alveoli. During the foetal period, the movements of those muscles which are associated with breathing are observable. They prepare the respiratory muscles for breathing. At birth, most of the fluid in the respiratory system is expelled through the mouth and nose. The first inspiration fills the respiratory system with air and the remnant of liquid is absorbed by the epithelial cells.