Chapter 24: The Respiratory System
Chapter 24: Respiratory System – Embryological Developmentby John F. Neas
The respiratory system begins to develop early during the embryonic period as a derivative of the primitive gut. The development involves ectoderm and endoderm. The upper respiratory system (nose and pharynx) develops at the same time as the lower respiratory system (trachea and lungs), but it is best to discuss their development separately because they involve different germ layers.
Upper Respiratory System
Animals with controlled mobility typically exhibit bilateral symmetry and cephalization. Cephalization means that the cephalic (head) end of the organism differentiates structurally and functionally from the rest of the body. Thus, the head develops a concentration of nerve cells and sense organs. In human beings, cephalization becomes apparent early in development. One of the first events is the development of the nasal cavity at three and one-half to four weeks of embryonic life. A region of thickened ectoderm called the olfactory (nasal) placode develops on the anterior and inferior part of the head. The placode invaginates to form the olfactory pit that extends posteriorly to connect with the foregut. The foregut develops from endoderm and later develops into the pharynx.
The oral cavity develops at the same time as the nasal cavity, and a thin oronasal membrane separates the two cavities for a short time. The oronasal membrane ruptures during the seventh week, producing a single, large oronasal cavity.
The primary palate begins to develop toward the end of the fifth week from two medial swellings of the maxillary processes that merge to form a triangular median part that develops into the primary palate.
The nasal septum begins to develop from a vertical plate that extends inferiorly from the roof of the nasal cavity. At about the same time, mesodermal shelves grow horizontally across the oronasal cavity. The hard palate develops when these horizontal shelves undergo intramembranous ossification to produce the palatine processes of the premaxillae, maxillae, and palatines. The palatine raphe indicates the line of fusion of these separate bony elements. The incisive foramen in the midline is a landmark between the primary and secondary palates. The posterior portion of the hard palate does not ossify. Rather, it forms a muscular arch, the soft (membranous) palate, with the uvula, the final part of the palate to develop.
Lower Respiratory System
After approximately three and one-half weeks of development, a shallow laryngotracheal groove develops along the midventral floor of the foregut endoderm near the level of the last pharyngeal arch. The groove gradually deepens and, by the fourth week, it becomes a blind outgrowth, or diverticulum, that extends caudally and anterior to the esophagus. This diverticulum is the laryngotracheal bud.
As the laryngotracheal bud grows, it differentiates into the future larynx and the lower respiratory system. The proximal end of the bud maintains a slit-like opening into the pharynx called the glottis. The middle portion of the bud will become the trachea. The distal end divides (bifurcates) into two lung buds that grow into the right and left bronchi and the lungs.
The developing lung buds divide as they grow and elongate, and the resulting divisions subdivide. This process of repeated branching continues until there is a complete system of bronchial tubes within the lower respiratory tract. The smooth muscle, cartilage, and connective tissues of the bronchial tubes and the pleural sacs of the lungs develop from mesenchyme (mesoderm). This supporting tissue of the lungs begins to develop at about eight weeks.
After the sixth month, the closed terminal portions of the tubes dilate and become the air sacs, or alveoli, of the lungs. By the end of the sixth month, there are approximately one million terminal branches, and the conducting passageways are complete to the level of the bronchioles. Each bronchiole produces several hundred alveoli during the next three months, and this process continues for a variable period after birth.
The lungs are not sufficiently developed to permit survival outside the womb until about twenty-six weeks of development. Thus, premature infants born before this time require respiratory equipment to live.
Coelom, Pleural Cavities, Pleura
The ventral body cavity develops from the intraembryonic coelom and has walls of skin, connective tissue, bone, muscles, and a serous membrane. Collectively, the organs within the ventral body cavity are the viscera, or visceral organs. The muscular diaphragm divides the coelom into an upper thoracic (chest) cavity that contains the heart and lungs and a lower abdominopelvic cavity that contains the intestines, liver, spleen, stomach, pancreas, kidneys, and reproductive organs.
By 26 days of development, the human embryo has three incompletely separated body cavities: the pericardial cavity, the peritoneal cavity, and two small pericardioperitoneal canals. The pericardioperitoneal canals provide a communication between the primitive pericardial and peritoneal cavities. Membranous partitions (folds) that develop during the firth and sixth weeks at the upper and lower ends of the pericardioperitoneal canals separate the pleural cavities from the pericardial cavity and the pleural cavities from the peritoneal cavity.
The diaphragm develops between the fourth and tenth weeks of prenatal life from four structures: septum transversum, pleuroperitoneal membranes, mesoesophagus (dorsal mesentery of the esophagus), and muscular components of the body wall.
The pleuroperitoneal membranes grow medially from the body wall and encroach on the pleuroperitoneal canals. They fuse with the mesoesophagus and with the dorsal part of the septum transversum posterior to the esophagus. During fusion, the septum transversum extends into the other parts, thus forming the entire muscle of the diaphragm and completing the partition between the thoracic and abdominopelvic cavities.
This microscopic image shows the general structure of the trachea in a fetal human being. A series of C-shaped rings of hyaline cartilage (C) support the tracheal mucosa (M) and bands of smooth muscle, called the trachealis muscle (T), join the free ends of the rings posteriorly.
This micrograph illustrates a late stage in development of the human lungs. Visceral pleura, with an outer lining of mesothelium (Me), covers the surface of the lungs and passes into the parenchyma of the lungs as fibrous septa (S). (Some lymphatic vessels (L) are present in the septa.
The smallest diameter passages of the conducting system are the terminal bronchioles (T). Each terminal bronchiole divides into short respiratory bronchioles (R) that have thinner walls. These, in turn, divide into alveolar ducts (AD) that open along their length into alveolar sacs and alveoli. The alveoli in the adult lung are distended spaces filled with air, but here they are filled with fluid. The photomicrograph also shows a large segmental branch of the pulmonary artery (A) in close association with a tertiary bronchiole (B). Note the prominent spiral layer of smooth muscle (M) layer in the bronchioles. V = pulmonary vessel.
Development of the Palate
Lower Respiratory System
Coelom, Pleural Cavities, Pleura