Embryology Flashcards
Cartilage
There are three types of cartilage: hyaline, fibrocartilage and elastic. Only fibrocartilage has a (small) blood supply, and elastic cartilage never ossifies, though the others may. The pinna is formed from elastic cartilage, as is the epiglottis. Hyaline cartilage is tough, but heals by formation of fibrous tissue.
Deep fascia is supplied by the same sensory nerves as the overlying skin, and is very sensitive. Ligaments are non-elastic, permitting them to hold joints together. There are three types of cartilage: hyaline, fibrocartilage and elastic. Only fibrocartilage has a blood supply, and elastic cartilage never ossifies, though the others may.
Branchial arches
The first pouch gives rise to the auditory tube, middle ear, and mastoid antrum. The second pouch forms some of the tympanic cavity, tonsillar crypts, and palatine tonsil. The third branchial pouch gives rise to the inferior parathyroids and the thymus. The fourth pouch gives rise to the superior parathyroids and possibly some of the thymus. The fifth pouch forms the ultimobranchial body, which gives rise to the parafollicular C cells of the thyroid.
The fourth branchial (pharyngeal) arch gives rise to the cricothyroid muscle and all intrinsic muscles of the palate except the tensor palatini, and the thyroid and epiglottic cartilages.
The sixth arch gives rise to all intrinsic muscles of the larynx except the cricothyroid (hence the different nerve supply in the adult) and cricoid, arytenoid, corniculate and cuneiform cartilages.
It is persistence of the sixth branchial arch artery (as the ductus arteriosus) that holds the left recurrent laryngeal nerve lower than the right.
Periosteum
The periosteum contains blood vessels that supply the underlying bone (by traversing the Haversian canals) and is also osteogenic, allowing new bone formation in response to stress, fracture, etc. It is very sensitive if injured. When it is subcutaneous, nerve supply comes from the cutaneous nerves to the overlying skin. Deep bones are supplied by branches from motor nerves to the adjacent muscles. Bone may form by intramembranous ossification (direct ossification of fibrous tissue) or endochondral ossification (where a pre-existing hyaline cartilage model of bone is gradually converted to bone). The clavicle is unusual in that it forms by intramembranous ossification, like the bones of the skull. Most long bones form by endochondral ossification.
The periosteum is a very thin sheath of connective tissue that encourages bone growth and development, and delivers blood and nutrients to the bones. The inner cambium layer is highly cellular and is composed of mesenchymal progenitor cells, differentiated osteogenic progenitor cells, osteoblasts, and fibroblasts, in a sparse collagenous matrix. These cells promote the elongation of bones during development and also assist in healing fractures.
Rathke’s pouch
Rathke’s pouch arises from the ectoderm of the mouth pit and goes on to form the anterior lobe of the pituitary. The posterior lobe forms from a downward projection of neurons from the hypothalamus. The hyoid originates from the second and third branchial arches. The trigeminal is the nerve of the first arch, the facial the second, the glossopharyngeal the third arch, and the vagus the fourth and sixth.
Meckel’s cartilage develops from the mesoderm of the first branchial arch. It forms the maxilla, palate, incus and malleus.
Mesoderm
The kidneys develop from the intermediate strip of mesoderm, and the neural tube is formed from the notochord, which originates as ectodermal cells.
Paraxial mesoderm lies nearest the midline
(Kidneys do not develop from this)
Pancreas
The pancreas develops from two growths, one in the ventral and one in the dorsal mesogastrium. Both growths have ducts—the ventral bud contains the main duct, and the dorsal contains the accessory. The dorsal bud is bigger, however its duct, in the most part, ends up anastomosing with the main duct of the ventral bud. The ventral bud has to rotate from right to left behind the developing foregut to meet the dorsal bud; this is due to asymmetrical growth of the foregut walls.
Septum transversum
The septum transversum is a mass of cranial mesenchyme that forms the diaphragm and ventral mesentery of the foregut (that becomes the lesser omentum, peritoneum of the liver, and the falciform ligament).
Congenital heart defects
The most common congenital cardiac defect is patent ductus arteriosus. The ductus arteriosus connects the left pulmonary artery to the arch of the aorta, and diverts oxygenated blood into the systemic circulation from the right side of the heart in the foetus. It is formed from the left sixth aortic arch in embryonic development. Failure to close results in a left-to-right cardiac shunt, leading to dyspnoea, pulmonary hypertension and eventual Eisenmenger syndrome and cardiac failure if untreated.
Aortic coarctation is a congenital narrowing of the aorta, seen in association with Turner syndrome and other congenital syndromes.
An aberrant right subclavian artery arises beyond the origin of the left subclavian from the arch of the aorta and passes behind the oesophagus.
Fallot’s tetralogy has four components: ventricular septal defect, overriding aorta, pulmonary stenosis, and right ventricular hypertrophy.
Transposition of the great vessels involves any abnormal arrangement of the great vessels, with or without rearrangement of the atria and ventricles as well.
Deep Fascia
The deep fascia encloses the limbs and body wall, varying greatly in thickness. It forms structures like the iliotibial tract. It is absent in the face and the ischioanal fossae. It is always anchored to the periosteum of any bone it passes directly over. It is supplied by the same sensory nerves as the overlying skin, and is very sensitive.
Limb plexusus
Limb plexuses are always formed by the anterior rami of spinal nerves. The enlargements of the spinal cord at cervical and lumbar levels result from increased cell bodies in the anterior horns, which go on to supply the muscles of the arms and legs. Anterior divisions of limb plexuses supply flexors, and posterior divisions supply extensors.
Alpha fibres from the anterior horn cells of the spinal cord supply the extrafusal fibres, and smaller gamma cells supply the intramural spindle fibres. Around 40% of the fibres in a nerve to a muscle are afferent fibres, important in proprioception. Anterior divisions of limb plexuses supply flexor muscles, and posterior divisions supply extensors.
Kidneys
The kidney develops in stages, beginning as the pronephros, mesonephros and then metanephros. The mesonephros forms the mesonephric duct, which goes on to form the bladder.
The ureter buds from the caudal end of the mesonephric duct and proceeds up to form the collecting system of the kidney. The metanephros forms the nephrons of the kidney.
The kidney thus develops in the pelvis and migrates cranially to its adult position high in the retroperitoneum. It takes its blood supply initially from the median sacral artery, then segmentally from the common then internal iliac arteries and aorta as it moves. The early arteries to the kidney degenerate, but the ureter retains its segmental supply.
Abdominal content development
Physiological herniation of the gut persists until the 10th week, at which point it is reduced back into the abdomen by the growing gut walls. Failure of this process can result in omphalocele or gastroschisis. Meckel’s diverticulum is found in around 2% of adults; it is the remains of the vitellointestinal duct, a connection between the yolk sac and the midgut lumen that usually disappears by weeks 5–6 of gestation. Due to rotation of the gut, the hindgut vessels lie anterior to other structures of the posterior abdominal wall (aorta, gonadal arteries, ureters, vena cava).
Umbilical veins
The left umbilical vein bypasses the portal circulation for the most part by sending its oxygenated blood via the ductus venosus to the inferior vena cava. The ductus venosus closes shortly after birth and fibroses to form the ligamentum venosum, which is continuous with the ligamentum teres, the fibrosed intra-abdominal portion of the umbilical vein.
Branchial arch arteries
The first and second arch arteries disappear early in development, leaving only the maxillary and stapedial arteries. The third branchial arch artery forms part of the carotid artery. The fourth branchial arch artery on the right forms the right subclavian artery and on the left, the arch of aorta. The sixth artery forms the ductus arteriosus and otherwise disappears; the fifth disappears entirely.
Cardinal veins
The right anterior cardinal vein forms the right brachiocephalic vein. The posterior cardinal vein forms the azygos vein.
