Week 5

mesodermal derivative, not endodermal like most of the intra-abdominal viscera
Septation of the heart chambers
1. development of cushion tissues
  1. endocardial cells in the atrioventricular and outflow tract regions undergo epithelial-to-mesenchymal transformation that invades extracellular matrix, proliferates, and differentiates into connective tissue
2. septation of atria
  1. 2 embryonic partial muscular septa fuse
    1. septum primum and septum secundum
  2. spina vestibuli joins septum primum and contributes to atrial septum
3. atrioventricular septum
  1. septum primum elongates forming 2 major endocardial-derived cushion tissues
    1. dorsal endocardial cushions
    2. ventral endocardial cushions
4. truncoconal septum growth begins
  1. cardiac neural crest cells contribute to truncoconal septum
  2. ascending aorta and pulmonary trunk outflow are separated
Atrioventricular valves begin formation
1. right tricuspid valve
2. left bicuspic valve
3. reinforced by chordae tendinae and papillary muscles
remodeling of venous inflow tract to produce vena cava, coronary sinus, and pulmonary veins
realignment of heart
1. align developing left atrioventricular canal with left atrium and ventricle and likewise for the right side of the heart
2. left ventricle brought into alignment with truncus arteriosus
trabeculation continues
formation of atrioventricular valves begins
formation of semilunar valves begins
1. prevent backflow from aorta and pulmonary trunk
Conotruncal swellings appear
cranial folding occurs
1. oropharyngeal membrane carried to presumptive facial region
2. heart brought from cranial region to thoracic cavity
3. septum transversum is caudal to heart
rotation of stomach occurs
expansion of dorsal mesentery forms greater omentum
spleen develops from mesenchymal condensation of dorsal mesogastrium (splanchnopleuric mesoderm) of the lesser sac
1. hematopoetic/lymphoid organ
central tendon of diaphragm forms from septum transversum
1. myoblasts differentiate within septum transversum and become innervated by spinal nerves at C3-C5
  1. these fibers join together to form paired phrenic nerves
primary intestinal loop created because growing ileum lengthens much more rapidly that the abdominal cavity
1. cranial limb gives rise to most of the ileum
2. caudal limb will become the ascending colon and transverse colon
3. herniates into umbilicus and rotates 90 degrees counterclockwise
Pericardium development
1. pleuropericardial folds of somatopleuric mesoderm ingress from lateral body walls
  1. folds have 3 layers consisting of mesenchyme sandwiched between 2 epithelial layers
    1. inner serous pericardium
    2. fibrous pericardium
    3. outer serous pericardium
after formation of the head fold, septum transversum divides coelom into pericardial, peritoneal, and two narrow canals called pericardioperitoneal canals or the future pleural cavities
1. ventral edge of the septum becomes fixed to the anterior body wall at the 7th thoracic level
2. dorsal connection to the esophageal mesenchyme becomes fixed at the 12th thoracic level
3. pleuropericardial folds and splanchnopleuric mesoderm creates parietal pleura surrounding developing lungs
diaphragm incorporates derivatives of 4 embryonic structures
1. septum transversum
2. pleuroperitoneal membranes
3. mesoderm of the body wall
4. esophageal mesenchyme
Ventral and dorsal pancreatic buds join
1. exocrine and endocrine functions
cranial-most mesonephric tubules disappear by end of week 5, leaving 20 pairs in levels L1-L3
3 primary ventricles further divide
1. prosencephalon
  1. telencephalon
    1. forms cerebral cortex, olfactory bulb and olfactory tract, commissural system, and lateral ventricles
  2. diencephalon
    1. forms pinneal gland and nucleus habenulae, thalamus, hypothalamus, posterior lobe of pituitary gland, and optic vesicles
2. mesencephalon
  1. incorporated into brainstem
  2. forms large tracts of neurons between forebrain and brainstem
  3. tectum is divided into 4 colliculi that convert auditory and visual information into spatial maps
  4. cerebral aqueduct part of ventricle system for CSF
  5. houses some cranial nerve motor nuclei
  6. substantia nigra important for motor control, learning, addiciton; affected in Parkinson's disease
  7. red neurons or motor nuclei for forelimbs
3. rhombencephalon
  1. metencephalon
    1. pons
      1. relays signals that link spinal cord and cerebral cortex with cerebellum
    2. cerebellum
      1. center for balance and postural control
  2. myelencephalon
    1. medulla oblongata that houses many cranial nerve nuclie and serves as a relay center between the spinal cord and higher brain centers
      1. also regulates respiration, heartbeat, reflexes, etc.
pontine flexure begins to folds metencephalon onto myelencephalon
motor neurons begin outgrowth first in the cervical region and later at more caudal levels
formation of dorsal root ganglia
1. derived from neural crest cells
ventral root ganglia begin to grow
dorsal and ventral roots come together to form spinal nerves
1. cervical spinal nerves are cranial to the vertebrae
2. more caudal spinal nerves are caudal to the vertebrae
development of the rib cage
1. costal processe of thoracic vertebrae begin elongation to form ribs
  1. grow toward ventral end
2. costovertebral joint forms late in the 5th week
sympathetic ganglia at levels T1-L4 begins to form
1. preganglionic fibers exit spinal column via ventral root
  1. entry to chain ganglia via white ramus
2. post-ganglionic fibers rejoin spinal nerves
  1. travel towards target organs via gray rami
  2. travel vertically up the chain to higher levels
lumenal cells of optic cup differentiate into neural retina
1. outer layer
  1. photoreceptors
  2. send axons to retinal ganglia
2. inner layer
  1. forms retinal ganglia
  2. sends axons through optic nerve to the thalamus
outer ear beings to develop
1. develops from 1st and 2nd pharyngeal arches
Rathke's pouch elongates and becomes constricted at its attachment to oral epithelium and comes into contact with infundibulum