Embryology, Weeks 6-8
- Article Author:
- Mary Donovan
- Article Editor:
- Marco Cascella
- Updated:
- 10/16/2020 10:56:37 PM
- For CME on this topic:
- Embryology, Weeks 6-8 CME
- PubMed Link:
- Embryology, Weeks 6-8
Introduction
In human embryology, weeks six through eight are characterized by the growth and differentiation of tissues into organs. This process is known as organogenesis and occurs from weeks three through eight; the embryonic period. During week three, the process of gastrulation occurs, which establishes three distinct cell layers; the mesoderm, endoderm, and ectoderm. These are the primary germ cell layers from which organs arise during organogenesis.[1]
In particular:
- The endoderm forms organs of the gastrointestinal and respiratory systems, as well as the thymus, parathyroid, bladder, and urethra.
- The ectoderm is responsible for the development of the skin and skin appendages, the nervous system, and portions of sensory organs.
- The mesoderm forms the circulatory system and blood, lymphatic system, bone, cartilage, muscles, and many internal organs. For example, the kidney, spleen, ureters, and adrenal cortex are all derived from mesoderm.[2]
At the end of week eight, organ systems have developed and are ready for further maturation. By week nine, the fetal period begins and involves the growth and differentiation of anatomical structures. The fetal period lasts until birth.
Development
Cardiovascular System
The first organ system to develop during organogenesis is the cardiovascular system. The heart has established its four chambers by four weeks of development, whereas week six involves cardiac outflow separation and descent of the heart (and lungs) into the thorax. The separation divides the truncus arteriosus into the ascending aorta and pulmonary artery; this occurs via spiraling of the aorticopulmonary septum. Anatomically, the aorta and pulmonary appear to wrap around each other superior to the heart. That appearance is the result of embryologic spiraling. The aorticopulmonary septum may also be referred to as the spiral or conotruncal septum.
Lung Development
Lung development occurs from the embryonic period through the fetal period and continues up to birth. In particular, lung growth begins in early embryonic development when right and left lung buds are formed from an initial outpouching, the respiratory diverticulum. The buds enlarge and branch to form the respiratory tree. The appearance of the visceral and parietal pleura takes place during weeks five through seven. Both types of pleura arise from mesoderm. The visceral pleura covers the developing bronchial tree, and the parietal pleura covers the internal chest wall. Pleuroperitoneal membranes form and fuse with the diaphragm, which separates the pleural and peritoneal body cavities. Closure of the pleuroperitoneal canal by these membranes takes place by approximately week seven.[3]
Gastrointestinal System
Weeks six through eight are also critical for the development of the gastrointestinal system. The midgut undergoes physiologic herniation through the umbilicus around week six, but this event may be delayed up until week ten. This physiologic process happens because the size of the abdominal cavity is too small to accommodate the enlarging gastrointestinal tract. Herniation provides ample space for the rapidly enlarging midgut. After herniation, the midgut undergoes three rotational events totaling 270 degrees of rotation. The first rotation consists of 90 degrees in a counterclockwise direction around the superior mesenteric artery. This helps establish the appropriate arrangement and placement of the bowel; the ileum is brought to the right side of the body. The second rotation occurs during 10 weeks of gestation and consists of 180 degrees in a counterclockwise direction. The midgut returns to the body cavity at the end of 10 weeks. Finally, the third rotation of 180 degrees in a counterclockwise direction places the cecum on the right side.
In early embryonic development, the lumen of the duodenum is occluded by epithelium. Weeks six through eight are important for establishing the lumen's patency as the duodenum expands in size.[4] The anal opening is established by the breakdown of the cloacal membrane during week seven.
The pancreas is endodermal in origin and develops by growing dorsal and ventral pancreatic buds. The buds begin as outgrowths of the duodenum. Week seven is significant because the dorsal and ventral buds fuse at this time.[5] Additionally, the ventral pancreatic bud undergoes rotation around the duodenum by week six. It rotates for 180 degrees in a clockwise direction.[4] These embryologic mechanisms are important for proper pancreatic development; congenital malformations may occur in the absence of such processes, which will be discussed later.
Furthermore, the liver undergoes rapid growth during this time. Its first appearance is during the third week of gestation; it undergoes rapid growth during weeks five through ten. The hepatic artery appears at week eight. The liver is endodermal in origin.[6]
Central Nervous System
The neural tube closes around week four and is the early derivative of the brain and spinal cord. During weeks five through eight, the CNS undergoes the development of its vesicles, which are embryologic precursors to different structures of the brain. The forebrain, midbrain, and hindbrain all develop from vesicles. These three structures are also known as the prosencephalon, mesencephalon, and rhombencephalon, respectively. The prosencephalon later develops into the diencephalon and telencephalon. The diencephalon gives rise to the thalami, hypothalamus, optic cups, and neurohypophysis, while the telencephalon grows to surround the diencephalon, midbrain, and hindbrain. The mesencephalon forms the aqueduct of Sylvius, superior and inferior colliculi, and tegmentum. The rhombencephalon gives rise to the fourth ventricle as well as the metencephalon, a structure that eventually develops into the pons and cerebellum.[7]
Other Organs
Many other organs develop during weeks six through eight, including the pituitary gland, thymus, and adrenal cortex. At week seven, the embryo assumes a characteristic C-shape. At week seven, the ocular retina also begins to develop. The upper and lower limbs continue to grow. Also, facial structures such as the nostrils, eyelids, outer ears, lip, and palate develop, and at week seven, the head and face contours begin to emerge.
Cellular
Cellular processes are highly regulated throughout organogenesis. For example, the CNS requires precise cellular pathways to be followed for proper organ system development. Part of the dorsal ectoderm becomes the neural ectoderm, and their columnar appearance distinguishes the cells. The neural tube forms during early development and serves as an embryonic precursor to the CNS. The process by which the neural tube is formed from the neural plate is called neurulation. The neural tube has closed by four weeks of development, and the first neurons of the human body begin to appear. The neural tube forms the brain anteriorly and thespinal cord posteriorly.
During week seven, cells of the ventricular zone of the brain start making neurons of the cortical plate. The ventricular zone is a proliferative cell layer in the brain that surrounds the ventricles and contains neural stem cells for neurogenesis. Neurogenesis describes the formation of new neurons and their incorporation into the CNS. After new neurons are made, they undergo specific pathways of migration and differentiation. These pathways allow for the creation of new structures and continued CNS growth and development.[8]
Biochemical
During organogenesis, the fetus is most susceptible to exposure to teratogens. A teratogen is defined as any chemical, infectious agent, or other environmental exposure that can disrupt normal fetal development and lead to congenital abnormalities. For example, the most common teratogen is alcohol exposure in utero, potentially causing fetal alcohol syndrome. This condition can present with growth retardation, neurobehavioral abnormalities, and dysmorphic facial features. The facial abnormalities may manifest in the fetus as short palpebral fissures, a thin upper lip, and a smooth philtrum.
One contributing biochemical process is the ability of ethanol to cause oxidative stress and produce free radicals. This suppresses oxidative phosphorylation and leads to a buildup of reduced nicotinamide adenine dinucleotide (NADH). Also, it induces apoptosis of neural crest cells, which are critical for the formation of fetal structures such as the nervous system.[9]
Molecular
Many precise molecular events guide normal organ system development. The gastrointestinal system, for instance, arises from the foregut. Key genes called Hox genes are involved in specific molecular pathways for gut development.
Hox gene expression is highly regulated and plays a role in determining the fate of different parts of the gut. For example, expression of Hoxa3 and Hoxb4 genes have been found in the foregut, while the expression of Hoxc5 occurs in the midgut, and Hoxa13 genes are expressed in the hindgut. This pattern of gene expression contributes to the molecular differentiation of the digestive tract.[10]
Function
Organogenesis is important for the concurrent development of multiple organs and organ systems. Organs arise from the endoderm, ectoderm, and mesoderm; the three primary germ cell layers are established during gastrulation. Each of these layers is derived from the epiblast. By week eight, organogenesis is complete. The fetus appears human-like and is prepared to undergo further growth and differentiation.
Mechanism
Key embryologic mechanisms during weeks six through eight are necessary for proper organ development. Dysfunction during these mechanisms can result in congenital anomalies. The pancreas is one example of an organ that may be disrupted by failed embryologic events. Pancreas divisum is the most common congenital defect involving the pancreas. It occurs due to failure of fusion of the dorsal and ventral pancreatic buds during the seventh week of gestation. Pancreas divisum causes two separate ductal systems to persist rather than one.
When the dorsal and ventral pancreatic buds fuse, the pancreatic accessory duct may degenerate or become a less functional pancreatic duct. Without fusion of the pancreatic buds, the accessory duct ends up draining the majority of the pancreas. By contrast, the normal anatomy of the pancreas would cause most drainage to occur through the main pancreatic duct. The majority of patients with pancreas divisum are asymptomatic. However, this anomaly can present in a young child with recurrent episodes of pancreatitis. It should be considered in the differential diagnosis when there are no other known etiologies for pancreatitis in the child.
Another possible congenital malformation of the pancreas is annular pancreas. The embryologic mechanism by which this occurs is from a failure of rotation of the ventral pancreatic bud. In normal development, the ventral pancreatic bud undergoes rotation with the duodenum. Lack of rotation may cause the ventral pancreatic bud to encircle the duodenum and possibly constrict it. The major complication that may result is small bowel obstruction. Similar to pancreas divisum, many patients with annular pancreas are asymptomatic. Patients who develop symptoms may present with features of a small bowel obstruction such as bilious vomiting, abdominal pain and distension, and the inability to have a bowel movement.
Testing
On transvaginal ultrasound, the fetal heartbeat may be detected beginning around six weeks. The presence of a fetal heartbeat helps confirm the viability of a pregnancy. A fetal pole may also be detected at six weeks of gestation. The fetal pole is the first sign of the developing fetus that appears on imaging. It can be described as a thickened, hyperechoic structure adjacent to the yolk sac. The fetal pole will continue to grow into the developing fetus.
Pathophysiology
As previously stated, cardiac outflow separation occurs around week six. The truncus arteriosus becomes divided into an ascending aorta and pulmonary artery via spiraling of the aorticopulmonary septum. Failure of spiraling of the aorticopulmonary septum results in a congenital heart defect known as transposition of the great arteries. In this condition, the ascending aorta and pulmonary trunk are flipped, creating a right to left shunt.
The aorta arises from the right ventricle, and the pulmonary trunk arises from the left ventricle. As a result, physiologic changes occur in the ventricles. The high pressure of the aorta will cause right ventricular hypertrophy. The low pressure of the pulmonary artery will cause left ventricular atrophy. This condition is incompatible with life unless a shunt is present, which allows the mixing of oxygenated and deoxygenated blood.[11]
Clinical Significance
Among the most common congenital defects in the United States is cleft lip or palate, which may occur together or separately. A cleft lip occurs when the upper jaw or gum contains an abnormal opening. Clinical presentation varies; in severe cases, the gum may be completely divided. However, minor cases may only involve a small notch where the jawbones are separated. Lip formation occurs during weeks four through seven. Similarly, cleft palate involves an abnormal opening in the roof of the mouth. It occurs due to the failure of midline fusion of the lateral palatal shelves by week ten. Treatment involves surgical repair.[12]
Omphalocele is a rare birth defect often associated with other congenital anomalies, such as trisomies 13, 18, and 21. Recall that the midgut herniates through the umbilicus during weeks six to seven. By week ten, the midgut returns to the abdominal cavity. Failure of the midgut to return to the abdomen results in a congenital malformation known as omphalocele. An omphalocele presents clinically in a neonate with abdominal contents protruding through the umbilicus. They may appear as intestinal loops or involve other abdominal organs such as the liver. A thin, transparent sac surrounds the abdominal contents.[13]
In contrast, gastroschisis is a birth defect that occurs due to a failure of fusion of the lateral folds of the anterior abdominal wall. Gastroschisis also presents at birth, but the abdominal contents protrude through an abdominal wall defect next to the umbilicus rather than through the umbilicus. Also, the protruding abdominal contents are not covered by a sac in gastroschisis.