Fifth Through Eighth Weeks of Stages Fetal Development /Weeks five through eight of gestation are characterized by the development of the major organ systems, including the circulatory, nervous, lymphatic, and gastrointestinal systems. During this time, the embryo is extremely susceptible to the effects of teratogens.
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.[rx] 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.[rx] Additionally, the ventral pancreatic bud undergoes rotation around the duodenum by week six. It rotates for 180 degrees in a clockwise direction.[rx] 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.[rx]
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.[rx]
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.[rx]
Fifth Through Eighth Weeks of Development
Weeks five to eight of gestation develops the major organs, including the circulatory, nervous, and gastrointestinal systems.
Key Points
Being susceptible to the effects of teratogens is high during embryonic development.
At week five, the brain, spinal cord, vertebrae, heart, vasculature, and gastrointestinal tract begin to develop.
During weeks six to seven, the embryo grows from 4 mm in length to 9 mm and begins to curve into a C-shape. The fetal heart bulges, develops further, and begins to beat in a regular rhythm. Rudimentary blood begins to move through the blood vessels. The neural tube, which forms the brain, closes.
During weeks six and seven, the limb buds form. The eyes, mouth, and ear structures begin to form. The initial differentiation of the tissues that will become the spleen, gallbladder, pancreas, liver, kidneys, stomach, and lungs occurs.
By week eight of gestation, the lungs begin to form, as well as the lymphatic system. The main development of the external genitalia begins, and the brain continues to develop. The arms and legs have grown longer, and the foot and hand areas can be clearly distinguished.
Key Terms
- chorionic membrane: One of the membranes that exist during pregnancy between the developing fetus and mother. The chorionic villi emerge from the chorion, invade the endometrium, and allow transfer of nutrients from maternal blood to fetal blood.
- Gestational age: The time that has passed since the onset of the last menstruation.
- Embryonic age: Measures the actual age of the embryo or fetus from the time of fertilization.
- teratogens: An agent, such as a virus, a drug, or radiation, that causes a malformation of an embryo or fetus.
Weeks five through eight of gestation are characterized by the development of the major organ systems, including the circulatory, nervous, lymphatic, and gastrointestinal systems. During this time, the embryo is extremely susceptible to the effects of teratogens.
Gestational age is the time that has passed since the onset of the last menstruation, which occurs two weeks before the actual fertilization. Embryonic age measures the actual age of the embryo or fetus from the time of fertilization. Thus, the first week of embryonic age is already week three counting with gestational age. The number of the week (used here) is one more than the actual age of the embryo/fetus. For example, the embryo is 0 whole weeks old during the first week after fertilization.
Week 5
At week five, the brain, spinal cord, vertebrae, heart, vasculature, and gastrointestinal tract begin to develop.
Week 6–7
Human embryo at seven weeks: An embryo from an ectopic pregnancy, still in the oviduct. This embryo is about five weeks old (or from the seventh week of menstrual age). The heart is the dark spot at the center of the image, bulging out of the embryo.
During weeks six and seven, the embryo grows from four millimeters in length to nine millimeters and begins to curve into a C-shape. The fetal heart bulges, develops further, and begins to beat in a regular rhythm. Rudimentary blood begins to move through the main embryonic blood vessels, connecting to the yolk sac and the chorionic membrane of the placenta.
The arm and leg buds, which will grow into the full limbs over the rest of development, become visible. The neural tube, which forms the brain, closes. The brain then develops into five areas and some cranial nerves are visible.
The eyes, mouth, and ear structures begin to form. The initial differentiation of the tissues that will become the spleen, gallbladder, pancreas, liver, kidneys, stomach, and lungs occurs.
Week 8
By week eight of gestation, the embryo measures 13 millimeters in length. The lungs begin to form, as well as the lymphatic system. The main development of the external genitalia begins, and the brain continues to develop.
The arms and legs have grown longer, and the foot and hand areas can be clearly distinguished. The hands and feet have fingers and toes, but may still be webbed.
Human embryo from an ectopic pregnancy: An embryo from an ectopic pregnancy, located in the part of the uterus to which the fallopian tube is attached. The features are consistent with a developmental age of seven weeks (the ninth week of menstrual age).
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