Human Anatomy and Physiology
Embryonic Development
Britni Schlachter Rasmussen University BSC2347/L: Anatomy & Physiology II Lab Prof. Elizabeth St. Louis
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Embryogenesis
The embryonic phase lasts from the end of the first week through the eighth week after conception. It is recognized as the entire first trimester of development. The entire embryonic phase is over in 7 weeks. (Wakim et al., 2021).
There are three distinct phases outlining embryonic development. The cleavage process is the initial cell division of the zygote after conception. Gastrulation, neurulation, and organogenesis are the cellular arrangements an embryo undergoes. These processes begin the development of major organ systems within the body.
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Weeks 1 8 after conception (first trimester)
4 Distinct phases: – Cleavage – Gastrulation – Neurulation – Organogenesis
Cleavage Process
Week 1 after conception
Occurs within the fallopian tube
Rapid mitotic cellular division
The resulting structure is called a morula
Blastocoel forms in the center resulting in a blastula
Cleavage phase ends, implantation begins
Inside the egg, the zygote undergoes rapid mitotic cell division, resulting in smaller and smaller cells called blastomeres. Identical chromosomes are produced inside each new cell. The resulting structure, consisting of 16-32 cells, is called a morula, named after the Latin word for raspberry which it resembles. This structure is short-lived. Water begins to flow into the structure, forcing the cells outward and forming a cavity in the center called a blastocoel. Cell division continues until hundreds of cells are present, however the blastula is still no larger than the original zygote. The egg is much larger than the normal human cell, so the cell division has now resulted in blastomeres that are the size of normal cells which are already destined to become specific tissues (Houp, 2021).
During this phase, the embryo starts to develop distinct cell layers, and the development of organs and organ systems begins (Wakim et al., 2021).
The cleavage phase ends with the blastula, and implantation occurs.
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Placenta & Umbilical Cord Formation
Occurs during implantation phase
Flexion places the embryo into the classic C shape with yolk sac on the left side
Placenta consists of both embryonic and maternal tissues
Amnion forms a sac that surrounds the embryo
Provides essentials to the developing embryo throughout the pregnancy and is delivered with the fetus at birth
During the implantation phase, hormonal changes prepare the uterus for the attachment of the blastula as it arrives from the fallopian tube. Upon arrival, the embryo undergoes flexion in various directions that place it into the classic C shape, with the yolk sac ending up on the left lateral side. The outer chorion membrane becomes highly vascularized and attaches to the uterine wall to begin the formation of the placenta. The amnion layer thickens and expands around the embryo. Fluid fills the amnion and the embryo becomes suspended in the protective fluid (Houp, 2021).
The placenta is a lifeline that develops between the embryo and mother. It allows the transfer of substances between them. The amniotic cavity is surrounded by a membrane called the amnion, which forms as a sac around the developing embryo. The yolk sac nourishes the early embryo, and the chorion develops into the fetal portion of the placenta. The placenta and umbilical cord will provide the embryo (and eventually the fetus) with nutrients, oxygen, antibodies, hormones, and other needed substances from the mother. It also passes waste products (such as urea and carbon dioxide) from the offspring to the mothers blood for her to process and excrete (Wakim et al., 2021).
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Gastrulation Process
Begins late in week 2 after conception
Archenteron forms
Blastula folds to create 3 layers -Ectoderm -Mesoderm -Endoderm
Gastrulation is the first phase of cellular migration that arranges cells into place to form three embryonic germ layers. These three layers are destined to give rise to all body structures and organ systems (Houp, 2021).
The first phase of gastrulation begins late in the second week after conception, only after the blastula is fully formed. The blastula, made up of one layer, folds inward and gives rise to two new layers. The resulting gastrula will consist of three layers: the ectoderm, mesoderm, and endoderm.
Eventually, these layers will form different organs and systems within the body of the fetus. The inner layer ( the endoderm) will form cells of many internal glands and organs, including the lungs, intestines, thyroid, pancreas, and bladder. The middle layer (the mesoderm) will form cells of the heart, blood, bones, muscles, and kidneys. The outer layer (the ectoderm) will form cells of the epidermis, nervous system, eyes, inner ears, and many connective tissues (Wakim et al., 2021).
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Blastopore formation
The final phase of gastrulation is the development of the primitive gut that will eventually become the gastrointestinal tract. A hole develops in the side of the embryo. This primary opening will eventually become the anus. The hole continues to burrow through the embryo until another opening develops. This secondary opening will become the mouth. The completed tunnel through the endoderm is called the digestive tube. The completion of the digestive tube marks the completion of the gastrulation phase (Wakim et al., 2021).
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The final phase of gastrulation
The formation of a primitive digestive tract
Neurulation
The final stage of early embryonic development
Occurs during weeks 3 and 4 after conception
Neural groove becomes neural tube
Neurulation, the beginning of nervous system development, is recognized as the final stage of early embryonic development.
It begins when a structure of differentiated cells called a neural plate forms from the ectoderm. After the neural plate is formed, a buckling occurs in it, forming a depression known as the neural groove. This groove deepens and begins to fold inward until its borders converge. The convergence of the borders results in the formation of a neural tube. Most of the neural tube will eventually become the spinal cord. The neural tube also develops a bulge at one end, which will later become the brain (Houp, 2021).
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Spina Bifida
Spina bifida is a term used to describe a fetal abnormality that occurs during the embryonic neurulation phase. When edges of the neural plate fail to seal along the posterior or dorsal region, it leaves openings called neuropores that allow the internal components of the spinal canal to protrude through the vertebrae, and in severe cases through the muscle and skin layers, leaving them completely exposed to the outside world. There are several forms and severities of spina bifida and all are capable of causing a range of issues after birth, including still birth (Hosick, 2019). The chart above reviews different forms of this birth defect.
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A major complication that can occur during the neurulation phase of embryonic development
Describes several types of abnormalities that result from incomplete closure of the neural tube
Cardiogenesis
The heart is the first functional organ to develop
Heartbeats begin by day 21-23 after conception
The heart is fully formed by the end of the eighth week after conception
Blood vessels begin developing in the mesoderm during the third week after conception. The heart begins as two endocardial tubes that develop, migrate, and fuse together a few days later. This is called the heart tube. The heart tube begins to beat and pump blood around day 22 even as it continues to develop. Within 1-2 days the heart tube has developed 5 distinct regions that will develop into the chambers and septa of the fully-formed heart by the end of week 8 (Wakim et al., 2021).
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Organogenesis
Most organs begin to develop between weeks 3 and 8 after conception
Growth will continue during the fetal phase and after birth
Embryonic phase is complete at the end of week 8 after conception
Week 5 Week 6 Week 7 Week 8
– Pharyngeal arches Inner ear Arm buds Liver Pancreas Spleen Gallbladder Eyes and nose Leg buds Hands Kidneys Stomach Lungs Longer arms and legs Digits Lymphatic system Sex organs Hair follicles Nipples External ears All other essential organs Heartbeat and limb movements detectible by US
Organogenesis is the generation of organs from the germ layers. Several organs begin to develop simultaneously during the third and eighth weeks after conception. The chart above lists some of the organs that begin to develop during specific weeks of the embryonic phase. By the end of week 8 after conception, once all of the primitive organs have begun to emerge, the embryonic phase is complete and the being is now considered a fetus. The fetal heartbeat and movement is now visible by ultrasound (Wakim et al., 2021).
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Risks to Embryo Development
Nutritional deficiencies
Alcohol consumption
Tobacco use
Illicit drug use
Viral exposure
Radiation exposure
Inhalant anesthesia
Hereditary conditions
The embryonic development phase is a delicate one. Malformation usually begins during early development. Deformities may arise from inherited mistakes in the genetic code or from the harmful influence of external factors such as those listed above (Houp, 2021).
Malformation usually begins during early development. Genetic defects or harmful environmental exposures during this stage are likely to have devastating effects on the developing organism. They may cause the embryo to die and be spontaneously aborted (also called a miscarriage).
If the embryo survives and goes on to develop and grow as a fetus, it is likely to have birth defects. In some cases, the fetus may survive and have no apparent outward abnormalities, however, exposure to some of these factors, such as alcohol, are linked to behavioral problems and cognitive disorders later in life. Others, like cigarette smoking, are linked to low birth weight, heart defects, and chronic illnesses like asthma (Wakim et al., 2021).
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References
Hill, M. A. (2020, May 14). Timeline human development. Embryology. Retrieved June 4, 2021,from https://embryology.med.unsw.edu.au/embryology/index.php/Timeline_human_development
Embryo. (2021). Columbia Electronic Encyclopedia, 6th Edition, 1.
Houp, K. H. (2021). Embryonic development. Salem Press Encyclopedia of Science.
Wakim, S., & Grewal, M. (2021, May 30). Embryonic Stage. Retrieved June 5, 2021, from https://bio.libretexts.org/@go/page/17805
Hosick, H. L. . P. D. (2019). Spina bifida. Magills Medical Guide (Online Edition).
