Embryonic Development Britni Schlachter Rasmussen University BSC2347/L: Anatomy & Physiology II Lab Prof. Elizabeth St. Louis 1 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. 2 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. 3 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 ...

1. Embryonic Development
Britni Schlachter
Rasmussen University
BSC2347/L:
Anatomy & Physiology II Lab
Prof. Elizabeth St. Louis
1
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.
2
Weeks 1 – 8 after conception (first trimester)
4 Distinct phases:

2. - 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

3. (Wakim et al., 2021).
The cleavage phase ends with the blastula, and implantation
occurs.
3
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

4. 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 mother’s blood for her to process and
excrete (Wakim et al., 2021).
4
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

5. 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

6. 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

7. 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 5Week 6Week 7Week 8- Pharyngeal arches
Inner ear
Arm buds
Liver
Pancreas
Spleen

8. GallbladderEyes and nose
Leg buds
Hands
Kidneys
Stomach
Lungs
Longer arms and legs
Digits
Lymphatic system
Sex organsHair 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

9. 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/Tim
eline_human_development
Embryo. (2021). Columbia Electronic
Encyclopedia, 6th Edition, 1.

10. 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. Magill’s
Medical Guide (Online Edition).
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