Human Embryo Development

Investigatory Project for Class 12

Student
Information:
Name:
R.Ratchitha
Class: XII
School:
BHISK
 Introduction
Human development is one of the most exciting
topics to study not only as a medical student, but
also for our fundamental understanding of the
human body. Of all health issues in Medicine,
fertility and reproduction is a topic that will affect
everyone. Now a days, the interest in human
development is widespread largely because of
curiosity about our beginnings and a desire to
improve the quality of human life. The process by
which a baby develops from a single cell is
miraculous and few events are more exciting than
a human birth.

Human development is a continuous process

that begins when an ovum is fertilized by a
sperm. Cell division, growth, differentiation,
and even cell death, transform the fertilized
ovum into a multicellular human being.
Although important maturational changes
continue to occur during the postnatal period
(infancy, childhood, adolescence and even
adulthood), the formation of the organ
systems occurs between fertilization and
birth, the prenatal period. The four initial
stages of development are the morula stage,
the blastula stage, the gastrula stage and the
neurula stage.
An embryo is an early stage of development
of a multicellular diploid eukaryotic organism.
In general, in organisms that reproduce
sexually, an embryo develops from a zygote,
the single cell resulting from the fertilization
of the female egg cell by the male sperm cell.
Embryogenesis is the process by which the
embryo forms and develops. In mammals, the
term refers chiefly to early stages of prenatal
development, whereas the term fetus and
fetal development describe later stages.
Embryogenesis starts with the fertilization of
the egg cell (ovum) by a sperm cell,
(spermatozoon).
 HISTORIC
EMBRYOLOGY

o Around 1400 BC Egyptians referred to the

placenta and its importance as the seat of
the external soul.
o Hippocrates and Aristotle are recognized
as the first true embryologists.
o Hippocrates believed that the embryo
began development by extracting
moisture and breath from the mother and
he identified a series of condensations and
fires that were responsible for the
development of bones, belly, and
circulation in the embryo and fetus. He
also supported the view that the human
fetus gained nourishment by sucking
blood from the placenta.
o Aristotle studied embryos of different
organisms by opening bird eggs at
different stages of development and
dissecting mammalian and cold-blooded
embryos. He observed that young
embryos of different species all possessed
universal characteristics and that as the
embryos aged, differentiating
characteristics arose.
o Galen’s main contribution to embryology
was his steadfast belief that the umbilical
cord was necessary for respiration.
o Albertus Magnus reawakened scientific
embryology. Albert believed that women
had seeds and that seeds coagulated,
much like cheese, after encountering male
seeds. When a coagulated seed contacted
menstrual blood, the seed now had the
nutrition necessary for proper
development.
o Nicholas Stensen in 17th century
demonstrated that the human female
ovary was homologous to the ovaries in
previously studied oviparous animals.
o The mammalian egg was finally seen and
recognized as a single cell; the idea of the
recapitulation theory began to take shape.
o Scottish surgeon John Hunter showed that
the maternal and fetal circulations were
distinct physiologies.
o Karl Ernst von Baer and Heinz Christian
pander proposed the germ layer theory of
development; von Baer discovered the
mammalian ovum, in 1827.
 o The span of gestation and the cause of
the birth were discovered by Jena, Gustav
Fischer in 1897.

Pregnancy is roughly divided into 3

stages known as trimesters of about 3
months each:

first trimester – conception to 12 weeks.

 From week 1 to 4
 From week 5 to 8
 From week 9 to 12
 Week 13

second trimester – 13 to 27 weeks.

 From week 14 to 17
 From week 18 to 21
 From week 22 to 25
 From week 26 to 27

third trimester – 28 to 40 weeks.

 From week 28 to 30
 From week 31 to 34
  From week 35 to 38
 From week 39 to 42

FIRST TRIMESTER
From week 1 to 4:

The weeks of pregnancy are dated from the

first day of the last period. This means that in
the first two weeks the body will be preparing
for the ovulation as usual. Amazingly, during
the third week the child's sex and all its
inherited genetic characteristics. The
developing baby now called a zygote. After
fertilization, the ball of cells, now an embryo,
the embryo burrows into the lining of the
womb (Implantation). During the fourth week
the embryo splits into two parts. One half will
become the placenta. In the other half, the
embryo itself continues to grow, and a sheet
of cells has just begun to create the neural
tube, where the baby's brain, spinal cord and
backbone will ultimately form.

From week 5 to 8:

During the fifth week, the embryo ticker will

start beating for the first time. The embryo
now has three distinct layers: the ectoderm,
the endoderm, and the mesoderm. Starting
from the sixth week until birth, the embryo
heart will beat about 150 times a minute. And
in the 6th week the brain hemispheres are
forming, and brain waves can now be
recorded. During the 7th week the embryo
develops distinct facial features. The nerve
cells in the baby's brain are growing at an
amazing rate - 100,000 cells per minute. At 8
weeks, the fetus is about one-half an inch
long (1.1cm). Facial features such as
developing ears, eyelids, and nose tip are
present.

From week 9 to 12:

During the 9th week the reproductive organs

are beginning to form, along with some other
key organs, like the pancreas and gallbladder.
Until 10th week the baby was classified as an
embryo, but by the end of the 10th week the
baby will be classified as fetus. Paddle-like, or
webbed, hands and feet will now separate
into fingers and toes, bones will begin to
harden, and his kidneys are now producing
urine. From the 10th and 11th weeks the
fetus will start to inhale and exhale small
amounts of amniotic fluid. At 12 weeks, the
fetus has grown to about 2 inches (4.4cm) in
length and may begin to move by itself. The
fingers and toes are discernible, and the fetal
heartbeat may be audible by Doppler
ultrasound.

Week 13:

During this week the fetus arms will lengthen

to be proportionate with his body, and the
fetus will be able to stick thumb in the mouth.
Also, by now, all the essential organs and
systems have formed.

A visual summary of the first trimester

 SECOND TRIMESTER
From week 14 to 17:
In 14th week if the fetus is a boy, the prostate
is forming, and if the fetus is a girl, her
ovaries are moving down into her pelvis. In
15th week the Lanugo will be formed which is
the first ultrafine, downy hair, now covers his
back, shoulders, ears, and forehead. It helps
fetus retain body heat, but once he gains
enough fat to do the job, this hair will fall off.
In the 16th week the fetus’s delicate skeleton
continues to harden from rubbery cartilage to
bone. The umbilical cord has fully matured. In
the 17th week the fetus starts plumping up
this week, as body fat is deposited under his
skin and sweat glands develop.

From week 18 to 21:

In the 18th week the bones and nerves in ears
are developed enough to function, the fetus
can hear all sorts of sounds including blood
coursing through the umbilical cord and the
mother’s heartbeat. In the 19th week Vernix
caseosa begins to coat the skin. This coating
helps regulate body temperature and protects
from the amniotic fluid. At 20th weeks the
developing baby is about 6 inches long and
may weigh about 10 ounces. The baby may
begin to make movements that the mother
can feel at about 19 to 21 weeks; this baby
movement is termed "quickening".

From week 22 to 25:

During 22nd week the baby’s skin becomes
opaque but will remain wrinkly, red, and
covered in vernix. Also, this week, the baby’s
fine-tuning his sense of touch. Billions of brain
cells will develop and control the baby’s
movements and life functions. Surfactant is
being produced, a substance that enables the
air sacs to inflate and the lungs to fully
expand. At 24 weeks, the baby may weigh 1.4
pounds. The baby's inner ear canals are
developed at 24 weeks, so researchers
speculate the baby can sense its position in
the uterus. During the 25th week the baby's
first bowel movement will form in the large
intestine. The thick, dark poop is called
meconium and it's typically excreted shortly
after the baby born.

From week 26 to 27:

The baby settles into a distinct sleep-wake
pattern. And the baby has been hiccupping
every once in a while, for several weeks, and
the parent might actually feel these baby
hiccups, which are triggered by the
involuntary movements of the baby’s
diaphragm. But as the baby's trachea is filled
with fluid instead of air, fetal hiccups don't
make a sound.

A visual summary of the second trimester

 THIRD TRIMESTER
From week 28 to 30:

Until now, the baby's eyelids have been fused

shut, but in 28th week the baby will begin to
open and close the eyes. And the irises are
now filled with pigment. At 28 weeks, the
baby normally weighs about 2 and one-half
pounds and has developed to the point that if
the baby is birthed prematurely for any
reason, the chances are good that the infant
will survive. From week 30 the baby will gain
about a half pound each week. Now that all of
the major body systems are in place and
functioning and needs padding to protect and
insulate the baby’s organs.

From week 31 to 34:

In the week 31 there is a development in the
reproductive organs. If the fetus is a male,
then the testicles will descend into the
scrotum through the inguinal canal. If the
fetus is female, then the clitoris will be
formed. By 32 weeks all the major organs are
fully functioning except the lungs, which need
just a bit more time to mature completely. In
the week 34 the protective vernix caseosa
begins to thicken. The bones in baby's skull
aren't fused yet. That allows them to shift as
his head squeezes through the birth canal.
They won't fully fuse until adulthood.

From week 35 to 38:

Now, the unborn baby should be positioning
the head facing down towards cervix and
vagina. And the baby tries to accumulate all
the fat which helps the baby to maintain the
body temperature and store energy. In the
week 37 the umbilical cord begins passing to
the baby in preparation for delivery. The
baby’s hearing, eyesight is improving each
day. His fingers are also becoming more
coordinated.

From week 39 to 42:

By now the baby has likely reached the birth
weight (6-9 pounds) and length (18-22
inches). At 39 weeks, the baby will be
considered full-term infant. The placenta
continues to supply the baby with nutrients
and antibodies until the parturition.
A visual summary of the first trimester
Comprehensive Visual of
Embryonic Development
 SOME RECENT FINDINGS
Stresses and strains on the human fetal
skeleton during embryonic development:
“Mechanical forces generated by fetal kicks
and movements result in the stimulation of
the fetal skeleton in the form of stress and
strain”. Size and location of the kidneys
during the fetal period: “The level of the left
kidney was higher than the level of the right
kidney in the fetal period. The posterior
surface relations to the ribs showed certain
ascendance during gestation, corresponding
to the vertebral levels. The offspring of the
women with obesity during their pregnancy
are exposed to an altered intra-uterine
environment. A subsequent influence on the
cardiovascular development during fetal life is
assumed. Estero(E4) is a natural estrogen
synthesized during pregnancy by the human
fetal liver, and the physiological role of this
hormone is unknown. Biologists have that E4
has anti-thrombotic properties.
 CONCLUSION
Embryonic development has fascinated
scientists and philosophers from ancient
culture to the present day. This project
explores embryonic development. It is very
amazing and interesting to learn about the
human development and how our bodies
came into being. Where development
describes the growth of humans throughout
the lifespan, from conception to death. This
project has helped to know the logical basis
for understanding the overall organization of
the human body. It has become increasing
clear that the key features of embryonic
development have remained for the most part
unaltered the evolution. While animals show
obvious differences in appearance, most of
their genes are well preserved, demonstrating
roughly similar structure and function.
However, despite years of dedicated research,
much remains to be discovered on the
formation of gametes (the sex cells),
fertilization, and the subsequent development
of the embryo.
 BIBLIOGRAPHY
Websites:
https://www.pampers.com/en-us/
pregnancy/pregnancy-calendar/first-
trimester
https://www.parents.com/pregnancy/signs/
symptoms/a-pregnancy-symptom-timeline/
https://www.pampers.com/en-us/
pregnancy/pregnancy-calendar/second-
trimester
https://www.pampers.com/en-us/
pregnancy/pregnancy-calendar/third-
trimester
https://embryo.asu.edu/pages/history-
embryology-1959-joseph-needham
https://www.onhealth.com/content/1/
fetal_development_stages
https://www.sciencedirect.com/