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HUMAN
EMBRYOLOGY
(Conception to Birth)
MRS. BUDZINSKI
TOPICS IN SCIENCE
Gametes join in fertilization

In grade 9 you learn about how sperm and eggs are produced through
meiosis.

Sperm are produced in the testicles by a process called spermatogenesis

Eggs (Ovum) are produced in the ovaries by a process called oogenesis.

Males start producing sperm when they hit puberty, producing millions a
day

Females are born with all of the eggs they will ever have. 1-3 eggs start to
mature during each menstrual cycle.

The gestation period lasts 266 days from fertilization to birth
Gamete Production
Gametes

In order for conception to occur, sperm and eggs need to come together.

During sexual intercourse, the male will ejaculate almost 180 million sperm
per mL.

It only takes 1 sperm to reach and fertilize the egg to make a baby, but the
female reproduction system is a scary and hostile place for sperm, and most
never reach the egg.
Fun Sperm Facts
1.
A male is considered infertile if he
releases 20 million per mL of sperm or
less.
2.
On average, it takes 5-65 minutes for
sperm to reach the fallopian tube
which is where fertilization generally
takes place. It only takes about 3
minutes in cows.
3.
Sperm typically stays in your body for
48 hours after sex, however, "it can
hang around for as long as five days to
a week," depending on how "friendly"
your cervical mucus is.
Gestational Age

Most obstetricians go by the gestational age (menstrual age).

Your gestational age actually begins on the 1st day of your last
period. The abbreviation for this would be (LMP), which
stands for last menstrual period.

With this method the average length of pregnancy is 40
weeks. So pregnancy week 1 would be the week of your
period, and you would not conceive for another two weeks or
so.

The reason why most doctors use this method is because
many women do not know with certainty the exact day that
they conceived their baby. But most women can remember
the first day of their last period.
Let the Division Begin!
• Week 1 to Week 8 are
considered the embryonic
period of development.
• Week 9 to week 37 or birth
are considered the fetal
period of development.
• The first month after birth
is the neonatal period of
development.
Gestational Age – 1 week (ie. 1 weeks after the day of your last period)
1 Week Pregnant

It is only 1 week after your period. Your uterine lining is thickening as
part of your regular cycle.

An ovary is going to start maturing a follicle which will turn into a
mature egg, ready for the sperm.
Gestational Age – 2 week (ie. 2 weeks after the day of your last period)
Fertilization
Only 1 sperm will be allowed to enter the egg.
 The egg will instantly put up a barrier, not allowing any
other sperm to enter.
 The sperms tail will break off as it is not needed anymore,
and the genetic DNA of the sperm will combine with that of
the egg, giving the developing zygote all of its genetic
material.
 When the egg and sperm combine, it is called a zygote.

The zygote divides into two cells, and
then keeps dividing every twelve to
twenty hours as it continues traveling
down the fallopian tube into the
uterus.
Cleavage follows fertilization

Gametes are produced by meiosis, but the baby grows and cells divide
with mitosis.

Cleavage is a series of rapid mitotic divisions (without cell growth)

The two-celled zygote divides repeatedly until a ball of 32 cells is formed
called a Morula (Day 3-4 after fertilization)

At this point, the morula is still moving down the fallopian tube towards
the uterus.

By day 5 after conception, the cluster of
divided cells is referred to as a blastocyst.
The blastocyst must implant itself into the
endometrium, or the thickened inner wall
of the uterus, where it can continue to
develop into an embryo.
Gestational Age – 4 week (ie. 4 weeks after the day of your last period)

Continued divisions make the hollow blastula

These few cells are pluripotent (have the potential to become ANY of the
220 types of cells in the human body).

These are embryonic stem cells

It is during the blastula phase that the egg will implant in the uterus.
(Around day 8)- this is when pregnancy really starts.
Week 4 Pregnant

Your body starts to produce the Human Chorionic
Gonadotrophin (HCG) hormone, which you can be able to
detect in your urine if you take a pregnancy test. The
hormone promotes the secretion of the progesterone
hormone by the ovaries, thus preventing the shedding of
the uterus lining.

It is this hormone that leads to some early pregnancy
symptoms. (Breast tenderness, nausea, vaginal bleeding
(spotting))
Extraembryonic Membranes

These membranes develop from the germ layers, but are NOT part
of the embryo (they are lost at birth)

They lie outside of the embryo & provide protection and
nourishment

Four exist in terrestrial vertebrates:

Chorion, amnion, allantois (stores nitrogenous waste in reptiles), &
yolk sac (not found in humans, but yolk aids in formation of RBC

Organogenesis (development of the organs and organ systems)
begins with the nervous system when the ectoderm turns into the
neural grove.
Implantation video
Gastrulation


At the end of the cleavage stage, cells making up the blastula
move about and surface proteins help cells recognize each
other
The gastrula is formed, which consists of 3 “germ layers”

Endoderm

Mesoderm

Ectoderm
video
Gestational Age – 5 week (ie. 5 weeks after the day of your last period)
5 Weeks pregnant

This marks the beginning of the embryonic period.

It is during this week and week 6 that many women find out they are
pregnancy because they have missed their periods, and all early
pregnancy symptoms start to show.

Deep in the uterus your embryo is growing at a furious pace. At this
point, he's about the size of a sesame seed, and he looks more like a tiny
tadpole than a human.

He's now made up of three layers — the ectoderm, the mesoderm, and
the endoderm — which will later form all of his organs and tissues.
The Three Layers

Ectoderm


Mesoderm


The neural tube —baby's brain, spinal cord, nerves, and backbone will
sprout. This layer will also give rise to his skin, hair, nails, mammary and
sweat glands, and tooth enamel.
His heart and circulatory system. This week, in fact, his tiny heart begins to
divide into chambers and beat and pump blood.) The mesoderm will also
form your baby's muscles, cartilage, bone, and subcutaneous (under skin)
tissue.
Endoderm

Lungs, intestines, and rudimentary urinary system, as well as his thyroid,
liver, and pancreas. In the meantime, the primitive placenta and umbilical
cord, which deliver nourishment and oxygen to your baby, are already on
the job.
Gestational Age – 6 week (ie. 6 weeks after the day of your last period)
Early pregnancy symptoms

Nausea and/or vomiting (morning
sickness)

Having a full feeling in your lower
abdomen

Breast tenderness or soreness

Food cravings or new dislikes

Frequent Urination

Heightened sense of smell

Heartburn


Fatigue or Tiredness
Darkening of the areolas (area
surrounding your nipples)

Abdominal Cramping and/or spotting

Constipation

Irritability or Mood Swings

High Basal Body Temperature (BBT)

Headaches

Metallic taste in your mouth

Dizziness or Fainting

Increased saliva production

Missing your menstrual cycle/period

A positive pregnancy test

Bloating or Weight Gain
6 Weeks Pregnant

Growth is rapid this week. Just four weeks after conception, the neural
tube along your baby's back is closing and your baby's heart is
pumping blood. The neural tubes undergo cellular processes that form
certain nerves tissues, tissues of the central nervous system, skeletal
structures and muscular components of the head and neck.

The openings of neural tube close; this anatomical change is very
central since it can lead to the development of spina bifida, a serious
abnormality related to the neural tube defects.

Basic facial features will begin to appear, including passageways that
will make up the inner ears and arches that will contribute to the jaw.
Your baby's body begins to take on a C-shaped curvature. Small buds
will soon become arms and legs.
• Right now your baby is about .08
to .16 of an inch, or 2 to 4mm
long. This is the average
measurement from crown-torump.
Gestational Age – 7 week (ie. 7 weeks after the day of your last period)
7 Weeks Pregnant

During this week, the embryo continues to grow in a curved shape.
Facial features and the brain are rapidly developing compared to the
rest of the body parts.

The embryo has a large forehead as the eyes lenses, the tiny nostrils and
the inner ear are forming. The arm and legs buds continue to form as the
ends of the upper arm take the shape of paddles. Retina and tongue start
to develop.

Uncoordinated muscle movement occurs, but nothing can be felt yet.

The pancreas is completely developed and ready to produce hormones
which will help the baby assimilate the nutrients and grow faster.

By the end of this week, the embryo has grown to about 9 to 10mm, a
size that is little bigger than that of the pencil eraser.

A major event happening during week 5 is the separation
between the baby’s and the mother’s blood. This doesn’t mean
the embryo begins an independent life inside the uterus – he’s
still connected to the mom’s body through the umbilical cord
and placenta, but his body begins producing its own blood.
Gestational Age – 8 week (ie. 8 weeks after the day of your last period)
8 Weeks Pregnant

During this week, there is a rapid development of the upper limbs.
The cartilages and bones are developing. The buds start to form
cartilages, which will later develop into bones of the arms and legs;

The facial structures continue to develop to form the outer ear. Eyes
become prominent. The nose and the upper lips begin to form as well.

The trunk of the embryos body begins to straighten.

By the end of the week, your baby is about 11 to 14 mm and is
weighing around 1 gram, about the size of a pea.

Reproductive organs start developing during pregnancy week 6
(Gestation 8) so it’s only a matter of days until you’ll be able to find out the
baby’s sex.

The intestines continue their development and the tubes connecting the
lungs and throat are growing and begin to form really tiny branches.
However, it will take no less than 36 weeks for the baby’s lungs to mature
and work by themselves.
Gestational Age – 9 week (ie. 9 weeks after the day of your last period)
9 Weeks Pregnant

The tiny embryo measuring less than one millimeter in the first day of
life is now almost 15 millimeters long, about the size of a blueberry.

Also, the cartilages continue developing and the baby’s intestine
starts moving out of the umbilical cord. Once they reach the baby’s
abdomen, he or she starts drinking amniotic fluid.

The baby’s kidneys start working so this is another important event
taking place this week.

Fetal development continues by the end of pregnancy week 9 with the
complete development and separation of the four unique chambers
inside the baby’s little heart.
The babies arms can bend at the elbows and it is
constantly kicking and moving around.
Gestational Age – 10 week (ie. 10 weeks after the day of your last period)
10 Weeks Pregnant

The neck begins to form and to become visible while your
baby’s eyelids begin to function to protect the developing
eyes.

The upper and lower limbs continue to grow as the fingers and
toes develop and become separated.

By the end of the week, the fetus resembles distinctly human,
though the head is still large compared to the body.

The heart is now fully-formed and the rate of its heart beats
around this time is between 130 times to 180 times a minute;
this heart rate decreases gradually as the pregnancy
progresses.

At this time, you could be able to see the upper lip and the
tiny nostrils in the nose.
Gestational Age – 11 week (ie. 11 weeks after the day of your last period)
11 Weeks Pregnant

During this week, most of the major organs system will develop to some
great extent while the growth of the head slows. In essence, your baby’s
external genitalia begin to develop to form either a penis or clitoris.
Other internal organs like the lungs, intestines, kidneys and liver are
developing and the red blood cells are forming in these organs.

During this week, the placenta is completely developed to support most
of the vital role of hormone production. By now, your baby will be about
2 inches or 50 millimeters long and weighs almost 1/3 ounce or 8 grams.

The connections between the nerves and the other organs enable him to
coordinate his movements, although the baby’s not really aware of what
he’s doing.

Little movements are felt by mom.
12 Weeks Pregnant

As the uterus rises above the pubic bones, your baby bump will become
more visible.

The baby weights around 14 grams and measures almost 5 centimeters.

The baby’s intestines are completely developed, the urinary system works
perfectly – yes, your little boy or girl is able to produce the first pee and
poop, but this won’t happen until you give birth.

The genitals keep developing and sometime this week your baby’s mouth
will start moving and making sucking gestures.

Your baby is developing fingernails.
This marks the end of 1st trimester
video

Another event happening by pregnancy week 12 is represented by the
movement of the baby’s intestine from the umbilical cord into his
abdominal cavity.

Also, the baby’s eyes move from the sides to the front of the head, so his
face looks perfectly human now. However, the two layers of skin covering
his tinny body are still transparent and coated with a very fine down.