Download Embryology - Dr Magrann

EMBRYONIC DEVELOPMENT
► Embryology – study of the origin and development of single individual
This is an amazing process that one cell can grow into an entire organism is 9 months!
Remember, typical (diploid) cells of the body have 46 chromosomes; and each gamete
has 23 chromosomes.
PARTURITION is the length of the pregnancy. The average length of parturition is 280
days after the last menstrual period.
► Prenatal period
 Embryonic period – first 8 weeks
 Fetal period – remaining 30 weeks
FERTILIZATION
► Events leading to fertilization
 Sperm binds to the egg
 Fusion of oocyte and sperm plasma membranes
 Enzymes prevent any other sperm from binding to the egg
 Fertilization – chromosomes of male and female gametes join
SPERM
This is the simplest and smallest cell in the body. It consists of a FLAGELLUM (to
swim), a NUCLEUS (to carry DNA), and an ACROSOME (for fertilization.
The function of the sperm is to allow the nucleus to reach the egg.
EGG
The egg has to support the embryo for a few weeks. Eggs are so large they can be seen
with the naked eye. It contains nutrients = YOLK.
Here are the relative sizes of the egg to the sperm:
Around the egg is the CORONA RADIATI. When the sperm reach the egg, millions of
them break open their acrosome and release enzymes to digest the corona radiate to get
into the egg. The first sperm to do this causes the egg to prevent any other sperm from
getting in. It is impossible for more than one sperm to fertilize one egg.
The egg begins rapid cell division = CLEAVAGES. This occurs as the egg is going
from the fallopian tube to the uterus. It takes one week to get to the uterus.
DAY 1: The egg is one cell = ZYGOTE.
DAY 3: The egg has begun to divide = MORULA. It enters the uterus in this stage.
DAY 7: The egg is thousands of cells = BLASTOCYST, which begins to implant into
the uterus (week one) by burrowing into it like a parasite.
DAY 60: The egg has now developed into a FETUS
Fertilization occurs on about day 14 of the menstrual cycle.
Implantation occurs on about day 21 of the menstrual cycle.
BLASTOCYST
The blastocyst contains two cell types:
1. Outer cells are the =TROPHOBLAST, which forms the PLACENTA
2. The inner cells are the INNER CELL MASS, which becomes the EMBRYO
and surrounding structures.
The trophoblast cells secrete a hormone = hCG (human chorionic gonadotrophin). This
hormone maintains the corpus luteum, which makes the progesterone that maintains the
uterine lining to grow. If no hCG, there will be menses.
hCG is the hormone which is measured in a pregnancy test. It will be in sufficient
quantities to be measured within about one week after a missed period.
INNER CELL MASS
Sometimes two inner cell masses develop = IDENTICAL TWINS. These have the
same embryo, same genetic make-up. Occurs randomly. If more than one egg becomes
fertilized = FRATERNAL TWINS.
If the inner cell mass doesn’t completely separate = CO-JOINED TWINS, which are
always identical. They can be joined anywhere. Sometimes they are joined at the chest
and share a heart, or are joined at the head and share meninges and a blood supply. This
can be detected in utero by ultrasound.
The first known co joined twins were from Siam, so they were called Siamese twins.
They married two sisters and had lots of kids. One died two days before the other one.
SPONTANEOUS ABORTION (Miscarriage)
Can occur for many reasons:
1. Implantation did not occur (woman never knows she’s pregnant)
2. Not enough hCG (menses occurs)
3. Developmental problem in fetus  death in utero
4. Not enough estrogen  lining breaks down
5. Improper implantation
6. Improper placenta
There are many other reasons as well. No one knows the rate of miscarriages, but they
guess that 70% of fertilized eggs get miscarried, and about 30% of implanted eggs
miscarry.
STRUCTURE OF PLACENTA
The placenta is derived from the TROPHOBLAST, as the embryo burrows into the uterus.
The trophoblast gives rise to the CHORION, which have CHORIONIC VILLI that
burrow into the uterus. The capillaries within a chorionic villus of the placenta
contain blood from the fetus ONLY, not the mother. Therefore, this tissue can be used
for genetic testing for birth defects.
The chorion surrounds the embryo at first. With time, it disintegrates in one area and
grows in another area. The part of the chorion that survives is the PLACENTA (12
weeks). Before 12 weeks, it is called the chorion.
PLACENTA
This allows oxygen and nutrients to diffuse from the mother to the fetus, and waste
products to diffuse from the fetus to the mother. This requires a lot of surface area.
The placenta is derived from the embryo, not the uterus.
The uterus has lots of blood vessels which empty into these spaces so the placenta is
surrounded by blood. There is still a membrane separating the capillary bed and
umbilical vein. The umbilical artery is BLUE, and the vein is RED. The UMBILICAL
CORD connects the placenta to the fetus.
The purpose of the placenta is to make sure there is enough O2 and nutrients for the fetus.
Other things can diffuse across the placenta, including drugs, viruses, alcohol, and
tobacco products.
The placenta can implant anywhere in the uterus. If it implants really low and covers part
of the cervix = PLACENTA PREVIA. This requires a C-section. The ultrasound can
check for the location of the placenta.
DEVELOPMENT OF THE EMBRYO
The embryo develops from the INNER CELL MASS.
Within the trophoblast there forms a two-layered structure:
1. EPIBLAST (shown in blue)
2. HYPOBLAST (shown in yellow)
Then, some of the epiblast cells move up, covering the inside of the trophoblast. Some of
them move between the epiblast and the hypoblast. This creates a three-layered embryo
through a process called GASTRULATION. The blastocyst is now called a
GASTRULIN.
The outer layer is the AMNION.
The other three layers make up the PRIMARY GERM LAYERS, which give rise to all
the other structures of the body:
1. ECTODERM  hair (nails, sweat glands), and nervous system
2. MESODERM  connective tissue and muscle, bones, and blood
3. ENDODERM  the GI tract (kidney, urinary bladder)
At this time, the gastrulin is like three sheets of paper (blue, pink, yellow).
Then a series of folds occurs:
The anterior fold = HEAD FOLD
The posterior fold = TAIL FOLD
Then all three layers form a TUBE.
The AMNION is filled with AMNIOTIC FLUID, which surrounds the embryo.
The function of the fluid is:
1. Cushions embryo
2. Allows the embryo to float so it can develop in three dimensions
3. Allows room for the embryo/fetus to move
The amnion ruptures just before birth = WATER BREAKING.
Sometimes it doesn’t tear, so the doctor goes in with a crochet hook device to puncture it.
In the fluid are fetal skin cells which can be gathered by an AMNIOCENTESIS.
This is only helpful in identifying chromosomal problems, not birth defects.
So here we see the tube in longitudinal and in cross section. The tube is held in place by
the mesentery. A mesoderm layer lines the inside of the amnion and surrounds the tube
which will form the GI tract, but there are no organs yet. At this time, it has just been 221/2 weeks after fertilization. Over the next few months, the organs will form in a
process called ORGANOGENESIS.
Two processes in organogenesis:
1. DIFFERENTIATION: specialization of cells. One cell will develop into the
stomach, and another cell will develop into the small intestine.
2. MORPHOGENETIC MOVEMENT: groups of cells move together to form distinct
structures. The first system to form is the NERVOUS SYSTEM = NEURULATION.
It is derived from the ECTODERM.
Cells in the posterior ectoderm form NEURAL FOLDS, which eventually becomes the
NEURAL TUBE. It now looks like this:
The hollow space will form the VENTRICLES and CENTRAL CANAL.
Sometimes the anterior part of the spinal cord doesn’t fuse = ANENCEPHALY
(“without a brain”). The head ends at the eyebrows, and there is no cerebellum. The
brainstem still works, so the heart beats for a few days. This can be detected by
ultrasound.
A more common condition is when the posterior portion of the spinal cord doesn’t close;
this is SPINA BIFIDA. The baby can be paralyzed from the waist down. If the cord
closes but the skin there doesn’t close, the meninges stick out, and needs to be repaired
surgically. If the ectoderm closes, but there is a mole there at the base of the spinal cord,
it may indicate there was a slight problem in closing up the tube.
The anterior part of the neural tube  brain
The posterior part of the neural tube  spinal cord
OTHER ORGAN SYSTEMS
GI TRACT
The tube starts to form pouches for the lungs, liver, pancreas, intestines, etc.
One area outside of the tube forms a cell mass that becomes the heart.
FETAL CIRCULATION
The heart develops from TUBES that fuse and twist to form the heart.
By three weeks:
Neural tube has formed
Heart begins to form
GI starts to fold.
The heart starts to pump during the fourth week.
How long has it been since the mother missed her period? Just one week.
The nervous system and cardiovascular system has already started developing.
Therefore, it’s too late to stop drinking and smoking AFTER you find out you’re
pregnant!
UMBILICAL VESSELS
There are two umbilical arteries, branches off the internal iliac arteries.
There is one umbilical vein, which goes up the midline of the body between the 2 lobes
of the liver, and enters the inferior vena cava. When the blood reaches the heart, it is
already oxygenated, so the fetus does not need to use its lungs yet.
MUSCULOSKELETAL SYSTEM
This develops from the mesoderm from mesodermal cells on either side of the spinal
cord. They are formed into blocks called SOMITES (cubes of mesoderm). Each somite
gives rise to muscles, bones, nerves, and blood vessels.
Somites are found in repeating units, one after another, each one developing into bone,
muscles, nerves, and blood vessels. We are segmented…just like earthworms!
Some of the muscles form LIMB BUDS.
LIMB DEVELOPMENT
The limb buds initially look like paddles. At about 16 weeks of gestation, an enzyme
dissolves the tissue between the fingers and toes, and the webbing disappears, carving out
the shape of the hands and fingers. Sometimes they don’t divide properly, causing
webbing between the fingers or toes.
FACE DEVELOPMENT
At 40 days, the EYES of the embryo are lateral, on the sides of the head. The
NOSTRILS are widely separated, and the mouth is extremely broad. Then these
structures move towards the midline to form the face. It looks human at 10 weeks, but it
is distinctly human at 12 weeks.
The PALATE is forming from two separate bones. The LIPS fuse to form the
PHILTRUM. If there is no fusion = CLEFT LIP or CLEFT PALATE.
Cleft palate is seen in 1/700 births. The oral cavity is open to the nasal cavity.
It is associated with alcoholism; its incidence goes up to 1/200 in alcoholics.
Fetal Alcohol Syndrome = eyes are slightly wider apart because the process didn’t form
properly.
DEVELOPMENT OF GENITALS
This begins at eight weeks.
Both the male and female look identical at first.
Females stay the same, and males have fetal testosterone which causes a change.
The males undergo a closure of the opening and the penis enlarges. The two folds fuse at
the midline. The testes descend a few months later.
Male and female fetuses can first be distinguished by their genitalia at 3 months.
PROBLEMS WITH MALE DEVELOPMENT
Hypospadias: In 1/300 boys the urethra doesn’t close all the way, or is opened on the
body of the penis. This does not occur in females.
Lack of fetal testosterone: the male child has female genitals. Then at puberty, the adult
testosterone kicks in, and they become normal men. There is a village in the Dominican
Republic that has this occur fairly often. They call it pinte waves (“balls at 15”).
The Olympics use genetic tests to make sure the females are really females.
The pills for men that prevent baldness block testosterone. A pregnant mother is not
allowed to even touch these pills, or it will absorb through her skin and cause
developmental problems in the child’s genitals.
Birth Defects
A TERATOGEN is any chemical, physical, or biological agent that induces birth
defects. THALIDOMIDE was a medicine used for morning sickness in the 1960’s and
1970’s, until it was found to cause the babies to be born without arms and legs. About
20,000 babies in 46 countries were affected.
FETAL ALCOHOL SYNDROME from the mother drinking alcohol is the most
common cause of mental retardation in the United States.
The most common birth defects involve the heart and circulation.
Embryonic development takes place up to 2 months; after that it starts to look human, and
is called a fetus. All that’s left is growth and differentiation. Most organ systems are
fully formed and ready to function in the fetus by the sixth month, except the respiratory
system, which matures later.
Babies that are born prematurely have immature lungs. One thing that does not
develop until the ninth month is the slippery SURFACTANT secretion in the lungs.
As the fetus grows, it triggers a hormone in the mother called OXYTOCIN, which
causes contraction of smooth muscles of the endometrium. It is not know why this
happens; maybe the amount of fat deposition in the baby, or the amount of surfactant.
As the uterus contracts, the cervix dilates and effaces (gets thinner). In a normal birth,
the baby is born, then the placenta separates and is delivered about 45 minutes later.
The mother will lose about 1 liter of blood, but she has 71/2 – 8 liters now anyway,
instead of the usual 5 liters.
CESAREAN is an incision just above the pubic symphysis, through the rectus
abdominus muscle, and through the uterus, and deliver the baby that way. She can still
give birth vaginally next time. The old way was to make an incision through the linea
alba, but it doesn’t heal as well, and she always has to deliver by C-section after that.
Most C-sections are unnecessary, but the doctor doesn’t know which ones are
unnecessary.
There are many fads in birthing children. The doctor doesn’t need to be in the room, but
one should be next door. Water births are a dumb idea because humans do not naturally
give birth under water, and we are air-breathing creatures. Also, the mother usually
defecates as the baby is being born, and the baby will be exposed to the feces in the
water. There is also some blood lose during delivery, and a small amount of blood will
color the water just as red as a large amount of blood, so there is no way to measure if
there is excess blood loss.
Development does not end at birth.
The digestive system develops for 1 year after birth. The intestine can absorb (not digest)
whole proteins for one year. Breast milk has antibodies from the mother, so as long as
the baby is breast feeding (up to one year), the baby is immune to all the infectious
diseases that the mother is immune to.
The skeletal system develops until 14-18 years.
The nervous system brain doesn’t have gyri and sulci until age 16. The brain of a 14 year
old is smoother.
Development continues through puberty. After that, they are physically mature.
► Puberty in girls begins no earlier than 7-8 years old for girls (average is 10 years),
and 9 years old for boys (average is 12 years).
► Puberty earlier than that is called Precocious Puberty.
Precocious Puberty
► Can start as early at 6 months – 5 years old.
► There may be an underlying medical problem that caused the precocious puberty.
► Because puberty started early, it will end early, and growth in height stops when
puberty ends.
► The maturation process can be stopped by hormone therapy to help the child
achieve a more normal height.
After puberty, it is not considered maturation, but AGING.
The aging process is also continuous. Male gorillas get silver hair and they are
considered mature enough to rule their tribe. Male humans get white hair and they are
considered too old. It’s a matter of perspective.
The maximum heart beat per minute steadily decreases by about 1 beat a year. A 20 year
old might have a heart beat of 100 per minute, but a 40 year old might have a heart rate of
80 beats per minute. There are no health issues with this.
SENESCENCE (“Senile”) is deterioration.
It begins at age 70 or so. There is deterioration in function.
An 80 year old man does not have the same kidney filtration as a 40 year old. This may
be an issue because they can’t clear drugs as well.
Smooth muscle of the large intestine may not work as well  constipation
Aging is maturation. Senescence is deterioration. Wrinkles don’t affect the health, so they
are part of maturation. Sebaceous glands that secrete less oil, drying out the skin, can be
senescence.