Embryonic Development

Embryonic Development


This is an amazing process that one cell can grow into an entire organism is 9 months!



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.

The relative sizes of the egg to the sperm

IDENTICAL TWINS is when one fertilized egg develops into two masses. These have the same embryo, same genetic make-up. Occurs randomly. If more than one egg becomes fertilized = FRATERNAL TWINS.

If the identical twins don’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. These two brothers had two heads but one pair of legs. They married two different women and had lots of kids. One died two days before the other one.


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 a membrane filled with nutrients= YOLK SAC

When the sperm reach the egg, millions of them break open their acrosome and release enzymes to digest the outer layer of the egg to get in. 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. Fertilization occurs in the fallopian tube or the uterus. The fertilized egg travels down to the uterus where it should implant in the wall. If there is no implantation, menses (shedding of the uterus lining) occurs.

The egg begins rapid cell division. 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 fertilized egg is called aZYGOTE. It is only one cell.

DAY 7: The egg is thousands of cells =BLASTOCYST, which begins to implant into the uterus by burrowing into it like a parasite.

Fertilization occurs on about day 14 of the menstrual cycle.

Implantation occurs on about day 21 of the menstrual cycle.

The blastocyst secretes a 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.

First and Second Months

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. The first thing to develop is the nervous system; it starts developing in the third week! By the end of the second month, all the major organ development is done, except for such things as the lungs and liver which mature later.

Third and Fourth Months

Eyelashes, eyebrows, hair on the head, and fingernails appear. Development of genitals 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. By the third to fourth month it is possible to distinguish males from females. By the end of the fourth month, the fetus is 6” in length and weighs 6 oz. OVERHEAD

Fifth through Seventh Months

The mother begins to feel fetal movement. The fetal heartbeat is loud enough to be heard when a physician applies a stethoscope to the mother’s abdomen.

During these months, the eyelids open fully. At the end of this period, the fetus is 12” and weighs 3 lb. It is possible that if born now, the baby will survive. OVERHEADS

Eighth and Ninth Months OVERHEADS

As the end of development approaches, the fetus usually rotates so that the head is pointed toward the cervix. However, if the fetus does not turn, then the likelihood of a

BREECH BIRTH (rump first) may prescribe a cesarean section. At the end of nine months, the fetus is about 20” long and weighs about 7 lb. OVERHEAD


This allows oxygen and nutrients to diffuse from the mother to the fetus, and waste products to diffuse from the fetus to the mother. The

UMBILICAL CORDconnects 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. Another thing that can cross the placenta are vitamins. There are new studies that prenatal vitamins are causing vitamin toxicity in the fetus, resulting in brain dysfunction. It’s better to just eat right or take just a regular multivitamin at the most.


This is the sac filled with fluid that the embryo sits in. 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.


As the uterus contracts, the cervix dilates. In a normal birth, the baby is born head first, then the placenta separates and is delivered about 45 minutes later. Expulsion of the placenta is called theAFTERBIRTH. The mother will lose about 1 liter of blood, but she has 2 extra liters now anyway.

CESAREAN is an incision through the abdominal wall to deliver the baby that way. She can give birth vaginally next time. 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.


By three weeks:

Nervous system is forming

Heart begins to form

GI system starts to form.

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!



Can occur for many reasons. No one knows the rate of miscarriages, but they guess that 70% of fertilized eggs get miscarried, and about 30% of implanted eggs miscarry.

As the fetus grows, sometime part of the spinal cord doesn’t close; this is SPINA BIFIDA. The baby can be paralyzed from the waist down.

CLEFT LIP or 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 is when the fetus has become damaged from the mother drinking alcohol during pregnancy.Sometimes just one day of drinking (if it’s at a significant point in the development) will cause this. The child will have mental retardation and the eyes are slightly wider apart.



Without 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 itpinte 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.

Development of a baby 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. Breast feeding longer than one year will no longer be providing any antibodies to the baby.

Puberty in girls begins inwardly around 9 years old, and boys start at 11, but the outward signs don’t show for a couple years.

The skeletal system develops until 14-18 years, except the pelvic bones in girls don’t finish growing until age 25; that’s why it’s better to wait until then before giving birth.

Parts of the nervous system and brain do not even start to develop until age 16. Development continues through puberty. After that, a person is considered physically mature.


At least 1 in 16 newborns has a birth defect, either minor or serious.

Only 20% are due to heredity. That means 80% of birth defects can be prevented.

Proper nutrition is a must, especially needed is enough folic acid.

X-rays must be avoided since they cause mutations and leukemia.

Toxic chemicals, such as pesticides also cause mutations.

Cigarette smoke not only contains carbon monoxide but also other toxic chemicals.

Babies horn to smokers are often underweight and subject to convulsions.

Pregnant Rh negative women should receive an Rh immunoglobulin injection to prevent the production of Rh antibodies. These antibodies can cause nervous system and heart defects. This is only a concern if the baby is Rh positive.

Sometimes, birth defects are caused by disease-causing organisms.

Females can be immunized before the childbearing years for rubella (German measles), which in particular causes birth defects such as deafness.

The AIDS virus in a pregnant woman causes the baby to have AIDS and mental retardation. When a mother has herpes, gonorrhea, or Chlamydia, which are STD’s that infect the vagina, newborns can become infected as they pass through the birth canal, but they can be delivered by cesarean section.

Pregnant women should not take any type of drug except prescribed by a doctor. Certainly, illegal drugs, such as marijuana, cocaine should be completely avoided. “Cocaine babies” now makeup 60% of drug-affected babies. Cocaine babies have visual problems, lack coordination, and are mentally retarded.

The drugs aspirin, caffeine (present in coffee, tea, and cola), and alcohol should be severely limited. Babies born to women who drink while pregnant are apt to have fetal alcohol syndrome (FAS). These babies have decreased weight, height, and head size, with malformation of the head and face, and have mental retardation.

Sex hormones, including birth-control pills, can cause abnormalities of the sex organs.

The morning sickness pill called thalidomide is well known for having caused deformities of the arms and legs in children born to women who took the drug. Of course, this medicine was taken off the market when they realized this, but sometimes you still see these kids who have grown up; I’ve seen a lot from Mexico.