Fertilization and Embryonic Development

Embryonic Development

• Week 1-8

• Mother support structure forms

• Embryo tissues and organs from

Fetal Development

• Week 9-Birth

• Organs function → form systems

• Fetus body grows rapidly

Journey of Egg and Sperm

1. Egg is released from ovary into the oviduct

2. This occurs during day 14, triggered by LH

3. Zygote takes four days to reach the uterus but it must be fertilized within 12-24 hours of its release

4. Sperm must reach the egg within the oviduct

5. Sperm enter the woman’s vagina, travel through the cervix, into the uterus and then into the oviduct

6. Most sperm do not make contact with the egg

The Egg

• Has a membrane that limits sperm entry to one sperm

• Membrane must be dissolved by enzymes in the acrosome head of sperm

• Sperm nucleus, containing DNA is inserted

• Egg and sperm nuclei fuse

• All organelles, including mitochondria, are provided by the egg

Timing

• Eggs require fertilization within 24 hours of ovulation

• Sperm can survive within the female reproductive tract for 5 days

• A woman therefore can become pregnant if sperm are deposited 5 days before ovulation or on the day after ovulation

• This works out to a woman’s true fertility period of approximately 6 days

Zygote

• Days 1-3, Cells 1-8

• A 2n=46 diploid fertilized egg that has divided up into 8 cells

• Occurs up to 3 days after fertilization

Morula

• Day 3-4

• A 2n =46 diploid fertilized egg that has divided up into from 16 -64 cells

• Process is called cleavage

• Occurs from between 3-4 days after fertilization

Blastocyte

• Day 5

• A 2n = 46 diploid fertilized egg that has divided up into 70 - 100 cells

• Occurs 5 days after fertilization

• Outer cell = trophoblast

• Outer layer forms chorion, which eventually transforms into placenta

• Inner layer forms amniotic sac, which surrounds the fetus

• Inner cells form the embryo blast, forming the embryo

Implantation

• blastocysts attached to endometrium

• Completed day 10-14

Hormones During Implantation and Early Pregnancy

1. HCG - human chorionic gonadotrpin

   • secreted by trophoblast

   • maintains the corpus luteum past its normal degeneration

   • causes the corpus luteum to continue secreting progesterone and some estrogen

2. Estrogen

   • Supports endometrial and uterus growth and inhibits FSH

3. Progesterone

   • Prepares endometrium from implantation, reduces uterine contractions, lowers the maternal immune response and inhibits LH

• Positive Feedback - hCG supports corpus luteum’s secretion of estrogen and progesterone

• Negative Feedback - hCG inhibits the secretion of LH. This stops ovulation and teh degeneration of corpus luteum

Gastrulation

• Day 14-21

• blastocytes inner cell mass begins to be separated from the outer trophoblast

• separated in a fluid filled space celled the amniotic cavity

• inner cell mass differentiates into 3 germ cell layers

  • Ectoderm (outer): skin, nervous system, pituitary gland, eyes

  • Mesoderm (middle): muscle, bones, circulatory system, inner skin, kidneys, spleen

  • Endoderm (inner): digestive tract, bladder, liver, gallbladder, pancreas, thymus thyroid gland

Neurolation

• Day 20-21

• ectoderm tissues fold over forming a neural tube

• origin of brain and spinal cord

• organ development begins

Development of Major layers of Trophoblast - Outer Layer: Placenta

• Weeks 3-8

• Formed from fusion of outer trophoblast and endometrium

• fusion results in vili that exchange gases and nutrients but not blood cells

• placenta is connected to the fetus through two arteries and one vein

• Within the umbilical cord

Development of Major layers of Trophoblast - Middle Layer: Chorion

• formed from mesoderm

• source of chorionic vili that extend into endoderm and the associated blood vessels

• allows for an exchange of materials with the mother

• also produce hCG. this maintains the corpus luteum

Development of Major layers of Trophoblast - Inner Layer + Structure

• Inner Layer: Amnion

  • inner layer that develops into the fluid filled amniotic sac

  • this insulates the embryo

  • protects from infection, dehydration, impact and temperature change

• Inner Structure: Yolk Sac

  • Initial source of blood fro the embryo

  • this is eventually absorbed to become the fetal digestive tract

• Inner Structure - Allantois

  • Membrane that forms the blood vessels that enter the umbilical cord

1st Trimester: 1-13

• Embryo is 7mm

• 4 chambered heart has formed

• anterior brain formed

• limb buds

• sucking reflex evident

• Rapid increase in hCG secretions by chorion

  • supports corpus luteum

  • corpus luteum secretes progesterone and smaller amounts of estrogen to maintain endometrium, both limit FSH and LH productions

2nd Trimester: 14-26 Weeks

• 57mm fetus

• all organs have formed

• soft hair called lanugo covers the body

• eyelids and lashes formed

• Progesterone and estrogen production shifts to the placenta

3rd Trimester: 27-40 Weeks

• Infant -530mm long and weights 3400g

• baby grows rapidly organs further develop

• last to mature are the digestive and respiration systems

• progesterone levels continue to increase until birth

• progesterone and estrogen production only from placenta

Key Points of Placenta

• NO direct blood exchange between mother and fetus

• nutrients, oxygen and wastes are exchanged through separate capillaries

• capillaries normally allow only these natural body substances to be exchanged