Fertilization and Embryonic Development

Effects of Aging Male Female Decrease in size and weight of testes Decrease in sperm production Prostate gland enlarges and increase in cancer Impotence is age-related Female Menopause Decrease in size of uterus and vaginal wall thins Age related increase in breast, uterine, ovarian cancer genetic defects in children of older women linked to aging follicles.

Fertilization The union of male and female gametes to form a single cell (the zygote) that contains 23 chromosomes from each parent Occurs in the fallopian tube The ovum moves slowly, and only remains viable for 12 - 24 hours. The sperm must move more quickly, but many get “lost” or do not make it.

The zygote travels down the fallopian tube in 3 - 5 days During this time, it undergoes cleavage - equal divisions of cells without any increase in size ( cells of the zygote become progressively smaller)

When it reaches the uterus (~ day 6), a fluid filled space has formed within the mass of cells - it is now called a blastocsyt Composed of an outer layer (will become the chorion and amnion) + an inner mass of cells (will become the embryo)

cleavage

Implantation occurs when the blastocyst nestles into the endometrium (day 10 - 14) The woman is said to be pregnant at this point

Embryonic Development In humans, the zygote becomes an embryo after ~4 days It is called an embryo until the end of the eighth week of pregnancy, after which it is called a fetus

E + P inhibit the release of FSH and LH After implantation, levels of progesterone and estrogen must remain high in order to prevent menstruation The PROBLEM: E + P inhibit the release of FSH and LH LH is required to sustain the corpus luteum, which releases E + P Something else is needed to maintain the corpus luteum

The outer layer of the blastocyst (chorion) produces the hormone human chorionic gonadotropic hormone (hCG) hCG maintains the corpus luteum for the first 3 months of pregnancy. After this time, the placenta maintains hormone levels A pregnancy test identifies levels of hCG in the urine

During the second week, the inner cell mass of the blastocyst (remember - this is what becomes the embryo) changes It flattens into a disk-shaped structure called the embryonic disk which differentiates into 3 distinct layers (the primary germ layers) The process of forming these layers is called gastrulation The blastocyst is now called the gastrula

Ectoderm, mesoderm, endoderm (The mesoderm forms between the endoderm and ectoderm shortly after they form)

The ectoderm will differentiate into the skin and nervous system Mesoderm - muscles, bones, kidneys, reproductive organs Endoderm - lining of digestive system, liver, pancreas

Extraembryonic Structures By 4 weeks, several structures are in place that support the developing embryo

The embryo floats in a fluid-filled sac surround by two membranes Outer layer (chorion) becomes the fetal portion of the placenta The chorionic villi are projections that increases the surface area for exchange of materials between mother and fetus Inner layer - amnion

Between the amnion and the embryo is the amniotic cavity - a fluid-filled sac that insulates the embryo and protects it from infection, dehydration, trauma, and temperature fluctuation The yolk sac is suspended from the abdominal area of the embryo - produces the first blood cells and contributes to the formation of the digestive tract

The allantois forms the foundation for the umbilical cord (blood vessels and space for exchange of wastes)

The Placenta By about 10 weeks, the placenta is fully developed The placenta attaches the embryo to the uterine wall The fetal portion of the placenta is the chorion tissue The maternal portion is blood vessels/pools of blood from the mother

Fetal and maternal blood freely exchange nutrients, oxygen, wastes, hormones, drugs, some viruses and antibodies Generally, bacteria can not cross

At about 4 months, the placenta begins to produce estrogen and progesterone (remember - hCG sustains the corpus luteum for the first 3 months)

The Umbilical Cord Forms at the end of the eighth week Connects the fetus to the placenta Two arteries carry oxygen-depleted blood away from the fetus One vein brings oxygen-rich blood to the fetus

Timeline SUMMARY Day 6: blastocyst Day 10 - 14: implantation Second week: cells begin to specialize Fourth week: extraembryonic structures have developed Eighth week: All the body organs are formed, embryo is distinctly human