Fertilization Sperm are ejaculated from the penis into the vagina just below the cervix and then swim through the uterus into the Fallopian tubes and they join with an egg = internal fertilization The egg is surrounded by a thick protective layer that contains sperm binding sites When a sperm binds to the egg, enzymes in a sac on the head of the sperm (acrosome) break through the protective layer of the egg The sperm enters the egg and their nuclei rupture and join A fertilized egg = zygote The zygote begins to divide by mitosis forming a ball of cells that attaches itself to the wall of the uterus

Fertilization and Implantaton Section 39-4 Uterine wall Blastocyst Morula 4 cells 2 cells Zygote Ovary Fallopian tube Day 3 Day 2 Day 1 Day 4 Fertilization Day 0 Day 7 Implantation of blastocyst Egg released by ovary

Early Development Cleavage is the first major phase of embryonic development, it occurs in the fallopian tubes It is the rapid succession of cell divisions (Mitotic) It creates a multicellular embryo from the zygote NO growth occurs cells divide in given space After 4 days, the embryo is a ball of about 50 cells = morula As the morula grows, a hollow fluid-filled cavity forms in the center when this occurs it is now called a blastocyst After 6-7 days, the blastocyst attaches itself to the wall of the uterus by secreting enzymes that digest a path into the soft tissue = implantation

Cleavage and Blastocyst Formation Figure 20-2

Embryonic Development Gastrulation is the second major phase of embryonic development The cells at one end of the blastula move inward Organs start to form after gastrulation Embryonic tissue layers begin to differentiate into specific tissues and organ systems

Gastrulation 3 layers = primary germ layers because all organs and tissue will be formed from them Ectoderm (outer) – becomes skin and nerves Mesoderm (middle) – becomes everything else Endoderm (inner) - becomes digestive, excretory, and respiratory systems Membranes develop that surround, protect, and nourish the developing embryo = amnion and chorion

Structures of early development

Placenta After 3 weeks of development, the nervous and digestive systems have begun to form The chorion has grown into the placenta Placenta = connection between mother and developing embryo supplying nutrients and oxygen and eliminating carbon dioxide and other wastes (respiration, nourishment, and excretion) The placenta separates the mother’s and embryo’s blood, but gases, food and wastes can diffuse across it After 8 weeks of development, embryo = fetus After 3 months of development, fetus has umbilical cord connecting fetus to placenta, most organs are fully formed (including muscles) and the fetus can reflex Amnion has grown into amniotic sac , filled with amniotic fluid that cushions and protects developing fetus

Later Development After 6 months of development, tissues are more complex and specialized, skeleton formed, heartbeat strengthened, soft hair grows over skin, and fetus continues to increase in mass The fetus may be able to survive outside the uterus with life-supporting equipment Last 3 months of development increase the fetus’ chances of survival The fetus doubles in mass, lungs and other organs change to prepare for life outside uterus

Critical Times…

Childbirth After about 9 months of development, the fetus if ready for birth Oxytocin from the pituitary gland affects large involuntary muscles in the uterine wall, stimulating contractions = labor Cervix expands until large enough to pass the head of the baby Amniotic sac breaks and fluid rushes out of the vagina = “water-breaking” Uterine contractions become stronger and more frequent, forcing baby out through the vagina The baby cries or coughs when outside the body, riding the lungs of fluid, so breathing can begin immediately The blood supply through the umbilical cord dries up and it is cut leaving a scar on the baby (navel/belly-button) After birth, the placenta and empty amniotic sac are expelled by a final series of uterine contractions called the afterbirth

Labor Dilation Stage Begins with onset of true labor Cervix dilates Fetus begins to shift toward cervical canal Highly variable in length, but typically lasts over 8 hours Frequency of contractions steadily increases Amnionic membrane ruptures (water breaks)

Labor

Labor Expulsion Stage Delivery Begins as cervix completes dilation Contractions reach maximum intensity Continues until fetus has emerged from vagina: Typically less than 2 hours Delivery Arrival of newborn infant into outside world

Labor Figure 20-11

Labor Placental Stage Muscle tension builds in walls of partially empty uterus Tears connections between endometrium and placenta Ends within an hour of delivery with ejection of placenta, or afterbirth Accompanied by a loss of blood

Labor Figure 20-11

After Childbirth The baby begins to lead an independent existence (respiration, excretion, maintaining homeostasis) The mother’s pituitary gland secretes the hormone prolactin, which stimulates the production of milk in the breast tissue Breast milk contains all of the nutrients the baby needs for growth and development during the first few months of life

Multiple Births? Fraternal twins = when 2 eggs are released and fertilized by 2 sperm during the same cycle Not identical because different eggs fertilized by different sperm Identical twins = when 1 zygote splits producing 2 embryos Identical because contain genetic information from same egg and same sperm

Multiple Births Conjoined Twins Siamese twins Genetically identical twins Occurs when splitting of blastomeres or of embryonic disc is not completed

Reproductive Technologies have made it possible to: fertilize and egg outside the female body called In vitro fertilization.  introduce semen inside the female body without sex called artificial insemination. Implant a fertilized egg into the uterus : also done by artificial insemination. Hormone therapy: provide female hormones to woman who can no longer make them. Risky.