17 Aging and Development Flashcards
Fertilization
The union of egg and sperm (ovum, ova) to form a zygote
Zona Pellucida
outer matrix surrounding the egg
Corona Radiata
outside the zona pellucida are few residual follicular cells from the ovarian follicle, called the corona radiata
Steps of Fertilization
- several sperm penetrate the corona radiata (but not the zona pellucida. First sperm to penetrate the zona pellucida triggers changes in the zona so others cant get in
- Sperm’s acrosomal enzymes digest a portion of the zona pellucida
- one sperm binds to and fuses with the egg’s plasma membrane
- sperm nucleus enters the egg
What prevents more than one sperm from entering the egg?
- The egg’s plasma membrane depolarizes from 265mv to 10mv, which helps prevent more sperm from attaching to the egg.
- Vesicles (cortical granule) within the egg releases enzymes that causes the zona pellucida to become impenetrable to other sperm
Development of embroyo after sperm and egg nuclei fuse
- Cleavage: cells undergo division (2,4,8, etc) without the embryo increasing in size
- Growth: cells continue to undergo division and size of embryo increases
- Morphogenesis: the embryo begins to take shape as cells migrate to specific locations
- Differentiation: cells take on specific structure and function (the nervous system is the first visible system fomed)
Extra-embryonic Membranes
- Chorion: forms the placenta, the organ that provides the embryo with nourishment and oxygen and rids it of waste
- Allantois: give rise to the urinary bladder, and the blood vessels of the umbilical cord that carry blood between the fetus and placenta
- Yolk sac: contains many blood vessels and where blood cells first form (there is a little yolk in humans, in contrast to eggs of other animals)
- Amnion: contains amniotic fluid that surrounds, cushions, and protects the embryo
Stages of Development
- pre-embryonic development - from moment of egg fertilization until end of 1st week
- embryonic development - from begin of 2nd week after fertilization until the end of the 2nd month
- fetal development - the 3rd throught the 9th month of development
- development after birth - stages of life including infancy, childhood, adolescence and adulthood
Pre-embyonic development
(1st week of development after fertilization)
- Cleavage: cell division that incrases the number of cells
- Morula: compact ball of embryonic cells
- Early blastocyst: inner cell mass that becomes the embryo covered by a layer of cell that becomes the chorion
- Implantation: embyo embeds into the uterine endometrial lining on approximately day 6 after fertilization
Embyonic Development: Week 2
- Pregnancy begins after implantation of fertilized egg
- Human Chorionic Gonadotropin (HCG) secreted by chorion. LIke LH, HCG maintains the corpus luteum in the ovary, which secretes progesterone to maintain the thick endometrium of the uterus - HCG is measured by the pregnancy test
- Inner cell mass becomes the embryonic disk that will go through gastrulation to become 3 primary germ cell layers of the embyo (endoderm, mesoderm, and ectoderm)
Ectoderm
(outer layer)
epidermis of skin, epithelial lining of oral cavity and rectum, nervous system
Mesoderm
(middle layer)
skeleton, muscular system, dermis of skin, cardiovascular system, urinary system, reproductive system, outer layers of respiratory system and digestive system
Endoderm
(inner layer)
epithelial lining of digestive tract and respiratory tract; associated glands of these sytems, epithelial lining of urinary bladder
Embryonic Development: Week 3
Nervous system begins to develop from ectoderm - is first system to be seen
The posterior neural tube will become the spinal cord and brain
Development of the hear begins from mesoderm
Embryonic Development: Weeks 4 & 5
4th week: embryo is appx 1/16 inch long. Chorionic villi and umbilical cord form. Limb buds form (later develop into legs and arms)
5th week: head enlarges. Eyes, ears and nose become prominent
Embryonic Development: 6-8 weeks
Embryo begins to look human
Movement begins
All organ systems are established
1.5” long and weigh about the same as an aspirin tablet
Fetal Development: month 3 and 4
hair develops, head slows in growth, body size catches up.
Cartilage begins to be replaced by bone
Able to distinguish female from male
Stethoscope will detect heartbeat
By the end of the month 4, the fetus is about 6 inches long and weighs roughly 6 ounces
Fetal Development: month 5-7
Fetal movement can be felt by the mother
Fetus in “fetal” poition
Eyelids are fully open
Fetal size increased to about 12 inches and roughly 3 pounds
Fetal Development: months 8 and 9
Weight gain is about a pound per week
Fetus usually rotates upside down so the head is pointed towards the uterine cervix
At the end of fetal development just prior to birth, the fetus weighs about 7.5 lbs and roughly 20 inches long
Preventing Birth Defects
• Get periodic physical exams during pregnancy
• Maintain good health habits: proper nutrition,
adequate sleep and exercise
• Avoid smoking, alcohol, drug abuse
• Avoid having x-rays
• Avoid certain medications and supplements (many drugs cross the placenta from mother into the fetus)
• Avoid sexually transmitted diseases or know if you have one so it can be treated
Placenta
Secretes estrogen and progesterone - maintain thick endometrium and prevent new follicles in ovary from maturing
Fetal Circulation
- The placenta functions as the fetal respiratory center for CO2 and O2exchange (as well as exchange of nutrients and wastes)
Mother’s uterine arteries carry oxygenated blood to the uterine endometrium.
- Fetal hemoglobin has a higher attraction for O2 than adult hemoglobin, so O2 diffuses from the mother’s blood across the chorionic villus of the placenta, into fetal blood vessels (no blood connection between fetus and mother).
• Oxygen-rich fetal blood from placenta into umbilical vein fetal liver ductus venosus inferior vena cava fetal right atrium.
- Most of this oxygenated blood is “shunted” through the foramen ovale of the atrial septum. Right atrium left atrium left ventricle aorta.
• Most of the oxygen-poor fetal blood entering the right atria (that did not go to the placenta) enters the pulmonary artery, but is “shunted” back into the aorta through the ductus arteriosus. Most blood entering the right atria is shunted away from the fetal lungs (which are not working!).
Movement of O2
Umbilical vein takes O2 rich blood from placenta
Imbillical arteries take O2 low blood to placenta
Fetal Ciculation: changes at birth
- Lungs suddenly inflate with air and begin to work for breathing, increasing blood flow into pulmonary artery. Gas exchange in lungs begins, O2 rich blood from lungs enters left side of heart via pulmonary veins
• This increased flow and pressure in left atria closes the foramen ovale – which stops shunting O2 - poor blood from right atria into left atria
- Ductus arteriosus between pulmonary artery and aorta closes, stopping that shunt
• Ductus venosus at liver, as well as umbilical arteries and veins close. Umbilical cord vessels stop working. Cord is cut.
