15.1 Flashcards
What are the 2 main periods of prenatal development?
The embryonic period and the fetal period
When does the Embryonic period of development take place?
The first 8 weeks, or the first 2/3rds of the first trimester
What happens in the embryonic stage of development?
Cells divide and become redistributed. tissues and organs form, as do structures that support and nourish the developing embryo
When does the Fetal period of development take place?
From the start of the 9th week till birth
What happens in the fetal period of development?
The body grows rapidly and organs begin to function and coordinate to form organ systems
What is fertilization?
The joining of male and female gametes that join to form a single cell with 23 chromosomes from each parent
How many days after ovulation does it take the ova to reach the uterus?
4 days
How many hours after ovulation must an egg be fertilized?
12 to 24 hours
When and where must the sperm meet the egg?
During the early part of its movement through the oviduct
Where must sperm go once its it ejaculated?
It must makes its way through the cervix and then through the uterus and finally the oviduct and into the egg
Why don’t most sperm survive the journey?
Because of the naturally acidic environment of the vagina and other go the wrong way
What is the membrane of the egg surrounded?
A thin clear layer of protein and carbohydrates called zona pellucida
What is the Zona Pellucida surrounded by?
Several jelly like layers of follicle cells that loosley adhere to one another called the Corona Radiata
What were the Corona Radiata to an egg?
A source of nourishment when it was in an ovarian follicle
What happens when sperm meets the corona radiate?
The sperms enzyme containing acrosome releases its contents
What do the enzymes released by the Acrosome do?
The enzymes digest a path through the corona zona pellucida
What happens once a sperm enters the egg?
The eggs plasma membrane depolarizes, preventing other sperm from binding and entering
What is a Zygote?
The single cell that is created when an egg and sperm join
What happens within 30 seconds of a zygote forming?
The 0.1 mm zygote divides by mitosis until it forms eight cells and so one
What is cleavage?
The process of cell division by a zygote with out the enlargement of cells
What is the morula stage?
When a zygote is a sphere of 16 cells
How many days after fertilization does the morula reach the uterus?
Within 3 to 5 days after fertilization
What happens as the Morula begins to fill with fluid?
2 different groups of cells form
What is the Blastocyst stage?
The stage after the Morula fills with liquid and 2 cell groups form
What is one of the cell groups in the Blastocyst?
The trophoblast, which forms the outer layer
What does the Trophoblast develop in?
A membrane called the Chorion
What does the Chorion in turn develop into?
The placenta
What is the Placenta?
The structure that provides nutrients and oxygen and removes wastes from developing offspring
When does the other group of cells within the blastocyst form?
The Inner Cell Mass (embryoblast)
What does the Inner cell mass develop into?
The embryo itself
How many does after fertilization does the blastocyst attach itself to the endometrium?
Between the fifth and seventh day after fertilization
What does the Trophoblast do after the Blastocyst attaches itself to the endometrium?
The Trophoblast cells secrete enzymes that digest some of the tissues and blood vessels of the endometrium to allow the blastocyst to sink into the uterine wall
What is implantation?
The nestling of the blastocyst into the endometrium
On what day of fertilization does implantation take place?
By day 10-14
What does the Trophoblast secrete after implantation?
A hormone called human chorionic gonadotropin (hCG)
What does hCG do?
It has the same effects as as LH and maintains the corpus luteum past the time it would otherwise degenerate
What happens as a result of the Corpus Luteum being maintained?
It continues to secrete estrogen and progesterone which maintains the endometrium
What are the levels of hCG over pregnancy?
hCG continues at a high level for about two months then declines to a low level by the end of four months
How long does the Corpus Luteum remain intact?
Throughout the entire pregnancy
What secretes hormones after the first trimester?
The placenta which secretes estrogen and progesterone
What is the Amniotic cavity?
The space between the inner cell mass and trophoblast that fills with fluid and the baby will develop
What happens to the inner cell mass as the amniotic cavity forms?
The inner cell mass flattens into a disc like structure called the embryonic disc
What is the embryonic disc supported by?
A short stalk which connects the blastocyst to the endometrium
What are the 3 layers of the embryonic disc?
The outer ectoderm which is closer to the amniotic cavity
The inner endoderm
And the middle layer called mesoderm
What is Gastrulation?
The process of forming the 3 layers of the embryonic disc
What are the primary germ layers?
The 3 layers of the embryonic disc
What is the name of the developing embryo once it develops the primary germ layers?
The Gastrula
What does Gastrulation mark the start of?
Morphogenesis
What is Morphogenesis?
The series of events that form distinct structures of the developing organism
What is differentiation?
The cellular process that enable cells to develop a particular shape and to perform specific functions
What happens between the 3rd and 8 weeks of a developing embryo?
Different rates of cell division in the primary germ layers cause tissues to fold it distinct patterns
What happens on the third week of development?
A thickened band of mesoderm cells develop along the back of the embryonic disk
What does the mesoderm form?
A rod like structure called the notochord which will eventually form the baby’s back and be the framework for the skeleton
Which layer of the three layers does the nervous system form from?
The ectoderm
What are the three layers of the embryonic disk?
Ectoderm (outer)
Endoderm (Inner)
Mesoderm (middle)
What is Gastrulation?
The process of forming the three layers of the embryonic disc
What is the collective name of the 3 layers of the embryonic disc?
The primary germ layers
Why is the formation of the primary germ layers important?
Because the cells, tissues, and organs of the body are derived from the primary germ layers
What does the mesoderm form?
A baby’s back and forms a rod like structure called the notochord
Which primary germ layer does the nervous system come from?
The ectoderm
How does the spinal chord form from the primary germ layers?
First, the cells along the surface above the notochord (the ectoderm) begin to thicken and then folds develop on each side of a groove along the surface. And then the folds fuse and become a tube
What is the tube that fuses around the notochord called?
The neural tube
What is Neurulation?
The process of forming the neural tube
By what day of development does the heart start beating?
Day 18
What happens after the fourth week of prenatal development?
Rapid growth and differentiation
What happens during the fourth week?
Blood cells start to form and fill vessels
Distinct head and facial features form
What happens during the 5th week of development?
The embryo’s head is very large in comparison with its body
They eyes open
Cells in the brain are differentiating very quickly
The cells in the embryo are 1.3 centimeters
What happens in the sixth week of development?
The brain continues rapid development
The limbs lengthen and flex slightly
The gonads are starting to produce hormones
What happens during the 7th and 8th week of development?
The embryo has distinct human characteristics
The organs are formed and the nervous system is starting to coordinate body activity
A skeleton of cartilage has formed
Eyes are developed but the lids are now closed
Genitalia is forming
What happens by the end of the 8th week?
The embryo is about the size and mass of a paper clip and about 90 percent of the organs and other structures that make up the human body are established
What are extra-embryonic membranes?
An intricate system of membranes that are external to the embryo
What are the extra-embryonic membranes?
Allantois
Amnion
Chorion
yolk sac
What are the extra-embryonic membranes responsible for?
Protection
Nutrition
Respiration
Excretion of the embryo
What are Chorionic Villi?
Finger like projection that extend from the chorion into the uterine lining that establish the beginnings of the placenta
What is a placenta?
A disk shaped organ that is rich in blood vessels
How is the embryo attached to the uterine wall?
By the placenta
What is the placenta the site of?
Metabolic exchange
When is the placenta fully developed?
By about 10 weeks
What is Amnion?
A transparent sac that develops from the cells of the embryonic disk which encloses the embryo completely and is penetrated by the umbilical cord
What is the purpose of amniotic fluid?
Protects the embryo from trauma and temperature fluctuations
What is the Yolk Sac?
A small sac that is suspended from the abdominal area of the embryo
What does the Yolk Sac do?
Contributes to the formation of the digestive tract and produces the first blood cells and the future egg or sperm cells
What does the Allantois do?
Forms the foundation of the umbilical cord
What happens to the Allantois do during the second month?
The allantois degenerates and the remainder becomes part of the urinary bladder
What is the Chorion?
The outermost membrane that encloses all other extra-embryonic membranes
Which part of the placenta comes from the embryo?
The chorion
What does the closeness of the mother and embryo permit?
Nutrients and oxygen to diffuse from the mothers circulatory system to the baby and for wastes to leave the baby’s circulation
What does the placenta not filter out?
Alcohol
Drugs
Nicotine
What is the umbilical cord?
A rope like structure that leads from the belly button of the fetus to the centre of the placenta
What do the two arteries of the placenta do?
Transport oxygen depleted blood from the fetus to the placenta. It also contains on vein which brings oxygen rich blood to the fetus
