Embryonic Period - 15.1 Flashcards
What do biologists divide prenatal birth into?
Three trimesters - three months
What are the two main periods of prenatal development?
Embryonic period and Fetal period
What period is the embryonic period?
First two thirds of the trimester - first Eight weeks
What happens during the embryonic period?
*Cells divide and become redistributed
* Tissues and organs forms
* Structures that support and nourish embryo develop
What period is the fetal period?
The remaining third of the first trimester and all the second and third trimesters - month weeks through to birth
What happens during the fetal period?
Body grows rapidly and organs begin to function and coordinate to form organ systems
What is fertilzation?
The joining of male and female gametes (sperm and egg) to form a singe cell that has 46 chromosomes - 23 chromosomes from each parent
What is another name for fertilization?
Conception
How long does it take egg to reach uterus?
About 4 days
When should the egg be fertilized to prevent it from losing its capacity to develop?
Within 12 or 24 hours of its released
Do all the sperms reach the egg?
No, some go to the wrong oviduct or are destroyed by the acidic environment of the vagina
What is the plasma membrane of the egg ?
Zona pellucida - clear layer of protein and carbohydrates
What layer surrounds the zona pellucida?
Corona radiata - several jelly like layers of follicle cells that loosely adhere to one another - cells were a source of nourishment for the egg when it was an ovarian follicle
What allows the sperm to penetrate corona radiata and and zona pellucida?
The acrosome (head) that contains enzymes
What happens when one sperm penetrates the egg?
The egg plasma membrane depolarizes, preventing other sperm from penetrating it
What happens within 12 hours of the sperm nucleus entering the egg?
The sperm cell nucleus and egg cell nucleus disappear - zygote - has 46 chromosomes, 23 from each parent cell
What happens within 30 hours of being fertilized?
The egg begins to divide - cleavage
What happens when a zygote is a sphere of 16 cells?
Morula
When does morula reach the uterus?
Three to five days after fertilization?
What are the two different cell groups that form when the blastocyst becomes filled with fluid?
The trophoblast and the inner cell mass
Where does the fluid that fills the blastocyst come from?
Diffuses from the uterus
What is the outer layer of the blastocyst?
Trophoblast
What does the trophoblast develop into?
The chorion
What will the chorion develop into?
Develops to form part of the placenta (provides nutrience and oxygen, removes wastes from the developing fetus).
What is the group of cells that forms inside the blastocyst?
Inner cell mass
What does the inner cell mass develop into?
Develops into the embryo itself
What happens between the fifth and seventh day after fertilization?
blastocyst attaches to the endometrium
How does the trophoblast implant into the endometrium?
It secretes enzymes that digest some of the tissues and blood vessels of the endometrium
What is implantation?
The nestling of the blastocyst into the endometrium
When is implantation complete?
By the tenth of fourteenth day
What hormone does the trophoblast secrete?
hCG - human chorionic gonadotropin
What does hCG do?
It maintains the corpus luteum - secretes estrogen and progesterone continuously, maintaining the endometrium and preventing menstruation
When does hCH production decline?
It declines to a low level at the end of four months - Corpus luteum is less important after the first trimester - placenta secretes enough estrogen and progesterone to maintain corpus luteum
What happens during the second week?
A space forms between the inner cell mass and the trophoblast (amniotic cavity) - will fill with fluid where baby will develop
What surrounds the amniotic cavity?
The amnion
What happens as the amniotic cavity forms?
Inner cell mass flattens into the embryonic disk
What supports the embryonic disk?
A short stalk that connects the blastocyst with the endometrium
What are the three layers of the embryonic disk?
Endoderm, mesoderm, and ectoderm
What is the process of forming these layers?
Gastrulation - primary germ layers
What is the developing embryo called
Gastrula
What marks the beginning of morphogenesis?
Gastrulation
What is morphgenesis?
The series of events that form distinct structures in the developing organisms
What is differentiation?
The cellular process that enables cells to develop a particular shape and perform a specific function - that are different from the functions of other cells
Why are the primary germ layers important?
All the cells, tissues, and organs of the body are derived from them
What occurs during the third and eighth week?
The organs form
What occurs during the 3rd week?
A thickened band of mesoderm develops along the back of the embryonic disk - these cells will become the babies back - and come together to form the notochord
* notochord - basic framework of the skeleton
How does the nervous system develop?
Develops from the ectoderm that is located just above the notochord
How does the ectoderm form the CNS?
*Cells along the surface of the notochord begin to thicken
* Folds develop on each side of a groove on its surface
* Folds fuse and become a tube (neural tube) which develop into the brain and spinal cord
When does the heart start beating?
On about the 18th day
What happens in the fourth week?
- Time of rapid growth and differentiation
- blood cells form and fill with developing blood vessels
- lungs and kidneys take shape
- small buds (legs and arms appear)
- Distinct head is visible (ears, eyes, and nose)
What happens in the fifth week?
- Head is very large compared to the body
- Eyes open but they do not have eyelids or irises
- Cells in the brain are differentiating very quickly
What happens in the sixth week?
- Brain continues rapid development
- Limbs lengthen and slightly flex
- Gonads start producing hormones that will influence the development of external genetelia
What happens in the seventh and eight week?
*Embryo has distinct human characteristics - organs are formed - nervous system is starting to coordinate movement
- Skeleton cartilage has formed
- Eyes are well developed - eyelids close them
- Nostrils are developing - plugged with mucus
- External genetelia is developing
*Embryo is size of paper clip
What are the extra embryonic membranes?
Allantonis, amion, chorion, and yolk sac
What are the extra embryonic membranes responsible for?
Protection, nutrition, respiration, and excretion
What is the amnion?
- Transparent sac that develops from cells of the embryonic disk
- it is penetrated only by the unbilical cord
- amnion becomes filled with fluid (amioniotic fluid) - that protects baby from trauma and temperature fluctuations, allows freedom of movement, and prevents limbs from sticking to the body
What is the allantois?
- Forms the foundation of the umbilical cord
- During the second month, most of the allantois degenerates
- remainder becomes part of the urinary bladder
What is the yolk sac?
- Sac that is suspended from the abdominal area of embryo
- contributes to the formation of the digestive tract and produces the first blood cells and the future egg or sperm cells
- has no nutritive function
What is the chorion?
- Outermost membrane
- encloses all the other extra embryonic membranes as well as the embryo
- Forms the fetal portion of the placenta
What establishes the beggininggs of the placenta?
The chorionic villi that extend into the uterine lining
How is the embryo attached to the uterine wall?
By the placenta
When is the placenta fully developed?
After about 10 weeks - it is now 600g
When does the yolk sac shrink and the amniotic sac enlarge?
Newar the end of eighth weeks
What does the umbillical cord contain?
Two arteries (transport oxygen depleted blood from fetus to placenta)
One vain (brings oxygen rich blood to the fetus)
Why does the unbillical cord have natural twists?
Because the unbilical vein is larger than the arteries are
