Week 3 - Embryology Flashcards
What is a conceptus
The developing offspring
What is a zygote
fertilized egg
What is the gestation period
time from the last menstrual period until birth (280 days)
What is an embryo
conceptus from week 3 through to week 8
What is a foetus
conceptus from week 9 till birth
What is pregnancy
Events that occur from fertilization until the infant is born
What is embryology
The branch of biology that studies the formation, growth and development of an embryo from a fertilized egg
What is week 1
Preimplantation period
What occurs in week 1
- While travelling through the urterine tubes the conceptus undergoes cleavage up to 16 cells
- the conceptus reaches the uterus and now becomes a more solid structure called the morula
- The cells in the morula continue to divide until 100 cells and forms a tightly bounded cell called a Blastocyst
- The blastocyst consist of outer trophoblast cells and the inner cell mass
- After about 5 days the blastocyst “hatches” from the surrounding zona pellucida in preparation for implantation
What is cleavage
the rapid mitotic cell division in the conceptus - first to 16 cells then 100 cells. During this the size of the conceptus remains constant
What is the morula
The conceptus during week 1 when the cells have now been loosely grouped and looks more compacted into a solid mass
What is a Blastocoel
The final cell structure of week 1
Structure
- Trophoblast Cells
The cells which form the outer shell and will develop into the chorionic sac and the foetal portion of the placenta
- Inner Cell Mass
The centrally located cells which is destined to become the embryonic disc. At this stage the cells are totipotent meaning that they could become any cell type in the human body
What is week 2
Implantation and Blastocyst differentiation
What occurs in week 2
- The blastocyst implants into the lining of the uterus forming the syncytiophoblast
- The inner cell mass forms a 2 layered embryonic disc (epiblast and hypoblast)
- (The blastocysts starts to organise into layers called the extraembryonic membrane which supports and protects the growing embryo including:)- might not be till gastrulation
- Amnion
- Yolk Sac
- Chorion
Explain conceptus implantation
- Implantation occurs in week 2 and is where the blastocyst comes in contact with the uterine wall and adheres to it via its trophoblast cells.
- If successful the trophoblast fuse together forming the syncytiophoblast which allows passage of blastocyst into endometrial wall.
- In response the uterine mucosa rebuilds itself and envelops the blastocyst
- The trophoblast cells secrete hCG which is a hormone that increases the production of progesterone and estrogen to suppress the menstruation cycle
In week 2 what does the inner cell mass turn into
The inner cell mass forms a 2 layered embryonic (bilaminar) disc (hypoblast and epiblast).
Also creates Yolk sac on the hypoblast side and the amniotic cavity on the epiblast side.
What are the layers of the embryonic disc (week 2)and what will they form
Epiblast - will form entire embryo and an epithelial layer lining the amniotic cavity (between bilaminar disc and trophoblast’s outer wall)
Hypoblast -nourishes the embryo and turns into the placenta (yolk sac - chorion)
What are the different components of the extraembryonic membrane
- Amnion
- Yolk Sac
- Chorion
What is the amnion
- Transparent sac formed from epiblast cells
- grows around the embryo and fills with amniotic fluid from material blood and later foetal urine
- this provides a buoyant environment that protects the embryo from trauma
- it acts as a thermal regulator
- allows freedom of movement and prevents parts from fusing together
What is the yolk sac
- a part of the extraembryonic membrane
- supplies some nutrients until the placenta takes over
- Formed from the hypoblast
- provides primitive blood circulation
- later the yolk sac forms the allantosis which is a primitive excretory duct that becomes apart of the embryo
- together the stalks of the yolk sac and allantois establishes the outer structure of the umbilical cord
What is the chorion
-apart of the extraembryonic membrane
- the membrane that surrounds all others and helps to form the placenta
- protects the conceptus from attack from the mother’s immune cells
What process occurs in week 3
Gastrulation and Notochord formation
What is gastrulation
-Formation of the 3 germ layers
Explain what occurs in week 3
- The 2 layered embryotic disc becomes a 3 layered disc (endoderm, Mesoderm, Ectoderm)
- During cell formation of the disc layers the embryo take on an oval shaped disc form and creastes a raised edge called the privative streak
- Notochord forms which runs down the longitudinal axis
Explain the formation of the 3 layered embryotic disc
- Cells differentiate at the primitive streak and migrate through it
- Cells displace the hypoblast cells and form the endoderm layer
- Cells loosely fill before the endoderm and form the mesoderm
- The remaining cells from the epiblast form the ectoderm
- The mesoderm layer are less organized and exist as a loosely connected cell community
- The endoderm and Ectoderm layers form tightly connected epithelial sheets of tissue