Flashcards in Embryology Deck (33):
LO E1.2 Discuss the changes taking place as the fertilised human ovum (zygote) travels down the Fallopian (uterine) tube to the uterus
- Oocyte is released from the ovary
- Travels along the Fallopian (uterine) tube
- Is fertilized by sperm in the ampulla
- Fertilised oocyte is called the zygote
- Ideal site for implantation is the posterior uterine wall
LO 1.3 Define a zygote
Zygote - A fertilised ovum/oocyte
LO 1.3 Define Cleavage
Cleavage - The dividing of cells into two masses known as blastomeres. First cleavage happens ~30 hours after fertilisation (resting period)
LO 1.3 Define Zona Pellucida
The glycoprotein ‘shell’ to prevent polyspermy
LO 1.3 Define Morula
The result of cleavage of the fertilised oocyte, each cell is Totipotent
LO 1.3 Define Ovary
The female reproductive organ producing oocytes/eggs for fertilisation
LO 1.3 Define Fallopian tube
A tube the oocyte travels along to reach the uterus. The egg is fertilised at the ampulla of the fallopian tube.
LO 1.3 Define Uterus
Female reproductive organ where implantation of the zygote and growth of the embryo occurs
LO 1.3 Define Blastocyst
Formed from the compaction of cells that make up the morula
LO 1.3 Define Trophoblast
(Outer cell mass) will later form support structures for the embryo (placenta)
LO 1.3 Define Embryoblast
(Inner cell mass) will later become the bilaminar disk
LO 1.3 Define Implantation
The attachment of the Blastocyst to the wall of the uterus at day 6 – 7
LO 1.3 Define Cytotrophoblast
(Derivative of Trophoblast) Placental membrane around the yolk sac
LO 1.3 Define Syncytiotrophoblast
(Derivative of Trophoblast) Cells that invade maternal sinusoids (irregular blood vessels) resulting in uteroplacental circulation.
LO 1.4 Discuss the formation of the Blastocyst and the initial stages of implantation (week 1)
Week 1 – Compaction (Day 4)
Blastomeres making up morula compact to form the Blastocyst
Embryoblast (Inner cell mass)
Trophoblast (Outer cell mass)
Week 1 – Hatching (Day 5)
- Blastocyst hatches from the zona pellucida
- No longer constrained and free to enlarge
- Can now interact with the uterine surface to implant
Week 1 – Implantation Begins (Day 6–7)
- The conceptus now has 107 cells.
- 8 will make the embryo
- 99 will begin the development of the foetal membrane
LO 1.6 Describe the formation of the embryonic disc in the inner cell mass and initial cell differentiation within it
Week 2 – “The week of two’s”
Both an embryo and a placenta start to be formed. At the earliest stages the placenta has priority.
Embryoblast -> Epiblast, Hypoblast
Trophoblast -> Syncytiotrophoblast, Cytotrophoblast
The bilaminar disk and amniotic cavity are formed.
Implantation is interstitial (invasive, surrounded and embedded). The uterine epithelium is breached and the conceptus implants within the uterine stroma.
Week 2, Day 9
Rapid development of Syncytiotrophoblast
Primitive yolk sac formed by Heuser’s membrane spreading across the Blastocyst cavity. Yolk sac membrane is in contact with the cytotrophoblast layer.
Week 2, Day 11
Primitive yolk sac membrane is pushed away from the cytotrophoblast layer by an acellular extraembryonic reticulum.
Reticulum is later converted to extraembryonic mesoderm by cell migration
Week 2, Day 12
Maternal sinusoids invaded by syncytiotrophoblast.
Lacunae become continuous with sinusoids.
Uteroplacental circulation begins.
Uterine stroma prepares for support of the embryo.
Week 2, Day 13
Formation of secondary yolk sac
Pinches off from the primary yolk sac
A.k.a. Definitive yolk sac
Week 2, Day 14
Spaces within the extraembryonic mesoderm merge to form the chorionic cavity
The Connecting Stalk suspends the embryo and its cavities in the chorionic cavity.
The connecting stalk is a column of mesoderm, and is the future umbilical cord.
Bleeding around now can be confused with menstrual bleeding.
By the end of the second week
- The conceptus has implanted
- The embryo and its two cavities (Amniotic cavity and Yolk sac) are
- Suspended by a connecting stalk in a
- Supporting Sac (Chorionic Cavity)
Describe possible implantation defects
Implantation Defects - Implantation at inappropriate sites can cause problems.
Ectopic Pregnancy – Implantation at site other than uterine body (most commonly fallopian tube). Can quickly become a life-threatening emergency.
Placenta Praevia – Implantation in lower uterine segment. Placenta grows across cervical opening. Requires C-section delivery
The Epiblast and hypoblast. Derived from the embryoblast. This will later become the trilaminar disk with the formation of the primitive streak.
A layer from which other developed tissues originated from
The outermost layer of cells giving rise to the epidermis and nerve tissue
The innermost layer of cells giving rise to the gut lining
The outermost surface during early development
The innermost surface during early development
The middle germ layer giving rise to muscle, bone and more
A narrow groove that develops on the dorsal surface of the Epiblast. It has an important role in the orientation of the embryo, determining the front and the back.
The reorganisation of the germ layers into the ectoderm, mesoderm and endoderm and hence establishing the origin of all tissues.
The three germ layers. Ectoderm, mesoderm and endoderm.
What are the Ectodermal Derivatives
(Hair, nails, lens and cornea of eye, mouth and anus lining)
Brain and spinal cord
Retina and iris of eye
What are the Mesodermal Derivatives
Smooth, skeletal and cardiac muscle
Blood and blood vessels
What are the Endodermal Derivatives
Lining of the digestive tract
Glands associates with digestion
(E.g. liver, pancreas)
Other gut derivates
When is the Embryonic Period
Weeks 3 to 8
- Period of greatest change
- All major structures and systems are formed
- The most perilous for the developing child
Describe week 3 of develoment
Week 3 – “Week of threes”
(with the establishment of the 1. Amniotic cavity
chorionic cavity by end of week 2) 2. Yolk sac
3. Chorionic cavity
Three “germ” layers – rudimentary lineages from which all others will arise
This is the process establishing the three “germ” layers and hence the origin of all tissues.
Trilaminar disc is formed and the axes observed in an adult are set.
- Dorsal/ ventral
This starts with the appearance of the primitive streak (primitive node with primitive pit)
Cellular rearrangement occurs
Primitive Streak, node and pit
- At the end of the second week the epiblast is a uniform disk.
- In the third week the primitive streak appears on the dorsal surface of the epiblast.
- The streak is a narrow groove with bulging edges
- Primitive node is located at the cranial end of the streak.
- Primitive pit is located at the centre of the node
- As gastrulation proceed and the three germ layers are established, the primitive streak regresses
- Development proceeds in a cranial/rostral to caudal direction.