Embryology Flashcards
(27 cards)
What is a Morula?
A morula is a cluster of 16 or more cells (held together by tight junctions) formed when cells have divided to maximise contact with each other.
What stage is a morula formed?
Day 4
What is a blastocyst?
A blastocyst is the first sign of cellular differentiation. It is made of:
- An inner cell mass (epiblast and hypoblast) - this will form the embryo
- An outer cell mass (trophoblast, tropho = ‘eat/nourish’ which is what the placenta does). - this will form the placenta
The blastocyst also has a fluid-filled blastocyst cavity.
What happens when the blastocyst runs out of nutrients?
When the blastocyst starts to run out of nutrients, the trophoblast secretes enzymes which break down the zona pellucida to allow for implantation.
When does implantation occur?
7-12 days after fertilisation
What is implantation?
The interaction between the implanting embryo and the endometrium of the uterus
Which cells implant first? What happens when these implant?
Trophoblast cells (as these are on the outside and so will encounter the endometrium first). These trophoblasts will then form a cytotrophoblast and a syncytiotrophoblast which will invade the stroma of the endometrium - allowing attachment.
Following implantation, a bilaminar disc forms. What are the layers of the bilaminar disc and what does it do?
The layers are the Epiblast and the Hypoblast. These structures allow formation of extra-embryonic membranes.
What are the 3 extra-embryonic membranes at the point of week 2/implantation? What is each of these membranes function?
- The Amnion = has a cavity filled with amniotic fluid and gives protection for the embryo
- The Yolk Sac = Important for nutrient transfer in weeks 2-3 and disappears after week 20.
- The Chorion = Forms the foetal component of the placenta
What are the 3 3’s of gastrulation
Occurs in week 3
Forms 3 layers (ectoderm, mesoderm, endoderm)
Has 3 important structures (primitive streak, notochord, neural tube)
What is the first event of gastrulation?
Formation of an invagination into the epiblast. This invagination is called the Primitive Streak.
What occurs at the primitive streak?
Cells move into the invagination/primitive streak and begin to differentiate, forming the 3 layers (ectoderm, mesoderm and endoderm).
There is also formation of the notochord and the neural tube
Where does most muscle formation occur?
In the mesoderm layer.
What are the regions of the mesoderm?
Notochord, paraxial mesoderm, intermediate mesoderm, lateral plate mesoderm, extraembryonic mesoderm
When does the mesoderm differentiate?
Days 17-21
What are the 3 main regions of the mesoderm? What is the function of each region?
- Paraxial Mesoderm - Lies adjacent to the notochord and neural tube. It forms the somites in the embryo.
- Intermediate Mesoderm - Forms the Genitourinary System
- Lateral Plate Mesoderm - Split by a cavity into 2 layers (visceral layer and parietal layer)
Where does skeletal muscle arise from?
Paraxial mesoderm
Where does smooth muscle (of the gut) arise from?
Visceral Lateral Plate Mesoderm
Where does smooth muscle (of the pupil, mammary and sweat glands) arise from?
Ectoderm
Where does Cardiac Muscle arise from?
Visceral Lateral Plate Mesoderm
What is a Somite?
A block of Paraxial Mesoderm which gives rise to skeletal muscles
What is the process of somite formation?
Somites form alongside the neural tube in a craniocaudal sequence from day 20 at roughly 3 pairs a day until the end of week 5 (this rate is used to accurately determine embryo age).
What controls the patterning of somites?
The Notochord influences somite formation.
- Genes (FGF family, Wnt and Notch) also control somites via the clock and wave mechanism.
What is the clock and wave mechanism?
Where genes (FGF, Wnt and Notch) tell cells to switch between permissive and non-permissive states in a timed fashion (clock).
Then, a wave of factors sweeps along the length of the embryo and interacts with the cells that are permissive at the right time and the right area
