Life Before Birth Flashcards
What is embryology?
The study of development from the stage of fertilisation to the foetal stage (about 9 weeks later).
What are the three stages of life before birth and when do they occur?
The preimplantation stage (week 1)
The embryo stage (weeks 2 - 8)
The foetal stage (weeks 9 - 38)
When does cleavage of the zygote occur?
Day one
How are the mitotic divisions of the zygote in the cleavage stage different from normal cell division?
The mitotic division is not accompanied by growth, so the cell number increases but the overall size of the embryo stays the same
Why is it important that the mitotic divisions of the zygote during cleavage are not accompanied by growth?
So that the embryo does not increase in overall size so that it can fit through the uterine tube
What is the name of the glycoprotein layer surrounding the plasma membrane of the oocyte?
Zona pellucida
What is the purpose of the zona pellucida?
It prevents premature implantation of the embryo
What is the morula and on what day, approximately, does the morula form?
This is when the cells of the embryo maximise their contact, forming a cluster of cells held together by tight junctions. It forms on roughly day four
What are the two layers of cells in the blastocyst and what is the function of each?
Cells of the inner cell mast will form the embryo
A circle of outer cells belong to the trophoblast and will form the placental structures
How does fluid get into the blastocyst cavity?
It leaks through the zona pellucida
At what stage does the embryo hatch from the zona pellucida?
The blastocyst stage
When does implantation of the embryo take place?
Between days 7 and 12
What cells are the first to implant in the endometrium?
Trophoblast cells
What are the two types of placental cells that trophoblasts will differentiate into?
Cytotrophoblasts
Synctiotrophoblasts
What are ectopic pregnancies?
Pregnancies in which the oocyte implants anywhere other than the uterus
When does the bilaminar disc form?
Week 2
What occurs during the formation of the bilaminar disc?
The inner mast cells of the embryo form two layers of cells - the epiblasts (sits on top) and the hypoblast (sits below)
Describe the amnion membrane.
It is continuous with the epiblast and persists right until birth. It’s cavity is filled with amniotic fluid and is useful in protecting the embryo
Describe the yolk sac.
It is continuous with the hypoblast and is important in nutrient transfer in weeks 2 and 3 and blood cell formation. It disappears around week 20
Describe the chorion.
It is the trophoblast and extra-embryonic mesoderm which forms the foetal component of the placenta
What happens to the chorionic cavity as the amnion expands?
The am ion expands until it completely lines the chorion and so the chorionic cavity disappears
Gastrulation occurs in week three. What is gastrulation?
This is when a trilaminar embryo is formed from the bilaminar epiblast
What are the three germ layers?
Ectoderm
Mesoderm
Endoderm
How are three layers formed from two in gastrulation?
The cells of the epiblast migrate towards the centre of the primitive streak and migrate down and outwards to create a middle layer in a process called invagination. The top layer becomes the ectoderm, the middle the mesoderm and the bottom the endoderm.
Give examples of cell/tissue types that arise from the ectoderm.
Skin cells Epithelial cells of the mouth and anus Cornea and lens of eye Nervous system Sensory receptors of the epidermis Adrenal medulla Tooth enamel Epithelium of pineal and pituitary glands
Give examples of cell/tissue types that arise from the mesoderm.
Notochord Skeletal system Muscular system Excretory system Circulatory and lymphatic systems Reproductive systems Dermis of the skin Lining of the body cavity Adrenal cortex
Give examples of cell/tissue types that arise from the endoderm.
Epithelial lining of digestive tract/respiratory system
Lining of uretha, urinary bladder and reproductive system
Liver
Pancreas
Thymus
Thyroid and parathyroid glands
During what week does the embryo roll up so that it is no longer a flat sheet but a closed cyclinder with ectoderm on the outside and endoderm on the inside?
Week four
What signalling centre controls the formation of the neural tube?
Notochord
What structure does the notochord eventually become in vertebrates?
The nucleus pulposes of the intervertebral discs
Which molecule will prevent the formation of the neural tube when active?
BMP4
Describe the formation of the neural tube?
The appearance of the notochord and mesoderm induces the overlying ectoderm to thicken and form the neural plate - the cells of this plate form the neuroectoderm
After the plate is induced, the cells lengthen and the lateral edges elevate which forms the neural folds with a depressed midregion called the neural groove
The folds approach each other in the midline and fuse to form the neural tube which sinks into the embryo and the overlying ectoderm repairs
What key signals are involved in inactivating BMP4 to allow neural tube formation to occur?
Noggin
Chordin
When does the anterior neuropore close?
Day 25
When does the posterior neuropore close?
Day 27
The floorplate of the developing neural tube produces its own SHH and this is responsible for the development of which structures?
Motor neurons
Why are dorsolateral hinge points absent in the upper spine?
There is greater amounts of SHH which inhibits noggin so that noggin cannot inhibit BMP2
What condition results from failure to close the anterior neuropore?
Anencephaly
What condition results from failure to close the posterior neuropore?
Spina bifida
Why do dorsolateral hinge points develop in the lower spine?
Because in this region of the spine there is less SHH so there is less inhibition of noggin which leads to noggin inhibition of BMP2 that allows the hinge point to form
What two other genes are required for SHH signalling?
Patched
Smoothened
Describe the relationship between SHH, patched and smoothened?
Smoothened normally starts a signalling pathway which leads to the activation of Gli genes which are involved jn cell fate determination
When SHH is not bound to patched then patched eill inhibit smoothened. When SHH is bound to patched, SHH inhibits patched’s inhibition of smoothened
What type of fat is required by SHH to become active?
Cholesterol
Which protein allows release of SHH and the establishment of a concentration gradient which allows its action as a morphogen?
Dispatched
What kind of defect results when only one copy if SHH is present?
Holoprosencephaly
