Fertilisation to trilaminar embryo Flashcards
Explain the process of fertilisation
Fertilisation usually occurs in the ampulla of the uterine tube
- Firstly, the sperm binds in a human specific interaction with the zona pellucida glycoprotein (ZP3)
- Next, acrosomal enzymes are released from the sperm head and the sperm digests it’s way into the egg
- The egg and the sperm plasma membrane fuse and the sperm’s contents enter the egg
- Sperm entry into the egg triggers the completion of meiosis 2 and the release of cortical granules by the oocyte which prevent another sperm getting in
What is the name given to the first set of cell division and what happens?
The first set of cell division is called cleavage. During this, the zygote undergoes mitotic division which subdivide the fertilised egg into many smaller daughter cells called BLASTOMERES. Up to 8 cell stage, blastomeres are known as totipotent
What does it mean if a cell is totipotent?
Totipotent means that cells are capable of becoming all cells in the body INCLUDING embryonic and placental cells
What is mosaicism and how does it occur?
Mosaicism is when some cells have an abnormal chromosome number and other being normal. It occurs when an individual has 2 or more cell lines that are from a single zygote but that have different chromosome complements. It usually occurs due to a mutation or mitotic nondisjunction during cleavage
What is mitotic nondisjunction?
Mitotic nondisjunction usually occurs in the early division of the zygote and it is when separation of the chromosomes don’t occur and both members of a pair move into one cell
What happens during the compaction of morula?
This is when the cells are confined in the zona pellucida and they maximise available space by coming into closer contact with each other and then begin to form cell junctions. The outer cells (which are also in contact with the zona pellucida), form extensive gap junctions. Fluid also enters through the ZP
Explain the structure of a blastocyst
Once cells have a recognisable central cavity, they are known as a blastocyst. Also, the outer layer of the blastocyst becomes single cell thickness called a trophoblast and there are inner mass cells which form the embryo itself
Why does the blastocyst need to hatch out of the protective zona pellucida?
The blastocyst hatches out to allow growth and so that the embryo can make direct contact with the endometrial lining of the uterus, ready for implantation
When and where does implantation usually occur?
Implantation typically happens by day 6 and it occurs on the POSTERIOR or anterior uterine wall
What is the corpus luteum?
This is a structure that is important for the maintenance of uterine development and allows implantation and pregnancy by releasing hormones (e.g estrogens and progesterone)
What happens at day 7.5/8?
Around this time of development, the blastocyst is partly embedded in the endometrial stroma. 2 things happen here.
The trophoblast differentiates into two layers:
1. Cytotrophoblast - inner layer, mononucleated cells
2. Syncytiotrophoblast - outer, multi-nucleated zone without distinct cell boundaries. The syncytiotrophoblast also produce human chorionic gonadotropin which is the basis for pregnancy tests and stimulates corpus luteum
The embryo also organises into 2 layers:
- Epiblast - layer of high columnar cells, dorsal surface of embryo
- Hypoblast - small cuboidal cells, ventral surface of embryo
What happens at day 9?
At day 9, there is the formation of 2 cavities:
1. Amniotic cavity (epiblast)
2. Primitive yolk sac (hypoblast)
As well as this, implantation is complete and closure by fibrin coagulum occurs. The vacuoles appear in the syncytiotrophoblast and unite to form lacunae
Explain the events that occur at day 12?
- Establishment of the uteroplacental circulation
- Maternal blood flows into the lacunae to diffuse to the embryoblast
- Extraembryonic mesoderm develops and eventually degenerates to form the chorionic cavity
What happens at day 13?
- Further development of chorionic cavity and presence of connecting stalk (which is eventually the umbilical cord)
- Second wave of hypoblast cells migrate to form the definitive (secondary) yolk sac
What happens once the embryo is implanted?
- Uteroplacental circulation forms as the syncytiotrophoblast extends and contacts maternal blood supply
- Amniotic cavity is present above the bilaminar embryonic disc and definitive yolk sac
- Chorionic cavity has developed and encloses the entire embryo
What is an ectopic pregnancy and name some sites?
This is when implantation occurs in a location outside of the uterus. Some sites this may occur is in the ovary, peritoneum and at various locations in the uterine tube
If an implantation occurs at the internal OS, what is this called and what is the consequence of this?
Placenta previa. It can cause severe, even life threatening bleeding in the second part of pregnancy and during delivery
Explain what the most characteristic event occurring during the third week of gestation is
Gastrulation - This is the process which establishes all 3 primary germ layers in the embryo
- This process begins with the formation of the primitive streak. The primitive streak appears in the midline at the caudal end of the epiblast. The cranial end of streak = primitive pit + node
- Body axes are established by the presence of the primitive streak
- Epiblast cells migrate towards primitive streak and move inward (invagination)
- Hypoblast cells are replaced by definitive endoderm
- Other cells come to lie between the epiblast and new endoderm to form mesoderm
- Cells remaining in the epiblast become ectoderm
What are the 3 primary germ layers?
- Ectoderm (outer layer)
- Mesoderm
- Endoderm (inner layer)
What is the fate of Ectoderm?
Ectoderm becomes the epidermis of the skin - hair, nails, associated glands. Also involved in formation of the nervous system - brain, spinal cord and peripheral nerve
What is the fate of Mesoderm?
Mesoderm becomes divided into 3 regions either side of the notochord:
- Paraxial mesoderm - forms axial skeleton, voluntary muscle, parts of dermis (back)
- Intermediate mesoderm - forms urogenital systems such as kidneys and ureter, gonads and associated structures
- Lateral plate - divided into somatic layer and visceral layer. Somatic layers forms most of the dermis, lining of body walls and parts of limbs (connective tissue and bone). Visceral layer forms cardiovascular systems, mesothelial covering of organs and smooth (involuntary) muscle
What is the fate of Endoderm?
Endoderm forms lining of gut tube, lining of respiratory tract and lining of bladder and urethra
