Embryology - Fertilisation to Trilaminar Embryo Flashcards
In humans, what does the sperm first bind to during fertilisation?
The zona pellucida glycoprotein (ZP3) - which tells cells to release calcium
What type of enzymes are released from the sperm head?
Acrosomal enzymes
When the egg and sperm plasma membrane fuse what happens?
The sperm contents enter the egg
What 2 things does sperm entry trigger?
Completion of meiosis II and release of cortical granules by oocyte
During the first 3 divisions of a zygote, the blastomeres are thought to be what potency?
Totipotent
What is the innermost layer of cells around the ovum called?
Corona radiata
What happens to the plasma membranes of sperm and oocyte when they bind?
They become continuous
What do cortical granules block?
Polyspermy
What is mosaicism?
When an individual has two or more cell lines with different chromosome complements eg those with Trisomy 21 may have some healthy cells and so with an extra chromosome
What causes mosaicism?
Mitotic nondisjunction which can occur in early division of the zygote
What is a morula?
16+ blastomeres
As the blastomeres divides what confines them?
Zona pellucida
How do blastomeres maximise available space?
By coming into closer contact with each other and beginning to form cell junctions
Outer cells touching the zona pellucida form what?
Extensive gap junctions
What enters through the zona pellucida?
Fluid
For implantation what does the embryo interact directly with?
The endometrial lining of the uterus
How long after fertilisation does implantation occur?
Day 6
Where does implantation normally occur?
The posterior or anterior uterine wall
What is the trophoblast?
The outer layer of cells of the blastocyst which provide nutrients (and become a large part of the placenta)
What does the trophoblast divide into?
Cytotrophoblast (made of individual cells) and syncytiotrophoblast (single multinucleated cells)
What 2 cell layers does an embryo organise into?
Epiblast (dorsal surface of embryo) and hypoblast (ventral surface of hypoblast)
What does the syncytiotrophoblast produce?
Human chorionic gonadotropin (hCG) which is the basis for a pregnancy test and stimulates corpus leteum (hormone secreting structure)
What is the blurring of a cell’s edges known as?
Compaction
On day 9 post fertilisation what two cavities are formed?
Amniotic cavity (epiblast) and primitive yolk sac (hypoblast) - note these layers were formed previously but now are forming cavities
When implantation is complete what structure closes the embryo?
Fibrin coagulum
How is lacunae formed?
By vacuoles in the syncytiotrophoblast uniting
What 2 things happen on Day 12 of the bilaminar embryo formation?
The uteroplacental circulation is established (maternal blood flows into the lacunae and diffuses to embryoblast) AND the extraembryonic mesoderm develops and eventually degenerates - chorionic cavity
What 2 things happen on Day 13 of the bilaminar embryo formation?
Further development of the chorionic cavity and presence of connecting stalk (which becomes umbilical cord) AND a second wave of hypoblast cells migrate to form the definitive yolk sac
How does the uteroplacental circulation form?
Syncytiotrophoblast extends and contacts the maternal blood supply
Where is the amniotic cavity and definitive yolk sac in relation to the bilaminar embryonic disc?
The amniotic cavity is above the disc and the sac is below
What encircles the entire embryo?
The chorionic cavity
What is the primitive streak?
The faintest sign of embryonic existence in the fertilised ovum
Where does the primitive streak appear?
In midline at the caudal end of epiblast
What end of the primitive streak are the primitive pit and node?
Cranial
What does the presence of the primitive streak establish?
The body axes
What is gastrulation?
The movement of epiblast cells through the primitive streak to form the three primary germ layers (trilaminar embryo)
During gastrulation what does the epiblast become?
Ectoderm
What are the fates of the ectoderm?
Epidermis of skin (hair, nails, associated glands) or nervous system (brain, spinal chord, peripheral nerves)
What are the fates of the mesoderm?
Divided into 3 regions either side the notocord - paraxial mesoderm (medial), intermediate mesoderm and lateral plate mesoderm (has a somatic layer and a visceral layer)
What are the fates of the paraxial mesoderm?
Axial skeleton, voluntary (skeletal) muscle or parts of dermis (back)
What are the fates of the intermediate mesoderm?
Urogenital systems - kidney and ureter or gonads and associated structures
What are the fates of the lateral plate mesoderm (somatic)?
Most of dermis, lining of body wall, parts of limbs
What are the fates of the lateral plate mesoderm (visceral)?
CV system, mesothelial covering of organs, smooth (involuntary) muscle
What are the fates of the endoderm?
Lining of the gut tube, respiratory tract or bladder and urethra
