Embryology - Fertilisation to Trilaminar Embryo

Question 1

In humans, what does the sperm first bind to during fertilisation?

Answer 1

The zona pellucida glycoprotein (ZP3) - which tells cells to release calcium

Question 2

What type of enzymes are released from the sperm head?

Answer 2

Acrosomal enzymes

Question 3

When the egg and sperm plasma membrane fuse what happens?

Answer 3

The sperm contents enter the egg

Question 4

What 2 things does sperm entry trigger?

Answer 4

Completion of meiosis II and release of cortical granules by oocyte

Question 5

During the first 3 divisions of a zygote, the blastomeres are thought to be what potency?

Answer 5

Totipotent

Question 6

What is the innermost layer of cells around the ovum called?

Answer 6

Corona radiata

Question 7

What happens to the plasma membranes of sperm and oocyte when they bind?

Answer 7

They become continuous

Question 8

What do cortical granules block?

Answer 8

Polyspermy

Question 9

What is mosaicism?

Answer 9

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

Question 10

What causes mosaicism?

Answer 10

Mitotic nondisjunction which can occur in early division of the zygote

Question 11

What is a morula?

Answer 11

16+ blastomeres

Question 12

As the blastomeres divides what confines them?

Answer 12

Zona pellucida

Question 13

How do blastomeres maximise available space?

Answer 13

By coming into closer contact with each other and beginning to form cell junctions

Question 14

Outer cells touching the zona pellucida form what?

Answer 14

Extensive gap junctions

Question 15

What enters through the zona pellucida?

Answer 15

Fluid

Question 16

For implantation what does the embryo interact directly with?

Answer 16

The endometrial lining of the uterus

Question 17

How long after fertilisation does implantation occur?

Answer 17

Day 6

Question 18

Where does implantation normally occur?

Answer 18

The posterior or anterior uterine wall

Question 19

What is the trophoblast?

Answer 19

The outer layer of cells of the blastocyst which provide nutrients (and become a large part of the placenta)

Question 20

What does the trophoblast divide into?

Answer 20

Cytotrophoblast (made of individual cells) and syncytiotrophoblast (single multinucleated cells)

Question 21

What 2 cell layers does an embryo organise into?

Answer 21

Epiblast (dorsal surface of embryo) and hypoblast (ventral surface of hypoblast)

Question 22

What does the syncytiotrophoblast produce?

Answer 22

Human chorionic gonadotropin (hCG) which is the basis for a pregnancy test and stimulates corpus leteum (hormone secreting structure)

Question 23

What is the blurring of a cell’s edges known as?

Answer 23

Compaction

Question 24

On day 9 post fertilisation what two cavities are formed?

Answer 24

Amniotic cavity (epiblast) and primitive yolk sac (hypoblast) - note these layers were formed previously but now are forming cavities

Question 25

When implantation is complete what structure closes the embryo?

Answer 25

Fibrin coagulum

Question 26

How is lacunae formed?

Answer 26

By vacuoles in the syncytiotrophoblast uniting

Question 27

What 2 things happen on Day 12 of the bilaminar embryo formation?

Answer 27

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

Question 28

What 2 things happen on Day 13 of the bilaminar embryo formation?

Answer 28

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

Question 29

How does the uteroplacental circulation form?

Answer 29

Syncytiotrophoblast extends and contacts the maternal blood supply

Question 30

Where is the amniotic cavity and definitive yolk sac in relation to the bilaminar embryonic disc?

Answer 30

The amniotic cavity is above the disc and the sac is below

Question 31

What encircles the entire embryo?

Answer 31

The chorionic cavity

Question 32

What is the primitive streak?

Answer 32

The faintest sign of embryonic existence in the fertilised ovum

Question 33

Where does the primitive streak appear?

Answer 33

In midline at the caudal end of epiblast

Question 34

What end of the primitive streak are the primitive pit and node?

Answer 34

Cranial

Question 35

What does the presence of the primitive streak establish?

Answer 35

The body axes

Question 36

What is gastrulation?

Answer 36

The movement of epiblast cells through the primitive streak to form the three primary germ layers (trilaminar embryo)

Question 37

During gastrulation what does the epiblast become?

Answer 37

Ectoderm

Question 38

What are the fates of the ectoderm?

Answer 38

Epidermis of skin (hair, nails, associated glands) or nervous system (brain, spinal chord, peripheral nerves)

Question 39

What are the fates of the mesoderm?

Answer 39

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)

Question 40

What are the fates of the paraxial mesoderm?

Answer 40

Axial skeleton, voluntary (skeletal) muscle or parts of dermis (back)

Question 41

What are the fates of the intermediate mesoderm?

Answer 41

Urogenital systems - kidney and ureter or gonads and associated structures

Question 42

What are the fates of the lateral plate mesoderm (somatic)?

Answer 42

Most of dermis, lining of body wall, parts of limbs

Question 43

What are the fates of the lateral plate mesoderm (visceral)?

Answer 43

CV system, mesothelial covering of organs, smooth (involuntary) muscle

Question 44

What are the fates of the endoderm?

Answer 44

Lining of the gut tube, respiratory tract or bladder and urethra