Embryology

Question 1

What is nondisjunction?

Answer 1

Failure of sister chromatids to separate during cell division
Can lead to trisomy

Question 2

What happens during fertilisation?

Answer 2

Sperm bind to zona pellucida
Acrosomal enzymes are release by the head of the sperm (digests into the oocyte)
Sperm and oocyte plasma membranes fuse
The oocyte completes meiosis 2 and releases cortical granules (making ZP impenetrable)

Question 3

What are the stages of embriogenesis?

Answer 3

Gametogenesis
Fertilisation
Cleavage
Gastrulation
Embryonic folding
Organogenesis

Question 4

What does the trophoblast split into?

Answer 4

Cytotrophoblast

Syncytiotrophoblast

Question 5

What does the ‘inner cell mass’ split into?

Answer 5

Epiblast layer

Hypoblast layer

Question 6

What are the 3 primary germ layers?

Answer 6

Ectoderm
Mesoderm
Endoderm

Question 7

What is the fate of the ectoderm?

Answer 7

Epidermis of the skin

Nervous system

Question 8

What is the fate of the mesoderm?

Answer 8

Paraxial mesoderm- axial skeleton, skeletal muscle and part of dermis of the back

Intermediate mesoderm- urogenital systems

Lateral plate mesoderm

• somatic part: most of dermis, lining of body wall and contributes to limbs
• visceral part: cardiovascular system, smooth muscle, mesothelium

Question 9

What is the fate of the endoderm?

Answer 9

Lining of gut tubes, respiratory tract and bladder/urethra

Question 10

during weeks 0-3 of development it is known as?

Answer 10

conceptus/embryo

Question 11

during which weeks is it known as an embryo?

Answer 11

0-3 weeks

Question 12

During which weeks is the embryonic period?

Answer 12

weeks 3-8

Question 13

during which weeks is it known as the fetal period?

Answer 13

weeks 9-40

Question 14

what happens during gametogenesis?

Answer 14

The gametes are formed.

Mitosis happens first then meiosis.

Question 15

what is mitosis?

Answer 15

Cell division to produce two identical daughter cells

Question 16

what is meiosis?

Answer 16

cell division to produce 4 haploid cells called gametes

Question 17

how many chromosomes does a gamete have?

Answer 17

half: 23

Question 18

the formation of male gametes is known as?

the formation of female gametes is knowns as?

Answer 18

spermatogenesis

oogenesis

Question 19

How long does the production of a male gamete take and what is the production rate?

Answer 19

each cycle takes 64 days

production rate = 6.5 million sperm per gram of testicular tissue per day

Question 20

when do primary oocytes begin meiosis?

Answer 20

by week 28-30 of development, they then lie dormant till puberty

Question 21

what is trisomy 21 and how is it caused?

Answer 21

an additional copy of chromosome 21 causing down syndrome.

caused by nondisjunction

Question 22

what is fertilisation?

Answer 22

Fusion of the sperm and oocyte to form a zygote

Question 23

what is a zygote?

Answer 23

a diploid single celled embryo

Question 24

what happens during cleavage?

Answer 24

rapid mitotic cell division with no overall increase in size/volume

Question 25

what happens during gastrulation?

Answer 25

formation of the germ layers (in weeks 3) and the body axes are established

Question 26

what occurs on day 0?

Answer 26

fertilisation

Question 27

what happens during fertilisation?

Answer 27

the sperm binds with the zona pellucida glycoprotein.
the head of the sperm releases acrosomal enzymes to digest into the egg
the egg and sperm membrane fuse and the sperm contents enter the egg.

Question 28

what is the name of the enzyme the sperm releases to digest its way into the egg?

Answer 28

acrosomal enzyme

Question 29

what does sperm penetration cause?

Answer 29

completion of meiosis 2 and release of cortical granules by the oocyte to make the zona pellucida impenetrable

Question 30

where does fertilisation usually occur?

Answer 30

in the ampulla of the uterine tube

Question 31

where does cleavage happen?

Answer 31

as the egg makes its way through the uterine tubes to get to the uterus.

Question 32

up to what stage is the zygote totipotent?

Answer 32

up to the 8 cell stage blastomere.

Question 33

what is mosaicism?

Answer 33

a condition where an individuals cells don’t all contain identical chromosomes.

Question 34

how is mosaicism caused?

Answer 34

by mitotic nondisjunction in a zygote which already has trisomy 21 - resulting in some cells being normal and some having trisomy.

Question 35

what is a morula

Answer 35

16+ blastomeres

Question 36

what are the first two layers and cavity formed called?

note this is still contained inside the zona pellucida

Answer 36

the outside layer = trophoblast
inside layer = embryoblast
Blastocyst cavity

Question 37

by which day does implantation occur?

Answer 37

day 6

Question 38

where does implantation occur?

Answer 38

the posterior uterine wall - normally

Question 39

the epiblast makes up which surface of the embryo?

Answer 39

dorsal

Question 40

the hypoblast makes up which surface of the embryo?

Answer 40

ventral

Question 41

when is implantation complete?

Answer 41

day 9

Question 42

which two cavities are formed on day 9?

Answer 42

the amniotic cavity and the primitive yolk sac

Question 43

what happens on day 12?

Answer 43

uteroplacental circulation is established and extraembryonic mesoderm develops and degenerates forming the chorionic cavity

Question 44

when is the primitive streak formed?

Answer 44

week 3

Question 45

explain where the following cavities are in relation to the bilaminar embryonic disc:

• amniotic cavity
• definitive yolk sac
• chorionic cavity

Answer 45

amniotic cavity = above the disc
definitive yolk sac = below the disc
chorionic cavity = encircles the entire embryo

Question 46

where is the primitive pit and node?

Answer 46

the caudal end of the primitive streak

Question 47

what does the epiblast become?

Answer 47

the primary germ layers