Early Embryonic Development

Question 1

Briefly describe the formation of an embryo

Answer 1

Sperm fuses with the ovum
This forms a zygote
Blastocyst
Bi laminar disc
Tri laminar disc
Embryo

Question 2

What forms the zygote ?

Answer 2

Spermatozoon and secondary oocyte (ovum)

Question 3

What is the function of the acrosome of the sperm ?

Answer 3

Contains enzymes required for the penetration of the zone pellucida of the oocyte.

Question 4

Where are 2ndary oocytes found ?

Answer 4

In the ovary, arrested in metaphase 2 of meiosis 2

Question 5

Describe what the cytoplasm of the oocyte is surrounded by.
(inner to outer)

Answer 5

Plasma membrane
Zona pellucida
Corona radiata

Question 6

What is ovulation ?

Answer 6

The discharge of usually a single secondary oocyte from the ovary as a result of increase in luteinising hormone (LH)

Question 7

What type of cells are found in the corona radiata ?

Answer 7

Cumulus oophorus cells

Question 8

When is Meiosis 2 completed ?

Answer 8

When the secondary oocyte is fertilised
The oocyte matures to become ovum

Question 9

Describe the steps in ovulation

Answer 9

The ends of the uterine tubes come into close contact with the ovary during ovulation.

Fimbriae of uterine tubes ‘sweep’ the ovulated ovum into the uterine tube.

Peristaltic waves of uterine tube musculature bring the 2ndary oocyte into the ampulla of the uterine tube.

Question 10

What is fertilisation ?

Answer 10

Fusion of the haploid sperm and haploid oocyte to form a diploid zygote.

Question 11

Where does fertilisation usually take place ?

Answer 11

Ampulla region of the uterine tube

Question 12

What must spermatozoa undergo to acquire fertilisation capability ?

Answer 12

Capacitation
Acrosome reaction

Question 13

Describe spermatozoon penetration

Answer 13

Phase 1 : Capacitation (passing through the corona radiata)

Phase 2 : Acrosome reaction (penetration of the zone pellucida)

Phase 3 : Penetration of the oocyte membrane by one spermatozoon, which loses its own plasma membrane.

Question 14

What enzyme penetrates the corona radiata ?

Answer 14

Hyaluronidase
Interaction with ZP3 receptor

Question 15

Describe fertilisation

Answer 15

Penetration of corona radiata - by hyaluronidase

Interaction with ZP3 receptor

Acrosome reaction - digests zone pellucida

Membrane fusion

Completion of oocyte meiosis

Release of the contents of cortical granules :

• Degradation of ZP3 to prevent second penetration

Question 16

Cleavage

Answer 16

Zygote to Morula

Chromosomes become arranged on the spindle, split longitudinally and move to opposite poles.

Question 17

What happens once the zygote has reached the 2 cell stage ?

Answer 17

It undergoes a series of mitotic divisions, increasing in the number of cells.

These cells, which become smaller with each cleavage division, are known as BLASTOMERES.

Question 18

How many cells are in the morula ?

Answer 18

16-18 cells
takes 72 hours

Question 19

Describe the structure of the morula

Answer 19

Inner cell mass - EMBRYOBLAST
which is surrounded by
Outer cell mass - TROPHOBLAST

Question 20

What does the embryoblast of the morula give rise to ?

Answer 20

Embryo

Question 21

What does the trophoblast of the morula give rise to ?

Answer 21

Contributes to the placenta

Question 22

Where do the cavities of the morula form ?

Answer 22

Cavities form in inner cell mass, filled with fluid.

Question 23

What does a single blastocyst cavity give rise to ?

Answer 23

The single blastocyst cavity will become the primitive yolk sac.

Question 24

Around what day does the blastocyst reach the uterine cavity ?

Answer 24

On day 6-7, the blastocyst reaches the uterine cavity.

Question 25

What happens on day 6-7 ?

Answer 25

The blastocyst reaches the uterine cavity. Trophoblast cells over the embryo blast begin to implant into the epithelial cells. Host/graft and graft/host reactions are normally suppressed.

Question 26

How do the cells in the embryoblast differentiate ?

Answer 26

Under the influence of fibroblast growth factors (FGFs) at the early blastocyst stage, cells in the embryoblast differentiate into epiblast and hypoblast cells. Scattered cells segregate ----> Bilaminar disc

Question 27

What happens on day 8 ?

Answer 27

On day 8, the blastocyst is partially embedded in the endometrium of the uterus.

Question 28

What does the trophoblast differentiate into ?

Answer 28

In the area over the embryoblast, the trophoblast differentiates into the inner layer of - CYTOtrophoblast, and an outer zone of - SYNCYTIOtrophoblast (secretes beta-HCG)

Question 29

What does the syncytiotrophoblast secrete and what is the importance of this ?

Answer 29

Secretes - Beta-HCG This is used to detect pregnancy

Question 30

What and where is the amniotic cavity ?

Answer 30

A small cavity appears within the epiblast called amniotic cavity.

Question 31

What happens at day 15 ?

Answer 31

Around day 15, Primitive streak appears as a thick structure on the epiblast : - In the midline - Near the caudal region of the bilaminar embryonic disc - Defines the body axes

Question 32

Where is the primitive streak ?

Answer 32

IN the midline. Near the caudal region of the bilaminar embryonic disc

Question 33

What does the primitive streak define ?

Answer 33

The body axes

Question 34

Name the parts of the primitive streak

Answer 34

Primitive : - pit - node - groove

Question 35

What does the cranial end of the primitive streak contain ?

Answer 35

Primitive node which contains the primitive pit

Question 36

What is the groove in the primitive streak called ?

Answer 36

Primitive groove

Question 37

What happens at day 16 ?

Answer 37

Gastrulation Bilaminar disc ----> Trilaminar disc

Question 38

Name the 3 layers of the trilaminar disc

Answer 38

Ectoderm Mesoderm Endoderm

Question 39

What is the ectoderm formed from ?

Answer 39

Epiblast

Question 40

What is the endoderm formed from ?

Answer 40

Hypoblast

Question 41

Describe invagination

Answer 41

Cells of the epiblast migrate through the primitive streak, detach from the epiblast, displace hypoblast and create a third layer.

Question 42

Describe what happens on day 17

Answer 42

Cranial and caudal folding Notochordal plate forms a notochord

Question 43

What is a notochord ?

Answer 43

A cylindrical structure which gives instructions to other cells/tissues. The notochord acts as a temporary axial skeleton for the embryo.

Question 44

What does the paraxial mesoderm give rise to ?

Answer 44

Somites - Sclerotome - Myotome - Dermatome Head mesenchyme

Question 45

What does the lateral plate mesoderm give rise to ?

Answer 45

Walls of the body cavities and serous membranes

Question 46

What does the intermediate mesoderm give rise to ?

Answer 46

The urogenital system

Question 47

Ectopic pregnancy

Answer 47

Implantation of blastocyst in an abnormal location IN the ovary or anywhere along the path of migration into the uterine cavity.

Question 48

What is the most common ectopic site ?

Answer 48

Tubal pregnancies (95%)

Question 49

What do ectopic pregnancies lead to ?

Answer 49

Can lead to spontaneous tubal abortion or … Can lead to maternal mortality if undetected after intraperitoneal rupture of the uterine tube and its vessels.

Question 50

When does the likelihood of tubal rupture and haemorrhage increase ?

Answer 50

From the 2nd through the 5th months Risk of maternal death