Embryology

Question 1

Define germinal age

Define embryonic age

Answer 1

Time since last menstruation

Time since fertilisation (germinal age - 2 weeks)

Question 2

Define:
Germinal period
Embryonic period
Foetal period

Answer 2

Germinal period: fertilisation - end of the 2nd week
Embryonic period: start of the 3rd week - end of the 8th week
Foetal period: start of the 9th week - birth (38 weeks)

Question 3

What are the stages of fertilisation?

Answer 3

Oocyte (secondary - paused at M2) is released from the ovaries and carried to the ampulla by fimbriae
Sperm will fertilise the egg in the Fallopian tubes (ampullary region), instigating the remaining meiosis of the oocyte and eventually fusion of the chromosomes producing a diploid (tototent) zygote
The zygote will move from the Fallopian tubes to the uterus dividing as it does so to form a morula (8 cell bundle)

Question 4

Describe the formation of a blastocyst from a morula?

Answer 4

Morula —> blastocyst

Forms a fluid filled cavity called the blastocoel cavity

Question 5

What changes also happen to a blastocyst to prepare it for implantation?

Answer 5

Loses its zona pellucida (allows implantation into the endometrium of the uterus)
Inner cell mass differentiates into the embryoblast

Question 6

How does a blastocyst implant?

Answer 6

Trophoblast (and endometrium) release enzymes that devour the endometrium of the uterus
Blastocyst embeds itself into the endometrium

Question 7

What other changes does the blastocyst undergo after implantation?

Answer 7

The trophoblast differentiates itself into the inner cytotrophoblast and the outer synctiotrophoblast
The embryoblast differentiates itself into the epiblast and the hypoblast forming a bilaminar embryonic disc
An amniotic cavity forms in the embryoblast between the epiblast and the cytotrophoblast

Question 8

How is the exocoelomic cavity formed?

Answer 8

The hypoblast forms an exocoelomic membrane around the inner surface of the cytotrophoblast
Blastocoel cavity becomes the Exocoelomic cavity (primitive yolk sac)

Question 9

How does the embryo develop a uteroplacental circulation?

Answer 9

Holes appear in the synctiotrophoblast (lacunae), these join to form lacunae networks
Capillaries in the uterine wall dilate to become maternal sinusoids
Enzymes from the synctiotrophoblast destroys the lining of sinusoids and glands allowing maternal secretions to enter the lacunae networks

Question 10

How does the chorionic cavity form?

Answer 10

Rapid growth of the trophoblast creates space around the embryoblast
New cells form between the cytotrophoblast and the embryoblast forming the extraembryonic mesoderm
Cavities forming in the the extraembryonic mesoderm fuse to become the chorionic cavity

Question 11

What are the final stages of development of a blastocyst?

Answer 11

The exocoelomic cavity (primitive yolk sac) eventually pinches off leaving the secondary yolk sac (embryoblast)
The chorionic cavity increases and the BED connects to the trophoblast by the connecting stalk (extra-embryonic mesoderm) for histiotrophic nutrients

Question 12

What is the difference between histiotrophic nutrition and haemotrophic nutrition?

Answer 12

Histiotrophic nutrition: nutrition from the trophoblast

Haemotrophic nutrition: nutrition from the maternal blood

Question 13

What is the normal implantation site of the blastocyst?

Answer 13

Upper posterior wall of the uterus

Question 14

What is ectopic pregnancy?

What is placenta previa?

Answer 14

Pregnancy at an alternative site in the uterus (most commonly in the Fallopian tubes)

Placenta previa is when the placenta blocks off the cervix

Question 15

What are the stages in the embryonic period?

Answer 15

Gastrulation
Neuralation
Segmentation
Folding

Question 16

What is gastrulation?

Answer 16

The formation of the trilaminar embryonic disc from the bilaminar embryonic disc

Question 17

How is the trilaminar disc formed?

Answer 17

Epiblast cells migrate to the primitive streak, invaginate down the primitive groove and displace the hypoblast layer to create a third layer.
The primitive node (area at the front of the primitive streak) moves caudally to cranially forming the notochord

Question 18

How is the buccoharyngeal and cloacal membrane formed?

Answer 18

Formed when the epiblast and the hypoblast come into contact with each other
Buccopharyngeal: future mouth
Cloacal: future anus

Question 19

What do the layers of the trilaminar embryonic disc form?

Answer 19

Ectoderm: external structures (skin, CNS)
Mesoderm: supporting tissues (bone, cartilage, muscle)
Endoderm: internal structures (respiratory tract, GI tract)

Question 20

How is left-right sidedness coordinated by the primitive node?

Answer 20

Ciliated cells

Question 21

What is neurulation?

Answer 21

Formation of the neurectoderm from the ectoderm

Question 22

How is the neurectoderm formed?

Answer 22

Signals from the notochord cause the thickening of the ectoderm forming a neural crest
Neural crest joins forming a neural tube and paraxial mesoderm forms 31 somites

Question 23

What are somites?

Answer 23

Blocks of mesoderm cells around a cavity

Question 24

What is segmentation?

Answer 24

Organisation of mesoderm into somites and then degeneration of somites into different tissues

Question 25

What happens during organised degeneration of somites?

Answer 25

Somites breaks down:
Ventral wall becomes sclerotome
Dorsal wall becomes dermomyotome
Dermomyotome eventually separates into dermatome (outside) and myotome (inside)

Question 26

What do sclerotome, myotome, dermatome eventually form?

Answer 26

Sclerotome: bone
Myotome: muscle
Dermatome: skin

Question 27

What is folding?

Answer 27

Process of pulling all the compartments into the right place to form and forming a new cavity (primordium of the GI tract)

Question 28

What does folding achieve?

Answer 28

Draws together the edges of the trilaminar disc forming a ventral body wall with the connecting stalk
Suspends embryo within amniotic sac

Question 29

At the end of the embryonic period what has happened?

Answer 29

Nervous system has started forming
Segments have appeared with specific functions
The embryo has been folded with everything in the right place