embryology 2 - fertilization to trilaminar

Question 1

fertilization - 4 steps

Answer 1

1) Sperm binds in a human specific interaction with the zona pellucida glycoprotein (ZP3)
2) Acrosomal enzymes released from sperm head; the sperm digests its way into egg.
3) Egg and sperm plasma membrane fuse and sperm contents enter the egg
4) Sperm entry triggers: - completion of meiosis 2 (fully matures) - release of cortical granules by oocyte (no other sperm can enter)
* typically occurs in the ampulla of the uterine tube

Question 2

cleavage - first cell divisions

Answer 2

• The zygote now undergoes a series of mitotic divisions which subdivide the fertilised egg into many smaller daughter cells called blastomeres. Up to the 8 cell stage blastomeres are thought to be totipotent

Question 3

what does totipotent mean

Answer 3

can form any cell including embryonic or placental

Question 4

mosaicism

Answer 4

• Individuals with two or more cell line with different chromosome complements

Question 5

when does mitotic nondisjunction occur

Answer 5

can occur in early division of the zygote

Question 6

nondisjunction

Answer 6

when cell splits into 2 daughter cells it splits unevenly

Question 7

what is a morula

Answer 7

16+ blastomeres

Question 8

what are the blastomere cells confined in

Answer 8

within the zona pellucida, outer protective lining around cell

Question 9

compaction of the morula

Answer 9

• Maximise available space by coming into closer contact with each other and begin to form cell junctions = start to adhere to each other
• Outer cells (in contact with ZP) form extensive gap junctions
• Fluid enters through ZP

Question 10

blastocyst

Answer 10

consists of trophoblast, blastocyst cavity, inner cell mass and the zona pellucida

Question 11

trophoblast

Answer 11

forms placenta

Question 12

inner cell mass

Answer 12

forms embryo

Question 13

blastocyst cavity

Answer 13

collection of fluid between cell masses

Question 14

what is the embryonic pole

Answer 14

the eventual site of implantation into the uterus

Question 15

why is it important for the blastocyst to hatch out the ZP

Answer 15

allows the embryo to directly interact with the endometrial lining of uterus for implantation

Question 16

when does implantation occur

Answer 16

• Typically, by day 6 on posterior or anterior uterine wall. Cell pole attaches

Question 17

corpus luteum

Answer 17

in ovary, forms every time an egg is released, it is a temporary endocrine gland. Secrets progesterone which tells the uterus to keep the thick endometrium lining for implantation. Eventually it gives up which is how periods are started

Question 18

bilaminar embryo - day 7.5

Answer 18

• Trophoblast divides into Cytotrophoblast and Syncytiotrophoblast
• Embryo organizes into 2 layers – epiblast and hypoblast. Amniotic cavity begins to form
• Epiblast forms dorsal surface of embryo, hypoblast is ventral
• Syncytiotrophoblast produces hCG, bases of pregnancy test and tells Corpus luteum to keep producing Progesterone

Question 19

day 9

Answer 19

• implantation complete (closed by fibrin coagulum)
• Vacuoles appear in the syncytiotrophoblast and unite to form lacunae (eventually sets up blood supply from mother to baby)
• Formation of 2 cavities – amniotic cavity (made by epiblast) primitive yolk sac (hypoblast)

Question 20

day 12

Answer 20

• Establishment of the uteroplacental circulation
• Maternal blood flows into lacunae to diffuse to the embryoblast
• Extraembryonic mesoderm develops (outside embryo) and eventually degenerates forming the chorionic cavity

Question 21

day 13

Answer 21

• Further development of the chorionic cavity and presence of connecting stalk (eventually umbilical cord).
• Second wave of hypoblast cells migrate to form the definitive (secondary) yolk sac

Question 22

features of the implanted embryo

Answer 22

• Uteroplacental circulation forms as syncytiotrophoblast extends and contacts maternal blood supply
• The amniotic cavity is present above the bilaminar embryonic disc and the definitive yolk sac below
• The chorionic cavity has developed and encircles the entire embryo

Question 23

ectopic pregnancy

Answer 23

• Implantation in a location outside the uterus.
• Dangerous and potentially life-threatening.
• Abnormal sites = ovary, uterine tube, peritoneum

Question 24

formation of primitive streak

Answer 24

• Week 3
• Primitive streak appears in midline at caudal end of epiblast
• Cranial end of Primitive streak = Primitive Pit and Node
• Sets up body rotation

Question 25

gastrulation

Answer 25

• Movement of epiblast cells through the primitive streak to form
  the three primary germ layers
• Body axes are established by presence of primitive streak
• Hypoblast cells replaced by definitive endoderm
• Cells migrate through the primitive streak between the epiblast and
  the definitive endoderm

Question 26

difference in layers between bilaminar and trilaminar

Answer 26

2 germ layers become 3 layers. its then classified at trilaminar

Question 27

what are the 3 germ layers

Answer 27

ectoderm, mesoderm, endoderm

Question 28

ectoderm

Answer 28

becomes epidermis of skin and nervous system

Question 29

mesoderm (MD)

Answer 29

divides into 3 regions

Question 30

what are the 3 regions of the mesoderm

Answer 30

paraxial MD, intermediate MD, lateral plate MD (which then splits into 2, somatic layer and visceral layer)

Question 31

paraxial mesoderm

Answer 31

becomes axial skeleton, skeletal muscles, part of dermis

Question 32

intermediate mesoderm

Answer 32

urogenital system

Question 33

lateral somatic layer MD

Answer 33

most of dermis. lining of body wall, parts of limbs

Question 34

lateral visceral layer MD

Answer 34

cardiovascular system, lining covering organs, smooth muscles

Question 35

endoderm

Answer 35

lining of gut tube, lining of respiratory, lining of bladder/urethra

Question 36

what is a notochord

Answer 36

• Mesoderm also has a notochord which is only present in embryos, releases hormone (sonic hedgehog) which tells tissue what patterns to form in