LEC 13: Embryology II - 08.26.14

Question 1

morula

Answer 1

• “ball of cells”
• after 4-5 divisions of the zygote, morula forms
• approximately 12-16 cells, or blastomeres
• ends with formation of blastocyst

Question 2

Zygote gives rise to three (3) tissue layers

Answer 2

1. ectoderm
2. mesoderm
3. endoderm

Question 3

embryonic stem cells

Answer 3

early on, zygote has potential to become anything in body

• cells are totipotent
• with each division of cells, some potential is lost

Question 4

When does the zona pellucida disintegrate

Answer 4

• zona pellucida disintegrates as blastocyst progresses from early to late stage blastocyst (2 days post-fertilization)
• only after zona pellucida disintegrates do cells/blastocyst grow considerably
• zona pellucida made cells progressively smaller after each division
• zona pellucida disintegrates to allow embryo to implant

Question 5

What are the parts of the blastocyst

Answer 5

A. embryoblast (inner cell mass)

B. degenerating zona pellucida

C. blastocyst cavity

D. trophoblast (outer cell mass)

Question 6

Where does morula formation occur

Answer 6

Fallopian tube, towards isthmus section of tube

(remember: fertilization and formation of zygote generally occurs in ampulla)

Question 7

What are the 2 major parts to the blastocyst

Answer 7

1. embryoblast (inner cell mass)
2. trophoblast (secretes early pregnancy factor, which shows up in pregnancy tests)

Question 8

When does implantation occur

Answer 8

• 5-7 days after fertilization, at blastocyst stage

Question 9

Describe the parts of the blastocyst as it begins to implant

Answer 9

A. Endometrial epithelium

B. Inner cell mass (embryoblast)

C. Trophoblast

E. Embryonic pole

F. Blastocyst cavity

G. Syncytiotrophoblast

H. Inner cell mass (embryoblast)

I. Cytotrophoblast

J. Hypoblast (primary endoderm)

K. Blastocyst cavity

Question 10

What two (2) layers derive from the trophoblast

Answer 10

1. cytotrophoblast (inner layer)
2. syncytiotrophoblast (outer layer)

Question 11

syncytiotrophoblast

Answer 11

multinucleated protoplasmic mass in which no cell boundaries can be observed; invades epithelium of endometrium to allow implantation of embryo

Question 12

What two (2) layers derive from embryoblast (inner cell mass)

Answer 12

1. epiblast (thicker layer, high columnar cells related to amniotic cavity)
2. hypoblast (small cuboidal cells adjacent to exocoelomic cavity)

Together, epiblast and hypoblast make up embryonic disc

Question 13

Besides epiblast and hypoblast, what else derives from embryoblast

Answer 13

Amniotic cavity

• small space appears in embryoblast during implantation of blastocyst
• amnioblasts separate from epiblast to form amnion (encloses amniotic cavity)

Question 14

What forms from epiblasts

Answer 14

amnioblasts (surround amniotic cavity)

Question 15

Amnion and amniotic cavity

Answer 15

• during implantation of embryo in uterine wall, small space appears in embryoblast
• amnioblasts separate from epiblast and form amnion
• amnion encloses amniotic cavity

Question 16

Label the parts of the bilaminar embryo

Answer 16

A. Syncytiotrophoblast

B. Amnion

C. Bilaminar embryonic disc

D. Exocoelomic membrane

E. Exocoelomic cavity

F. Cytotrophoblast

Question 17

Formation of primary yolk sac

Answer 17

• blasocyst cavity becomes exoceolomic cavity
• exoceolomic membrane and cavity become primary yolk sac

Question 18

What does the primary yolk sac endoderm give rise to

Answer 18

• extraembryonic mesoderm
• surrounds yolk sac, bilaminar disc, and amniotic cavity

Question 19

Formation of secondary yolk sac

Answer 19

• spaces form in extraembryonic mesoderm
• spaces coalesce to form an extraembryonic coelom
• part of primary yolk sac is pinched off
• remaining extraembryonic endoderm forms secondary (definitive yolk sac)

Question 20

What is the function of the definitive (secondary) yolk sac

Answer 20

• early transfer of nutrients
• early hematopoietic (blood) development
• provides primordial germ cells (gives rise to germ cells for embryo)

Question 21

prechordal plate

Answer 21

• develops as a thickened circle of hypoblasts in a localized area of the bilaminar embryo
• determines head region of the embryo
• future site of the mouth

Question 22

Describe the three (3) fates of epiblasts

Answer 22

1. ectoderm of amnion
2. embryonic ectoderm
3. primitive streak

Question 23

Describe the fate of hypoblasts

Answer 23

1. endoderm of yolk sac

Question 24

What is the primary difference between epiblasts and hypoblasts (both arise from embryoblasts)

Answer 24

epiblasts = contribute to formation of embryo

hypoblasts = do not contribute to any parts of embryo (none of the 3 fetal layers)