LEC 13: Embryology II - 08.26.14 Flashcards Preview

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Flashcards in LEC 13: Embryology II - 08.26.14 Deck (24):


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




Zygote gives rise to three (3) tissue layers

1. ectoderm

2. mesoderm

3. endoderm



embryonic stem cells

early on, zygote has potential to become anything in body

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


When does the zona pellucida disintegrate

  • 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



What are the parts of the blastocyst

A. embryoblast (inner cell mass)

B. degenerating zona pellucida

C. blastocyst cavity

D. trophoblast (outer cell mass)


Where does morula formation occur

Fallopian tube, towards isthmus section of tube

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


What are the 2 major parts to the blastocyst

1. embryoblast (inner cell mass)

2. trophoblast (secretes early pregnancy factor, which shows up in pregnancy tests)


When does implantation occur

  • 5-7 days after fertilization, at blastocyst stage


Describe the parts of the blastocyst as it begins to implant

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


What two (2) layers derive from the trophoblast

1. cytotrophoblast (inner layer)

2. syncytiotrophoblast (outer layer)




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


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

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


Besides epiblast and hypoblast, what else derives from embryoblast

Amniotic cavity

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


What forms from epiblasts

amnioblasts (surround amniotic cavity)


Amnion and amniotic cavity

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


Label the parts of the bilaminar embryo

A. Syncytiotrophoblast

B. Amnion

C. Bilaminar embryonic disc

D. Exocoelomic membrane

E. Exocoelomic cavity

F. Cytotrophoblast


Formation of primary yolk sac

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


What does the primary yolk sac endoderm give rise to

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


Formation of secondary yolk sac

  • 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)


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

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


prechordal plate

  • 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


Describe the three (3) fates of epiblasts

1. ectoderm of amnion

2. embryonic ectoderm

3. primitive streak


Describe the fate of hypoblasts

1. endoderm of yolk sac


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

epiblasts = contribute to formation of embryo


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