Embryology: Week 2 Of Development

Front 1

Blastocyst Formation

Back 1

About 4 days after fertilization, shortly after morula enters uterus, spaces form in the morula --> coalesce into one fluid-filled space called the blastocystic cavity inside the morula. As fluid increases in blastocystic cavity, separates blastomeres (cell mass) into 2 parts:

-Inner cell mass: forms embryo (if this divides, we get monozygotic (identical) twins)

-Trophoblast: layer of cells surrounding the cavity which helps form placenta and contributes to extraembryonic mesoderm

Front 2

Germ layers and anatomy during week 2

Back 2

Germ layers have cellular characteristics; week 2 is time of intense cellular proliferation

1) Bilaminar embryo: inner cell mass divides into epiblast (ectoderm) and hypoblast (endoderm).

2) 2 fluid filled sacs: amniotic sac from epiblast and yolk sac from hypoblast

3) Bilaminar embryonic disc = area of contact; sits between 2 sacs and gives rise to whole body

Front 3

Prechordal Plate

Back 3

1. A rostral (head-direction) area of tissue-affinity, tight apposition between endoderm (hypoblast) and ectoderm (epiblast)

2. Key organizer of head, site of future mouth

Front 4

Connecting Stalk

Back 4

1. Brige of extraembryonic mesoderm from trophoblast to embryo

2. Route of future umbilical blood vessels.

Front 5

Day 8 of development

Back 5

1. Trophoblast differentiates into cytotrophoblast & syncytiotrophoblast

2. Embryoblast (inner cell mass) differentiates into epiblast (ectoderm) and hypoblast (endoderm). Formation of amniotic cavity

Front 6

Day 9

Back 6

1. Blastocyst attaches, embeds and closes defect (plug) in endometrium

2. Primitive yolk sac (endoderm) forms

3. Further differentiation of epiblast (future cells of embryo) and hypoblast (endoderm) (cells mostly displaced laterally into extraembryonic membranes

Front 7

Days 11-12

Back 7

1. Begin development of uteroplacental circulation: Penetration of syncytiotrophoblast erodes into endothelium of maternal capillaries, forming sinusoids and lacunar spaces with the trophoblast that fill with maternal blood

2. Cavitation of extra-embryonic mesoderm and formation of chorionic cavity

3. Somatopleuric mesoderm (lines cytotrophoblast and amnion) and extra-embryonic splanchnopleuric mesoderm (covers yolk sac)

Front 8

Day 13

Back 8

1. Formation of primary stem villi (a core of cytotrophoblast and outer syncytiotrophoblast)

2. Involution of primary yolk sac and appearance of definitive yolk sac

3. Expansion of chorionic cavity and establishment of the chorion, including chorionic plate

4. Amniotic cavity begins as fluid filled spaces coalesce into one large cavity. Amnioblasts cells of lining are derived from ectoderm

Front 9

Trophoblastic Hypoplasia

Back 9

Lack of differentiation (syncytium only)

Front 10

Trophoblastic hyperplasia

Back 10

a. Non-invasive, constituting a hydatidiform mole. Believed to result from only paternal DNA participating in zygote development.

b. Malignant, leading to a chorioepithelioma (i.e., a chorion epithelioma) (epithelioma = abnormal growth of epithelium)

Front 11

Differentiated layers of trophoblast

Back 11

Differentiation occurs after fertilization

1) An inner layer of cytotrophoblast (cellular trophoblast). Inner cell mass differentiates into epiblast (ectoderm) and hypoblast (endoderm)

2) An outer layer of syncytiotrophoblast (syncytium) consisting of a multinucleated protoplasmic mass in which no cell boundaries can be observed; this will form the fetal placenta