Developmental Period: Week 1&2

Question 1

What type of cell is released during ovulation?

Answer 1

Secondary oocyte ✅

Question 2

What is the function of the zona pellucida?

Answer 2

Attract sperm, prevent polyspermy, block abnormal implantation 🧲🛡️

Question 3

Where does fertilization occur?

Answer 3

In the ampulla of the fallopian tube 🚺

Question 4

What happens on Day 1 post-fertilization?

Answer 4

Zygote is formed; it’s a single-cell structure covered by zona pellucida 🥚

Question 5

What is the ideal cell count in a morula?

Answer 5

16 cells (can also be 12/32/58) 🔢

Question 6

When does the advanced morula enter the uterine cavity?

Answer 6

Day 4 🛬

Question 7

What is the approximate cell count of an advanced morula?

Answer 7

32 cells usually (range: 16–64) ✅

Question 8

When does blastocyst formation start?

Answer 8

End of day 4

Question 9

What happens on day 2 post fertilization?

Answer 9

Zygote for the first time cleaves into 2 cells

Question 10

What happens on day 3 post fertilization?

Answer 10

Multicellular stage/morula stage 16 celled
12/16/32/58

Question 11

When does the blastocyst hatch out of the zona pellucida?

Answer 11

Day 5 ⏳

Question 12

When does attachment to the endometrium begin?

Answer 12

Day 6 📌

Question 13

When is implantation completed?

Answer 13

Around Day 12 (±1 day) 🧷

Question 14

When is uteroplacental circulation established?

Answer 14

Day 12 ± 1 🫀🌐

Question 15

What is the correct sequence of fertilization reactions?

Answer 15

Acrosome → Cortical → Zona

Question 16

What is the function of the zona reaction?

Answer 16

It prevents polyspermy by hardening the ZP membrane 🚫🧬

Question 17

What triggers the cortical reaction?

Answer 17

Fusion of sperm and oocyte → Ca²⁺ release → cortical granules 💣

Question 18

What happens during zona reaction?

Answer 18

Sperm binds ZP → loses acrosomal cap → enzymes released to penetrate ZP.

Question 19

What structure helps in attachment and penetration into endometrium?

Answer 19

Syncytiotrophoblast 🔥

Question 20

Which trophoblast component has cell boundaries?

Answer 20

Cytotrophoblast 🧱

Question 21

Which day does blastocyst hatch and implantation begin?

Answer 21

Day 5 ✅

Question 22

What cells form the amniotic cavity and yolk sac?

Answer 22

Epiblast → Amniotic cavity

Hypoblast → Yolk sac 🥚🌊

Question 23

What appears in syncytiotrophoblast by Day 9?

Answer 23

Lacunae → start of uteroplacental nutrition 🩸

Question 24

What does the trophoblast become during Day 6–8?

Answer 24

Syncytiotrophoblast & Cytotrophoblast 🎭

Question 25

What is the blastodisc, and what are its components?

Answer 25

The blastodisc is a bilaminar disc formed from the inner cell mass (embryoblast). It consists of two layers: Epiblast (dorsal) → forms the amniotic cavity Hypoblast (ventral) → forms the primary yolk sac 🧬 Blastodisc appears around Day 9.

Question 26

What are the functions of embryoblast and trophoblast in the blastocyst?

Answer 26

Embryoblast (inner cell mass) → Forms the embryo proper Trophoblast (outer cell mass) → Helps in implantation and forms the placenta Later divides into: Cytotrophoblast (cell boundaries) Syncytiotrophoblast (invasive, multinucleated)

Question 27

What are the two layers of the trophoblast, and what do they do?

Answer 27

Cytotrophoblast: Cellular layer (has boundaries) Forms the core of chorionic villi → becomes placenta Syncytiotrophoblast: Multinucleated, no boundaries Invades endometrium Forms finger-like projections and lacunae

Question 28

What are the two layers of the embryoblast, and what do they become?

Answer 28

Epiblast (dorsal): Forms amniotic cavity Will later give rise to all three germ layers Hypoblast (ventral): Forms primary yolk sac 🧬 Together they form the bilaminar embryonic disc → Blastodisc

Question 29

What are the two cavities that appear during Week 2?

Answer 29

Amniotic cavity: Located above epiblast (dorsal) Yolk sac cavity: Located below hypoblast (ventral)

Question 30

What are the main components of a blastocyst?

Answer 30

Inner cell mass (Embryoblast) → forms the embryo Outer cell mass (Trophoblast) → forms the placenta Blastocoele: fluid-filled cavity Formed at the end of Day 4, matures by Day 5

Question 31

What is the fate of epiblast cells?

Answer 31

The epiblast is the true embryonic layer — it gives rise to: All three germ layers via gastrulation: Ectoderm Mesoderm Endoderm The amnion (lining of amniotic cavity) Eventually, the entire embryo 📅 Starts differentiating around Day 14 📌 High-yield tip: Epiblast = "Everything Baby" 👶✨

Question 32

What is the fate of hypoblast cells?

Answer 32

The hypoblast contributes to: The primary yolk sac (transient structure) Extraembryonic mesoderm (via signals) Not involved in forming the actual embryo 🧬 Gets displaced during gastrulation by migrating epiblast cells. 📌 Mnemonic: Hypoblast = "Helps but doesn't become baby" 🙅‍♂️

Question 33

What are the two layers formed from the embryoblast?

Answer 33

Epiblast (dorsal, columnar) → forms embryo and amnion Hypoblast (ventral, cuboidal) → forms yolk sac, contributes to extraembryonic tissues

Question 34

What does the Double Bleb Sign indicate on early pregnancy ultrasound?

Answer 34

It shows two fluid-filled sacs: Amniotic cavity Yolk sac cavity → Seen in normal intrauterine pregnancy (early USG)

Question 35

What cell layers line the amniotic and yolk sac cavities?

Answer 35

Amniotic cavity floor → Epiblast Yolk sac roof → Hypoblast