Lecture 34- Human Development During First 2 Weeks

Question 1

How long is the human gestation period from fertilization? What about from last menstrual cycle?

Answer 1

266 days from fertilization

280 days mentrual cycle

Question 2

Obstetricians commonly split the gestation period into what?

Answer 2

Trimesters

Question 3

Where specifically does fertilization occur?

Answer 3

The ampula of the Fallopian tube

Question 4

What are the cells inside the morula termed, and when does the morula form?

Answer 4

They are called blastomeres; at day 3 the morula forms

Question 5

Describe the early blastocyst, and state which day it forms

Answer 5

Inner cell mass called embryoblast with fluid filled blastocoel (blastocyst cavity) sorrounded by trophoblast and fertilization membrane Day 4

Question 6

What is the difference between the implanting blastocyst and early blastocyst? And on which day does the implanting blastocyst form?

Answer 6

• The fertilization membrane (zona pellucida) has degenerated and disappeared in order for the trophoblast to implant into the uterus
• Day 7 (Implantation takes place on day 6 and completes on day 12)

Question 7

Where exactly does the blastocyst implant?

Answer 7

On the posterior wall of the body of the uterus

Question 8

What happens at day 8?

Answer 8

• The trophoblast at the embryonic pole (close to embroyblast) differentiated into 2 layers
• Syncytiotrophoblast (at top and continuous with endometrium)
• Cytotrophoblast (below syncytiotrophoblast)
  the embryoblast differentiates into the
• Epiblast
• Hypoblast below
• A small cavity in the epiblast called amniotic cavity appears

Question 9

What happens at day 9?

Answer 9

• Amniotic cavity expands
• embryo is now a bilaminar disk (Epiblast and Hypoblast thicken)
• Exocoloemic cavity (primary yolk sac) forms, and the Exocoelomic membrane, made from the Hypoblast, surrounds it (it is continuous with the Hypoblast)

Question 10

What happens at days 11 and2?

Answer 10

• The blastocyst is completely embedded in the endometrial stroma and the uteroplacental circulation has been established.
• The exoocoelmic membrane, primary yolk sac, and the embryoblast detach (migrate away) from the cytotrophoblast (like an island) creating the Extraembryonic mesoderm
• Chorionic cavities start forming on day 12 Bilaminar disk now termed embryonic disk.

Question 11

What happens in the last day in the second week?

Answer 11

• The chorionic cavities fuse together a large, unified, chorionic cavity, that covers the primary yolk sac and the amniotic cavity; the part where it does not cover is called the connecting stalk (will letter form the umbilical chord
• This separates the extraembryonic mesoderm into 2 layers; somatic layer (lining the cytotrophoblast from outside) and splamthic layer (or visceral layer) which spurrounds the primary yolk sac
• this visceral layer will except pressure to small part of primary yolk sac to detach and migrate near the cytotrophoblast lining
• previous primary yolk sac now termed secondary yolk sac

1. Cavities within The syncytiotrophoblast form the intervellous space, which get filled with maternal blood
2. Fetal blood vessels develop in the chorionic villi which his sorrounded by intervillous space

​^ the chorionic villi which remains form the placenta

Question 13

What is the rule of 2?

Answer 13

A way to memorize in week 2 that

• the trophoblast differentiates into 2 layers
• the embryoblast differentiates into 2 layers
• the extras,Byronic mesoderm splits into 2 layers
• there are 2 yolk sacs (prim and sec)
• 2 cavities form

Question 14

What is ectopic pregnancy?

Answer 14

Pregnancy where implantation does not occur in posterior wall of uterus

sach as in the Fallopian tube

This is dangerous to mother and the embryo is aborted

Question 15

What are the 2 types of twins?

Answer 15

• Dizygotic (from 2 eggs) -dissimilar
• ‘monozygotic (from 1 zygote that got separated (2 inner cell masses in blastocyst) identical twin