Early Human Development

Question 1

What does the Pre- embryonic period involve?

How long does it take? what are the stages of development in this period

Answer 1

First 2 weeks after fertilization

Zygote, morula and blastocyst

Question 2

Zygote

Answer 2

1 cell, diploid
- undergoes rapid mitotic divisions called cleavage divisions (just mitosis)

Question 3

Morula

Answer 3

72 hours

solid ball of 16-32 cells (cells called blastomeres)
SAME size as ovum/zygote (huge cellsO

Question 4

Blastocyst

Answer 4

(5 days)

Features:

Blastocyst cavity/blasocel: Fluid filled cavity develops

embryoblast (inner cell mass)
- cells will divide and differentiate (undergo changes to become more specialized) to form the embryo

Trophoblast (made of trophoblast cells)
- cells will divide and differentiate to form the chorion (membranes supports development of baby then form placenta)

blastocyst moves from uterine tube and attaches to the endometrium of uterine

Question 5

Blastocel/Blastocyst cavity

Answer 5

blastocyst (5 days) develops a fluid filled cavity called the Blastocel or blastocyst cavity

Question 6

Embryoblast

Answer 6

inner cell mass

Cells will divide and differentiate (undergo changes to become more specialized) to form the embryo

Question 7

Trophoblast

Answer 7

outside

made up of trophoblast cells

cells will divide and differentiate to form the chorion then form the placenta
Provides nutrients to developing embryo

Question 8

Chorion

Answer 8

Made by trophoblast
membranes that supports the development of the baby then form placenta

Question 9

Blastomeres

Answer 9

cells that make up morula (solid ball of 16-32 cells)

Question 10

Implantation

Answer 10

blastcyst moves from uterine tube and attaches to endometrium of uterine wall

5-7 days after fertilization

Question 11

Stages of development

Answer 11

Pre embryonic period
Zygote, Morula, Blastocyst
- Implantation
Blastocyst -> uterine tube -> attaches itself to endometrium of uterine wall
- After implantation
Embryoblast (apart of the blastocyst) -forms-> bilayer embryonic disc (epiblast and hypoblast)

Embryonic period
Epiblast cells -> 3 germ layers (ectoderm, mesoderm, endoderm)
After epiblast will disappear
Remaining embryonic membranes form

Embryonic membranes
Chorion, amnion, yolk sac, and Allantois

Fetal period
Growth and maturation of organ occurs

Question 12

Zygote

Answer 12

1 cell diploid
Undergo rapid mitotic divisions called cleavage divisions

Question 13

Cleavage divisions

Answer 13

Involving zygote, rapid mitotic divisions

Question 14

Morula

Answer 14

72 hours
Solid ball of 16-32 blastomeres
Same size as ovum/egg

Question 15

Blastocyst

Answer 15

5 days
Features
Blastocyst cavity
Embryoblast (inner cell mass)
Trophoblast (made of trophoblast cells)

Question 16

Blastocyst cavity

Answer 16

Fluid filled cavity a part of the blastocyst in the pre embryonic period

Question 17

Blastocyst cavity

Answer 17

Fluid filled cavity

Question 18

Trophoblast

Answer 18

Cells divide and differentiate to form chorion (membrane that supports development of baby) then placenta

Question 19

Implantation

Answer 19

(5-7 days after fertilization)
The blastocyst will go through the uterine tube and attach itself to the endometrium of uterine wall

Question 20

After implantation

Answer 20

Embryoblast (inner cell mass from blastocyst) forms a embryonic disc bilayer composed of

Each layer goes through mitosis
1) epiblast
2) hypoblast

Question 21

Epiblast

Answer 21

Formed by the Embryoblast
A part of the embryonic bilayer (occurs after implantation)

Undergoes mitosis to form amnion with fluid filled amniotic cavity, and the 3 germ layers (triliminar embryonic disc = forms the embryo)

Question 22

Hypoblast

Answer 22

Undergoes mitosis to form the yolk sac

Question 23

Embryonic period

Answer 23

3 germ layers formed by the epiblast cells under going mitosis and differentiation
- once the 3 germ layers form the epiblast disappears

Germ layers
Ectoderm
Mesoderm
Endoderm

During this period..
Major organ systems almost completely formed
- heart begins to beat
- limb buds differentiate
- remaining embryonic membranes form

Question 24

Ectoderm

Answer 24

A germ layer formed by the epiblast and during the embryonic period
Form Nervous and epidermis of skin

Question 25

Mesoderm

Answer 25

A germ layer formed by the epiblast and during the embryonic period
Forms muscles, cardiovascular system dermis and most bones

Question 26

Endoderm

Answer 26

A germ layer formed by the epiblast and during the embryonic period

Form Epithelial linings of digestive, respiratory, urinary, reproductive systems and their associated glands

Question 27

What are the embryonic membranes

Answer 27

Chorion

Amnion

Yolk sac

Allantois

Question 28

Chorion

Answer 28

Derived from trophoblast cells (apart of the blastocyst)
Surrounds all embryonic structures
Chronic villi: projections of chorion containing blood
- vessels that will form the fetal part of placenta

Question 29

Amnion

Answer 29

Derived from epiblast (epiblast is apart of the embryonic disc bilayer formed by the Embryoblast = inner cell mass) during pre embryonic period forms amniotic cavity

Surrounds embryo/fetus with amniotic fluid
- function of fluid is for shock as portion (when mom is walking lots and baby is moving) and temp regulation

Question 30

Yolk sac

Answer 30

Derived from hypoblast during the pre embryonic period

Forms part of gut (gastrointestinal tract)

Produces earliers blood cells and vessels (heat beats at 6 weeks)

Source of primoridial germ cells -migrate-> gonad = produce egg and sperm (will become reproductive cells when gonads develop)

Question 31

Allantois

Answer 31

Out of pocket yolk sac
Forms the umbilical cord and the urinary bladder

Question 32

Fetal period

Answer 32

9th week to end of 40th weeks

Growth and maturation of organ occurs