Unit 5 (2.0): Early Human Development

Question 1

What happens during fertilization and development?

Answer 1

Haploid sperm and haploid oocyte (egg) come into contact with one another forming a diploid zygote

Question 2

What happens during pre-embryonic development?

Answer 2

• series of developmental occurrences leading up to zygote becoming an embryo
• diploid zygote begins as a single cell and divides by mitosis to produce many cells = cleavage divisions
  
  • these divisions increase the number of cells,
    producing a solid ball of 16-32 cells called a
    morula
  • each cell is called a blastomere
  • the overall size of the morula is the same as
    the zygote but instead of one cell, there are
    many small cells

Question 3

What happens during development (gestation)?

Answer 3

As the morula divides further, cells rearrange themselves and a blastocyst is produced
- blastocyst has a fluid filled cavity called a blastocoele (blastocyst cavity)

Composed of:
1. trophoblast cells that surround the blastocyst and eventually become the chorion (provides nourishment to the developing embryo)

2. An embryoblast which will become the embryo (all called the inner cell mass - cells will differentiate to become the embryo)

Question 4

What happens during implantation?

Answer 4

Implantation = the attachment of the blastocyst to the endometrium of the uterus
- embryoblast develops into the embryonic disc following implantation

Embryonic disc consists of the:
1. epiblast layer: will give rise to the fetus and will also give rise to the amnion
2. hypoblast layer: will give rise to the yolk sac

Question 5

What happens during embryonic development?

Answer 5

Epiblast layer develops into 3 germ layers:
1. ectoderm: will go on to form the nervous system and the epidermal layer of the skin
2. mesoderm: will go on to form the muscle, bone, blood vessels, and the dermis
3. endoderm: will go on to form the epithelial lining of the digestive tract, respiratory tract, urinary tract, reproductive tract, and the associated glands

• all major organ systems have completely developed
• heart begins to beat
• brain begins to develop
• limb buds begin to differentiate

Question 6

What is the first of the 4 embryonic membranes?

Answer 6

1. Amnion
   - forms from the epiblast layer
   - fluid-filled cavity that acts to surround and cushion the developing embryo and fetus form bumps and other disturbances

Question 7

What is the second of the 4 embryonic membranes?

Answer 7

2. Yolk sac:
   - forms from the hypoblast layer
   - produces the early blood cells and the germ cells

Question 8

What is the third of the 4 embryonic membranes?

Answer 8

3. Chorion
   - forms from trophoblast cells
   - becomes the fetal portion of the placenta
   - surrounds all of the embryonic membranes

Question 9

What is the fourth of the 4 embryonic membranes?

Answer 9

4. Allantois:
   - projection off of the yolk sac
   - form the umbilical cord and urinary bladder

Question 10

What happens during fetal development?

Answer 10

• referred to as the fetal period
• period of growth and maturation of organs