Human Reproduction and Development
February 13, 2002

Readings: Ch 38: p 651-654, 656-657
Great illustrations and quiz!

"One of life's great dramas: the development of an offspring in the image of its parents"
- Cecie Starr

I. What's the use of Sexual Reproduction?

Sexual reproduction: a costly process

Why bother, then?

The genetic diversity that results from sexual reproduction increases the possibility that the offpsring will survive and pass its genes on to the next generation, offsetting the biological "cost".

II. Stages of Animal Development (please know the terms mentioned in the Learning Objectives!)

How does a one-celled zygote become a multicellular organisms with tissues, organs, and other specialized cells and structures? [Besides Lots and Lots of mitosis!]?
Most animals, including humans, go through 6 stages of embryonic development

1. Gamete formation (n + n): eggs and sperm are produced in specialized tissues

2. Fertilization: the plasma membrane of the egg and sperm fuse; then the nuclei fuse = 2n

Zygote (2n) - the first cell of the new animal

The Moment of Fertilization


3. Cleavage: mitotic cell divisions begin, converting the zygote to a multicellular organism

Morula - solid ball of cells
Blastula - hollow, fluid-filled ball of cells

Cleavage - 1 cell becomes manyHow you looked as a blastula....

4. Gastrulation: major cellular reorganization into 2 or 3 tissue (germ) layers:

Gastrulation into 3 germ layers

Gastrula: Early embryo with 3 tissue layers. All cells have the same DNA; however, different cells now begin to "turn on" (or "express") different genes:

5. Organ formation: sets of cells become specialized for a specific function - heart, liver, brain

6. Tissue specialization: cells within organs rearrange and specialize into tissues

Successful development at each stage depends on successful completion of the stage before!

III. Human reproduction (or, ...pregnancy happens...) (For a great site, see The Visible Embryo Project)

All the basic steps of early embryo development are the same in all higher animals (but our babies are much cuter...)

1. Gamete formation (meiosis; we discussed this already)

2. Fertilization: the union of egg and sperm

3. The first days and weeks after conception:

Day 1: first cleavage - 1 cell becomes 2

Day 2: second cleavage - 4-cell stage

Day 3: 6-12 cell stage - can test at this stage for genetic diseases if done by IVF

Day 4: 16-32 cell stage - solid ball of cells - morula

Day 5: Solid morula develops into hollow, fluid-filled blastula

The embryo will develop from the inner cell mass, or embryonic disc

Day 6 -7: Blastocyst attaches to the endometrium and burrows in: implantation

The blastocyst starts to secrete HCG - human chorionic gonatotropin

Days 10 - 14: Pregnancy becomes established

At the end of this stage, a woman will have just missed her period!

Day 14

Days 15 - 21: Emergence of the vertebrate body plan

Day 19


Week 3 - Week 8 (Embryo) - Development of all organ systems

Day 22: Cardiac cells (early heart) begins to beat

Day 24


Week 4

Day 48: First brain waves can be detected

Week 4-8 is whan all the major organ systems of the body are formed and when most teratogens have their greatest effect

Week 8

Week 9 - Week 40: (Fetus):

After 12 weeks or so, the baby's development is largely "finished" - except brain and lung development

The fetus just spends much of the 2nd and 3rd trimesters just growing (and doing various flip-turns and kicks inside the amniotic fluid)


Week 16

 Week 20 - Love that thumb!  Week 40 - Partruition


III. Teratogens and pregnancy:

A great website: Teratogens: Protecting your Unborn Baby



1. Explain the 6 stages of development common to all animals.
2. What is a gamete? fertilization? A zygote?
3. Explain the major features in the formation of the human embryo at the days mentioned above.
4. What is the diference between a morula, a blastula, and a grastrula?
5. What is the primitive streak? What tissues are produced from the 3 germ layers ectoderm, endoderm and mesoderm?
6. What is the inner cell mass and what is its significance?
7. What structures form from the neural groove, the somites, and the pharangeal arches?
8. Name at least 4 teratogens.