Human Reproduction and Development
February 13, 2002
Readings: Ch 38: p 651-654, 656-657
CD-ROM: 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
- Special reproductive cells and structures must be constructed
- Courtship behavior, hormones, colors, scents must be developed
- Reproductive timing ‚ finding and recognizing mates, sperm and egg available at the same time - must occur
- The developing embryo and young must be cared for, often for years or decades!
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:
- Ectoderm: skin, nervous system
- Endoderm: lining of gut and internal organs
- Mesoderm: muscles, bones, heart
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:
- Cell differentiation: different cells begin to express tissue-specific or organ-specific genes to make heart, brain, etc
- Morphogenesis: these cells then undergo cell divisions, cell migration, or programmed cell death (PCD), for example, to allow fingers to form
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!
Days 15 - 21: Emergence of the vertebrate body plan
Week 3 - Week 8 (Embryo) - Development of all organ systems
Day 22: Cardiac cells (early heart) begins to beat
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 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 20 - Love that thumb!||Week 40 - Partruition|
III. Teratogens and pregnancy:
A great website: Teratogens: Protecting your Unborn Baby
- The early processes of development all need to occur at a precise time and place - that is why poor maternal nutrition, fever, drugs, alcohol, (particularly in the 1st trimester) all have potentially serious effects of the developing baby.
- Baby with FAS
- However, ALSO remember that ~9,000 human babies are born EVERY HOUR of every day (a net increase of 2.5 babies per second, or about 80 million every year!*) - and the vast majority are healthy and perfectly formed, even sometimes under adverse conditions. So don't worry!
- *Click here for the current US and world population (We will cover this in much greater detail later in the semester)
1. Explain the 6 stages of development common to