Human Reproduction and Development
March 3, 2004


Readings: Ch 38: p 644-649, 652-653, 666-673, 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

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 Development of an Animal Embryo: (please know the terms mentioned in the Learning Objectives!)

How does a one-celled zygote - the product of fertilization - 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: Successful development at each stage depends on successful completion of the stage before!

1. Gamete formation (n + n): eggs and sperm are produced in specialized tissues (We already discussed Meiosis!)

2. Fertilization: the union of egg and sperm

-Occurs in Fallopian Tubes (usually)
- 200 million sperm to one egg! (sometimes 2, 3 or more eggs!).

-When 1 sperm penetrates the zona pellucida (clear zone around egg), cortical granules are released that prevent any other sperm from penetrating the egg


- After fertilization, the egg and sperm and nuclei fuse, and a new diploid human zygote results (2n) - the first cell of the new animal...


The Moment of Fertilization

3. Cleavage: The first days and weeks after conception: mitotic cell divisions begin, converting the zygote to a multicellular organism

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

-Stimulates estrogen and progesterone to prevent menstrual flow
-Causes "morning sickness" in some women...
-Pregnancy test measures the amount of this hormone

 

Days 7-10: 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
 

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 to become different organs.

Days 10 - 14: Pregnancy becomes established

-Fluid filled amniotic cavity starts to form
-Yolk sac starts to form (will make blood cells, germ cells)
-Embryo starts to form from embryonic disc
-
Chorion (placenta) starts to form

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

Days 15 - 21: Emergence of the vertebrate body plan

Primitive streak starts to form - this is the site of gastrulation (formation of the 3 tissue layers - ecto, endo, and mesoderm)

Neural groove (future spinal cord and brain) begins to form
Somites (bands of tissue that will become muscles and bones) begin to form
Pharangeal arches (future face, neck, mouth, nose) begin to form

Days 21 on...
Week 3 - Week 8 (Embryo) - Development of all organ systems

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

Day 24 embryo shown at right




If the embryo makes it this far...it is usually off to a good start! [Quote:]
"The first 3 weeks are the most hazardous periods in your life. Roughly one third to one half of all fertilized zygotes never make it beyond this point. Sadly, many women will have at least one miscarriage in their childbearing years. Some of the critical errors that can cause a miscarriage are:
* Inheritance of a defective set of chromosomes. Errors in meiosis (called nondisjunctions) can produce an egg or sperm that has an abnormal number of chromosomes or broken chromosomes. This is almost always lethal. About half of the early miscarriages in humans are afflicted with this kind of random chromosomal defect.
* Errors in mitosis after ferilization. A nondisjunction in a dividing blastula may produce one abnormal cell -- but since the blastula has so few cells, that means a significant fraction of the embryo is defective, preventing further development.
* Implantation errors. Human embryos have to nestle down in a good home, in the uterus. If the mother's hormones are not just right, that can prevent implantation, and the otherwise healthy zygote may be sloughed away. In addition, 0.5 - 1% of all pregnancies are ectopic: the zygote tries to implant in the wrong place, most often in the fallopian tubes. This is always fatal for the embryo, and has the potential to be fatal for the mother."

And now for the rest of development... Here come the organs!
Week 4:
How you looked at 1 month past conception....

Week 4

Week 7 / 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

And then, before you know it...

 

 Week 20 - Love that thumb!

 Week 36 - 40 - Partruition....


III. Teratogens and pregnancy:

A great website: Teratogens: Protecting your Unborn Baby


Objectives:

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.