Biology 124 Lab
As you have learned in a previous lab, meiosis creates haploid gametes. When a sperm gamete fertilizes an egg gamete a single-celled, diploid zygote is created. This event is the first step in development and is known as fertilization. This single-celled zygote will then undergo development in a process shared by such diverse organisms as starfish, chickens and humans.
The similarities in development across taxa is remarkable and implies strong evolutionary connection
The similarity in the developmental process across taxa is remarkable and implies evolutionary ancestry across these taxa.
An egg being swarmed by sperm for fertilization
The second step in development is cleavage. In cleavage our single celled zygote
will undergo mitosis and divide (cleave) into two cells and continue to divide and multiply into more and more identical cells.
A zygote cleaving into two identical sister cells
This ball of sister cells will eventually form a hollow ball of cells known as the blastula.
The hollow ball of identical cells known as the blastula
The blastula will then undergo the third step in development, gastrulation, when the identical sister cells begin to differentiate (begin to change toward their intended function) and form three germ layers: the ectoderm, the mesoderm and the endoderm. The cells of the newly formed ectoderm will become the cells of the skin and other surface cells. The mesoderm will become the cells of connective tissue and the endoderm will become the cells of the inner organs.
A gastrula that has begun to differentiate
This differentiation of identical sister cells is one of the great mysteries of cell biology. How do identical cells know how and when to be different? How does one cell become a skin cell and its identical neighbor become a brain cell? They contain the same genetic information and the same structure and differ only in position in the blastula. There are many theories to explain the mystery of differentiation but it has yet to be solved.
The final step of development that we will discuss here is organogenesis.
A human fetus with recognizable organs
Like the name implies this is the process in which the germ layers will differentiate further. The ectoderm cells will further separate into skin cells, and cuticle cells and hair cells. The mesoderm will further divide into tendons, and muscles and bone. And the endoderm will divide into kidneys and heart and spleen. By this point we have the formation of recognizable fetus which will continue to refine its development into more and more specialized cells and cell function until a complete organism is created.
-What would happen if an undifferentiated embryo was split into
two pieces?
- How are Siamese twins formed?
- Why is gastrulation considered the most important event in a person's life?
- Draw and define a Blastula.
- Draw and define Gastrulation.
- If cells were separated at any time up to the blastula stage, what would be
the result? Why?
- Gastrulation occurs when the hollow ball of embryonic cells begins to fold in
on itself to form layers of cells. Which of the following is not one of
the layers of cells formed (endoderm; pachyderm; ectoderm; mesoderm).
- The formation of ___________ layers is the point at which cells begin to be
differentiate.
- What are the 4 stages of embryonic development and what happens at each stage?
- What occurs if an embryo in the earliest stages of development (before
gastrulation) is divided in two?
- What might be the result if gastrulation occurs in more than 1 place?
- Does the blood of the mother and embryo ever mix? Why is this important?
- Define Apoptosis.
- Are cells undergoing mitosis or meiosis during cleavage?
- Why can't someone with O blood receive A or B blood?