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Dorsal Lip of the Blastopore: Definition & Concept

Instructor: Erika Steele

Erika has taught college Biology, Microbiology, and Environmental Science. She has a PhD in Science Education.

You are the sperm that won the great race! Congratulations, Speed Racer. You are the victor that made to the final destination, and the first to penetrate the protective barriers surrounding the prize egg. Once the sperm penetrates the egg, the process of development is initiated where a single cell divides and undergoes the necessary changes to produce every single cell, organ, and tissue found in the animal. The blastopore lip initiates the formation of the spinal cord.

Sex Cells: Man Meets Woman

Figure 1: Gametes or sex cells are haploid. They only have half the genetic information to make another organism. Animals are diploid. To develop normally, the chromosome number has to be restored. When the gametes unite during fertilization, the chromosome number is restored
Fertilization restores diploid number

Animals are diploid; this means that their genomes normally contain two copies of each chromosome. The gamete or sex cell produced by males in animals is called a sperm. Female animals produce eggs. Gametes are haploid, since they contain half the number of chromosomes found in the organism's genome. In Figure 1, the haploid number is indicated by the letter n. The diploid number is indicated by 2n. Each contains the genetic instructions, or chromosomes_, to make a new organism, but except under certain conditions cannot make another organism. Let's just say sex cells are 'freaky'. Some organisms are capable of a phenomenon called parthenogenesis which just a fancy word for 'virgin birth'. An egg can undergo development without being fertilized by a sperm. The chromosome number is still restored to a diploid number, and the zygote develops in the same way it would normally develop.

Most organisms need to have their eggs fertilized to make a baby. When an egg and sperm cell reunite, the diploid number is restored, and a cell with the instructions to make a new organism is formed. This cell is called a zygote. As shown in Figure 1 above, the zygote has 2 copies of the genome. One copy came from the male or paternal parent in the sperm. The other came from the female or maternal parent in the egg. This zygote is a single cell that somehow gives rise to a whole new organism.

Zygote! Zygote! How Do Your Organs Grow?

Figure 2: A single celled zygote. Cells that result from copies of this single cell will give rise to an organism with multiple cells, tissues, and organs.
Single cell zygote

It is completely fascinating that the single cell resulting from the unification of a sperm and egg shown in Figure 2 can make every single cell in the human body. The process is called differentiation. Differentiation is dependent upon the animal's genome.

Figure 3: After fertilization, the zygote undergoes changes leading up to gastrulation which is vital to organ formation.
germ layer development

In the early stages of embryonic development the three germ layers must be formed as show in Figure 3. The ectoderm will become the skin, hair, and nervous system. The mesoderm will become bones, muscle, and connective tissue. The endoderm will become the digestive system and lungs. The process of forming the germ layers is called gastrulation. In order to begin gastrulation, the fertilized egg or zygote has to become develop into a gastrula. The steps of this transformation are described below.

Figure 4: The developmental stages of an early embryo.
Developmental stages

  • Morula. A morula is a solid ball of cells shown in panels B and C of Figure 4. These cells have not yet begun to differentiate. They have the potential to make any cell in the body. A morula typically contains 16-32 cells.
  • Blastula. The next stage is blastula stage, shown in panels D and E of Figure 4 . The blastula is sometimes called a blastocyst. A morula becomes a blastula when it is formed into a hollow ball and filled with water. The liquid inside the blastula is called a blastocoel_ indicated in panel D. The blastula stage is when cells first begin to become different. They will start to demonstrate genetically programmed characteristics or differentiate. The outer layer is called the trophectoderm and will form the placenta. The cells on the inside are called the inner cell mass (ICM). Both the trophectoderm and ICM are shown in panel E. The cells of the ICM can still make any cell of the body.
  • Gastrula. The next phase of development is called the gastrula shown in panel F of Figure 4. Gastrulation is the process by which the ICM of a blastula starts to form the three germ layers: the ectoderm, mesoderm, and endoderm.

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