11.12.3 Events of Gastrulation and Organogenesis

Question 1

Events of Gastrulation and Organogenesis

Answer 1

• In chordates (during organogenesis) the notochord develops from the mesoderm followed by the formation of the neural tube from the ectoderm.
• Two factors that influence cell differentiation are:
1. cytoplasmic factors-unequal distributions of cellular material during division.
2. cell-cell induction-certain cells signal the differentiation of totipotent cells
through cell secretions and cell-cell contact.
• Based upon a series of transplant experiments in the 1920s, Hans Spemann and Hilde Mangold determined that during gastrulation the cells at the dorsal lip of the blastopore are the primary inducers of cell differentiation.

Question 2

Review of the events following fertilization:

Answer 2

1. Cleavage: A zygote undergoes mitotic
   divisions forming a morula (solid ball of
   cells) that eventually changes into a
   blastula (hollow ball of cells.)
2. Gastrulation: The cells of the blastula
   rearrange to form three germ layers:
   ectoderm, mesoderm, and endoderm.
3. Organogenesis: Includes the processes
   of morphogenesis (change in appearance)
   and cell differentiation forming the beginnings
   of organs and tissues.

Question 3

note

Answer 3

• As a taxonomic group, the chordates share anatomical
  similarities including the presence of a notochord during
  embryological development. The notochord is a cartilaginous structure that establishes a dorsal axis in a developing embryo and may later take part in the formation of the vertebrae.
• A subphylum of the chordates, the urochordates (also called tunicates, pictured to the left), displays the primitive chordate plan without other anatomical features displayed by vertebrate chordates.
• During early organogenesis, formation of the notochord begins when neural ridges form on the ectoderm. Neural folds come together and fuse forming a hollow cylinder beneath the neural tube. The neural tube will eventually form the brain and the spinal cord.
• A cross section of a frog embryo during early organogenesis shows the notochord, which is formed from the mesoderm layer. The two ridges (neural folds) comprise the lateral edges of the neural plate. The neural folds will come together and form the neural tube.
• Some chordates, such as the tunicates, will lose their
  notochord, while others keep it for development of the
  vertebrae.
• Somites, blocks of cells derived from mesoderm, develop along the notochord. These will later give rise to segmental structures, such as the vertebrae and spinal muscles.

Question 4

totipotency

Answer 4

• The term totipotency describes the ability for each cell of an embryo to have the potential to form all parts of an organism. During embryonic development, cells of a zygote are totipotent for a certain length of time, awaiting specific chemical instructions to enable cell differentiation into various tissues and organs.
• Cytoplasmic factors and cell-cell induction are two
  controlling factors in cell differentiation.
• Review: The presence of yolk, nutrients stored in the egg, affects the pattern of cleavage and establishes polarity in the zygote.
• Cells may also concentrate different types of RNA and
  proteins at separate ends and establish polarity.

Question 5

note 2

Answer 5

• In the 1920s, German zoologist Hans Spemann and his
  student Hilde Mangold performed a series of experiments concerning induction during development.
• Induction is the ability of a group of cells to initiate
  differentiation in other cells.
• By performing a series of transplant experiments, Spemann and Mangold determined that the dorsal lip of the blastopore in the early gastrula has a key role in cell differentiation.
• They transplanted cells from the dorsal lip of one gastrula to a different location on another embryo.
• The transplanted cells induced the totipotent cells of the gastrula to form an entire second organism. Because of the results, Spemann and Mangold called the cells from the dorsal lip of the blastopore “the primary inducer.” The differentiation of totipotent cells as they reach the dorsal lip of the blastopore is a time-dependent phenomenon.

Question 6

The notochord establishes a dorsal axis in a developing embryo. What layer of germ cells form the notochord?

Answer 6

• mesoderm

Question 7

Organogenesis includes which two processes?

Answer 7

• morphogenesis and cell differentiation

Question 8

The work of Spelmann and Mangold found that the dorsal lip of the blastopore serves what role in embryological development?

Answer 8

• Primary inducer

Question 9

Identify the structures indicated by numbered blanks in the figure.

Answer 9

• 1. notochord; 2. somite; 3. neural plate

Question 10

Which of the following statements is a conclusion drawn from the experiments of Spemann and Mangold?

Answer 10

• The dorsal blastopore lip is involved in the initiation of gastrulation.

Question 11

During embryological development, what is the hollow ball of cells formed after cleavage called?

Answer 11

• blastula

Question 12

Gastrulation results in the formation of three primary tissue layers that will give rise to all of the organs and tissues of the body. Which of the following statements is correct?

Answer 12

• Ectoderm gives rise to nerve tissues.

Question 13

__________ includes the process of __________ and cell differentiation forming the beginnings of organs and tissues.

Answer 13

• Organogenesis, morphogenesis

Question 14

True or false?

The formation of the notochord in chordates causes a dorsal anterior-posterior axis in the embryo.

Answer 14

• true

Question 15

Which of the following describes a chordate?

Answer 15

• An organism that has a notochord at some point in its embryological development.