University Lab Notes: Embryonic Development, Laboratory Specimens, and Brain Models

Review Session: A review session for the upcoming lab exam is scheduled for Tuesday.
Lab Exam 3:
- Timing: To be held the week following the review session.
- Coverage: The exam includes all materials from arthropodied two up to embryo development, the brain, and the healthy embryo.
Research Paper: The student paper is due the same week as Lab Exam 3.

Model Organism: The starfish is used as the primary model for studying embryonic development.
Biological Classification: Both humans and starfish are classified as deuterostomes.
Early Development: Due to this shared classification, starfish embryos develop in a manner very similar to human embryos in the earliest stages.
Larval Specificity: While early stages are similar, the last three stages identified in the lab (the larvae) are specific to starfish and do not resemble human development.

The Unfertilized Egg:
- Visibility: Prior to fertilization, the nucleus and nucleolus are clearly visible within the egg.
Fertilization and Polyspermy Block:
- Mechanism: When sperm joins the egg, the zona pellucida expands outward.
- Function: This expansion occurs to block polyspermy, ensuring that only one sperm joins the egg and not multiple.
- Visual Identification: In the lab, this appears as a very thin outline of the zona pellucida.
The Zygote Stage:
- Protein Production: Immediately after forming the zygote, the embryo begins rapid cell production of large quantities of proteins required for development.
- Visual Distinctions: The high concentration of protein makes the interior of the zygote appear cloudy. While the nucleus is still present, it is obscured and no longer clearly visible. This lack of a visible nucleus, combined with the presence of the zona pellucida, identifies the stage as a zygote.

Definition of Cleavage: This is a period of rapid cell division where the cells split in half repeatedly without creating new cytoplasm. Consequently, the individual cells become smaller with each division.
Progression of Stages:
- Two-cell stage.
- Four-cell stage.
- Eight-cell stage: Because development is three-dimensional, some cells may be obscured behind others, so all eight may not be visible in a two-dimensional view.
The Morula Stage:
- Structure: A clumped ball of cells where the interior of the embryo is completely filled with cells.
- Identification: While it may appear as a solid ball in two dimensions, the characteristic feature is that it is not yet hollow.

Structure: The cells move toward the edges of the embryo, creating a hollow ball of cells, similar in structure to a soccer ball.
Visual Identification: Because the interior is hollow, more light passes through it, making the center appear lighter in color.
Anatomical Terms:
- Blastoderm: The outer edge or layer of cells in the blastula.
- Blastocoel: The hollow cavity inside the blastula (where "coel" refers to hollow).
Transitional Forms: There are transitional stages between the morula and blastula, but for the exam, only definitive examples of each will be used.

Gastrulation Process: The embryo begins to pinch inward on one side, forming a tube in the middle.
Archenteron: This is the name for the tube forming within the gastrula, which will ultimately become the primitive gut or digestive tract (gastrointestinal tract).
The Three Germ Layers: Once the gastrula forms, the embryo contains three distinct layers:
- Ectoderm: The outermost layer which develops into the skin.
- Endoderm: The innermost layer which develops into the digestive tract.
- Mesoderm: The middle layer which develops into the remainder of the body.

Preserved Specimens:
- Bats: Real specimens to be handled with care.
- Skunk: Real specimen.
- Cat: A mutant fetus specimen sourced from previous anatomy classes (formerly inside a pregnant cat).
- Dolphin Skull: Real specimen.
- Sea Lion Skull: Real specimen.
- Opossum: A model created "in-house" that was overstuffed during taxidermy, resulting in a "chubby" face.
- Rabbit: Real specimen with very soft fur; one leg is loose and only attached by the skin.
- Coyote Tail: Purchased from Amazon.
- Cow Radius: A bone from the upper part of a cow's arm.
- Deer Leg: A specimen provided to the lab.

Embryo Slides: These contain a mixture of all developmental stages on a single slide. Students may need to check multiple slides to find the clearest representation of a specific stage.
Amphioxus: Students should ignore the amphioxus on the slides, as it was covered in a previous session.
Brain Models:
- Two types are available: white models and pink models.
- The pink models are considered more complicated to read.
- The white models (sometimes referred to as the white trains) are easier to visualize and will be the version used for the lab exam.

Question: During the explanation of the tube forming in the middle of the embryo, a student asked if it was the "Dorsal neurochromatic."
Response: The instructor corrected this, clarifying that the tube is the archenteron, representing the digestive tract (gastrointestinal tract), which marks the gastrula stage.