Utilizing Playdoh in the Classroom to Construct a 3D Model Depicting Cellular Movements and Tissue Remodeling during Human Gastrulation, Early Organogenesis, and Embryonic Folding

ABSTRACT

In classroom studies of mammalian embryology, students must fully grasp the cellular and tissue remodeling needed to initiate gastrulation to ensure comprehension of forthcoming developmental processes such as tissue specification and organogenesis. However, quickly and completely communicating three-dimensional concepts such as gastrulation, neurulation, and embryonic folding through common two-dimensional tools such as PowerPoint is challenging for students because this method lacks the spatial orientation needed to fully understand development. Therefore, professors can utilize active learning approaches with 3D-modeling clay to aid students in visualizing developmental changes. 3D-modeling of the developmental processes focused on cell and tissue movements for the initiation of gastrulation and organogenesis is limited in published literature/videos. Therefore, this activity fills in the modeling gap by focusing on the detachment and movement of the epiblast cell through the primitive streak to generate the three germ layers, neural tube folding, cardiogenesis, and the anatomical position of the early brain and heart to drive embryonic folding. The usage of this hands-on learning tool will assist lecturers in preventing early gaps in knowledge while students first construct the model and allows for correction in misunderstandings by utilizing the complete model in discussions after construction.

INTRODUCTION

It is often difficult for undergraduates to grasp the cellular movements/tissue remodeling needed to initiate gastrulation, early organogenesis, and anatomy-driven embryonic folding (1). Active learning approaches with modeling clay can aid students in visualizing developmental/anatomical changes (2; https://www.swarthmore.edu/NatSci/sgilber1/DB_lab/Frog/frog_gast_model.html). Published activities/online videos outline early mitotic divisions to generate the blastocystic-inner-cell-mass (3) or early gastrula (https://www.youtube.com/watch?v=xRaxRpT6YuY), are detailed after gastrulation with an anatomical-focus (4; https://www.youtube.com/watch?v=rN3lep6roRI&t=932s), or aid in constructing vertebrate embryos after organogenesis completion (5). Notably, the developmental processes focused on cell and tissue movements for the initiation of gastrulation and organogenesis are limited in published artifacts. This protocol fills in the modeling gap with the following learning/content outcomes: (i) detachment and movement of epiblast cells through the primitive streak to generate the germ layers, (ii) neural tube folding, (iv) cardiogenesis, and (v) the anatomical position of the early brain and heart to drive embryonic folding.

This activity has been utilized in embryology and collegiate-level introductory cell biology courses. It should proceed in conjunction with traditional lecture over two 50-minute periods with a maximum class size of 50 (2 to 3/group). The first lecture is focused on modeling epiblast cell movement into the primitive streak, and the second is focused on neurulation, cardiogenic appearance, and embryonic folding. A detailed lecture plan including content suggestions, instructor-guided questions, helpful hints, and most common difficulties are mapped in the supplemental material. Instructors can gauge student understanding with quick in-lessen concept-check questions, by monitoring group discussions, with short, narrated student-generated videos (https://www.youtube.com/watch?v=QsXJrVQ1aQw), and/or with student peer-edits of another group’s videos documenting both clear interpretations and inaccuracies.

ACTIVITY PROTOCOL

Figure 1 and Figure 2 outlines the steps of the protocol.

FIG 1.

Migration of epiblast cells through the primitive streak to form germ layers. (A) Use a rolling tube or hands to flatten 3.5 by 5.0-in. elongated discs to represent the differentiation of epiblast (∼2 oz. blue) and hypoblast (∼2 oz. yellow) from the inner cell mass. Use a straw to lightly imprint cells, explaining that cell movements will be needed for development to proceed. (B) Review anatomical orientations. Stack the epiblast dorsal to the hypoblast. Ask, “Do we know cranial/anterior from caudal/posterior?” (C) Using a ruler, form the primitive streak (PS) by slicing only the epiblast layer at the posterior (caudal) end. (D to G) Ask students, “What changes could take place to transform from 2 layers to 3 layers?” The most common answer is mitosis. Discuss that triploblastic gastrulation requires both cell division and then cellular movements. (D) Firmly press the straw to remove/detach epiblast cells. (E) The epiblast cells migrate toward the PS. Ask the students, “Can the cells fit into or migrate down the PS?” (F and G) Flatten and reshape the detached columnar-shaped epiblast cells into bottle-necked forms (epithelial-mesenchymal transition labeled “EMT” in panel F) to fit into the PS (G). (H) The first set of epiblast cells that ingress the PS integrate into the hypoblast to form endoderm. To model endodermic transformation, collect the bottle-neck epiblast cells and smash them into the endoderm. Utilize the remaining yellow Playdoh to create and place bands on top of the epiblast cells to model the newly generated endoderm. (I) The next epiblast cell-wave to migrate down the PS fills in the dimensional space between the endoderm and epiblast to create mesoderm. To model mesoderm, use red Playdoh to create and place bands on top of the newly migrated epiblast cells that are dorsal to the endoderm. (J) Have students remove the epiblast layer and exchange the red-striped cells with a full mesoderm (∼2 oz. red) (the figure demonstrates the cells and only replaced half of the mesoderm for instructor visualization). The remaining epiblast becomes ectoderm to complete the trilaminar gastrula. The product from this figure will be utilized for the procedures in Fig. 2. The product can be stored in an air-tight container; alternatively, break the gastrula down and then have the students rebuild the gastrula by memory (with some instructor-guided directions) in the first 10 minutes of the next class period.

FIG 2.

Notochord induction of neuroectoderm/ectoderm, cardiac positioning, and embryonic folding. (A) Notochord provides structural support, marks the embryonic midline, and will provide chemical and physical interactions with the dorsal-lying ectoderm to specialize into neuroectoderm. Ask students, “If notochord induces the nervous system, where would it be placed relative to the PS?” Place a red rod-like notochord cranial to the PS. (B) Ask students, “Does all of the ectoderm physically touch the notochord?” Explain that only the ectoderm touching or near notochord will be induced into neural tissue (presumptive neural plate). The ectoderm not touching notochord becomes skin, hair, and nails. (C) Neural tube formation/neurulation initiates with the expansion or elongation of the neural plate via increased mitosis. Add 1 in. of blue Playdoh (∼1 oz.) to each side of ectoderm to create expanded lateral folds (not shown). Traditionally, the notochord uses physiochemical interactions to pull the neural plate downward, forming a groove. To represent this process, students should use a pencil/pen to press a neural groove into the neural-ectoderm. (D and E). Groove formation creates neural folds (D) and lateral edges which fold into a tube because of inward pressures exerted on the lateral edges (E). Instruct students to make sure the tube remains hollow. Discussion follows on the order in which the tube anatomically closes. While the neural tube is closing, the primitive streak will regress; model by sealing the cut ectoderm. After neural tube closure, the cranial region of the neural tube undergoes rapid proliferation for brain expansion. Place a large round brain (∼.5 oz. blue) at the cranial edge of the neural tube (E). (F) Along with neural tube formation, the cardiac system also develops. The cardiogenic crest is initiated by the migration of mesodermic cells around the nervous system. Shape a red crescent moon (∼.25 oz. red) and place it anterior and ventral to the brain to mimic formation of heart fields. The crest will fold into tubes and then loop into the heart (not modeled here). (G) Begin embryonic folding by instructing students to pick up the embryo to orient the head up and tail down. Ask them, “What did you observe?” The head should fall forward, tucking first, and then the tail meets the head, followed by the lateral edges folding to the midline. (H to J) Allow students to observe the final model and check for exposed bits of red and yellow (I and J) or lack of closure in the ectoderm (H), signaling developmental defects. Ask students to share findings and research potential clinical diagnosis for the germ layer issue.

CONCLUSION

Students quickly understand the morphogenic changes needed for successful development by advancing their 3D/4D understanding of early embryonic cell and tissue movements. Enhanced knowledge retention is evidenced by significantly increased exam averages (KM Ackerman, data not shown) and accurately scripted videos of theses developmental processes.