Upright Imaging of Drosophila Egg Chambers

Summary

The upright imaging method described in this protocol allows for the detailed visualization of the poles of a developing Drosophila melanogaster egg. This end-on view provides a new perspective into the arrangements and morphologies of multiple cell types in the follicular epithelium.

Abstract

Drosophila melanogaster oogenesis provides an ideal context for studying varied developmental processes since the ovary is relatively simple in architecture, is well-characterized, and is amenable to genetic analysis. Each egg chamber consists of germ-line cells surrounded by a single epithelial layer of somatic follicle cells. Subsets of follicle cells undergo differentiation during specific stages to become several different cell types. Standard techniques primarily allow for a lateral view of egg chambers, and therefore a limited view of follicle cell organization and identity. The upright imaging protocol describes a mounting technique that enables a novel, vertical view of egg chambers with a standard confocal microscope. Samples are first mounted between two layers of glycerin jelly in a lateral (horizontal) position on a glass microscope slide. The jelly with encased egg chambers is then cut into blocks, transferred to a coverslip, and flipped to position egg chambers upright. Mounted egg chambers can be imaged on either an upright or an inverted confocal microscope. This technique enables the study of follicle cell specification, organization, molecular markers, and egg development with new detail and from a new perspective.

Introduction

Study of Drosophila melanogaster has provided great insights into the genetic regulation of a wide range of phenomena. In particular, there has been extensive research on egg development, because oogenesis provides a tractable way to investigate many different developmental processes, including tissue patterning, cell polarity changes, cell cycle switching, and translational regulation^1,2,3,4. One important morphogenic event during oogenesis is the specification, acquisition of motility, and migration of a set of cells called border cells (reviewed in ⁵). Since cell migration is a key feature of animal morphogenesis, and because the gene....

Protocol

1. Dissection of Ovarioles

1. Transfer about fifteen 2-4-day-old female flies and a few males to a fresh fly food vial with added active dry yeast. Use cotton as a plug for the vials, and add a few drops of water to the cotton to keep the humidity high.
2. Place vial in a 25 °C incubator for 14-16 hr to maximize the number of stage 8-10 egg chambers.
   NOTE: Incubation time varies depending on temperature and desired stage.
3. Prepare dissection media, 0.1M KPO₄ and NP40 solutions as needed for the next day (see Materials).
4. Anesthetize the flies using CO₂, and place under a dissection microscope. Pipet s....

Results

The upright imaging method allowed us to see directly the organization of cells in the anterior follicular epithelium at stage 8. A general marker for follicle cell fate, the Eyes Absent (EYA) protein, as well as the nuclear DNA marker DAPI, showed even expression across this field of cells, and demonstrated that all cells could be seen with similar staining intensities (Figure 2B”). Proteins regulated in response to the cytokine UPD, however, showed variable patterns and expression levels. Polar cells r.......

Discussion

Here we describe a method to mount and image small, developing egg chambers from a end-on perspective. Common techniques for imaging egg chambers are optimized for lateral views and primarily allow precise visualization of medio-lateral follicle cells when stained with fluorescent antibodies. The use of Z-stacks or 3-D reconstructions aids in viewing multiple focal planes, but is still inadequate for sub-cellular resolution of the poles of elliptical egg chambers (Figure 2D). While this can be overcome p.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
0.1 M Potassium phosphate Buffer (KPO4 Buffer)			Add 3.1 g of NaH2PO4•H2O and 10.9 g of Na2HPO4 (anhydrous) to distilled H2O to make a volume of 1 L. The pH of the final solution will be 7.4. This buffer can be stored for up to 1 month at 4°C.
16% Paraformaldehyde aqueous solution, EM grade	Electron Microscopy Sciences	15710	methanol-free to preserve GFP fluorescence
30G x 1/2 inch needle	VWR	BD305106	Regular bevel
Active Dry Yeast	Genesee Scientific	62-103	To fatten female flies
Bovine Serum Albumin	PAA-cell culture company	A15-701	Used in NP40 Wash Buffer
Dumont #5 Forceps	Fine Science Tools	11295-10	Dumostar alloy, biologie tip; sharp tips are essential for ovariole dissection
DAPI (4′,6-Diamidino-2-phenylindole dihydrochloride)	Sigma Aldrich	D9542	5 mg/ml stock solution
Fetal Bovine Serum (FBS)	Life Technologies	16140-071	Added to supplement dissection medium (10%)
Glass culture tube	VWR	47729-576	14 ml
Glass Depression Slides	VWR	470019-020	1.2 mm Thick, Double Cavity
Glycerin Jelly	Electron Microsocpy Sciences	17998-10	Mounting media
Glycerol	IBI Scientific	IB15760	70% in PBS
IGEPAL CA-630	Sigma Aldrich	I3021-500ML	(interchangable for Nonidet P-40)  Used in NP40 Wash Buffer
Leica Fluorescent Stereoscope	Leica Microsystems
Leica SP5 Confocal Microscope	Leica Microsystems		40x/0.55NA dry objective
Micro spatula	VWR	82027-518	Stainless steel
Microscope Slide	VWR	16004-368	75x25x1 mm
NP40 Wash Buffer			50 mM TRIS-HCl, 150 mM NaCl, 0.5% Ipegal, 1 mg/ml BSA, and 0.02% sodium azide
Penicillin-Streptomycin-Glutamine	Life Technologies	10378-016	Added to supplement dissection medium (0.6X)
Petridish- Polysterine, sterile	VWR	82050-548	60 W x 15 H mm
Phosphate Buffer Saline	Sigma Aldrich	P3813-10PAK	10 packs of Powder
Potassium phosphate dibasic	Sigma Aldrich	P3786-500G	Used in 0.1 M KPO4 Buffer
Potassium phosphate monobasic	Sigma Aldrich	P9791-500G	Used in 0.1 M KPO4 Buffer
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Schneider’s Insect Medium	Life Technologies	11720018	With L-glutamine and sodium bicarbonate
Sodium azide	Sigma Aldrich	S2002-25G	Used in fluorescent antibody staining
Sodium chloride	Sigma Aldrich	S3014-500G	Used in NP40 Wash Buffer
TRIS-HCl	IBI Scientific	IB70144	1 M TRIS-HCl, pH 7.4
Volocity 3D Image Analysis Software	PerkinElmer		For processing confocal Z-stacks