Body Cavity Development Is Guided by Morphogen Transfer between Germ Layers

Abstract

The body cavity is a space where internal organs develop and are placed. Despite the importance of this internal space, how the body cavity forms specifically within the mesoderm remains largely unknown. Here, we report that upon the onset of dorsal mesodermal cell polarization and initial lumen formation, mesodermal cells form filamentous projections that are directed toward the ectoderm. This cell behavior enables the dorsal population of mesodermal cells to receive BMP7 that is expressed by the ectoderm. Suppression of ectodermal BMP7 diminishes mesodermal cell projection and results in the loss of body cavity development. The data reveal that body cavity induction depends on signaling factor transfer from ectoderm to mesoderm.

Keywords: BMP7; body cavity; ectoderm; filopodia; mesoderm.

Figure 1.. Body Cavity Formation Analysis by Cryohistology and Live Imaging Approaches

(A–D) Lateral plate mesoderm at the 0-somite stage (A), α-tubulin expression in red, Laminin1 deposition in green, F-actin deposition in white and nuclei in blue. At the 1-somite stage (HH7) (B), first cavities within the mesoderm can be detected (asterisk), and F-actin enrichment labels a demarcation line in the mesoderm along which cavity formation is initiated (arrow). (C and D) Mesoderm continues to form two layers. Scale bars: 50 μm. (E) Whole-mount three-dimensional reconstruction of an anterior chick embryo half at the 4-somite stage. The red square indicates the area of interest in the lateral plate where body cavity formation is occurring. The area is scanned along the dorsoventral axis in an embryo culture dish. Scale bar: 300 μm. (F) Embryo semiculture configuration. Scale bar: 300 μm. (G) Conventional dorsoventral scan displays distortions and low resolution along the z axis. Scale bar: 30 μm. (H) z axis is originally scanned along the anteroposterior axis, which results in enhanced resolution along the dorsoventral axis. Scale bar: 30 μm. A, anterior; D, dorsal; P, posterior; V, ventral; Ec, ectoderm; En, endoderm; Mes, mesoderm; So, so mites.

Figure 2.. Live Imaging Analysis of Mesodermal Cell Behavior during Body Cavity Formation and Immunohistochemistry

(A) Transverse optical slice through lateral plate mesoderm of a 1-somite stage GFP-expressing chick embryo. Mesodermal cells dynamically project long extensions toward the ectoderm during body cavity initiation. Scale bar: 100 μm. (B–D) Smaller cavities are formed within the mesoderm and subsequently fuse into larger cavities (asterisk). Time recorded is indicated. The mesoderm continues to project fine cellular extensions toward the ectoderm. (E–H) Three-dimensional rendering of cell interactions between mesoderm and ectoderm. Time recorded is indicated. Scale bar: 30 μm. (I–L) Color-coded timeframes depicting tissue silhouettes reveal the dynamics of cell projections over a period of 15 min. Time recorded is indicated. (L) Overlay of 4 frames of color-coded tissue silhouettes. White represents the starting frame, cyan the 5 min frame, purple the 10 min frame and red the 15 min frame. (M) Subarea showing mesodermal projections to the ectoderm in an overlayof4channels, with α-tubulin in red, Laminin1 in green, F-actin in white, and nuclei in blue. Mesodermal protrusions are based on F-actin and α-tubulin. Scale bar: 10 μm. (N–P) Separate channels overlay with blue nuclei. (N) Ectodermal Laminin1 deposition serves as the interface for mesodermal cells. (O and P) Mesodermal cell projections contain α-tubulin (O) and F-actin (P) and are oriented toward the ectoderm and display branching (white arrows). (Q) Size development of medial and lateral cavities over time in 2-somite embryos, with the section plane on the 1-somite level (accumulative data from 3 embryos, measurements per time point n = 6, total n = 18/time point). (R) Monitoring mesodermal cell activity in 5 min steps over a period of 45 min in a region the size of 0.01 cm² detects an average of 18 mesodermal projections per 5 min frame toward the ectoderm. (S) On average, individual mesodermal cell projections extend and retract over a period of 15 to 20 min with a length of up to 38 μm and an average length of 26.16 ± 5.06 μm (mean ± SEM). See also Videos S1 and S2.

Figure 3.. Mesodermal Cell Projections Receive BMP7 from the Ectoderm

(A) Construct expressing human BMP7 that carries a triple FLAG tag was used. (B) Chick embryos were cultivated on a filter paper. (C) hBMP7 construct was electroporated into the ectoderm ex ovo. (D) Fluorescently labeled control oligo shows the area of transfection (n = 10). (E) Red label of the control oligo is in the ectoderm, and green label of the FLAG tag is in the ectoderm and mesoderm. White staining represents F-actin deposition, and nuclei are in blue. Scale bar: 20 μm. (F) Single channel for FLAG tag shows distribution in ectoderm and mesoderm. (G) Single channel for transfection control oligo, which is only present in the ectoderm. (H) Single channel for F-actin. (I) Higher magnification reveals that actin-containing mesodermal cell projections carry the FLAG-tagged hBMP7 protein. Scale bar: 10 μm. (J–L) Single channels for FLAG tag, transfection control morpholino, and F-actin, respectively, as indicated. (M) Overexpression of control vector expressing a triple FLAG tag (green) in the ectoderm (n = 9). Exclusive ectodermal transfection is demonstrated by control oligo co-transfection in red. Mesodermal cell projections extend toward ectoderm but do not take up the FLAG tag. Scale bar: 20 μm. (N–P) Single channels, as in (J)–(L). (Q–T) Higher magnification reveals cell projections that have not received the FLAG tag expressed in the ectoderm. Scale bar: 10 μm. Fluorescent signals are as indicated. (Q′–T′) Negative control of anti-FLAG staining on tissue section treated with control morpholino without anti-FLAG primary antibody. (U–W) Overexpression of hBMP7 in the overlying ectoderm leads to enhanced body cavity formation (asterisk) (n = 6). Fluorescent signals are as indicated. Scale bar: 200 μm. See also Figures S1 and S2.

Figure 4.. Suppression of BMP7 in the Ectoderm Diminishes Body Cavity Development

(A) The ectoderm of a GFP-expressing embryo transfected with a red labelled morpholino. (B) Single image of live imaging analysis of GFP-expressing embryo with a red control morpholino (Scale bar in B,C: 50μm). (C) Ectoderm transfection with a red labelled morpholino against BMP7 leads to a reduction of mesodermal cell projections towards the ectoderm. (D) Montage of left and right side of an embryo section transfected with a red control morpholino in the ectoderm and stained for Laminin1 in green and F-actin in white. The body cavity has been formed (asterisk) (n= 24), Scale bar: 150μm. (E) Subarea of D in which body cavity formation has occurred (asterisk) and two separate epithelial mesodermal layers have been established. Somatic mesodermal cells display interactions with the ectodermal basal lamina by forming long cell projections which are intertwined with Laminin1 (Scale bar: 80μm). (F) Higher magnification without DAPI to enhance visualization of the fine filamentous cell interaction and the network of Laminin1 (Scale bar: 40μm). (G) Transfection with a morpholino directed against BMP7 (n=18). Body cavity formation is not discernable (Scale bar: 150μm) (montage of left and right side). (H) Subarea magnification reveals the lack of Laminin1 deposition in the dorsal mesoderm and the lack of cell projections towards the ectoderm. The mesoderm did not form two separate epithelialized layers (unpaired t-test control vs morpholino, p-value < 0,001), scale bar: 80μm. (I) Lack of mesodermal cell projections, lack of Laminin1 deposition and absence of tissue interaction between ectoderm and mesoderm (scale bar: 40μm). (J) Model for body cavity formation. 1 - BMP7 is expressed in the ectoderm overlying the native lateral plate mesoderm. 2 - Mesodermal cells are attracted to the ectoderm and form cell projections by which they interact with ectodermal cells and directly take up ectodermal BMP7. It is currently unknown if BMP is acting directly or indirectly in attracting mesodermal cells. 3 - The native mesoderm population is separated into dorsal and ventral subpopulations. 4 - Polarization and epithelialization to form the body cavity (asterisk). Ec: ectoderm, En: endoderm, Som: somatic mesoderm, Spm: splanchnic mesoderm. See also Figures S3 and S4 and Videos S3 and S4.