
5
isotropic elastic free energy [42, 48]
Fshell =Zd2rκ
2(∇2f)2+λ
2ϵ2
ii +µϵ2
ij ,(3)
where i, j ∈ {θ, ϕ}and using the Einstein summation
convention. The energy functional (3), accounts for
bending stiffness through a Helfrich-like bending term
that penalizes out-of-plane deformation f(positive when
pointing inwards), and the stretching of the membrane
through the nonlinear strain tensor ϵij . The 2D Lam´e
parameters λ, µ are proportional to the 2D Young’s mod-
ulus Y. The strain combines contributions from fand
from the in-plane deformation u(r) (SI Sec. IV). We also
allow for a preferred radius of curvature Rcof the shell
mismatched with the radius Rof the shell Rc≥R, which
in the large-F¨oppl-von K´arm´an (FvK) regime leads to a
strain tensor ϵij =1
2(∂iuj+∂jui+∂if∂jf)−δij f/Rc
(SI Sec. IV). Previous work [44, 45, 67] has shown the
NE to be stiffer than most biological membranes and to
be well described as a thin membrane of a 3D isotropic
elastic material with an effective 3D Young’s modulus
E≈1 kPa and thickness of h∼10 −100 nm (for a
more detailed discussion of limitations of fluid membrane
models, see SI Sec. IV 5), leading to a bending rigidity of
κ= 100−300 kTeq ≈10−18 J, where Teq is the room tem-
perature, and a stretching rigidity, captured by the 2D
Young’s modulus, of Y≈10−4N/m [43]. By construc-
tion, these moduli are approximately related through the
effective thickness h∼pκ/Y [48]. Note that Yis a fac-
tor of 103smaller than the stretching rigidity of a lipid
bilayer, potentially explained by the presence of ‘area
reservoirs’ in NEs and by transmembrane protein confor-
mational changes [44]. For a shell of radius R, one can
define the FvK number γ=Y R2/κ which describes the
relative propensity of the material to bend rather than to
stretch. Using the above values, we find that the NE has
a large FvK number γ∼104−106, comparable to that
of a sheet of paper or graphene [1]. Accordingly, the NE
is more amenable to bending than to stretching, and de-
formations are expected to appear as sharp wrinkles and
creases, in agreement with our observations (Fig. 1).
To compare the surface shapes and fluctuation pre-
dicted by Eq. (3) with our experimental data, we sim-
ulated the equilibrium Langevin PDE derived from this
free energy (see Methods and SI Sec. IV 4 for simulation
details). The simulations account for hydrodynamic cou-
pling and both passive and active fluctuations, which are
modeled by an effective temperature kTeff . Despite the
model’s minimal character and theoretical limitations of
Eq. (3) at long wavelengths where l→0 (SI Sec. IV), the
numerically obtained shapes (Fig. 2a) are qualitatively
similar to those in the experiments (Fig. 1d). In the
experimentally accessible range of low-to-intermediate
angular wave numbers 3 ≲l≲11, the angular spec-
tra extracted from the simulations at different ratios of
kTeff /κ ∈[0.05,0.5] (Fig. 2b) and experimental data
(Fig. 1e) also show an approximately similar decay, sug-
gesting that the minimal elastic shell model in Eq. (3)
captures relevant features of the NE, providing a basis
for further analysis and predictions.
A main feature of the experimentally measured spec-
tra is that both younger and older nuclei exhibit a sim-
ilar asymptotic power law decay in the limit of small
angular numbers l≤10 (Fig. 2c). To rationalize this
observation, we first note that the scaling behavior in
our experiments deviates from the basic linear response
theory predictions, which is expected because, even for
younger nuclei, the radial fluctuations ftypically exceed
the NE thickness h∼10−3R(Fig. 1c-f). More precisely,
for small fluctuations (f≪h≪R) and small thermo-
dynamic pressure (p≪pc= 4√κY /R2
c, where pcis the
critical buckling pressure of the sphere), linear response
theory predicts that the power spectrum Plexhibits a
plateau for l≤lcand falls of as l−4for l≫lcwith
a crossover value lc≈γ1/4pR/Rc(SI Sec. IV)[45, 48],
which is not seen in our experiments (Figs. 1e and 2c).
Indeed, classical shell theory [2] states that nonlinear ef-
fects become important when the out-of-plane deforma-
tions fbecome comparable to or exceed the shell thick-
ness h, which is generally the case in our data where
h≪f≪R(Fig. 1c,d,g). Nonlinear analysis of elastic
plates and shells has a long history [46, 52] and has seen
major advances in the last decade [42, 48], motivated
in part by the discovery of graphene [47]. As demon-
strated above, the FvK number of the NE is comparable
to that of graphene, so we can borrow and apply recent
theoretical results to understand the fluctuation spectra
of the NE. Specifically, a detailed renormalization group
(RG) analysis [48] of Eq. (3) showed that, for sufficiently
small plate fluctuations, elastic nonlinearities lead to a
modified asymptotic decay of Pl∝l−3.2, consistent with
our experimental and simulated data (Figs. 1e and 2b,c)
and with previous experiments in red blood cell spec-
trin networks [49]. Notably, earlier studies [42, 48, 52]
also predicted that the interplay of elastic nonlinearities
and fluctuations can cause the spontaneous collapse of
sufficiently large shells, suggesting a physical mechanism
that could contribute to the eventual breakdown of the
nurse cell NE when these cells donate their contents to
the oocyte [21, 60].
The previously mentioned RG methods can give rise
to divergences in large deformation regimes, where non-
linearities dominate the shell’s response (SI Sec. IV,
Fig. S9). To obtain an analytical prediction for the scal-
ing in the larger-deformation regime h≪f≪R < Rc,
relevant to older nuclei, we performed an asymptotic di-
mensional analysis that provides additional insight into
how NE wrinkling can be controlled. To that end, we
added to the elastic free energy Fshell an effective pres-
sure term Fp=−Rd2rpefff, where peff accounts for a
normal load, which may arise from osmotic pressure dif-
ferences or microtubule-induced local stresses. Denoting
by Lthe characteristic surface variation length scale and
omitting numerical prefactors that depend on details of
the adopted thin-shell modeling approach (SI Sec. IV),
one finds for shells of thickness h∼pκ/Y that the vari-