13.2 Lipid Vesicle Lamellarity

Lipid Vesicle Lamellarity describes the number of concentric lipid bilayers that make up the wall of a lipid vesicle. It is a structural classification axis that distinguishes vesicles built from a single bilayer from those built from several nested bilayers, and it directly shapes the internal architecture, volume, and exchange behavior of the vesicle.

Core Classes of Lamellarity

Unilamellar Vesicles

A unilamellar lipid vesicle is enclosed by exactly one lipid bilayer, producing a single aqueous lumen bounded directly by that bilayer. This is the simplest lamellarity class and is the closest structural analogue to the plasma membrane of a biological cell, which is why it is the preferred starting architecture for many synthetic cell constructs.

Oligolamellar Vesicles

An oligolamellar lipid vesicle contains a small number of concentric bilayers, more than one but not the large stack characteristic of fully multilamellar structures. Oligolamellar vesicles occupy an intermediate position between the single-shell simplicity of unilamellar vesicles and the densely layered architecture of multilamellar vesicles.

Multilamellar Vesicles

A multilamellar lipid vesicle consists of many concentric lipid bilayers arranged like the layers of an onion, with thin aqueous spaces separating each successive bilayer. This "onion-shell" organization results in a structurally complex vesicle with multiple nested compartments rather than a single unified lumen.

Structural Organization

Concentric Bilayer Arrangement

In multilamellar and oligolamellar vesicles, the bilayers are arranged concentrically around a common center, each bilayer forming a closed shell nested inside the one before it. This concentric organization is what gives rise to the layered, shell-like cross-section characteristic of higher-lamellarity vesicles.

Interlamellar Aqueous Spaces

Between each pair of adjacent bilayers in a multilamellar structure lies a thin aqueous space, distinct from the central lumen at the vesicle's core. These interlamellar spaces are physically separated from one another by the intervening bilayers, meaning solutes trapped in one interlamellar space are not automatically in contact with solutes in another.

Lamella Number as a Discrete Property

The lamella number, meaning the count of concentric bilayers, is a discrete integer property of a given vesicle. Lamellarity as a classification (unilamellar, oligolamellar, multilamellar) is derived directly from this underlying count.

The diagram contrasts a single closed shell, representing a unilamellar vesicle, against several concentric nested shells, representing a multilamellar vesicle.

Functional Consequences of Lamellarity

Effect on Internal Volume

Lamellarity strongly influences the usable internal volume of a vesicle. A unilamellar vesicle devotes essentially all of its interior to a single, continuous aqueous lumen, while a multilamellar vesicle divides much of its interior into a series of thin, separated interlamellar spaces, reducing the volume of the central lumen relative to the vesicle's overall size.

Effect on Molecular Exchange

The number of bilayers a molecule must cross to move between the external medium and the innermost lumen scales with lamellarity. In a unilamellar vesicle, exchange requires crossing only one bilayer, whereas in a multilamellar vesicle a molecule must traverse each successive bilayer and interlamellar space in turn, making exchange with the innermost compartment progressively slower and more restricted as lamella number increases.

Heterogeneity and Determination

Lamellarity Heterogeneity

A population of lipid vesicles produced under a given set of conditions is rarely uniform in lamellarity, typically containing a mixture of unilamellar, oligolamellar, and multilamellar vesicles. This heterogeneity is an intrinsic feature of many vesicle populations rather than an anomaly limited to specific preparation methods.

Lamellarity Determination

Determining the lamellarity of a given vesicle or vesicle population requires methods capable of resolving the internal shell structure, since lamellarity is not generally apparent from external size or shape alone. Accurate determination is important because lamellarity affects downstream properties such as internal volume and exchange behavior described above.

Relevance to Synthetic Cell Construction

Lamellarity Selection

When constructing a synthetic cell, the choice of lamellarity is a deliberate design decision, since unilamellar vesicles most closely approximate a natural cell's single-membrane boundary and typically maximize usable internal volume relative to vesicle size, while multilamellar vesicles introduce additional internal compartments and altered exchange properties that may or may not be desirable depending on the goals of the construct.