Passive voice with de (entouré de)

Normally, the passive voice uses par (by) to introduce the agent of an action. However, when a passive sentence describes a state of being, a condition, or a physical description rather than a dynamic action, French uses de (or d') instead of par.

The nucleus is surrounded by a membrane.
Le noyau est entouré d'une membrane.
The membrane is composed of a thick layer.
La membrane est composée d'une couche épaisse.

This structure is especially common with verbs that express composition, filling, or covering:

The cell is filled with particles.
La cellule est remplie de particules.
The wall is constituted of liquid.
La paroi est constituée de liquide.
The wall is lined with this substance.
La paroi est tapissée de cette substance.
The membrane is dotted with particles.
La membrane est parsemée de particules.
The nucleus is coated with an unknown substance.
Le noyau est revêtu d'une substance inconnue.