Nuclear envelope

The nuclear envelope is a critical double-membrane structure found in eukaryotic cells. It serves as a protective boundary that surrounds the nucleus and its genetic material. This envelope is composed of two distinct lipid bilayer membranes. These are known as the inner nuclear membrane and the outer nuclear membrane. The small gap between these two layers is called the perinuclear space. This space is typically quite narrow, measuring between 10 and 50 nanometers in width.

The structure of the envelope is complex and irregular. It features various protrusions and invaginations that can be seen using an electron microscope. The outer nuclear membrane is physically continuous with the membrane of the endoplasmic reticulum. While they are linked, the outer membrane contains much higher concentrations of certain proteins. One important group is the nesprin proteins, which are found in all mammals. These proteins help connect the cell's cytoskeleton to the nucleoskeleton. This connection is vital for positioning the nucleus and helping the cell sense mechanical forces.

Inside the envelope, the inner nuclear membrane performs different tasks. It encloses the nucleoplasm and is lined with a fiber network called the nuclear lamina. This lamina is a mesh of intermediate filament proteins that is 10 to 40 nanometers thick. The lamina provides structural strength to the nucleus and helps with chromatin function. The two membranes are connected to each other by nuclear pore complexes. These pores are approximately 100 nanometers across, with an inner channel about 40 nanometers wide.

There are roughly one thousand of these pores scattered across the envelope. They are made of proteins called nucleoporins that link the inner and outer membranes together.

During the cell cycle, the nuclear envelope undergoes dramatic changes. In the G2 phase of interphase, the membrane increases its surface area. It also doubles its number of nuclear pore complexes to prepare for growth. The way the envelope behaves during cell division depends on the type of eukaryote. Some organisms, such as yeast, undergo closed mitosis. In these cases, the nuclear membrane remains intact while spindle fibers form inside or penetrate it. However, in animals and plants, the membrane must break down during the prometaphase stage of mitosis.

In mammals, the breakdown of the nuclear membrane happens very quickly, often within minutes. This process is driven by M-Cdk proteins. These proteins phosphorylate nucleoporin polypeptides, causing them to be removed from the pore complexes. As the pores break apart, M-Cdk also phosphorylates the nuclear lamina. This causes the structural framework to disassemble, leading the membranes to break into small vesicles. Evidence from microscopy suggests the membrane is absorbed by the endoplasmic reticulum during this time. This allows the mitotic spindle fibers to access the chromosomes freely.

Scientists are still debating exactly how the membrane reforms during telophase. One theory is called vesicle fusion, where small pieces of the membrane join together to rebuild the envelope. Another theory suggests the re-shaping of the endoplasmic reticulum. In this model, the parts of the endoplasmic reticulum that absorbed the membrane simply wrap around the nuclear space again. Beyond mitosis, the membrane can also rupture in migrating mammalian cells. This rupture is likely caused by nuclear deformation and is repaired by protein complexes called ESCRT.

Studying the nuclear envelope is essential for understanding various health conditions. Mutations in the genes that code for inner nuclear membrane proteins can lead to diseases known as laminopathies. Furthermore, incorrect breakdown of the envelope has been seen in cancer cells. This can lead to genomic instability and the formation of micronuclei. Some researchers also look at the evolutionary origins of this structure. They propose that the nucleus emerged in a primitive eukaryotic ancestor. This may have been triggered by archaeo-bacterial symbiosis. One theory suggests the membrane evolved to protect the genome from reactive oxygen species produced by early mitochondria.