Cell Nucleus Ultrastructure and Chemical Composition

Nuclear Ultrastructure and Morphology

1. Nuclear Irregularity: Through electron microscopy (EM), it is observed that the morphology has a more irregular contour than seen in optical microscopy (OM). These irregularities are related to:

• Aging and cellular maturation
• Nuclear metabolism and exchange with the cytoplasm

The Nuclear Envelope Structure

2. Nuclear Envelope: The nucleus is surrounded by a double membrane formed by an inner and outer nuclear membrane that bind at the nuclear pore level.

• External Membrane: It has a thickness of 60 Å. It is in continuity with the cisternae of the MPU (Endoplasmic Reticulum) and, like it, is covered by ribosomes.
• Internal Membrane: It has a thickness of 60 Å. It relates to the chromatin and proteins or the nuclear fibrous lamina.

Between the two membranes is the perinuclear space with a thickness of 150-200 Å.

Nuclear Pores and Transport Dynamics

3. Nuclear Pores: These are complex structures that control the traffic between the nucleus and cytoplasm, and vice versa. The number of pores is proportional to the nuclear activity. They are dynamic structures because they appear and disappear depending on cellular activity. A longitudinal structure exists within the pore: the transporter or central pore complex, which has a diameter of 10 nm. This hole can sometimes swell up to 26 nm.

Annulate Lamellae and Nuclear Lamina

4. Annulate Lamellae: They are found in the cytoplasm near the nucleus. These cisternae (tanks) are similar to the Endoplasmic Reticulum (ER) and are perforated by pore complexes similar to those of the nucleus (core).

5. Nuclear Lamina (Print): This is an electron-dense layer attached to the inner nuclear membrane. It has a variable thickness ranging from 150-600 Å and is interrupted at the pores.

Chemical Composition of the Nucleus

The nuclear envelope consists of 30% lipids, 70% proteins, and RNA.

• A. Lipids: 90% are phospholipids and 10% are neutral lipids such as triglycerides (TG), cholesterol, and cholesterol esters.
• B. Proteins: In the outer membrane, there is glucose-6-phosphatase and two transport chains for electrons (e^-) which are not coupled to the respiratory chain.
• C. RNA: Ribosomal RNA (rRNA) passes through the pores.

At the perinuclear space level, Calcium (Ca) is present because there are intramembranous Calcium (Ca) channels in the inner membrane.

Biological Functions and Exchange

The primary function is the nuclear-cytoplasmic exchange. This transport occurs through the pores.

• Pores: They control the traffic of substances from inside to outside and vice versa. Ions and small molecules enter by passive transport. For active transport, which requires energy, proteins with a molecular weight (Pm) > 44,000 D are transported.
• Membrane: Ions, amino acids (AA), small proteins, nucleosides, nucleotides, hormones, and viruses pass through.
• Endoplasmic Reticulum (ER): Connected to the nuclear envelope through its cisternae.
• Annulate Lamellae: These carry ribonucleoproteins.

The system is also involved in the synthesis of proteins, glycosylation reactions of lipids and proteins, and performs or is involved in detoxification reactions.