The eukaryotic cell continued - The world under the microscope

The Golgi device[1]

The Golgi device, or Golgi complex, is found in all nuclear cells. The number of individual Golgi devices per cell can amount to several tens. In most cells, the various Golgi complexes are located together, often close to the nucleus, in an area where the centrioles, the cytocentre, are also located. The central part of the Golgi complex consists of a number of curved, closely spaced, flattened cisterns[2]. The complex is surrounded by a large quantity of small vesicles, which supply material from the RER and discharge it to various destinations in the cell. Internal transport between the Golgi cisterns probably also takes place by means of vesicle transport.

The polarity of the complex gives rise to a specific terminology: the cis side (cis = on this side, i.e. as seen from the RER) or convex side is the immature side of the Golgi device, while the trans side (trans = on the opposite side, as seen from the RER) or concave side represents the mature side. The last cistern on the trans side is called trans-Golgi network (TGN) and has a special task in transmitting material. A curious fact is that the membranes from cis to trans become slightly thicker, gradually reaching the thickness of the cell membrane.

Secretation products reach the cisterns of the Golgi complex in low concentration via transport vesicles, which cut off from the ER and merge with a cis-Golgi cisterne.

In the Golgi complex, the secretions pass through the successive cisterns in a fixed sequence, where they are further glycosylated and finally concentrated. The different cisterns have different enzyme contents. In the microscopic image, the concentration of products is mainly expressed on the trans-side by the increasing electron density of the condensing vacuoles, which pass into the even darker secretion granula.

In the TGN (trans-Golgi network), products are sorted for three destinations:

1. the lysosomes;

2. the cell membrane;

3. the extracellular environment.

This sorting and addressing concerns hydrolytic enzymes (lysosomes), membrane proteins (cell membrane) and secretion products (extracellular environment). The key to these sorting processes is the mannose 6-phosphate, which addresses products for the lysosomes.  Addressing for the cell membrane and for secretion is based on the absence of mannose 6-phosphate and on the possible adhesion of molecules to the membrane.

The lysosomes[1]

Lysosomes are 0.1 - 0.5 µm in size and are surrounded by a membrane. They are virtually round, heterogeneous in content and usually electron-tight. Several to hundreds of lysosomes can occur per cell. They contain hydrolitic enzymes (hydrolases), supplied from the Golgi apparatus, and form the digestive apparatus of eukaryotic cells. Many lysosomes can be found in macrophages and neutrophils. Material to be digested reaches the lysosomes via two different pathways: via endocytosis, which is called heterophagy, and via autophagy, in which the cell digests parts of its cytoplasm or organelles. Autophagy ensures a turnover of organelles and the rejuvenation of cell organelles. The lifespan of a mitochondrion in a liver parenchyma cell is about ten days, while the cell itself lives much longer.

There are about eighty different lysosomal hydrolases known. These hydrolases have an acidic pH-optimum (3½-5). The enzyme composition of lysosomes can vary between different cell types. Acid phosphatase is the marker enzyme. There are also ribonuclease, deoxyribonuclease, cathepsin B, D, H and L, sulfatases and glycuronidases. With these enzymes, the lysosome can break down a large number of molecules. The products of this digestion pass through the lysosomal membrane using transport proteins and are reused in the cell. The membrane of the lysosome protects the cytoplasm against the action of the hydrolases. The low pH-optimum of these enzymes is also a protective factor: the pH in the lysosomes is lower than that of the cytoplasm. In the membrane there is an ATP-dependent H+ ion pump, which maintains the low pH within the organelle. Any undigested residues remain in the lysosome, creating a residual body (Figure 1-5). These residual bodies can be found as yellowish-brown lipofuscinegranula, for example in cells with a long lifespan, such as neurons, myocardial cells and macrophages. The lysosomal enzymes are synthesized in the RER, and addressed in the Golgi complex in the form of mannose 6-phosphate. This addressing is recognized by the receptors in the TGN (trans-Golgi network).

Transport vesicles, carrying new hydrolases, provide a shuttle between the Golgi apparatus and the lysosomes. On the way back, they again transport free receptors to the Golgi area. The RER and Golgi complex are absent in cells that are no longer capable of protein synthesis, such as granulocytes from the blood, with the result that these cells cannot produce new hydrolases and consequently consume their lysosomes in case of repeated phagocytosis.

The nucleus[1]

The nucleus of each cell contains in the DNA the coded information for the synthesis of all proteins in all cells of the body. Other complex or polymeric compounds, such as carbohydrates, lipids and glycolipids, are created by the specific action of enzymes, which in turn are encoded by mRNAs. Each protein is encoded in one particular region of the DNA: a gene. By transcription[2] from DNA to messenger-RNA (mRNA), the DNA code is transcribed in a readable form and transported to the cytoplasm. The mRNA can be translated into protein using ribosomes, tranfer-RNA (tRNA) and with the use of energy (translation). The tRNA and the ribosomal RNA (rRNA), which builds up the ribosomes together with proteins, are also transcribed into the core of the DNA.

The nucleus is separated from the cytoplasm by the nuclear envelope, consisting of two parallel membranes, which form a perinuclear cistern (Figure 1-6).

On the inside of the core envelope are irregular collections of condensed chromatin, the heterochromatin. The inner sheet is reinforced with the lamina densa, consisting of laminin. The lamina densa is interrupted at the pores, as is the heterochromatin. The number of pores is related to the level of protein synthesis activity of the cell.

The outer leaf of the nucleus envelope can be connected to the RER. It contains ribosomes and thus participates in protein synthesis. The nuclear envelope can be seen as a specialized cistern of the RER. The nuclear envelope contains core pores with a diameter of 70 nm[2] for transport between the core and the cytoplasm (figure 1-7). These pores are provided with a combination of proteins with a special function and structure, the so-called pore-annulus (or nuclear-pore) complex. This complex controls the movement of molecules through the core pore.

[1] Junqueira L.C. en Carneiro J. (2004, tenth edition), ISBN 13: 9789035226715, Functionele histologie, Maarssen. Uitgeverij Elsevier. Chapter 3, 'De cel'.

[2] Wikipedia, de vrije encyclopedie, https://en.wikipedia.org/wiki/Main_Page