Mitochondria, Nucleus, Endoplasmic Reticulum, Golgi

  >   Rahul's Noteblog   >   Notes on Histology   >   Mitochondria, Nucleus, Endoplasmic Reticulum, Golgi

Mitochondria:

Mitochondria (chondriosomes) provide energy to cells for biosynthetic and motor activities via oxidative phosphorylation. Carbohydrates, fatty acids and amino acids are oxidized in mitochondria to carbon dioxide and water. Free energy released is used to convert ADP to ATP which is exported to the cytoplasm.

Outer Membrane of Mitochondria:

Outer membrane contains transport proteins that allow free permeability of molecules from the cytosol to the intermembranous space Inner membrane has cristae and F1 particles and is highly impermeable to ions. These features allow the development of an electrochemical gradient necessary to produce ATP. This is the chemiosmotic hypothesis. This will be discussed in biochemistry.

Mitochondrial Anatomy:

"Standard" mitochondrion is elongate and matrix space contains calcium carbonate crystals, an artifact of fixation for TEM. Divide by fission. Contain own DNA and RNA.

The nucleus is a double layered membrane punctuated with nuclear pore complexes.

The nucleus encodes genetic information from which the structures and functions of the organism derive. It controls and participates in the synthesis of polypeptides and proteins on which the cell structure is fabricated and through which the functions of the cell are expressed. All eukaryotic cells have a nucleus except those that have secondarily lost it, ex. RBC, platelets, lens fibers.

Inside a Mitochondrium:

• Euchromatin.

• Heterochromatin.

• Nucleoplasm.

• Histone proteins.

• Inner and outer nuclear membranes.

• Perinuclear cistern.

• Nucleolus.

• Nuclear pore complexes.

• Continuity of perinuclear cistern and ER lumen.

What is Pyknosis?

Pyknosis - densification of the chromatin, an early response to injury Karyorexus - nuclear membrane breaks down Karyolysis - chromatin itself breaks down LM appearance of the nucleus.

Nucleus:

Nuclear pore Complexes:

Nuclear pore complexes are not evenly spaced and can represent 3-35% of the nuclear surface or 3,000-4,000 pore complexes.

Contents of Nuclear Pore Complexes:

• columnar component that forms the bulk of the wall.

• transmembrane glycoprotein luminal component that anchors the complex to the nuclear membrane.

• fibrils on nuclear and cytosolic sides with a "cage" on the nuclear side.

The Nucleolus:

The nucleolus occurs in the nucleoplasm and is a ribonucleoprotein machine.

Barr Bodies:

One of the two X chromosomes in the somatic cells of females condenses early in embryonic development and remains heterochromatic throughout life. In polymorphonuclear leukocytes this "sex chromatin" is the "drumstick" or "Barr body."

Nuclear Lamina:

The fibrous dense lamina (FDL) is composed of nuclear lamins A, B and C interposed between heterochromatin andinner nuclear membrane.

Phosphorylation of lamins in prophase breaks down the nuclear envelope and dephosphorylation at telophase reconstitutes it.

A cytoplasmic nucleoskeleton of keratin and vimentin extends from the nucleus to the periphery. An inner nucleoskeleton is continuous with the FDL. They both help position the nucleus and give it shape.

Annulate Lamellae:

Annulate lamellae are common in germ and cancer cells and resemble arrays of nuclear membrane with pore complexes. May have a messenger and storage function in carrying information from the nucleus to other parts of the cell. Gene amplification occurs in oocytes whereby the nucleolar organizer region of chromosomes produces extra copies of rRNA for use during early development. This long lived rRNA may be stored in annulate lamellae.

Endoplasmic Reticulum:

Rough and Smooth Endoplasmic Reticulum:

Rough, granular endoplasmic reticulum (rough endoplasmic reticulum) (ergastoplasm) is characterized by confluence with the perinuclear cistern and the smooth, agranular endoplasmic reticulum (smooth endoplasmic reticulum). The membrane is studed with ribosomes and encloses the cisternal space. Elements of the rough endoplasmic reticulum may be flat, tubular or vesicular. Prime function is to synthesize proteins destined for export from the cell or intracellular utilization in contrast to proteins synthesized on free polysomes.

Rough endoplasmic reticulum stains basophilic due to the presence of ribosomes and is known as Nissl substance in neuron cell bodies.

Polysomes are strings of ribosomes attached to mRNA.

Free cytoplasmic polysomes Fixed, membrane-associated ribosomes on rough endoplasmic reticulum.

Ribophorins may prevent ribosomes from migrating to the smooth endoplasmic reticulum.

Functions of Smooth Endoplasmic Reticulum:

• Detoxify endogenous and exogenous lipid soluble materials andbull; synthesize lipids to replace damaged or old membranes.

• Synthesize lipid as a means of storing excess energy as cytoplasmic droplets.

Golgi Apparatus:

Golgi Complex:

• Golgi complex or dictyosome is a membrane complex of saccules found near the nucleus.

Functions of Golgi Complex:

• Major cellular membrane sorter in the cell.

• Covalent modification of proteins andbull; Formation of secretory vesicles .

Anatomy of the Golgi Complex:

• Cis-Trans relations in the Golgi.

• ER resident proteins are retrieved from the Golgi.

• Trans Golgi network (TGN) is the main cellular site of membrane sorting.

• Golgi saccules Are unique and distinct.

• Regulated secretion involves clathrin.

Clathrin:

Clathrin is a cytosolic protein that forms a lattice around pits and vesicles, and is involved in receptor mediated endocytosis and vesicular traffic from the trans Golgi network. Clathrin's role is to facilitate receptor transmission by a concentration and sorting process that initiates targeting to specific intracellular compartments.

Lysosomes:

Lysosomes: Principal site of intracellular digestion. ~40 acid hydrolases. Proton pump in membrane maintains pH of 5, optimum for hydrolytic enzymes.

Lysosome Functions:

Lysosomes are the meeting places in which streams of Intracellular traffic converge. Digestive enzymes delivered from rough endoplasmic reticulum via Golgi and substances to be digested come from 3 paths:

1.Phagocytosis brings in large particles and microorganisms.

2.Endocytosis. Endocytosed material is delivered to early endosomes. From here the endocytosed material passes to late endosomes where the material meets lysosomal hydrolases from the Golgi. Late endosomes have a pH of 6 and this is where Digestion begins. Mature lysosomes form from late endosomes and there is a further decrease in pH.

3. Autophagy disposes of obsolete cell parts.

Pinocytosis and Phagocytosis:

Pinocytosis:

• Cell drinking, continual ingestion of fluid.

Phagocytosis:

• Cell eating, ingestion of large particles.

Lysosomal Biogeneis:

Receptor mediated endocytosis (RME) involves clathrin and concentrates specific receptors to be internalized. Adaptins are molecules that recognize specific transmembrane proteins and link them to clathrin. Clathrin is generic, adaptins are different and mediate capture of different receptors.

Receptor-Mediated Endocytosis of Cholesterol:

1. Cholesterol is transported in blood bound to protein as LDL.

2. When cholesterol is needed for membrane synthesis a LDL transmembrane receptor is made and inserted into the plasmalemma.

3. LDL receptors are clustered by clathrin.

4. LDL is internalized and clathrin drops off.

5. LDL moves to early endosome then to late endosomes with low pH.

6. LDL receptors are recycled.

Peroxisome:

Membrane limited organelle present in almost all eukaryotic cells, also called microbodies. Contain catalase, and urate oxidase. Peroxisomes contain one or more enzymes that use oxygen to remove hydrogen ions from specific substrates forming hydrogen peroxide. Catalase utilizes hydrogen peroxide to to oxidize substrates including formaldehyde, alcohol, phenols and formic acid. These are water soluble compounds and cannot be detoxified by the smooth endoplasmic reticulum membrane, hence the peroxisome aids smooth endoplasmic reticulum in detoxification of various molecules.

Additional Reading:

Basic Histology

1. Introduction to Histology
2. Basic Cell Physiology
3. Actin, Microtubules, and Intermediate Filaments
4. Mitochondria, Nucleus, Endoplasmic Reticulum, Golgi
5. Epithelium (Epithelial Tissue)
6. Connective and Adipose Tissue
7. Types of Cartilage
8. Osteogenesis
9. Nervous Tissue
10. Muscle Tissue
11. Cardiovascular System
12. Blood and Hematopoiesis
13. Lymphoid Tissue
14. Digestive Tract I: Oral Cavity
15. Digestive Tract II: Esophagus through Intestines
16. Liver, Pancreas, and Gall Bladder
17. Respiratory System
18. Integument
19. Urinary System
20. Endocrine System
21. Male Reproductive System
22. Female Reproductive System
23. Eye and Ear

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