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Cell Components

Glycocalyx

Microvilli

Zonula Occludens

(Tight Junction)

Zonula Adherens

Desmosome

Figure 1-1-15. Apical Cell Surface/Cell Junctions

Connexon

7 nm

Figure 1-1-14. Gap Junction

Connexon

7 nm

Figure 1-1-14. Gap Junction

Apical (Free) Surface Specializations

Stereocilia are elongated

Microvilli microvilli found at the apices

Microvilli are apical cell surface evaginations of cell membranes that function to increase the cell of cells lining the epididymis, surface area available for absorption (Figure 1-1-15). A thick glvcocalyx coat covers them. The ductus deferens, and hair cells core of each microvillus contains actin microfilaments. It is anchored in the apical cell cytoplasm of the inner ear, where they to the terminal web, which itself is anchored to the zonula adherens of the cell membrane.

play a role in auditory sensation.

Glycocalyx

Flagella are longer than cilia but have the same

Microvilli microstructure; a prominent example is in the sperm, where the single flagellum

Zonula Occludens

(Tight Junction)

provides motility.

Zonula Adherens

Clinical Corre ate

Desmosome

Kartagener Syndrome

Absent or aberrant dynein arms are found in the ci lia and flagella of individuals suffering from Kartagener syndrome (a subset of immoti e ci la syndrome). Such individuals often have chronic sinusitis and bronchiectasis as well as infertility and, in some cases, situs inversus.

Figure 1-1-15. Apical Cell Surface/Cell Junctions

KAPUtf..

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Cilia

Cilia are apical cell surface projections of cell membrane that contain microtubules (Figures 1-1-16 and 1-1-17). They are inserted on centriole-like basal bodies present below the membrane surface at the apical pole.

Cilia contain two central microtubides surrounded by a circle of nine peripheral microtubule doublets. The peripheral doublets are fused so that they share a common tubule wall and form two subtubules, A and B. Adjacent doublets are connected to one another by nexin links (Figure 1-1-17).

Image copyright 1984 Llppincott Williams & Wllkins. Used with permission.

8 = Basal Body

IJ = Intermediate Junction

M = Microvillus

OJ = Occluding Junction

Figure 1-1-16. Cilia

Dynein arms

Spoke

Central singlet

Nexin link

Plasma membrane

Central sheath

Bridge

Figure 1-1-17. Structure of the Axoneme of a Cilium

Dynein arms

Nexin link

Spoke

Central singlet

Plasma membrane

Central sheath

Bridge

Figure 1-1-17. Structure of the Axoneme of a Cilium

Movement of Cilia

A pair of dynein arms is attached to each A subtubule. The arms bind to ATP and rearrange themselves so that a binding site for the B subtubule in the tip of the arm is exposed. The B

tubule interacts with the binding site, causing the arm to snap back and movement to occur.

Each cycle of a single dynein arm slides adjacent doublets 10 nm past each other.

Cilia move back and forth to propel fluid and particles in one direction. They are important in clearing mucus from the respiratory tract.

Chapter Summary

Cell components include the nucleus, cytoplasm, cytoskeleton, and cell surface. The Nucleus

The nucleus consists of a nuclear envelope, lamina, nucleolus and chromatin.

Phosphorylation of the lamina during prophase of mitosis initiates nuclear disassembly into small vesicles.

The nucleolus assembles ribosomes and synthesizes ribosomal RNA. The nucleolus has a fibrillar center that contains nontranscribed DNA

Chromatin is a complex of DNA, histone, and nonhistone proteins. DNA exists in three forms: B DNA, Z DNA, and A DNA. Histone proteins are positively charged and complex with DNA to form nudeosomes and solenoid fibers. Nonhistone proteins are neutral and perform diverse functions such as DNA repair, replication, transcription, and regulation of chromatin function. There are two forms of chromatin: euchromatin, which is transcriptionally active, and heterochromatin, which is transcriptionally inactive. Ten percent of chromatin is in the form of heterochromatin. The Barr body (inactive X chromosome) is heterochromatin.

The Cytoplasm

The cytoplasm contains ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, mitochondria, and matrix.

Ribosomes are composed of rRNA and protein. Large ribosomal units are synthesized in the nucleolus, whereas small ones are synthesized in the nucleus. Polysomes are formed from ribosomes associating with a single mRNA strand. There are two kinds of polysomes: free and membrane-bound. The former synthesize proteins destined for the nucleus, peroxisomes, or mitochondria. The latter form secretory proteins, lysosomal enzymes, and membrane proteins. Endoplasmic reticulum exists in two forms: smooth and rough. Smooth endoplasmic reticulum (SER) lacks ribosomes. It is involved in detoxification reactions-hydroxylation, via cytochrome P450, and conjugation. It forms glucose from glycogen via membrane-bound enzyme glucose-6 phosphatase and lipolysis by releasing fatty acid from triglyceride. Other products made here include phospholipids, lipoproteins, and sterols. SER in striated muscle is known as sarcoplasmic reticulum. Calcium ions are sequestered and released here. Rough endoplasmic reticulum (RER) contains ribosomes that synthesize proteins that are delivered to Golgi apparatus, lysosomes, and plasma membrane.

Lysosomes are classified as primary or secondary. The latter are formed by fusion of the former with either phagosomes or cellular organelles. Lysosomes contain approximately 60 hydrolytic enzymes, all of which are acidic. They degrade DNA, RNA lipids, glycoproteins, proteins, and phosphatases.

Peroxisomes are organelles that synthesize and degrade hydrogen peroxide, initiate p oxidation of very long-chain fatty acids, synthesize bile and exchange of phospholipid.

Mitochondria are organelles bounded by two membranes-an outer and inner membrane. The inner membrane contains enzymes for electron transport and oxidative phosphorylation.

0Continued)

Cell Components

Chapter Summary (continued)

Mitochondrial matrix contains dehydrogenases and mitochondrial DNA, which is always inherited from the mother. Thus, transmission of diseases of energy metabolism is from the mother. Intramitochondrial granules contain calcium and magnesium and are probably stored here. The cytoskeleton is a supportive network that contains microtubules, intermediate filaments, and microfilaments. Assembly of microtubules is important for spindle formation. Intermittent filaments contain tissue-specific proteins. Microfilaments are composed of actin.

Cell Surface

Important cell surface modifications include the basal and reticular lamina, tight junctions, desmosomes, gap junctions, microvilli, and cilia.

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