Histology and Cytology Lecture Notes Review

Comprehensive practice flashcards in Question and Answer format covering cell biology, organelles, inclusions, and nuclear histology from the Introduction to Cytology lecture series.

How is histology defined, and what are its primary investigative tools?

Histology (from histos + logus) is the science concerned with the microscopic study of normal tissues (systems, organs, tissues, and cells). Its primary tools are microscopes and stains.

In routine Hematoxylin and Eosin (H&E) staining, what colors do the nucleus and cytoplasm exhibit?

The nucleus is always blue (basophilic), whereas the cytoplasm may appear red (acidophilic) or blue (basophilic).

What are the three main constituents of the cytoplasm?

1. Cytosole (the fluid part)

2. Organelles (specialized structures essential for vital cellular processes)

3. Inclusions (non-essential structures that may be present or absent, such as lipids, glycogen, and pigments)

Which organelles are categorized as membranous?

The cell membrane, mitochondria, endoplasmic reticulum (rough and smooth), Golgi apparatus, lysosomes, and secretory vesicles.

Which cellular structures are classified as non-membranous?

Ribosomes, centrioles, cilia, flagella, filaments (actin, myosin, and intermediate filaments), and the cytoskeleton.

What microscopic appearance does the cell membrane exhibit under light microscopy and transmission electron microscopy?

Under light microscopy (LM), the cell membrane is not visible ($10\,nm$) unless stained with special stains like PAS or Ag (silver). Under electron microscopy (EM), it appears as two dark lines separated by a light line, known as a trilaminar appearance.

What are the three components of the cell membrane's molecular structure?

1. Phospholipid molecules arranged in two layers

2. Protein molecules (peripheral proteins and integral proteins)

3. Carbohydrate molecules attached to proteins or lipids (glycoproteins and glycolipids), which form the cell coat (glycocalyx)

What are the three primary functions of the glycocalyx (cell coat)?

Protection of the cell, cell recognition and adhesion, and acting as receptors.

What protein coats the cytoplasmic side of the cell membrane during receptor-mediated endocytosis?

Clathrin coats the cytoplasmic side at a coated pit, which then pinches off to become a clathrin-coated vesicle.

What are the primary functions of coated vesicles?

Protein uptake from the environment, cell membrane recycling, and transport of certain contents between organelles.

What clinical condition results from a defective cell membrane receptor for growth hormone?

Dwarfism, resulting from a lack of response due to the defective receptor.

What special stains are used to demonstrate mitochondria, and what colors do they produce?

Janus green (which stains mitochondria green) and iron hematoxylin (which stains mitochondria dark blue).

Why can mitochondria produce their own proteins and replicate independently?

Because the mitochondrial matrix contains its own DNA and ribosomes, allowing self-replication and independent protein synthesis.

What is the typical origin and presentation of mitochondrial defects?

Defective (mutant) mitochondria lead to a lack of energy, are usually of maternal origin, and manifest clinically as myopathy or neuropathy.

What structural feature binds ribosomes to the surface of the rough endoplasmic reticulum (rER)?

Ribophorins bind the ribosomes to the outer surface of the rER membranous sheets.

Why is rough endoplasmic reticulum (rER) responsible for cytoplasmic basophilia?

Because it is covered with abundant ribosomes, which are rich in RNA and bind basic dyes.

What clinical pathology is associated with defective rough endoplasmic reticulum (rER) in bone cells?

Weak bone structure.

What are the key morphological differences between smooth endoplasmic reticulum (sER) and rough endoplasmic reticulum (rER)?

Smooth ER is composed of branching and anastomosing tubules with no ribosomes on the surface, making it acidophilic. Rough ER consists of parallel flattened tubules and vesicles studded with ribosomes, making it basophilic.

What are the main functions of the smooth endoplasmic reticulum (sER)?

1. Synthesis of lipids and cholesterol
2. Synthesis of steroid hormones (e.g., cortisone)
3. Muscle contraction assistance by acting as a calcium pump
4. Detoxification of drugs and toxins
5. Glycogenolysis in liver cells
6. $\text{HCl}$ secretion in stomach cells
7. Intracellular pathway

What clinical manifestation occurs when smooth endoplasmic reticulum (sER) is defective in liver cells?

Neonatal jaundice.

What are the two faces of the Golgi apparatus, and what are their respective roles?

The convex (forming/cis) face receives transfer vesicles from the rER. The concave (mature/trans) face packages and releases secretory vesicles.

What is a 'negative Golgi image', and where is it characteristically observed in H&E sections?

It is an unstained or pale region adjacent to the nucleus where the Golgi apparatus is located, prominently seen in plasma cells because the surrounding cytoplasm is intensely basophilic.

What special stain is used to visualize the Golgi apparatus under light microscopy?

Silver stain ($\text{Ag}$).

In this self-assessment diagram of the Golgi apparatus, what does the pointed structure represent?

It represents a transfer vesicle containing raw protein arriving at the forming face of the Golgi complex.

What is the dual origin of lysosomes regarding their membrane and enzymatic contents?

Lysosomal membranes originate from the mature (trans) surface of the Golgi apparatus, whereas their hydrolytic enzymes are synthesized in the rough endoplasmic reticulum (rER).

Which histochemical stain is used to identify lysosomes?

Acid phosphatase.

What are the four varieties of secondary lysosomes?

1. Heterolysosome: primary lysosome fused with a phagosome
2. Multivesicular body: primary lysosome fused with a pinocytic vesicle
3. Autolysosome: primary lysosome fused with an autophagosome
4. Residual body: remnance remaining after digestion

What are the four major functions of lysosomes?

1. Intracellular digestion
2. After-death autolysis
3. Facilitation of fertilization
4. Hormone activation (e.g., converting thyroglobulin to thyroxin)

How do the membrane and enzymes of peroxisomes originate?

The single peroxisomal membrane originates from the rough endoplasmic reticulum (rER), while its enzymes are synthesized on free ribosomes.

What enzymatic activities and functions characterize peroxisomes?

They contain catalase and peroxide-forming enzymes (such as urate oxidase in the crystalline core) to form and degrade $\text{H}_2\text{O}_2$, break down $\beta$-chain fatty acids, and metabolize alcohol.

What are the three described patterns of cytoplasmic basophilia caused by ribosomes?

Focal, diffuse, and localized.

What are the diameters of the three main cytoskeletal elements: microfilaments, intermediate filaments, and microtubules?

• Microfilaments: $5\text{--}7\,nm$
• Intermediate filaments: $8\text{--}10\,nm$
• Microtubules: $24\,nm$

How does the stability of microtubules differ between the cytoplasm and the ciliary shaft?

Microtubules are dynamic in the cytoplasm, but they are stable within the shaft of cilia and centrioles.

What is the structural unit and protofilament assembly of a microtubule?

A microtubule consists of tubulin heterodimers (alpha-tubulin and beta-tubulin) arranged in $13$ parallel protofilaments forming a flexible, unbranched hollow cylinder.

Which drug arrests microtubule polymerization?

Colchicine.

What is the ultrastructural arrangement of microtubules in a centriole?

A wall composed of $9$ triplets of microtubules, totaling $27$ stable microtubules arranged as a cylinder.

What is the microtubule arrangement within the shaft (axoneme) of a cilium?

$9$ doublets and $2$ central singlets of microtubules, comprising a total of $20$ microtubules.

What causes immotile cilia syndrome, and what are its clinical consequences?

It is caused by immobility of cilia and flagella due to a deficiency of dynein protein (which is responsible for ciliary movement). It leads to male infertility and chronic respiratory tract infections in both sexes.

Match the following intermediate filament types to their specific cellular locations: Cytokeratin, Vimentin, Desmin, Neurofilaments, Lamin.

• Cytokeratin: epithelial cells
• Vimentin: connective tissue and muscles
• Desmin: muscle cells
• Neurofilaments: neurons
• Lamin: nuclear envelope

What is the structure and function of a proteasome?

It is a tiny, barrel-shaped non-membranous organelle composed of regulatory protein rings ($\alpha$-rings and $\beta$-rings) at both ends. It recognizes ubiquitin-tagged proteins to degrade foreign, misfolded, or non-functioning proteins.

Which special stains are used to demonstrate glycogen in liver cells?

Best's carmine (staining it red) and PAS (staining it magenta red).

How do fat globules appear in routine H&E sections compared to Sudan III staining?

In H&E, fat is dissolved during tissue preparation, leaving empty white spaces. In Sudan III, fat droplets stain bright orange.

Which cell types commonly contain the endogenous wear-and-tear pigment lipofuscin?

Nerve cells and cardiac muscle (heart) cells.

What are examples of endogenous and exogenous cellular pigments?

• Endogenous: hemoglobin, myoglobin, melanin, and lipofuscin
• Exogenous: carbon, dust, carotene, and tattoo pigment

Where is tattoo pigment primarily deposited within skin histology?

Inside macrophages located in the dermis.

What are the four main components of the nucleus?

1. Nuclear envelope
2. Chromatin
3. Nucleolus
4. Nucleoplasm

How are the outer and inner membranes of the nuclear envelope structurally distinct?

The outer membrane is continuous with the rough endoplasmic reticulum (rER), while the inner membrane is associated with nuclear lamins (intermediate filaments).

What type of protein is lamin, and what are its primary nuclear functions?

Lamin is a Type V intermediate filament (fibrous protein) associated with the inner nuclear membrane. It provides support to the nuclear envelope, regulates DNA replication, positions nuclear pores, and participates in disassembling and reforming the nuclear envelope during mitosis.

What structural and functional differences distinguish euchromatin from heterochromatin?

Euchromatin is extended, metabolically active, and appears pale under electron microscopy. Heterochromatin is condensed, transcriptionally inactive, and appears dark.

What are the four structural subregions of the nucleolus?

1. Pars granulosa (peripheral granular region containing ribosomal subunit precursors)
2. Pars fibrosa (central electron-dense fibrillar portion of rRNA and proteins)
3. Intra-nucleolar chromatin (DNA containing genes for rRNAs and tRNAs from chromosomes 13, 15, 20, and 21)
4. Amorphous matrix (pars amorpha; colloidal ground substance)

What is the primary function of the nucleolus?

Formation of transfer RNA (tRNA) and formation of ribosomal RNA (rRNA), which combines with proteins to form ribosomal subunits.

What is the function of the nucleoplasm?

It acts as a fluid medium facilitating the movement of molecules, particularly the three types of RNA (ribosomal, messenger, and transfer), from the nucleus into the cytoplasm.