Histology & Genetics
1. What this chapter covers, and how NEET PG actually tests it
Histology and genetics are rarely asked as pure description. They are asked as structure-to-disease reasoning.
A question does not usually ask which junction anchors keratinocytes to each other. It describes a patient with flaccid blisters and asks which protein the autoantibody targets.
The reasoning chain is always the same. A specific molecule has a specific job in a specific tissue, so losing it produces a specific and predictable failure.
Genetics is tested identically. The mechanism of inheritance is not the endpoint — the endpoint is a pedigree, a recurrence risk, or an explanation for why a family's disease behaves oddly.
This chapter covers four areas where NEET PG concentrates these questions: epithelia and cell junctions, collagen and connective tissue, modes of inheritance, and chromosomal disorders.
| In scope here | Deliberately out of scope |
|---|---|
| Cell junction types and their autoimmune/genetic diseases | Detailed staining protocols and microscopy technique |
| Collagen types, synthesis steps and the disease at each step | Neoplastic histopathology (see Pathology) |
| Mendelian, mitochondrial and non-classical inheritance | Population genetics and Hardy-Weinberg calculation (see PSM) |
| Aneuploidies, translocations and recurrence risk | Molecular diagnostic methodology beyond karyotype and FISH |
2. Epithelia and cell junctions
2.1 Four junction types, four different jobs
Epithelial cells are not simply stacked. They are bound by four structurally distinct junctions, each solving a different mechanical or physiological problem.
| Junction | Key protein | Function |
|---|---|---|
| Tight junction (zonula occludens) | Claudins, occludins | Seals the paracellular space, creating a permeability barrier |
| Adherens junction (zonula adherens) | E-cadherin, linked to actin | Belt-like mechanical adhesion, holds sheets together |
| Desmosome (macula adherens) | Desmoglein, desmocollin, linked to keratin | Spot-weld resisting shearing stress |
| Hemidesmosome | Integrin, BP180, BP230 | Anchors the basal cell downward to the basement membrane |
The direction of anchorage is the whole point. Desmosomes bind cell to cell; hemidesmosomes bind cell to basement membrane.
That single distinction generates one of the most reliably asked pairs in the exam.
2.2 Pemphigus versus pemphigoid — the junction predicts the blister
Pemphigus vulgaris is caused by IgG autoantibodies against desmoglein 3 (often with desmoglein 1).
Since desmogleins hold keratinocytes to each other, cells separate within the epidermis. The split is therefore intraepidermal, specifically suprabasal.
The basal cells stay attached to the basement membrane below, producing the classic "row of tombstones" appearance on histology.
Clinically this gives flaccid bullae that rupture easily, painful oral erosions early in the disease, and a positive Nikolsky sign.
Immunofluorescence shows a net-like or fishnet intercellular IgG pattern, because the target is between cells.
Bullous pemphigoid targets hemidesmosomal proteins BP180 and BP230 instead.
Since hemidesmosomes hold cells to the basement membrane, the entire epidermis lifts off as a unit. The split is subepidermal.
That gives tense bullae with an intact epidermal roof, far less mucosal involvement, and a negative Nikolsky sign, in an older patient.
Immunofluorescence shows a linear band of IgG and C3 along the basement membrane, because the target is a single flat plane.
Every clinical difference between the two follows from where the split is, and where the split is follows from which junction is attacked. Nothing here needs separate memorisation once that chain is in place.
2.3 Cilia and the consequences of losing motility
Motile cilia have a 9+2 microtubule arrangement, with dynein arms generating the sliding force that produces the beat.
Primary ciliary dyskinesia results from defective dynein arms, so cilia are structurally present but immotile.
Its consequences map directly onto every tissue that depends on ciliary movement.
Airway clearance fails, giving chronic sinusitis and bronchiectasis. Sperm flagella are immotile, giving male infertility. Fallopian tube cilia fail, raising ectopic pregnancy risk.
Situs inversus occurs in about half of cases because embryonic nodal cilia establish left-right asymmetry, and without motion that determination becomes random.
The triad of situs inversus, chronic sinusitis and bronchiectasis is Kartagener's syndrome, a subset of primary ciliary dyskinesia.
3. Collagen and connective tissue
3.1 Four types worth knowing cold
Collagen accounts for roughly a third of body protein, and NEET PG restricts itself to the types with clean disease correlates.
| Type | Where | Disease when it fails |
|---|---|---|
| I | Bone, skin, tendon, dentine, late wound | Osteogenesis imperfecta |
| II | Hyaline cartilage, vitreous, nucleus pulposus | Chondrodysplasias |
| III | Reticular fibres, blood vessels, granulation tissue | Vascular Ehlers-Danlos |
| IV | Basement membrane | Alport syndrome, Goodpasture syndrome |
A useful ordering is that types I to IV follow the sequence bone, cartilage, reticulin, basement membrane.
Type III's presence in early granulation tissue and blood vessels explains why vascular Ehlers-Danlos is the lethal subtype — arterial, bowel and uterine rupture, rather than merely stretchy skin.
3.2 Synthesis, and the disease attached to each step
Collagen synthesis is a sequence, and a different disease sits at almost every step. Learning the sequence therefore gives you several diseases at once.
Preprocollagen is translated with a Gly-X-Y repeating motif, where glycine occupies every third position because it is the only residue small enough for the triple helix core.
Hydroxylation of proline and lysine follows, catalysed by enzymes that require vitamin C.
Deficiency here is scurvy. Without hydroxylation the helix is unstable, giving poor wound healing, perifollicular haemorrhage, gum bleeding and subperiosteal haemorrhage in children.
Glycosylation then allows triple helix formation, producing procollagen.
A glycine substitution mutation at this stage is the usual basis of osteogenesis imperfecta, most often affecting COL1A1 or COL1A2.
Because glycine is the only residue that fits the helix core, replacing it anywhere disrupts the whole molecule — which is why most cases are autosomal dominant with a dominant-negative effect rather than recessive.
The clinical picture follows type I collagen's distribution: multiple fractures with minimal trauma, blue sclerae from thin sclera revealing the choroid, dentinogenesis imperfecta, and conductive hearing loss from ossicular fragility.
Blue sclerae with recurrent fractures in a child is also the classic mimic of non-accidental injury, which is why the distinction is clinically urgent.
After secretion, procollagen peptidases cleave the terminal propeptides to form tropocollagen.
Failure at this step gives classical Ehlers-Danlos, with hyperextensible skin and hypermobile joints.
Finally lysyl oxidase, a copper-dependent enzyme, cross-links tropocollagen into mature fibrils.
This is why Menkes disease, an X-linked defect of copper transport, produces brittle kinked hair, hypotonia and vascular tortuosity — copper deficiency disables cross-linking.
3.3 Type IV collagen and two glomerular diseases
Type IV collagen forms the basement membrane, and two very different diseases attack it.
Alport syndrome is a hereditary structural defect, most often X-linked, giving haematuria, sensorineural deafness and ocular abnormalities such as lenticonus.
Electron microscopy shows a basket-weave splitting of the glomerular basement membrane.
Goodpasture syndrome is instead an autoantibody against the alpha-3 chain of type IV collagen.
Because that chain is present in both glomerular and alveolar basement membranes, the disease produces haematuria with haemoptysis simultaneously.
Immunofluorescence shows a linear IgG pattern along the basement membrane, for the same reason bullous pemphigoid does — a flat, continuous target.
4. Modes of inheritance
4.1 Reading the pedigree signature rather than memorising disease lists
Each inheritance mode leaves a recognisable footprint on a pedigree, and the footprint is more useful than any list of diseases.
Autosomal dominant appears in every generation, affects both sexes equally, and shows male-to-male transmission. Affected parent to child risk is 50%.
Autosomal recessive skips generations, affects both sexes, and is commoner with consanguinity. Two carrier parents give a 25% affected risk.
X-linked recessive affects mainly males, transmitted through unaffected carrier mothers.
The decisive negative rule is that there is no male-to-male transmission, because a father gives his son a Y chromosome, not an X. A single father-to-son transmission on a pedigree excludes X-linkage entirely.
X-linked dominant affects both sexes but produces a distinctive asymmetry: an affected father transmits to all his daughters and none of his sons.
Mitochondrial inheritance is transmitted only by the mother, to all her children, since sperm mitochondria are not retained in the zygote.
4.2 Why mitochondrial disease is so variable
Each cell contains many mitochondria, and a mutation may be present in some but not others.
This mixture is heteroplasmy, and the proportion of mutant genomes varies between cells, tissues and individuals.
So severity varies widely within the same family, and the tissues affected first are those with the highest energy demand — brain, retina, skeletal and cardiac muscle.
Leber hereditary optic neuropathy, MELAS and MERRF all follow this pattern.
4.3 Anticipation and trinucleotide repeats
Some disorders become more severe, or present earlier, in successive generations. This is anticipation.
The mechanism is trinucleotide repeat expansion: the repeat tract is unstable during gametogenesis and tends to grow.
| Disease | Repeat | Notes |
|---|---|---|
| Huntington disease | CAG | Paternal transmission expands most |
| Myotonic dystrophy | CTG in DMPK | Maternal transmission expands most |
| Fragile X syndrome | CGG in FMR1 | Commonest inherited cause of intellectual disability |
| Friedreich ataxia | GAA | Autosomal recessive, unlike the others |
Fragile X is worth extra attention because it breaks the neat X-linked pattern. Carrier females can be mildly affected through skewed X-inactivation, and the disease shows a premutation state that expands on maternal transmission.
4.4 Imprinting, and why the same deletion gives two diseases
Some genes are expressed from only one parent's copy, the other being silenced. This is genomic imprinting.
A deletion at 15q11-13 therefore produces entirely different diseases depending on which parent contributed the deleted chromosome.
Paternal deletion gives Prader-Willi syndrome — hypotonia in infancy, then hyperphagia and obesity, hypogonadism and intellectual disability.
Maternal deletion gives Angelman syndrome, involving UBE3A — severe intellectual disability, ataxic gait, seizures and inappropriate laughter.
The same result arises from uniparental disomy, where both copies come from one parent. Maternal uniparental disomy gives Prader-Willi; paternal gives Angelman.
The lesson generalises beyond these two. When a question emphasises which parent transmitted a deletion, imprinting is the concept being tested.
4.5 Lyonization
In every female somatic cell one X chromosome is randomly inactivated early in embryogenesis, forming the Barr body.
The number of Barr bodies is always the total X count minus one.
Because inactivation is random but then clonally inherited, the proportion of cells expressing each X varies. Skewed inactivation is why some carrier females of X-linked recessive disease show mild symptoms.
5. Chromosomal disorders
5.1 Down syndrome — three mechanisms with very different counselling
Trisomy 21 is the commonest autosomal aneuploidy compatible with survival, and its three cytogenetic mechanisms matter mainly because they carry different recurrence risks.
| Mechanism | Frequency | Recurrence implication |
|---|---|---|
| Meiotic nondisjunction (47,XX or XY, +21) | ~95% | ~1% above the age-related risk; rises with maternal age |
| Robertsonian translocation | ~3-4% | Depends on which parent carries it |
| Mosaicism | ~1-2% | Low; phenotype often milder |
Most nondisjunction occurs in maternal meiosis I, which is the basis of the maternal age association.
Translocation Down syndrome is the one that demands parental karyotyping, because a balanced carrier parent has a substantially raised recurrence risk.
For a rob(14;21) carrier mother the recurrence risk is roughly 10-15%, while a carrier father's is around 1%.
A t(21;21) carrier is the extreme case: every viable conception is trisomic, so the recurrence risk is 100%.
Note the counterintuitive point that translocation Down syndrome is not associated with advanced maternal age, unlike the far commoner nondisjunction form.
Clinical features follow from the extra chromosome broadly: hypotonia, single palmar crease, upslanting palpebral fissures, endocardial cushion defects, duodenal atresia with a double-bubble sign, and a raised risk of acute leukaemia and early Alzheimer disease.
5.2 Turner and Klinefelter — counting Barr bodies
Turner syndrome is 45,X, and it is the only monosomy compatible with survival.
There is no Barr body, since only one X is present.
Features include short stature, streak ovaries with primary amenorrhoea, webbed neck from a fetal cystic hygroma, bicuspid aortic valve and coarctation of the aorta, and horseshoe kidney.
Gonadotropins are high while oestrogen is low, because the streak gonads cannot respond.
Klinefelter syndrome is 47,XXY, with one Barr body.
Features include tall stature with long limbs, small firm testes, gynaecomastia, and infertility from hyalinized seminiferous tubules.
Testosterone is low while FSH and LH are high, and FSH rises most because Sertoli cell damage removes inhibin feedback.
5.3 The severe autosomal trisomies
Edwards syndrome (trisomy 18) presents with clenched hands with overlapping fingers, rocker-bottom feet, micrognathia and a prominent occiput.
Patau syndrome (trisomy 13) presents with midline defects — holoprosencephaly, cleft lip and palate, and cutis aplasia — plus polydactyly.
Both carry very poor survival beyond the first year, in contrast to trisomy 21.
A workable memory hook links the initial to the age: Edwards to Election age (18), Patau to Puberty age (13).
Worked clinical vignettes
Q1. A 55-year-old presents with painful oral erosions and flaccid skin bullae that rupture with lateral pressure. Biopsy shows suprabasal acantholysis with a row of tombstones. Which protein is the autoantibody target?
Pick an option to check your answer.
Show explanation
Solution. Flaccid bullae, oral involvement, positive Nikolsky and an intraepidermal split all point to loss of cell-to-cell adhesion.
The junction holding keratinocytes to each other is the desmosome, whose target antigen in pemphigus vulgaris is desmoglein 3.
(a) BP180 is hemidesmosomal, giving a subepidermal split and tense bullae. (c) is Goodpasture's target. Answer: (b).
Q2. A 4-year-old has had three long-bone fractures after trivial falls, blue sclerae and mildly discoloured teeth. Which molecular defect is most likely?
Pick an option to check your answer.
Show explanation
Solution. Recurrent fractures, blue sclerae and dentinogenesis imperfecta together indicate osteogenesis imperfecta, a type I collagen disorder.
Glycine occupies every third position in the helix and is the only residue small enough for the core, so its substitution disrupts the whole molecule — giving the dominant-negative, autosomal dominant pattern seen clinically.
(a) Scurvy causes bleeding gums and perifollicular haemorrhage, not blue sclerae. Answer: (b).
Q3. A couple's first child has Down syndrome. Karyotype shows 46 chromosomes with a rob(14;21). The mother is found to be a balanced carrier. What is the approximate recurrence risk in a future pregnancy?
Pick an option to check your answer.
Show explanation
Solution. A normal chromosome count with translocation Down syndrome signals a Robertsonian translocation, which mandates parental karyotyping.
For a carrier mother the recurrence risk is roughly 10-15%, considerably higher than a carrier father's approximately 1%.
(d) applies only to a t(21;21) carrier, where every viable conception is trisomic. Answer: (b).
7. Common exam traps
- Mixing up pemphigus and pemphigoid. Pemphigus is desmosomal, intraepidermal, flaccid, Nikolsky positive, fishnet immunofluorescence. Pemphigoid is hemidesmosomal, subepidermal, tense, Nikolsky negative, linear immunofluorescence.
- Forgetting that type III collagen failure is the dangerous Ehlers-Danlos. Vascular type risks arterial, bowel and uterine rupture, not just skin hyperextensibility.
- Missing male-to-male transmission on a pedigree. A single father-to-son transmission excludes X-linked inheritance outright, whatever else the pedigree suggests.
- Assuming all mitochondrial carriers are equally affected. Heteroplasmy makes severity highly variable even between siblings.
- Reversing Prader-Willi and Angelman. Paternal deletion at 15q11-13 gives Prader-Willi; maternal deletion gives Angelman. If the question stresses parental origin, imprinting is the concept.
- Applying maternal age risk to translocation Down syndrome. Only the nondisjunction form carries the maternal age association.
- Counting Barr bodies wrongly. Barr bodies equal the number of X chromosomes minus one, so Turner has none and Klinefelter has one.
- Treating Friedreich ataxia as another dominant repeat disorder. It expands a GAA repeat but is autosomal recessive, unlike Huntington, myotonic dystrophy and Fragile X.
Summary
- Four junction types solve four problems: tight junctions seal, adherens junctions bind sheets, desmosomes spot-weld cell to cell, hemidesmosomes anchor cell to basement membrane.
- Pemphigus vulgaris targets desmoglein 3, splitting intraepidermally with flaccid bullae, positive Nikolsky and fishnet immunofluorescence.
- Bullous pemphigoid targets BP180 and BP230, splitting subepidermally with tense bullae, negative Nikolsky and a linear immunofluorescence band.
- Primary ciliary dyskinesia disables 9+2 dynein arms, producing bronchiectasis, sinusitis, infertility and random situs, of which Kartagener's is the situs inversus subset.
- Collagen types map cleanly: I bone and skin, II cartilage, III reticulin and vessels, IV basement membrane.
- Each collagen synthesis step has its own disease: vitamin C hydroxylation gives scurvy, glycine substitution gives osteogenesis imperfecta, peptidase failure gives classical Ehlers-Danlos, copper-dependent lysyl oxidase failure gives Menkes.
- Alport is a structural type IV collagen defect with basket-weave basement membrane; Goodpasture is an antibody against its alpha-3 chain, hitting kidney and lung together.
- Pedigree signatures beat disease lists. No male-to-male transmission means X-linked; an affected father transmitting to all daughters and no sons means X-linked dominant.
- Mitochondrial disease is maternally transmitted to all offspring, with heteroplasmy explaining the wide variation in severity.
- Anticipation reflects unstable trinucleotide repeats: CAG in Huntington, CTG in myotonic dystrophy, CGG in Fragile X, GAA in the recessive Friedreich ataxia.
- Imprinting means a 15q11-13 deletion gives Prader-Willi if paternal and Angelman if maternal, with uniparental disomy producing the same outcomes in reverse.
- Down syndrome is about 95% nondisjunction, 3-4% Robertsonian translocation and 1-2% mosaicism, and only the translocation form requires parental karyotyping.
- A rob(14;21) carrier mother has a roughly 10-15% recurrence risk, a carrier father about 1%, and a t(21;21) carrier 100%.
- Turner is 45,X with no Barr body, streak ovaries, coarctation and horseshoe kidney; Klinefelter is 47,XXY with one Barr body, small firm testes and raised FSH and LH.
- Trisomy 18 shows clenched overlapping fingers and rocker-bottom feet; trisomy 13 shows midline defects and polydactyly, both with poor first-year survival.
