Animal Tissues and Histology

Animal Tissues and Histology carries 4 of the 40 Zoology questions. The syllabus asks for the four primary tissues — epithelial, connective, muscular and nervous — with their structure, location and function, so almost every question is "which tissue lines/supports/forms this organ" or "which cell or fibre does this job". The marks sit in the epithelium table, the three cartilages and bone, the blood cells with their numbers, the three kinds of muscle with the sarcomere, and the parts of a neuron.

3.1 Epithelial, connective, muscular and nervous tissues: structure, location and function

A tissue is a group of similar cells, with their intercellular substance, that has a common origin and performs a common function. Histology is the study of tissues. Animals have four primary tissues.

Tissue	Germ layer	Cells and matrix	Main job	Examples
Epithelial	all three	cells packed close, almost no matrix, on a basement membrane	covering, lining, protection, secretion, absorption	epidermis, lining of gut, glands
Connective	mesoderm	few cells scattered in abundant matrix with fibres	binding, support, transport, storage, defence	areolar tissue, tendon, cartilage, bone, blood
Muscular	mesoderm (mostly)	elongated contractile muscle fibres	movement, contraction	skeletal, smooth, cardiac muscle
Nervous	ectoderm	neurons and neuroglia	receive, conduct and transmit impulses; coordination	brain, spinal cord, nerves

Epithelial tissue

Epithelium is a sheet of closely packed cells with negligible intercellular matrix, resting on a non-cellular basement membrane, that covers the body surface and lines its cavities, ducts and tubes.

• It arises from all three germ layers — epidermis (ectoderm), gut lining (endoderm), lining of blood vessels and kidney tubules (mesoderm).
• Avascular: no blood vessels enter it; nutrients diffuse from capillaries of the connective tissue below.
• Cells are polar (free apical and basal surfaces) and joined by cell junctions: tight junctions (stop leakage between cells), adhering junctions / desmosomes (cement neighbouring cells), gap junctions (cytoplasmic channels for ions and small molecules).
• High power of regeneration; free surfaces may bear microvilli (absorption), cilia (movement) or stereocilia (long immotile microvilli, as in the epididymis).
• The lining of blood vessels and the heart is called endothelium; of the body cavities (pleura, peritoneum, pericardium) mesothelium.

Epithelium	Structure	Location	Function
Simple squamous (pavement)	one layer of thin, flat, polygonal cells with a central flattened nucleus	alveoli of lungs, walls of blood capillaries and vessels (endothelium), Bowman's capsule, thin loop of Henle, serous membranes	diffusion, filtration
Simple cuboidal	one layer of cube-shaped cells, round central nucleus	kidney tubules (PCT with a brush border of microvilli), thyroid follicles, ducts of glands, surface of the ovary (germinal epithelium)	secretion, absorption
Simple columnar	tall cells, oval nucleus near the base; often with microvilli and goblet cells	lining of stomach, small intestine (brush border), gall bladder	secretion, absorption
Ciliated columnar	columnar cells with cilia	fallopian tubes (oviducts), small bronchioles, uterus, central canal of spinal cord	moves ova, mucus or fluid in one direction
Pseudostratified ciliated columnar	one layer, but cells of different heights with nuclei at different levels, so it looks layered; all cells touch the basement membrane; goblet cells	trachea, bronchi, nasal passages	traps dust in mucus, cilia sweep it up
Stratified squamous keratinised	many layers; only the basal (germinative) layer divides; surface cells dead, filled with keratin	epidermis of skin	waterproof, resists wear
Stratified squamous non-keratinised	many layers, surface cells living and moist	buccal cavity, pharynx, oesophagus, vagina, cornea	resists abrasion
Stratified cuboidal	two or more layers of cuboidal cells	ducts of sweat glands, salivary and mammary ducts	protection, secretion
Stratified columnar	several layers, columnar at the surface	part of the male urethra, conjunctiva, large ducts	protection
Transitional (urothelium)	4–6 layers; large dome-shaped (umbrella) surface cells that flatten when the organ is stretched	urinary bladder, ureters, renal pelvis	stretches without tearing

Glandular epithelium. Epithelial cells specialised for secretion form glands. A unicellular gland is the mucus-secreting goblet cell of the intestine and respiratory tract. Multicellular glands are classified three ways.

Basis	Type	Example
Where the secretion goes	exocrine — through a duct	salivary, sweat, sebaceous, gastric, mammary glands
	endocrine — ductless, into the blood (hormones)	thyroid, pituitary, adrenal
	heterocrine (mixed) — both	pancreas (enzymes + insulin), gonads
Mode of secretion	merocrine (eccrine) — secretion leaves by exocytosis, cell intact	salivary glands, pancreas, most sweat glands
	apocrine — the apical part of the cell pinches off with the secretion	mammary glands, sweat glands of the armpit
	holocrine — the whole cell breaks down to form the secretion	sebaceous glands
Shape	simple tubular / coiled tubular / branched tubular	intestinal crypts / sweat glands / gastric glands
	simple branched alveolar	sebaceous glands
	compound tubulo-alveolar	salivary glands, pancreas, mammary glands

Connective tissue

Connective tissues are the most abundant and widespread tissues. They arise from the mesoderm, have few cells in a large amount of matrix, are usually vascular (cartilage is the exception) and bind, support, protect, store and transport.

1Collagen fibres — thick, pink, wavy bundles2Elastic fibres — thin, dark, branching and straight3Fibroblast nucleus

The fibres

Fibre	Protein	Appearance	Property	Where
White (collagen)	collagen — the commonest protein of the body	wavy unbranched bundles	great tensile strength, little stretch; boiled → gelatin	tendons, dermis, bone, cartilage
Yellow (elastic)	elastin	single, branched fibres	stretch and recoil	ligaments, walls of arteries, lungs, pinna
Reticular	reticulin (a fine collagen)	delicate branched network	supporting framework (stroma)	lymph nodes, spleen, liver, bone marrow

The cells of loose connective tissue

Cell	Job
Fibroblast	the commonest cell; secretes fibres and matrix
Macrophage (histiocyte)	phagocytosis — engulfs microbes and debris
Mast cell	secretes heparin (anticoagulant), histamine (dilates vessels; allergy) and serotonin
Plasma cell	derived from B lymphocytes; secretes antibodies
Adipocyte (fat cell)	stores fat in one large droplet; nucleus pushed to the edge (signet-ring cell)
Melanocyte, lymphocyte	pigment; immunity

Classification of connective tissue

Group	Tissue	Structure	Location
Loose (proper)	Areolar	loose mesh of collagen and elastic fibres, all the cell types, semi-fluid matrix	under the skin, round blood vessels, nerves and muscles, in mesenteries — "packing tissue"
	Adipose	fat cells packed together; white fat (one droplet) and brown fat (many droplets, many mitochondria — makes heat)	subcutaneous layer, round kidneys, heart, mesentery, yellow bone marrow; brown fat in newborns and hibernating mammals
	Reticular	reticular fibres with reticular cells	lymph nodes, spleen, bone marrow
Dense	Dense regular	fibres in parallel bundles	tendons (white collagen; join muscle to bone, inelastic); ligaments (with yellow elastic fibres; join bone to bone, slightly elastic)
	Dense irregular	collagen fibres in all directions	dermis of skin, capsules of organs, periosteum, perichondrium
Supporting (skeletal)	Cartilage	chondrocytes in lacunae in a firm matrix of chondrin (chondroitin sulphate); covered by perichondrium; avascular	see the cartilage table
	Bone	osteocytes in lacunae in a hard matrix of ossein (collagen) and calcium salts	skeleton
Fluid	Blood	cells in fluid plasma	blood vessels
	Lymph	plasma-like fluid with lymphocytes, no RBCs or platelets	lymph vessels

Cartilage

Type	Matrix	Location
Hyaline — the commonest, glassy, bluish-white	homogeneous; fine collagen fibres invisible	articular ends of long bones, costal cartilages of ribs, rings of the trachea and bronchi, larynx, nasal septum; the embryonic skeleton
Elastic (yellow)	many yellow elastic fibres	pinna (external ear), epiglottis, Eustachian tube
White fibrocartilage	dense bundles of collagen fibres — the strongest	intervertebral discs, pubic symphysis, menisci of the knee
Calcified	hyaline cartilage hardened by calcium salts	suprascapula of the frog; heads of the humerus and femur of the frog

Bone is the hardest connective tissue. Its matrix is about two-thirds inorganic — mainly calcium phosphate as hydroxyapatite with some calcium carbonate — and one-third organic, chiefly the protein ossein. Acid dissolves the salts and leaves a flexible bone; burning removes the ossein and leaves a brittle one.

• Osteoblasts form bone; when trapped in their own matrix they become osteocytes, which sit in lacunae and reach each other through fine canaliculi. Osteoclasts resorb bone.
• Compact bone (shafts of long bones) of mammals is built of Haversian systems (osteons): a central Haversian canal carrying blood vessels and nerves, surrounded by concentric lamellae of matrix with lacunae between them. Transverse Volkmann's canals link neighbouring Haversian canals. Outside is the periosteum, inside the endosteum.
• Spongy (cancellous) bone (ends of long bones, flat bones) is a lattice of trabeculae with red bone marrow (makes blood cells) in the spaces; the marrow cavity of the shaft holds yellow (fatty) marrow in adults.

Feature	Cartilage	Bone
Matrix	chondrin, soft and flexible	ossein + calcium salts, hard and rigid
Blood vessels	absent (nourished by diffusion from the perichondrium)	present (Haversian and Volkmann's canals)
Cells	chondrocytes, 1–4 per lacuna	osteocytes, 1 per lacuna
Canaliculi	absent	present
Growth	from inside and from the surface	from the surface (and at growth plates)
Marrow	absent	present

Blood is a fluid connective tissue: about 55 % plasma and 45 % formed elements. Plasma is 90–92 % water and 6–8 % proteins — albumin (osmotic pressure), globulins (antibodies, transport) and fibrinogen (clotting) — with glucose, amino acids, salts, hormones and urea.

1Nucleus pushed to one side by the fat droplet2Adipocyte — the droplet-filled space

Element	Number per mm³	Structure	Life span	Job
RBC (erythrocyte)	5–5.5 million (men), about 4.5 million (women)	biconcave, no nucleus in mammals, about 7.5 μm; full of haemoglobin (12–16 g per 100 mL blood); made in red bone marrow	120 days; destroyed in the spleen (the "graveyard of RBCs") and liver	carry O₂ and some CO₂
WBC (leucocyte)	6000–8000	nucleated, colourless, amoeboid; can squeeze out of capillaries (diapedesis)	days	defence
Platelets (thrombocytes)	1.5–3.5 lakh	non-nucleated fragments of megakaryocytes of bone marrow	about a week	blood clotting

WBC	Type	% of WBCs	Nucleus	Job
Neutrophil	granulocyte	60–65 (most abundant)	3–5 lobes	phagocytosis of bacteria; first to arrive; pus is dead neutrophils
Eosinophil (acidophil)	granulocyte	2–3	bilobed	allergy and parasitic worm infections (rises in both)
Basophil	granulocyte	0.5–1 (fewest)	S-shaped or lobed	releases histamine, heparin and serotonin (inflammation)
Lymphocyte	agranulocyte	20–25	large round nucleus filling the cell	immunity: B cells (antibodies), T cells
Monocyte	agranulocyte	6–8	kidney- (bean-) shaped; the largest WBC	phagocytosis; becomes a macrophage in tissues

Muscular tissue

Muscle is made of long, contractile muscle fibres (cells) whose cytoplasm (sarcoplasm) is packed with myofibrils of the proteins actin (thin filaments) and myosin (thick filaments). The cell membrane is the sarcolemma; the endoplasmic reticulum, which stores Ca²⁺, is the sarcoplasmic reticulum. Muscles are mesodermal, except the muscles of the iris and the myoepithelial cells of glands, which are ectodermal.

1Cross-striations (alternating A and I bands)2One of the many peripheral nuclei of a single fibre

Feature	Skeletal (striated, voluntary)	Smooth (unstriated, involuntary, visceral)	Cardiac
Shape	long, cylindrical, unbranched	spindle-shaped, unbranched	cylindrical, branched
Nuclei	many, at the periphery (a syncytium)	one, central	one (sometimes two), central
Striations	prominent light and dark bands	absent	present, fainter
Intercalated discs	absent	absent	present (gap junctions and desmosomes)
Control	voluntary (somatic nerves)	involuntary (autonomic)	involuntary; myogenic — beats on its own
Contraction	fast, powerful, fatigues quickly	slow, sustained, no fatigue	rhythmic, never fatigues
Location	limbs, body wall, tongue, pharynx, diaphragm, attached to bones by tendons	walls of alimentary canal, blood vessels, urinary bladder, uterus, iris, ciliary body, arrector pili	wall of the heart (myocardium)

The sarcomere is the functional unit of a myofibril — the segment between two Z lines (Krause's membranes).

Band	Contains	Name	In contraction
A band	myosin (with overlapping actin at its ends)	anisotropic, dark	length unchanged
I band	actin only	isotropic, light; bisected by the Z line	shortens
H zone	myosin only, centre of the A band	Hensen's zone; bisected by the M line	shortens / disappears
Sarcomere	Z line to Z line (about 2–3 μm at rest)	—	shortens

Sliding filament theory (Huxley and Hanson, 1954). A nerve impulse releases Ca²⁺ from the sarcoplasmic reticulum; Ca²⁺ binds troponin, which shifts tropomyosin off the binding sites on actin; the myosin heads (ATPase) form cross-bridges, swing and pull the actin towards the M line. ATP is needed to detach each cross-bridge — after death ATP runs out and the muscles lock (rigor mortis). The filaments themselves do not shorten.

Red (slow, type I) fibres	White (fast, type II) fibres
much myoglobin, many mitochondria, rich blood supply	little myoglobin, few mitochondria
aerobic, slow, sustained contraction, resist fatigue	anaerobic (lactic acid), fast, brief, fatigue quickly
back muscles, flight muscles of birds	eye muscles, breast muscles of the domestic fowl

Nervous tissue

Nervous tissue is ectodermal and consists of neurons, the structural and functional units that conduct impulses, and neuroglia, which support them.

1Cell body (cyton) with its nucleus2Dendrite — branching, carries impulses towards the cell body3Nucleus of a neuroglial cell

The neuron

• Cyton (cell body, perikaryon) — a large nucleus with a prominent nucleolus; cytoplasm with Nissl granules (clumps of rough ER with ribosomes: protein synthesis) and neurofibrils. Mature neurons have no functional centrioles and do not divide.
• Dendrites — short, branched processes that carry impulses towards the cyton; they contain Nissl granules.
• Axon — usually one long process carrying impulses away from the cyton; it arises from the axon hillock (no Nissl granules), and ends in branches with synaptic knobs full of neurotransmitter vesicles.
• Myelin sheath — a fatty (lipoprotein) wrapping formed by Schwann cells in the PNS and oligodendrocytes in the CNS; the outer sheath of the Schwann cell is the neurilemma. Gaps in the myelin are the nodes of Ranvier, where the impulse jumps (saltatory conduction, up to about 120 m s⁻¹).

Neuron type	Processes	Where
Unipolar	one	early embryo; invertebrates
Pseudounipolar	one process that splits into two	sensory neurons of the dorsal root ganglia
Bipolar	one axon + one dendrite	retina, olfactory epithelium, inner ear
Multipolar	one axon + many dendrites (the commonest)	motor neurons, brain and spinal cord

By function neurons are sensory (afferent), motor (efferent) or interneurons (association neurons). Myelinated fibres form the white matter and the cranial and spinal nerves; non-myelinated fibres occur in the grey matter and the autonomic nervous system. A nerve is a bundle of nerve fibres wrapped in connective tissue (endoneurium, perineurium, epineurium).

Neuroglia — non-conducting supporting cells that make up about half the volume of nervous tissue and, unlike neurons, can still divide.

Glial cell	Job
Astrocytes	star-shaped; support neurons, form the blood–brain barrier, regulate ions
Oligodendrocytes	form myelin in the CNS
Schwann cells	form myelin in the PNS and the neurilemma
Microglia	phagocytic scavengers of the CNS (of mesodermal origin)
Ependymal cells	line the brain ventricles and central canal; help make CSF

Answering "which tissue" questions

Decide first whether the organ needs to diffuse (squamous), secrete or absorb (cuboidal/columnar), move something along (ciliated), resist wear (stratified) or stretch (transitional). The location follows from the job.

Formula and fact sheet

Item	Value or name
Basement membrane, no matrix, avascular	epithelium
Endothelium / mesothelium	lining of blood vessels / of body cavities
Brush border	PCT of kidney (cuboidal), small intestine (columnar)
Stereocilia	epididymis, vas deferens
Goblet cell	unicellular mucous gland
Heterocrine gland	pancreas, gonads
Holocrine / apocrine / merocrine	sebaceous / mammary / salivary, pancreas
Commonest cell of areolar tissue	fibroblast
Heparin and histamine	mast cells (and basophils)
Antibodies	plasma cells (from B lymphocytes)
Brown fat	thermogenesis — newborns, hibernating animals
Collagen / elastin / reticulin	white / yellow / reticular fibres
Tendon / ligament	muscle to bone / bone to bone
Cartilage matrix / bone matrix protein	chondrin / ossein
Bone salt	calcium phosphate as hydroxyapatite (about 2/3 of the matrix)
Bone-forming / bone-resorbing cells	osteoblasts / osteoclasts
Haversian and Volkmann's canals	compact bone of mammals
Blood	55 % plasma, 45 % cells; plasma 90–92 % water, 6–8 % protein
RBC	5–5.5 million/mm³; biconcave, no nucleus; 120 days; spleen = graveyard
WBC	6000–8000/mm³ — N 60–65, L 20–25, M 6–8, E 2–3, B 0.5–1 %
Platelets	1.5–3.5 lakh/mm³; megakaryocyte fragments; clotting
Haemoglobin	12–16 g per 100 mL
Skeletal muscle	striated, voluntary, multinucleate, unbranched
Smooth muscle	unstriated, involuntary, uninucleate, spindle-shaped
Cardiac muscle	striated, involuntary, branched, intercalated discs, myogenic
Sarcomere	Z to Z; A band constant; I band and H zone shorten
Ca²⁺ store / Ca²⁺ receptor	sarcoplasmic reticulum / troponin
Nissl granules	rough ER + ribosomes in cyton and dendrites; absent from the axon hillock
Myelin	Schwann cells (PNS), oligodendrocytes (CNS)
Saltatory conduction	nodes of Ranvier
Bipolar / pseudounipolar / multipolar	retina / dorsal root ganglion / motor neurons
Blood–brain barrier / phagocytic glia	astrocytes / microglia

Chapter summary

• A tissue is a group of similar cells with a common origin and function; the four primary tissues are epithelial, connective, muscular and nervous.
• Epithelium rests on a basement membrane, has almost no matrix, is avascular and arises from all three germ layers; it is named by layers and cell shape — squamous (alveoli, capillaries), cuboidal (kidney tubules, thyroid), columnar (stomach, intestine), ciliated (oviduct), pseudostratified (trachea), stratified squamous (skin, oesophagus), transitional (urinary bladder).
• Glands are exocrine, endocrine or heterocrine (pancreas), and secrete in merocrine (salivary), apocrine (mammary) or holocrine (sebaceous) fashion.
• Connective tissue has few cells (fibroblasts, macrophages, mast cells, plasma cells, fat cells) in abundant matrix with collagen, elastic and reticular fibres: areolar, adipose, tendon, ligament, cartilage, bone, blood and lymph.
• Cartilage is avascular with chondrin matrix — hyaline (joints, trachea), elastic (pinna, epiglottis), fibrocartilage (intervertebral discs); bone has ossein and hydroxyapatite, osteocytes in lacunae and Haversian systems.
• Blood: 55 % plasma; RBCs (5 million/mm³, 120 days, no nucleus), WBCs (6000–8000/mm³; neutrophils most, basophils fewest, monocytes largest) and platelets (clotting).
• Muscle: skeletal (striated, voluntary, multinucleate), smooth (spindle, involuntary) and cardiac (branched, intercalated discs, myogenic); in contraction actin slides over myosin, the A band stays constant and the I band and H zone shorten.
• Nervous tissue: neurons (cyton with Nissl granules, dendrites, axon, myelin from Schwann cells or oligodendrocytes, nodes of Ranvier) and neuroglia (astrocytes, oligodendrocytes, Schwann cells, microglia, ependymal cells).

Practice questions

Set A — Recall (25 questions)

1. The inner lining of blood vessels (endothelium) is made ofasimple cuboidal epitheliumbsimple columnar epitheliumcsimple squamous epitheliumdtransitional epithelium
2. The urinary bladder is lined byatransitional epitheliumbkeratinised stratified squamous epitheliumcsimple columnar epitheliumdpseudostratified epithelium
3. The trachea is lined byasimple squamous epitheliumbstratified squamous epitheliumcsimple cuboidal epitheliumdpseudostratified ciliated columnar epithelium
4. Cuboidal epithelium with a brush border of microvilli lines theatracheabproximal convoluted tubulecurinary bladderdepidermis
5. Sebaceous glands secrete by theamerocrine modebapocrine modecholocrine modedendocrine mode
6. Mammary glands are an example ofaapocrine glandsbholocrine glandscendocrine glandsdunicellular glands
7. The most abundant cell of areolar connective tissue is theamast cellbmacrophagecplasma celldfibroblast
8. Heparin and histamine are secreted in connective tissue byafibroblastsbmast cellscplasma cellsdadipocytes
9. A tendon connectsamuscle to bonebbone to bonecmuscle to muscledbone to cartilage
10. The pinna and the epiglottis are supported byahyaline cartilagebwhite fibrocartilagecelastic cartilagedcalcified cartilage
11. The intervertebral discs are made ofahyaline cartilagebwhite fibrocartilagecelastic cartilagedspongy bone
12. Haversian canals are found inahyaline cartilagebelastic cartilagecred bone marrowdcompact bone of mammals
13. Bone-forming cells are calledaosteoblastsbosteoclastscchondroblastsdfibroblasts
14. The protein of the bone matrix isachondrinbkeratincosseindelastin
15. The most numerous white blood cells in human blood arealymphocytesbmonocytesceosinophilsdneutrophils
16. The largest white blood cell is thealymphocytebmonocytecbasophildneutrophil
17. Antibodies are secreted byamast cellsbmacrophagescplasma cellsderythrocytes
18. The average life span of a human red blood cell isa120 daysb12 daysc60 daysd7 days
19. Intercalated discs are found inaskeletal musclebsmooth musclectendonsdcardiac muscle
20. Smooth muscle is found in theabicepsbwall of the intestinecheartdtongue
21. The functional unit of a myofibril is theasarcomerebsarcolemmacsarcoplasmdsarcoplasmic reticulum
22. The myelin sheath of peripheral nerve fibres is formed byaoligodendrocytesbastrocytescSchwann cellsdmicroglia
23. Nissl granules of a neuron are made ofamitochondriablysosomescGolgi bodiesdrough ER with ribosomes
24. Bipolar neurons are found in theamotor nerves of the spinal cordbretinacdorsal root gangliadcerebral cortex
25. The gaps between successive segments of the myelin sheath areaNissl bodiesbaxon hillockscnodes of Ranvierdsynaptic clefts

Answers and explanations — Set A

1. (c) — thin, flat cells allow exchange across capillary walls; cuboidal and columnar epithelia are too thick.

2. (a) — transitional epithelium stretches as the bladder fills; keratinised epithelium is confined to the skin.

3. (d) — the "respiratory epithelium": cells of different heights with cilia and goblet cells, all on one basement membrane.

4. (b) — PCT cells reabsorb most of the filtrate across a brush border. The trachea is ciliated, the bladder transitional.

5. (c) — the whole sebaceous cell breaks down to form sebum.

6. (a) — the tip of the secreting cell pinches off with the milk fat; sebaceous glands are holocrine.

7. (d) — fibroblasts secrete the fibres and matrix and outnumber every other cell type.

8. (b) — mast cells (like basophils) release heparin (anticoagulant) and histamine (inflammation).

9. (a) — tendons of dense white collagen fibres join muscle to bone; ligaments join bone to bone.

10. (c) — elastic fibres let the pinna and epiglottis bend and spring back.

11. (b) — collagen bundles make fibrocartilage the strongest cartilage, fit for the discs and pubic symphysis.

12. (d) — each Haversian system (osteon) of compact bone has a central canal with blood vessels; cartilage is avascular.

13. (a) — osteoblasts build bone; osteoclasts resorb it.

14. (c) — ossein (collagen) is the organic part of bone matrix; chondrin belongs to cartilage.

15. (d) — neutrophils are 60–65 % of WBCs; lymphocytes come second at 20–25 %.

16. (b) — the monocyte, with a kidney-shaped nucleus, becomes a tissue macrophage.

17. (c) — plasma cells are activated B lymphocytes.

18. (a) — about 120 days; worn-out RBCs are removed in the spleen and liver.

19. (d) — intercalated discs with gap junctions let the heart's fibres contract as one unit.

20. (b) — the gut wall's smooth muscle produces peristalsis; biceps and tongue are skeletal, the heart cardiac.

21. (a) — the sarcomere, from one Z line to the next, is the unit that shortens.

22. (c) — Schwann cells in the PNS; oligodendrocytes myelinate CNS fibres.

23. (d) — Nissl granules are rough ER studded with ribosomes, making proteins for the long axon.

24. (b) — retinal bipolar cells link the rods and cones with ganglion cells.

25. (c) — the nodes of Ranvier, where the impulse jumps in saltatory conduction.

Set B — Understanding (15 questions)

1. Epithelial cells receive their oxygen and nutrientsafrom capillaries running between the cellsbby diffusion from vessels in the connective tissue belowcfrom lymph vessels within the epitheliumdfrom the air directly
2. Transitional epithelium suits the urinary bladder becauseaits cells secrete mucusbit is keratinisedcit absorbs urinedits cells change shape and flatten when the bladder stretches
3. Which pair is correctly matched?aciliated columnar — fallopian tubebkeratinised stratified squamous — oesophaguscsimple squamous — PCTdtransitional — trachea
4. Which statement is true of cartilage but NOT of bone?aits matrix contains hydroxyapatitebit has Haversian canalscit is avascular and fed by diffusiondits cells lie in lacunae
5. During contraction of a skeletal muscle, which part of the sarcomere keeps its length?aI bandbA bandcH zonedthe whole sarcomere
6. Cardiac muscle works throughout life without fatigue mainly because itais under voluntary controlblacks myofibrilschas no nucleidhas many mitochondria and a rich blood supply
7. Red muscle fibres differ from white muscle fibres in havingamore myoglobin and mitochondriabfewer mitochondriacfaster but brief contractionda poorer blood supply
8. Myelinated fibres conduct impulses faster than non-myelinated fibres of the same diameter becauseathey have more Nissl granulesbmyelin itself carries the currentcthe impulse jumps from node to nodedthey lack a neurilemma
9. Blood is classed as a connective tissue because itacontains haemoglobinbis mesodermal, with cells in a fluid matrixcflows in vesselsdhas no fibres at all
10. Brown adipose tissue is abundant in newborn babies and hibernating mammals because itastores waterbsecretes insulincforms antibodiesdproduces heat by oxidising fat
11. The phagocytic cells of the central nervous system are theamicrogliabastrocytescoligodendrocytesdependymal cells
12. Basophils resemble mast cells in that bothamake antibodiesbcarry oxygencrelease histamine and heparindform myelin
13. Ligaments are more elastic than tendons because they contain moreacollagen fibresbyellow elastic fibresccartilagedcalcium salts
14. Mature neurons cannot be replaced by division because theyalack nucleiblack Nissl granulesclack mitochondriadhave no functional centrioles
15. Platelets areafragments of megakaryocytesbnucleated cells from the spleenca kind of lymphocytedRBCs without haemoglobin

Answers and explanations — Set B

1. (b) — epithelium is avascular; everything diffuses across the basement membrane from the connective tissue capillaries.

2. (d) — the dome-shaped surface cells flatten and slide, so the lining stretches without tearing.

3. (a) — cilia sweep the ovum along the oviduct. The oesophagus is non-keratinised, the PCT cuboidal, the trachea pseudostratified.

4. (c) — cartilage has no blood vessels; both tissues have cells in lacunae (d), and (a), (b) are bone only.

5. (b) — the A band is the length of the myosin filaments, which do not shorten; actin slides in, so the I band and H zone narrow.

6. (d) — abundant mitochondria and coronary blood keep cardiac muscle aerobic, and its long refractory period forces rest between beats. It is involuntary (a).

7. (a) — myoglobin stores O₂ and many mitochondria give aerobic, sustained contraction.

8. (c) — myelin insulates; ion exchange happens only at the nodes, so the impulse jumps (saltatory conduction).

9. (b) — like all connective tissues it arises from mesoderm and has cells in an abundant matrix (plasma); fibrinogen forms fibres on clotting.

10. (d) — brown fat cells have many mitochondria that release the energy of fat as heat (non-shivering thermogenesis).

11. (a) — microglia are the brain's macrophages, derived from mesoderm.

12. (c) — both have granules of histamine and heparin and act in inflammation and allergy.

13. (b) — ligaments carry elastic fibres and allow joint movement; tendons are almost pure collagen.

14. (d) — neurons are permanently out of the cell cycle; textbooks give the lack of functional centrioles as the reason.

15. (a) — megakaryocytes of the bone marrow break into platelets, which have no nucleus.

Set C — Application (10 questions)

1. A slide shows long, unbranched, cylindrical fibres with many peripheral nuclei and cross-striations. It was most probably taken from theaheartbtonguecstomach walldiris
2. A section shows cells in lacunae in a clear, glassy matrix with no visible fibres, surrounded by perichondrium. It was most probably taken from theapinnabintervertebral disccepiglottisdtracheal rings
3. A sarcomere 2.4 μm long has an A band of 1.6 μm. If it shortens to 2.0 μm, the total length of I band in the sarcomere becomesa0.4 μmb0.8 μmc0.2 μmd1.2 μm
4. In a count of 200 WBCs a technician finds 124 neutrophils, 50 lymphocytes, 14 monocytes, 10 eosinophils and 2 basophils. The count suggestsabacterial infectionbviral infectioncallergy or worm infectiondleukaemia
5. A person has 5 litres of blood with 5 million RBCs per mm³. If each RBC lives 120 days, about how many RBCs must be made each day?a2 × 10⁹b2 × 10¹¹c2 × 10¹³d4 × 10⁷
6. A tissue from the pubic symphysis has chondrocytes in lacunae and dense bundles of collagen fibres in its matrix. It isahyaline cartilagebelastic cartilageccompact bonedwhite fibrocartilage
7. A drug that blocks the release of Ca²⁺ from the sarcoplasmic reticulum wouldaprevent skeletal muscle contractionbspeed up contractionccause rigor mortisdaffect only smooth muscle
8. Muscles become stiff a few hours after death becauseaactin and myosin filaments shortenbcalcium is removed from the musclecno ATP is left to detach the myosin cross-bridgesdthe sarcolemma dissolves
9. A drug destroys the cell bodies of neurons lying in the dorsal root ganglia. The neurons affected areamultipolar motor neuronsbpseudounipolar sensory neuronscbipolar interneuronsdmultipolar autonomic neurons
10. A tissue has spindle-shaped cells, each with one central nucleus and no striations. Which activity depends on it?amovement of the limbsbthe heartbeatcblinking of the eyelidsdperistalsis of the intestine

Answers and explanations — Set C

1. (b) — these are skeletal fibres; the tongue is skeletal muscle, while the heart is cardiac and the stomach wall and iris smooth.

2. (d) — a glassy matrix without visible fibres is hyaline cartilage, as in the C-shaped tracheal rings. Pinna and epiglottis are elastic, the disc fibrocartilage.

3. (a) — the A band stays 1.6 μm, so I band = sarcomere − A band = 2.0 − 1.6 = 0.4 μm (it was 0.8 μm before).

4. (c) — eosinophils are 10/200 = 5 %, above the normal 2–3 %; neutrophils (62 %), lymphocytes (25 %), monocytes (7 %) and basophils (1 %) are normal.

5. (b) — 5 L = 5 × 10⁶ mm³, so total RBCs = 5 × 10⁶ × 5 × 10⁶ = 2.5 × 10¹³; ÷ 120 ≈ 2 × 10¹¹ per day.

6. (d) — collagen bundles in a cartilage matrix mean white fibrocartilage, the tissue of the pubic symphysis and intervertebral discs.

7. (a) — without Ca²⁺ troponin keeps the binding sites covered and no cross-bridges form; rigor (c) needs a lack of ATP, not of Ca²⁺.

8. (c) — rigor mortis: cross-bridges stay locked to actin because ATP is needed to release them; the filaments themselves never shorten (a).

9. (b) — the dorsal root ganglia hold the cell bodies of pseudounipolar sensory neurons; motor cell bodies lie in the grey matter of the cord.

10. (d) — smooth muscle drives peristalsis; limbs and eyelids use skeletal muscle and the heartbeat is cardiac.