Unit 6 · Zoology · 8 teaching hours · 28 min read

Animal Tissues

What the syllabus asks for 4

Epithelial tissue
Connective tissue
Muscular tissue
Nervous tissue

A tissue is a group of similar cells, together with their intercellular material, that share a common origin and do one job. The multicellular animal body is built from only four of them — epithelial, connective, muscular and nervous. Every organ you meet in the Human Biology unit, from the stomach wall to the eyeball, is these four tissues stacked in different proportions. Learn the four properly now and the rest of Zoology becomes description rather than memorisation.

What the examiner asks from this unit

Three things, every year. A labelled diagram of a named tissue as it looks under the microscope — the neuron, the T.S. of bone, the three muscle types. A difference table — tendon vs ligament, cartilage vs bone, striated vs cardiac vs smooth muscle. And a "where is it found and why" question, which wants structure linked to function, not a list. Practise drawing; a correct diagram with six labels is often the whole four marks.

6.1 Epithelial tissue

Epithelial tissue

Epithelium is a sheet of closely packed cells, with almost no intercellular matrix, resting on a non-cellular basement membrane, which covers the free surfaces of the body and lines its cavities, ducts and tubes.

Epithelium is the only tissue that arises from all three germ layers: the epidermis of the skin and the lining of the mouth and anus are ectodermal, the lining of the gut and its glands endodermal, and the lining of blood vessels, kidney tubules and coelom mesodermal.

Its general characters follow from its job of forming a boundary:

Cells are packed tightly and joined by cell junctions (tight junctions, desmosomes), so the sheet is continuous and leak-proof.
Intercellular matrix is negligible.
The sheet has polarity — a free (apical) surface and a basal surface cemented to the basement membrane, a thin glycoprotein layer secreted partly by the epithelium and partly by the connective tissue below.
It is avascular. No blood vessel enters an epithelium; food and oxygen diffuse in from the capillaries of the connective tissue beneath.
It has a very high power of regeneration, because it is the layer that gets worn away.

Epithelia are classified first by the number of cell layers, then by the shape of the cells at the free surface.

photograph — **Figure 1.** (a) Simple squamous epithelium in surface view (silver-stained whole mount, ×100): thin, flat, polygonal cells fit edge to edge like paving stones, and the silver outlines each boundary. Source: Berkshire Community College Bioscience Image Library / Wikimedia Commons (CC0)

Compound (stratified) epithelium

When the epithelium must resist wear, it becomes many cells thick. Only the deepest layer — the germinative or Malpighian layer — touches the basement membrane and divides; its daughter cells are pushed upwards, flatten, die and are shed. In the epidermis of the skin the dying cells fill with the protein keratin, giving a dry, waterproof, keratinised stratified squamous epithelium. In the mouth, oesophagus and vagina the surface stays moist and the epithelium is non-keratinised.

Transitional epithelium (urothelium) lines the ureter and urinary bladder. It has no true stratum corneum, and its cells slide over one another so the sheet can stretch: relaxed it looks 4–5 cells thick with dome-shaped surface cells, distended it looks 2–3 cells thick and flat.

Glandular epithelium

When epithelial cells specialise for secretion they sink into the connective tissue below and form a gland. A single cell doing this is a unicellular gland — the goblet cell of the gut and trachea. Many cells doing it together make a multicellular gland, tubular if the secretory part is a tube (intestinal crypts, sweat glands) or alveolar/acinar if it is a flask (salivary, sebaceous).

Glands are grouped by where the secretion goes — exocrine glands release through a duct, endocrine glands have no duct and pour hormones into the blood — and by how the cell releases it.

Mode	How the cell secretes	Example
Merocrine	by exocytosis; the cell stays intact	salivary gland, pancreas
Apocrine	apical part of the cell pinches off with the secretion	mammary gland
Holocrine	the whole cell disintegrates and becomes the secretion	sebaceous gland

**Figure 5.** Glandular epithelium: a unicellular goblet cell and the two commonest multicellular forms.

Type	Shape at free surface	Chief location	Main function
Simple squamous	flat, tile-like, central bulge	alveoli, Bowman's capsule, endothelium of vessels	diffusion, filtration
Simple cuboidal	cube, round central nucleus	kidney tubules, thyroid follicles, gland ducts	absorption, secretion
Simple columnar	tall, nucleus near the base	stomach, small intestine, gall bladder	secretion, absorption
Ciliated columnar	tall, with cilia	oviduct, bronchioles, ventricles of brain	moves mucus, ovum, CSF
Pseudostratified	tall and short cells, nuclei at two levels	trachea, nasal cavity, male urethra	protection, mucus movement
Stratified squamous	many layers, flat and dead at the top	epidermis, oesophagus, vagina, cornea	protection against wear
Transitional	2–5 layers, cells slide	ureter, urinary bladder	allows stretching
Glandular	secretory, sunk into the tissue	gut lining, all glands	secretion

"Pseudostratified" is not "stratified"

A pseudostratified epithelium is one cell layer thick. Every cell reaches the basement membrane, but the cells are of different heights, so their nuclei sit at different levels and the sheet only looks layered. A stratified epithelium has real layers and only the deepest touches the basement membrane.

Worked example 6.1 — magnification and real cell size

Problem. In a photomicrograph of the lining of the small intestine taken at a magnification of \times 1000, one columnar cell measures 40\ \text{mm} from base to free surface and 8\ \text{mm} across. (a) Find its real height and width in micrometres. (b) How many such cells lie side by side along 1\ \text{mm} of the intestinal lining?

Solution. Magnification is defined as

M = \frac{\text{image size}}{\text{actual size}} \;\Rightarrow\; \text{actual size} = \frac{\text{image size}}{M}

(a) Height:

\frac{40\ \text{mm}}{1000} = 0.040\ \text{mm} = 0.040 \times 1000\ \mu\text{m} = 40\ \mu\text{m}

Width: 8/1000 = 0.008\ \text{mm} = 8\ \mu\text{m}. (Recall 1\ \text{mm} = 1000\ \mu\text{m}.)

(b) Each cell is 8\ \mu\text{m} wide and 1\ \text{mm} = 1000\ \mu\text{m}, so

n = \frac{1000\ \mu\text{m}}{8\ \mu\text{m}} = 125\ \text{cells}

A cell 40\ \mu\text{m} tall and only 8\ \mu\text{m} wide is five times taller than it is broad — that is exactly what "columnar" means.

6.2 Connective tissue

Connective tissue

Connective tissue is a mesodermal tissue in which relatively few cells lie scattered in an abundant intercellular matrix of fibres and ground substance. It binds, supports, packs, transports and defends.

Connective tissue is the mirror image of epithelium: there the cells crowd together and the matrix is negligible, here the matrix does the work and the cells are far apart. Its properties are the properties of its matrix. Three kinds of fibre are woven into it:

White collagen fibres — thick, wavy, non-branching bundles of the protein collagen. Very strong, inelastic. Boiling turns them into gelatin.
Yellow elastic fibres — thinner, straight, branching fibres of elastin. They stretch to about 1.5 times their length and snap back.
Reticular fibres — fine branching collagen, forming the mesh of lymph nodes, spleen and bone marrow.

The commonest cells are the fibroblast (spindle-shaped; makes the fibres and ground substance), the macrophage or histiocyte (amoeboid; engulfs bacteria and debris), the mast cell (granular; secretes heparin, which stops blood clotting inside vessels, and histamine, which causes inflammation and allergy), the plasma cell (secretes antibodies) and the adipocyte (stores fat).

Group	Types	Distinguishing feature	Chief function
Loose connective	areolar, adipose, reticular	soft, cells many, matrix loose	packing, fat store, framework
Dense connective	tendon, ligament (regular); dermis (irregular)	fibres crowded and ordered	transmits or resists pull
Skeletal (supporting)	cartilage, bone	matrix solid	support, protection, movement
Fluid (vascular)	blood, lymph	matrix is liquid plasma	transport, defence

Loose connective tissue

Areolar tissue is the universal packing material — under the skin, between muscles, around nerves, blood vessels and organs. Its loose, jelly-like matrix lets tissue fluid and white blood cells move freely, which is why it is also the main site of inflammation.

Adipose tissue is areolar tissue in which fat-storing cells have multiplied. Each adipocyte is almost filled by a single fat droplet, so the cytoplasm is a thin rim and the nucleus is squashed against one side — the "signet-ring" appearance. It stores energy, insulates against cold and cushions the kidneys, eyeballs and soles of the feet.

Dense connective tissue

In dense tissue the fibres are packed so tightly that the cells are squeezed into rows between them. When the fibres all run one way the tissue is regular and resists pull along a single line — the tendon and the ligament. When they run in all directions it is irregular and resists pull from any direction — the dermis of the skin and the capsules of organs.

Feature	Tendon	Ligament
Chief fibre	white collagen	yellow elastic
Arrangement	straight, parallel bundles	branching, interlacing
Elasticity	almost none — inextensible	elastic, stretches and recoils
Colour when fresh	glistening white	yellowish
Cells	fibroblasts in rows between bundles	fibroblasts scattered
Connects	muscle to bone	bone to bone at a joint
Blood supply	poor, so it heals slowly	poor

Cartilage

Cartilage is a solid but flexible skeletal tissue. Its cells, the chondrocytes, sit singly or in twos inside fluid-filled spaces called lacunae, embedded in a firm matrix of the protein chondrin. The whole mass is wrapped in a fibrous sheath, the perichondrium, from which it grows and through which it is fed — because cartilage itself has no blood vessels.

Type	Matrix contains	Where found
Hyaline	chondrin only; clear, bluish, glassy	tracheal rings, nose tip, articular surfaces, embryonic skeleton
Elastic	a network of yellow elastic fibres	pinna of ear, epiglottis, eustachian tube
Fibrous (white)	dense bundles of collagen	intervertebral discs, pubic symphysis
Calcified	calcium salts deposited in the matrix	head of the humerus, suprascapula of frog

Bone

Bone is the hardest connective tissue. Its matrix, ossein, is about one-third organic protein — which gives toughness — and two-thirds inorganic salts, mainly calcium phosphate with some calcium carbonate, which give hardness. Bone cells, the osteocytes, lie in lacunae and keep in touch with one another and with the blood supply through hair-fine channels, the canaliculi.

In the compact bone of a mammal, this material is organised into cylinders. Each cylinder, an osteon or Haversian system, is a central Haversian canal carrying a blood capillary, a lymph vessel and a nerve, surrounded by 4–20 concentric lamellae of matrix with rings of lacunae between them. Haversian canals run lengthwise along the bone and are linked sideways by Volkmann's canals. The whole bone is sheathed in the periosteum.

Feature	Cartilage	Bone
Matrix	chondrin (protein), soft and flexible	ossein + calcium salts, hard and rigid
Cells	chondrocytes, 1–4 per lacuna	osteocytes, 1 per lacuna
Canaliculi	absent	present, linking lacunae
Blood supply	absent (avascular)	rich, through Haversian canals
Covering	perichondrium	periosteum
Growth	interstitial and appositional	appositional only

Cartilage heals slowly — and frog bone has no Haversian systems

Because cartilage has no blood vessels, its cells are fed only by diffusion through the matrix from the perichondrium. That is why a torn knee cartilage heals far more slowly than a broken bone. Second point, often asked: well-formed Haversian systems are a feature of mammalian compact bone. The bone of the frog and other lower vertebrates is nourished by diffusion and lacks Haversian canals.

Fluid connective tissue

Blood is a connective tissue whose matrix is liquid. About 55% of it by volume is plasma — 90–92% water, with plasma proteins (albumin, globulin, fibrinogen), glucose, salts, hormones and wastes — and about 45% is the formed elements.

Formed element	Normal count (per mm³)	Structure	Function
Erythrocyte (RBC)	4.5–5.5 million	biconcave disc, 7–8 μm, no nucleus in mammals	carries O₂ and CO₂ as oxyhaemoglobin and carbaminohaemoglobin
Neutrophil	4000–11000 WBC in total; 60–65% of them	3–5 lobed nucleus, fine granules	phagocytosis of bacteria
Eosinophil	2–3% of WBC	bilobed nucleus, coarse granules	allergy and parasite defence
Basophil	0.5–1% of WBC	lobed nucleus, large dark granules	secretes heparin and histamine
Lymphocyte	20–25% of WBC	large round nucleus, thin cytoplasm	antibodies and cell-mediated immunity
Monocyte	6–8% of WBC	kidney-shaped nucleus, largest WBC	becomes a macrophage in tissues
Thrombocyte (platelet)	150,000–400,000	cell fragment, no nucleus	blood clotting

Worked example 6.2 — counting red cells with a haemocytometer

Problem. Blood is diluted 1 : 200 and loaded into a Neubauer counting chamber. The central ruled square is 1\ \text{mm} \times 1\ \text{mm}, divided into 400 small squares, and the depth under the cover-slip is 0.1\ \text{mm}. A student counts 500 red cells in 80 small squares. Find the RBC count per mm³ and say whether the person is anaemic.

Solution. Area of one small square:

A = \frac{1\ \text{mm}^{2}}{400} = 2.5 \times 10^{-3}\ \text{mm}^{2}

Volume of blood over 80 small squares:

V = 80 \times (2.5 \times 10^{-3}\ \text{mm}^{2}) \times 0.1\ \text{mm} = 0.02\ \text{mm}^{3}

Cells per mm³ of the diluted blood:

\frac{500}{0.02} = 25\,000\ \text{per mm}^{3}

Multiplying by the dilution factor 200 to get back to whole blood:

25\,000 \times 200 = 5\,000\,000 = 5 \times 10^{6}\ \text{per mm}^{3}

Five million per mm³ lies in the normal range of 4.5–5.5 million, so the person is not anaemic.

6.3 Muscular tissue

Muscular tissue

Muscular tissue is a mesodermal tissue made of elongated cells, the muscle fibres, whose cytoplasm is packed with the contractile protein filaments actin and myosin, so that the cell can shorten and generate force.

A muscle fibre has its own vocabulary: the cell membrane is the sarcolemma, the cytoplasm the sarcoplasm, the endoplasmic reticulum the sarcoplasmic reticulum (it stores the Ca²⁺ that triggers contraction), and the contractile threads running the length of the fibre are the myofibrils. About 40% of the body mass of an adult is muscle, in three kinds.

Feature	Striated (skeletal)	Cardiac	Smooth (unstriated)
Shape of fibre	long, cylindrical, unbranched	cylindrical, branched	spindle-shaped (fusiform)
Striations	present, prominent	present, faint	absent
Nuclei	many, at the periphery	one (rarely two), central	one, central
Intercalated discs	absent	present	absent
Control	voluntary	involuntary	involuntary
Speed and fatigue	fast, powerful, tires quickly	moderate, rhythmic, never tires	slow, sustained, tires slowly
Location	attached to bones, tongue, diaphragm	wall of the heart only	gut, uterus, blood vessels, iris, bronchi

The sarcomere

Under high power, a myofibril shows repeating light and dark bands. The dark A band (anisotropic) is the length of the thick myosin filaments; the light I band (isotropic) contains only thin actin filaments and is bisected by a dark Z line. The H zone in the middle of the A band contains myosin only, and is crossed by the M line.

Sarcomere

A sarcomere is the segment of a myofibril between two successive Z lines. It is the functional (contractile) unit of striated muscle. At rest it is about 2.0–2.5 μm long.

**Figure 18.** One sarcomere, the unit between two Z lines. During contraction the filaments do not shorten — the actin slides over the myosin, so the I band and H zone narrow while the A band stays the same length.

Worked example 6.3 — how far does a muscle shorten?

Problem. A myofibril in the biceps is 2.4\ \text{cm} long and its sarcomeres are each 2.4\ \mu\text{m} long at rest. (a) How many sarcomeres lie end to end along it? (b) If every sarcomere shortens to 1.8\ \mu\text{m} during a full contraction, what is the new length of the myofibril and the percentage shortening?

Solution.

(a) Convert to the same unit: 2.4\ \text{cm} = 24\ \text{mm} = 24\,000\ \mu\text{m}.

n = \frac{24\,000\ \mu\text{m}}{2.4\ \mu\text{m}} = 10\,000\ \text{sarcomeres}

(b) New length = 10\,000 \times 1.8\ \mu\text{m} = 18\,000\ \mu\text{m} = 1.8\ \text{cm}.

\text{shortening} = \frac{2.4 - 1.8}{2.4}\times 100 = 25\%

Each sarcomere shortens by only 0.6\ \mu\text{m}, but because 10,000 of them are joined in series the whole fibril shortens by 0.6\ \text{cm}. That is how a change too small to see adds up to a visible movement.

6.4 Nervous tissue

Nervous tissue is ectodermal and is built from two kinds of cell: the neurons, which carry the messages, and the neuroglia, about ten times more numerous, which support, insulate, feed and defend them. Neurons are excitable — they respond to a stimulus by generating an electrical impulse — and conductive — they pass that impulse on. Mature neurons have lost the power to divide, which is why nerve damage is often permanent.

A typical motor neuron has three parts:

Cyton (cell body or perikaryon) — contains the nucleus, and granular patches of rough endoplasmic reticulum with ribosomes called Nissl granules, which make the neuron's proteins. It has no centriole.
Dendrites — many short, branched, tapering processes that carry impulses towards the cyton. They also contain Nissl granules.
Axon — a single long process arising from the cone-shaped axon hillock that carries the impulse away from the cyton. It has no Nissl granules. It ends in fine branches, the telodendria, each tipped by a synaptic knob.

In a myelinated fibre the axon is wrapped in a fatty myelin sheath made by Schwann cells, covered outside by the neurilemma. The sheath is interrupted every millimetre or so at the nodes of Ranvier. The impulse jumps from node to node — saltatory conduction — which is why a myelinated fibre conducts up to a hundred times faster than a bare one.

The direction rule

Dendrite = Delivers to the cell body. Axon carries away. At a synapse the vesicles are always on the axon side, so transmission across a synapse is strictly one-way.

Neurons are named by the number of processes leaving the cyton — unipolar (one process; in insect and embryonic nerve cells), bipolar (one axon and one dendrite; retina, olfactory epithelium) and multipolar (one axon and many dendrites; the commonest type, in the brain and spinal cord). By function they are sensory (afferent), motor (efferent) or association (interneurons).

The main neuroglia are astrocytes (anchor neurons to capillaries and help form the blood–brain barrier), oligodendrocytes (make myelin inside the brain and spinal cord), microglia (phagocytes of the nervous system) and ependymal cells (line the brain ventricles and circulate cerebrospinal fluid).

Synapse

A synapse is the functional junction between the synaptic knob of one neuron and the dendrite or cyton of the next. The membranes do not touch; they are separated by a synaptic cleft about 20 nm wide. The impulse crosses chemically: it makes the knob release a neurotransmitter such as acetylcholine, which diffuses across the cleft and excites the next membrane.

Worked example 6.4 — how long does a reflex take?

Problem. You step on a thorn in a Chitwan forest and pull your foot back. The sensory path from the foot to the spinal cord is 0.90\ \text{m} and the motor path back to the leg muscle is 0.80\ \text{m}; both are myelinated fibres conducting at 85\ \text{m s}^{-1}. The reflex arc contains 2 synapses, each causing a delay of 0.5\ \text{ms}. (a) Find the total reflex time. (b) How long would the same journey take along a non-myelinated fibre conducting at 0.5\ \text{m s}^{-1}?

Solution.

(a) Total length of nerve fibre = 0.90 + 0.80 = 1.70\ \text{m}.

t_{\text{conduction}} = \frac{1.70\ \text{m}}{85\ \text{m s}^{-1}} = 0.020\ \text{s} = 20\ \text{ms}

Synaptic delay = 2 \times 0.5 = 1.0\ \text{ms}. Total reflex time

t = 20 + 1 = 21\ \text{ms} = 0.021\ \text{s}

(b) Along a bare fibre:

t = \frac{1.70\ \text{m}}{0.5\ \text{m s}^{-1}} = 3.4\ \text{s}

The myelin sheath makes the response about 160 times faster — quite literally the difference between a scratch and a deep wound.

Do not mix up the two sheaths

The myelin sheath is the fatty insulating layer produced by the Schwann cell. The neurilemma is the thin outer living membrane of the Schwann cell itself, outside the myelin. Non-myelinated fibres still have a neurilemma — they simply lack myelin, and so have no nodes of Ranvier and conduct slowly.

Feature	Myelinated fibre	Non-myelinated fibre
Myelin sheath	present	absent
Nodes of Ranvier	present	absent
Conduction	saltatory, fast (up to about 120 m s⁻¹)	continuous, slow (about 0.5–2 m s⁻¹)
Found in	cranial and spinal nerves, white matter	autonomic nerves, grey matter

Chapter summary

A tissue is a group of similar cells with a common origin doing one job; the four animal tissues are epithelial, connective, muscular and nervous.
Epithelium: cells packed, matrix negligible, avascular, polar, on a basement membrane, high regeneration. Simple (squamous, cuboidal, columnar, ciliated, pseudostratified), compound (stratified squamous, transitional) or glandular.
Connective tissue: few cells, abundant matrix of collagen, elastic and reticular fibres. Loose (areolar, adipose), dense (tendon = collagen, muscle→bone; ligament = elastin, bone→bone), skeletal (cartilage, bone) and fluid (blood, lymph).
Cartilage: chondrocytes in lacunae in chondrin, avascular, covered by perichondrium. Bone: osteocytes in lacunae joined by canaliculi, matrix of ossein + calcium salts, organised into Haversian systems in mammals.
Blood = 55% plasma + 45% formed elements. RBC 4.5–5.5 million per mm³, WBC 4000–11000 per mm³, platelets 150,000–400,000 per mm³.
Muscle: striated (cylindrical, unbranched, many peripheral nuclei, voluntary), cardiac (branched, central nucleus, intercalated discs, involuntary, tireless), smooth (spindle, one central nucleus, no striations, involuntary).
The sarcomere, from Z line to Z line, is the contractile unit; on contraction the I band and H zone shorten but the A band does not.
A neuron has a cyton with Nissl granules, dendrites carrying impulses in, and one axon carrying them out. Myelin plus nodes of Ranvier give saltatory conduction; the synapse transmits chemically and in one direction only.

Practice questions

Group A — Multiple choice (1 mark each)

The tissue that arises from all three germ layers is [1]aconnectivebepithelialcmusculardnervous
Transitional epithelium is characteristic of the [1]atracheabsmall intestinecurinary bladderdoesophagus
Tendons are made chiefly of [1]ayellow elastic fibresbwhite collagen fibrescreticular fibresdchondrin
Which is absent in cartilage? [1]alacunaebchondrocytescblood vesselsdperichondrium
Intercalated discs occur in [1]askeletal musclebcardiac musclecsmooth muscledall three
During contraction of a sarcomere, the band whose length does not change is the [1]aI bandbH zonecA banddZ line
Nissl granules are absent from the [1]acytonbdendritecaxondnucleus
Heparin and histamine are secreted by the [1]afibroblastbmast cellcplasma celldosteocyte

Answers to Group A

1. (b) — epidermis is ectodermal, gut lining endodermal, endothelium mesodermal.

2. (c) — it must stretch as the bladder fills.

3. (b) — which is why a tendon is inextensible.

4. (c) — cartilage is avascular and is fed by diffusion from the perichondrium.

5. (b) — they are the junctions between adjacent cardiac fibres.

6. (c) — the A band equals the length of the myosin filament, which does not shorten.

7. (c) — the axon has no Nissl granules; this distinguishes it from a dendrite.

8. (b) — heparin prevents clotting inside vessels, histamine causes inflammation.

Group B — Short answer questions (4 marks each)

Define epithelial tissue and give four general characters of it. [4]
Distinguish between a tendon and a ligament under four heads. [4]
Draw a labelled T.S. of a Haversian system and name any four parts. [4]
Give four differences between cardiac muscle and smooth muscle. [4]
In a haemocytometer count, blood is diluted 1 : 200, and 420 red cells are counted in 80 small squares, each of area \frac{1}{400}\ \text{mm}^{2} under a depth of 0.1\ \text{mm}. Calculate the RBC count per mm³ and comment. [4]
Why is cartilage slow to heal while bone heals comparatively fast? [4]

Answers to Group B

2. Tendon — white collagen fibres, straight parallel bundles, inextensible, joins muscle to bone. Ligament — yellow elastic fibres, branching, elastic, joins bone to bone.

4. Cardiac: branched fibres, one central nucleus, intercalated discs present, found only in the heart, never fatigues. Smooth: spindle-shaped unbranched cells, one central nucleus, no discs, no striations, found in gut, uterus and blood vessels.

5. Volume over 80 small squares = 80 \times \frac{1}{400}\ \text{mm}^{2} \times 0.1\ \text{mm} = 0.02\ \text{mm}^{3}. Diluted count = 420/0.02 = 21\,000 per mm³. Whole blood = 21\,000 \times 200 = 4\,200\,000 = 4.2 million per mm³. This is below the normal range of 4.5–5.5 million, so the person is mildly anaemic.

6. Cartilage is avascular: its chondrocytes receive food and oxygen only by slow diffusion through the solid matrix from the perichondrium, and they divide slowly. Bone is richly supplied with blood through the Haversian and Volkmann's canals, and the periosteum contains osteoblasts that can lay down new matrix quickly, so a fracture unites in weeks.

Group C — Long answer questions (8 marks each)

(a) Classify epithelial tissue with one example and one location for each type. [5](b) Draw a labelled diagram of pseudostratified ciliated columnar epithelium and explain why it is not a stratified epithelium. [3]
(a) Describe the structure of a myelinated motor neuron with a labelled diagram. [5](b) What is a synapse? Explain how an impulse crosses it and why transmission is one-way. [3]

Answer outline to Group C question 2

(a) Cyton with nucleus, Nissl granules and no centriole; several branched dendrites conducting towards the cyton; a single axon arising at the axon hillock, covered by myelin made by Schwann cells, with neurilemma outside and nodes of Ranvier between successive Schwann cells; ending in telodendria tipped by synaptic knobs. Award marks for the labelled diagram.

(b) A synapse is the junction between the synaptic knob of one neuron and the dendrite or cyton of the next, separated by a cleft of about 20 nm. The arriving impulse opens Ca²⁺ channels in the knob; synaptic vesicles fuse with the presynaptic membrane and release acetylcholine, which diffuses across the cleft and binds receptors on the postsynaptic membrane, starting a new impulse. The enzyme cholinesterase then destroys the transmitter. Transmission is one-way because vesicles and transmitter exist only on the presynaptic side and receptors only on the postsynaptic side.

Practise more: Biology question bank