Histology

Question 1

Name the 3 layers of the integument.

Answer 1

Epidermis
Dermis
Hypodermis

Question 2

What are the epidermal derivatives?

Answer 2

Follicular structures - hair and feather

Glandular structures - sebaceous and sweat glands

Keratinous structures - hoof, nail, beak and claw

Question 3

Describe plantar skin.

Answer 3

• Hairless, no hair follicles
• No sebaceous or sweat glands associated with hair follicles.
• Thick layer of epidermis.

Question 4

Name and describe the 4 layers that keratinocytes are arranged in.

Answer 4

Basal layer – stratum basale. Single layer of cells anchored by semi desmosomes and attached move towards surface from basal layer, taking about 4 weeks.

Prickle cell layer – stratum spinosum. Thickest layer. Cells that begin to start making keratin.

Granular layer – stratum granulosum. Mya be multiple layers but not as thick as prickle layer. Dead cells with nothing but keratin within them. How many layers depend on location.

Keratinised layer – stratum corneum. Flake off.

Question 5

Name the 3 non-epithelial cells in the epidermis.

Answer 5

Melanocytes
Dendritic (Langerhans) cells
Merkel cells

Question 6

Describe the structure and function of melanocytes.

Answer 6

• Attached to basement membrane with anchoring filaments
• Located in basal epidermis
• Smaller than keratinocytes
• Surrounded by a clear space
• Easily confused with mitotic bodies
• Melanin is synthesised from tyrosine in melanosomes
• Keratinocytes gain melanin from melanocytes
• Small round black cells with a halo around through a light microscope

Question 7

Describe dendritic (Langerhans) cells.

Answer 7

• Dendritic, antigen-presenting cells
• Mobile and present throughout epidermis
• Dendritic processes extend between keratinocytes
• The cells are also present in the dermis, other epithelia and lymphoid organs

Question 8

Describe merkel cells.

Answer 8

• Intra-epidermal touch receptors, occur in groups, maybe in domes
• Synapses with large myelinated afferent fibres in dermis
• Highest population density is in hairless skin (digits and lips) and in hair follicles
• Not visible in routine preparations: needs immune-histochemistry or EM

Question 9

Describe the structure of the dermis.

Answer 9

• Superficially contains loosely arranged collagen, elastic fibres and fibroblasts
• Deeper layer may contain more densely arranged collagen and elastic fibres
• Numerous small blood vessels and nerves
• Thickness is highly variable, depending on site (thin on eyelids, thick on soles)
• Contains epidermal derivatives – hair follicles and sweat glands

Question 10

What is the structure of the hypodermis?

Answer 10

Variable thickness and composition, predominantly composed of loose adipose tissue arranged in lobules. Lobules are separated by thin bands of fibrous tissue (interlobular septae).

Question 11

What are 3 functions of subcuticular adipose tissue?

Answer 11

• Insulation
• Energy storage
• Shock absorption

Question 12

Describe the structure of hair follicles.

Answer 12

• Complex epidermal invaginations
• Hair unit: follicle, arrector pili muscles, sebaceous and sweat glands
• Sinus hairs: (whiskers) specialised tactile hairs, follicles are surrounded by cavernous blood spaces

Question 13

What are sebaceous glands?

Answer 13

• Develop as an outgrowth of the external root sheath
• Tarsal glands – enlarged sebaceous glands which open on the eyelid margin
• Holocrine glands

Question 14

Describe holocrine glands.

Answer 14

• Secretion (sebum) formed from disintegrated cells and discharges on to hair shaft
• Sebum is oily and coats the hair and skin surface
• Lost cells are replaced by mitosis of basal cells

Question 15

Describe the 3 portions of the eccrine/atrichial sweat glands.

Answer 15

1. Secretory portion
2. Ascending and initial intra-epidermal portion
3. Spiral intra-epidermal portion

Question 16

Describe the structure of apocrine/apitrichial sweat glands.

Answer 16

• Coiled tubular glands
• Secretory portion is located in the dermis and/or hypodermis
• Ductal portion is similar in appearance to an eccrine duct
• Secretion is into adjacent hair follicle

Question 17

What is the function of apocrine/epitrichial sweat glands?

Answer 17

Lubrication and scent and us under hormonal control

Question 18

Identify the histological features of a neonatal foal hoof.

Answer 18

• Peripole epidermis - soft horn
• Proliferation zone of periopic corium
• Dermal papilla in tubule
• Intertubular horn
• Proliferation zone of coronary corium
• Horn tubules
• Primary lamina
• Secondary lamina

Question 19

What does a collagenous capsule around a foreign body embedded in the foot tell you?

Answer 19

It has been there a long time

Question 20

Histologically the normal heart valve has a central interstitium and a covering (or lining) layer. Briefly describe these two components.

Answer 20

The interstitium of a heart valve is composed of ‘stromal’ cell types – smooth muscle cells, fibroblasts, myofibroblasts; also collagen protein. The lining layer is a composed of slender endothelial cells, known as endocardium, which is contiguous with the endothelium which lines the entire circulatory system.

Question 21

Describe the histological appearance of chronic inflammation.

Answer 21

Macrophages
Epithelial macrophages
Multi-nucleated giant cells
Lymphocytes

• Diffuse (leprous) pattern where cells are dispersed in sheets as a result of the TH2-biased immunologic response.
• Granuloma formation - cells in nodular pattern - TH1 biased immunologic response
• Which pattern develops depends on agent, immune response and mediators

Question 22

Describe the histology of granulomas.

Answer 22

• Small central cavitated area that is hard to see what is in there.
• The next wider zone looks less cellular because the cells are larger.
• Outer area looks darker and more cellular, where lymphocytes, tissues healing macrophages and plasma cells are.
• Outside this, there is more and more dispersed zone of lymphocytes.
• Then fibrous collagenous capsule.

Question 23

Describe the process of immunohistochemistry.

Answer 23

1. Obtain an antibody against the antigen of interest, either a highly specific monoclonal antibody or a purified polyclonal antibody, which will often have been raised by immunising a rabbit.
2. Rabbit antibody is then incubated with the tissue and binds specifically to the antigen of interest.
3. The next step is to incubate with a secondary antibody, which binds to the primary antibody, such as an anti-rabbit antibody. This will typically be labelled with a fluorescent compound.
4. This can be visualised using a fluorescent microscope, revealing in great detail in labelled calls, such is the case of an oligodendrocyte.

Question 24

What are the species differnces in histological appearance of the cerebral cortex.

Answer 24

Normal folded appearance of the cerebral cortex with sulci and gyri in most species that you are likely to come across, apart from rodents and non-mammalian vertebrates.

Question 25

What would you expect the primary visual cortex to have in histological appearance?

Answer 25

- A particularly thick and prominent lamina IV to receive input from the LGN. - But it will not have a very prominent lamina V, as it doesn’t have much of a projection to subcortical structures. - It does have a prominent lamina II, as cells in this area have ipsilateral projections to higher visual cortical areas.

Question 26

What would you expect the motor cortex to have in histological appearance?

Answer 26

- A less well-developed lamina IV and lamina II and much more prominent laminae III and V. - Lamina III provides output to contralateral cortex, potentially important for coordinating movements on each side of the body. - Lamina V contains the large pyramidal neurons that project to subcortical targets including some of the largest neurons in the body that project to spinal levels in the corticospinal tract.

Question 27

Describe cortical ischaemic damage.

Answer 27

Being able to recognise the normal appearance of the cortical laminae helps you to recognize when the lamination is disordered or absent as in this case of a dog with cortical ischemia following cardiac arrest during surgery. The central laminae are more susceptible to damage from ischemia than the deep and superficial laminae.

Question 28

Describe the structure of the components of the cerebellar cortex.

Answer 28

The overall structure is for the planar dendritic trees to be aligned in the same direction, with parallel fibres running perpendicular to the plane of the dendritic tree like wires between electricity pylons.

Question 29

Name the 6 layers of the mammalian cerebral cortex, superficial to deep.

Answer 29

1. Molecular layer 2. External granule layer (output to ipsilateral cortex) 3. External pyramidal layer (output to contralateral cortex) 4. Internal granule cell layer (input from thalamus) 5. Internal pyramidal layer (output to subcortical structures) 6. Multiform layer (output to thalamus)

Question 30

Does immunostaining reflect the number of pyramidal neurones present in the cortex?

Answer 30

No, because there are different subpopulations of pyramidal cell with different patterns of projections that express different types of neurofilament proteins. Also the staining is never 100% and some neurones that express neurofilament protein may not be stained.

Question 31

How would evolution increase the processing power of the cortex?

Answer 31

By increasing the number of cortical columns to process the information, i.e., increase the cortical area. This means greater folding of cortex to fit in the limited volume of a skull that can fit through the birth canal.

Question 32

Does the folding of the cerebral cortex observed in different species support your hypothesis?

Answer 32

You might expect the amount of cortical folding to be lowest in monotremes and then increase in placental mammals. The duck-billed platypus certainly has no folding of cortex, but the echidna, which is another monotreme, has extensively folded cortex, which goes to show that it not simply due to the closeness of phylogenetic relationship, but also to the selective pressures on individual species. But what do mammals do with all this cortical area? Birds are remarkably intelligent, despite lacking mammalian type neocortex.

Question 33

Name the layers of white matter.

Answer 33

1. White matter 2. Granule cell layer 3. Purkinje cell layer 4. Molecular layer 5. Meninges and blood

Question 34

What structure is dopamine released into and where does it go?

Answer 34

Dopamine is released into the pituitary portal circulation at the median eminence. It is conveyed by the pituitary portal circulation to the lactotrophs in the anterior pituitary.

Question 35

What are 2 hypothalamic nuclei containing oxytocin expressing neurones?

Answer 35

Paraventricular Supraoptic

Question 36

Describe the histology and fish lamellae.

Answer 36

- Ultra-thin layer of pavement cells that cover the lamellae not easily seen but pink stained nucleated erythrocytes flowing single file through the blood channel. - Brown stained pillar cells between erythrocytes. These cells have a H shaped profile in cross section. The prongs of the H overlap with the prongs of the pillar cells above and below to form the blood channels.

Question 37

What is the tissue type of the reptilian lung?

Answer 37

- It is lined by a typical pseudostratified respiratory epithelium, which has the normal functions of particle entrapment and clearance via mucus production and ciliary motion - Some of this respiratory-type epithelium is found within the upper part of the faveoli, which from elongated channels spanning the depth of the lung tissue. - Prominent rings of smooth muscle around the entrances to the faveoli

Question 38

What is the tissue type of the reptilian stomach?

Answer 38

- Stomach epithelium is simple columnar and is protected by mucous produced by the lightly stained neck cells, which are the equivalent of mucus producing goblet cells in the mammalian stomach. - Instead of having separate chief and parietal cells, reptilian stomach, like avian stomachs, have a single type of oxynopeptic cell that produce both pepsinogen and acid. These have prominent zymogen granules, similar to those found in chief cells in mammals.

Question 39

List the layers of cells of the reptilian stomach from outside inwards.

Answer 39

1. Serosa 2. Muscularis externa 3. Submucosa 4. Muscularis mucosa 5. Laminar propria 6. Gastric glands. Branched tubular glands. Cells contain zymogen granules and secrete acid and enzyme similar to avian. 7. Neck cells. Secrete mucus to protect gastric epithelium from digestion. 8. Columnar absorptive epithelium 9. Gastric pit 10. Lumen of gastric gland opening at gastric pit 11. Gastric lumen

Question 40

Describe the histological appearance of the avian parabronchi.

Answer 40

- Honeycomb like appearance of the wall of the parabronchi is due to the atria/hallway/entrance. - There are numerous holes in the atria, which are entrances to individual capillaries. - The atria help to create some turbulence in the bulk flow of air along the parabronchus.

Question 41

What is a consequence of thin endothelium of avian air capillaries?

Answer 41

But the thin diffusion barriers of the avian lung do make birds susceptible to pulmonary hypertension.

Question 42

Describe the histological cross section of the parabronchi.

Answer 42

Star shaped cross sections in the lung tissue through parabronchi. Points on the star are the atria that lead to air capillaries.

Question 43

What is the type of epithelium in the crop?

Answer 43

Pseudostratified ciliated columnar respiratory epithelium with mucus secreting cells.

Question 44

What is the type of epithelium from plicae and sulci in the proventriculus?

Answer 44

Columnar epithelium secreting mucus

Question 45

Compare the mammalian and avian duodenum.

Answer 45

- Mammal intestinal villi are longer - Avian crypts are shallower than mammals - Avian crypts are without Paneth cells - Lymphoid tissue is sparse in avian