Histology and Embryology

Question 1

Why is eyelid skin the thinnest in the body?

Answer 1

Attenuated dermis and hypodermic

Question 2

4 layers of keratinocytes in the epidermis from inner to outer and their progression

Answer 2

Stratum Basale/Germinativum- only ones to go through mitosis
Stratum Spinosum-Start degrading/losing organelles
Stratum Granulosum- Start degrading/losing organelles
Stratum Corneum-Dead, flattened. Shed every few weeks by desquamation.

Big spiders get crushed

Question 3

Cells of the epidermis

Answer 3

1. Keratinocytes (90%) He says just in spinosum and granulosum
2. Melanocytes. Come from neural crest cells, found among the basal cells.
3. Langerhans- white blood cell
4. Merkel Cells- Light touch receptors

Question 4

How does the epidermis get blood supply

Answer 4

It is avascular, gets blood from the underlying dermis.

Question 5

How many layers of the keratinocytes in the epidermis

Answer 5

Stratum Basale/Germinativum- Single layer of cuboidal cells
Stratum Spinosum- Prickle layer, muli layer
Stratum Granulosum- Thin
Stratum Corneum- Very thin, 3-4 layers

Question 6

Which layer of the normal skin is not in the eyelid

Answer 6

Stratum Lucidum, in between granulosum and corneum.

Question 7

Which cells of the epidermis can be cancerous

Answer 7

Melanocytes, basal, and merkel

Question 8

Rete pegs/Papillae

Answer 8

Epithelial extensions into underlying dermis. Improves adhesion of tissues.

Scar tissue doesn’t have RP. Less bond of the epidermis to the dermis.

Question 9

2 main layers of the dermis

Answer 9

Papillary- found more superficial between the rete pegs/papillae.

Reticular- Deep to rete pegs, above orbicularis oculi. Loose areolar tissue that stretches easily with edema or heme (swells easily). Contains lash follicles, blood vessels, lymphatics, and sensory nerves.

Question 10

Where is fat located in the eyelid

Answer 10

Eyelid lacks adipose tissue between the dermis and orbicularis, but there is the ROOF and SOOF behind the orbicularis. The fat is not associated with the skin.

Question 11

3 main germ layers inner to outer

Answer 11

Endoderm
Mesoderm
Ectoderm

Question 12

Mesoderm gives rise to

Answer 12

mesoderm invaded by neural crest cells –> Mesenchymal tissue.

Question 13

Ectoderm starts as a single layer, then what?

Answer 13

Splits into squamous epithelium (periderm) and basal layer. Will go through desquamation. At 11th week, the Spinosum and granulosum grow between the 2 layers to become the epidermis

Question 14

Which 2 germ layers form most the structures of the yelid

Answer 14

Surface ectoderm and mesoderm–> mesenchyme when invaded by neural crest cells

*There is signaling between the 2 layers. Closely associated.

Question 15

Surface ectoderm- responsible for the development of

Answer 15

The structures on the outside of the eye 
Conj 
Skin epithelium 
Hair follicles 
Gland of Zeiss 
Glands of Moll 
Meibomian glands

Question 16

Mesenchymal tissue is responsible for the development of

Answer 16

Tarsal plate 
Levator 
Orbicularis 
Septum 
Mullers

Question 17

First sign of eyelid fold (Superior and inferior lids) in embryo

Answer 17

6 weeks

Question 18

Embryonic stage timing and fetal stage

Answer 18

First 8 weeks is embryonic. Eyelid fusion occurs and marks the end of the embryonic stage at 8 weeks.

Question 19

What occurs at 7 weeks? 1 week after eyelid folds present

Answer 19

Invagination into the mesenchyme for start of punctum and canaliculi.

Question 20

What occurs at 8 weeks?

Answer 20

Eyelid fusion- filopodia with contractile elements extend, allowing for corneal coverage. Promotes K development.

Marks the end of the embryonic stage.

Question 21

Fetal development stage is marked by

Answer 21

Mesenchymal (mesoderm with neural crest cells) infiltration into the tissue.

Question 22

What gives rise to melanocytes

Answer 22

Mesenchyme. Melanocytes play a big role in cancerous lesions of the eyelid.

Question 23

Developments at 9, 11, and 12, 13, 14, 18, 20, 24 weeks

Answer 23

9 weeks- Orbicularis development begins
11 weeks- Start of tarsal plate, lash follicles, orbital septum, and meibomian glands.
12 weeks- Eyelashes start to develop and levator, tarsal plate noted.
13 weeks- Zeis and Moll developing
14- Distinct layers of lid and mullers muscle begins
18- Tarsal plate and vasculature are defined
20-Eyelid separation begins
24 weeks- Most tissues are evident and separated. All structures are present, just not fully developed.

Question 24

Levator has shared connective tissue (epimysium) with

Answer 24

SR

Question 25

How are Meibomian glands comparable to eyelash follicles?

Answer 25

They are called a follicle without a hair shaft. They develop with central epithelial cylinders with keratohyalin cells lining the inside of the gland.

Keratohyalin cells may have the ability to signal/upregulate later in life when we see MGD. This causes hyperkeratinization and scarring in MGD>

Question 26

What week are all structures are present, just not fully developed.

Answer 26

24

Question 27

When does eyelid separation begin and in what direction

Answer 27

Week 20

Begins anterior and nasal.

Question 28

What factors yield separation of the eyelids

Answer 28

Keratin expression, holocene expression from meibomian glands, and apoptosis

Question 29

When does blinking begin? How many blinks per minute?

Answer 29

30-33 weeks
6 blinks per minute.
Not that the tissue isn’t developed- but there is not a need to blink in the womb.

Question 30

Pre mature babies tend to have a high or low blink rate when born?

Answer 30

High, excessive

Question 31

Hemostasis

Answer 31

Stopping bleeding
Resolving bleeding at site of trauma while maintaining blood flow elsewhere.

Formation of a clot
Primary hemostasis- platelet aggregation plugs leaking vessel.
Secondary hemostasis- fibrin activation (solid)

Question 32

Formation of clot (2 stages)

Answer 32

Formation of a clot
Primary hemostasis- platelet aggregation plugs leaking vessel.
Secondary hemostasis- fibrin activation (solid)

Question 33

Intra-operative hemostasis/how to induce hemostasis

Answer 33

Topical- chemical agents that induce clotting

Mechanical- Use a clamp, pressure, bandaging allows for coagulation/platelet plug
*Arteries respond better to pressure since they can vasoconstrict

Cautery-heat to shrink the vessel wall. Induces thrombus.

Sutures/wall closure

Question 34

3 fluid compartments

Answer 34

Intracellular- within cells
Intravascular- in blood vessels
Interstitial- between intact cells (edema)
*Edema should be picked up by lymphatics.

Question 35

Fluid flow among/between compartments is influenced by

Answer 35

Tonicity changes. Changes in osmotic concentrations of ions. (Na+, K+, and Cl-)

Changes in oncotic gradient. Movement from intracellular to interstitial. Especially albumin.

Neuro-endocrine and cytokine response with anesthesia impact both gradients.