Histo Test 2 Cert

Front 1

Muscle fiber

Back 1

individual muscle cell

Front 2

structure of Skeletal muscle fiber

Back 2

multiple nuclei at periphery of the muscle fiber.

1-30 mm in length, 10-100 in diameter

Surrounded by external lamina(basal lamina)

Front 3

Bands

Back 3

A: entire myosin

H: myosin only

I: actin only

Z disc: actin

M line: myosin

Front 4

Triads

Back 4

one T tubule+ 2 terminal cistern(sarcoplasmic reticulum, smooth ER)

1: allow nervous impulse to reach all part of the fiber

2: release Ca2+

Front 5

Function of Connective tissue in skeletal muscle

Back 5

1 Transmit force of contraction to insertion

2 separate into compartments

Front 6

Endo/ peri/ epi- mysium

Back 6

endo: surround each muscle fiber

peri: surround group of fiber, divide into fascicles.

epi: dense connective, surround the entire muscle, continuous with tendon

Front 7

Cardiac m structure

Back 7

Cylindrical, branch, and form interwoven bundles

Front 8

Intercalated disc(junctions)

Back 8

1 Adherent junction

2 gap junction

Front 9

Neuron

Back 9

1 functional unit

2: cell body, dendrites, axon, and synapses

3 complex circuit

Front 10

supportive cell

Back 10

outnumbered

astrocytes, oligodendrocytes and microglia cells in the CNS; comprised of Schwann cells in the PNS

Front 11

Cell body

Back 11

nucleus: highly euchromatic

nissel substance

extensive cytoskeleton support

major synthetic region

Front 12

dendrite

Back 12

highly branched

nissel(like cell body)

form most receptor field

Front 13

axon

Back 13

usually one

devoid nissel

originated from axon hillock

first region is initial segment(rich in channels)

Front 14

pre-synaptic component

Back 14

distal of axon

terminal buton(contains mitochondria and vesicles)

Front 15

post synaptic

Back 15

thickening on its cytoplasmic site

Front 16

Microglia

Back 16

phagocytic cell

derived from mesoderm

Front 17

astrocyte

Back 17

Stellate morphology

1: physical and nutrient support

2: scar formation

3: maintain homeostasis

Front 18

Oligodendrocyte

Back 18

myelin in CNS

Front 19

Schwann cell

Back 19

surround axon

surround cell body in ganglion

Front 20

internode

Back 20

single myelin segment

Front 21

CNS vs PNS myelin

Back 21

CNS: one to many(40-50)

PNS: one to one

Front 22

Ganglia

Back 22

group of cell body in PNS

Front 23

Nerves

Back 23

bundle of axon in PNS

Front 24

Gray matter

Back 24

neuron body and dendrites in CNS

Front 25

White matter

Back 25

also called tracts

bundle of axon in CNS

Front 26

Connective tissue in PNS

Back 26

Endoneurium: surrounding the Schwann cell

peri: around a group of axon

epi: entire nerve

Front 27

Connective tissue in CNS

Back 27

Meninges

1 pia matter

2 arachnoid

3 dura matter

Front 28

Pia matter

Back 28

1 surface of the brain

2 accompany large blood vessel

Front 29

arachnoid

Back 29

1 separated from pia by trabeculae

2 contain blood vessels and CSF(produced by choroid plexus)

3 make the leptomeninges with pia matter

Front 30

Dura matter

Back 30

1 outer most

2 dense connective tissue

Front 31

Simple squamous

Back 31

j

Front 32

simple cuboidal

Back 32

j

Front 33

simple columnar

Back 33

j

Front 34

stratified squamous

Back 34

j

Front 35

stratified cuboidal/columnar

Back 35

j

Front 36

Pseudostratified columnar

Back 36

j

Front 37

transitional

Back 37

j

Front 38

Microvilli

Back 38

j

Front 39

Cilia

Back 39

Front 40

Cilia

Back 40

1 actively motive evagination of luminal plasmalemma, 2- 10 um long

2 axoneme(cilia, 2 w goblet cell, 3 w basal lamina)

3 microtubules 9+2

Front 41

Basal lamina

Back 41

Lamina densa+ lamina lucida

produced by epithelial cell

Type 4 collagen, laminin, proteoglycan

acellular, up to 100 nm

Front 42

Basement membrane

Back 42

Basal lamina + reticular lamina

reticular lamina: produced by connective tissue

Front 43

Cell junction

Back 43

Desmosome

Tight junction

Gap

Front 44

Desmosome

Back 44

help anchor cell to each other, but space maintained

Types:

spot desmosome: 15-30 intercelluar space, tonofilament

hemidesmosome: basal lamina

belt desmosome: zone of adhesion, 10-20, actin filament, terminal web

Front 45

Tight junction

Back 45

1: plasmalemmal fusion

2: functions: adhesion, exclusion, migration of protein.

Front 46

Gap junction

Back 46

1: 2-4 nm gap

2: metabolic and electrical coupling

3: connexons

Front 47

Endothelium- surface

Back 47

negative charge

- repel blood cell and platelet(-)

- attract to basement membrane(+)

Front 48

Endothelium - structure

Back 48

a. simple squamous epithelium lining blood vessels & heart – specialized to mediate and monitor exchange of molecules
b. rests on a basement membrane
c. linked by cell junctions – strongest in arterioles and loosest in venules
d. permeability barrier = endothelium + basement membrane
e. long-lived cells, though replaced by pre-existing cells by mitosis

Front 49

Fiber(blood)

Back 49

1 concentric lamellae

2 internal external elastic laminae

3 collagen

4 elastic fibers

5 smooth muscle

Front 50

Connective cell

Back 50

make fiber

phagocytosis

immunologic

vasoactive

Front 51

Vascular smooth m

Back 51

except in capillaries and some venules

1 in all three tunics

2 orientation: intima and adventitia, longitudinal

3 only thing in tunica media of elastic, main in tunica media of others

4 gap junction- spreads innervation

Front 52

Vasa vasorum

Back 52

small blood vessel > 1mm diameter

visible in tunica adventitia and outer media

intima and inner media: diffusion

Front 53

lymphatics

Back 53

visible in adventitia

Front 54

Nerves

Back 54

except capillaries

a. vasomotor sym of arterioles

b. afferent- baroreceptor and chemo in adventitia

Front 55

Elastic - general

Back 55

1/10 thin wall

high elastic

Front 56

Elastic - intima

Back 56

1/6

tight junction

loose CT

internal elastic lamina

Front 57

Elastic - media

Back 57

1. smooth m between elastic lamellae

2. outer elastic lamina indistinct

3. 40 - 70 elastic lamellae

4. Thickest

Front 58

Elastic/muscular - adventitia

Back 58

1. Thin, unorganized

2. Dense irregular

3. vasa vasorum, lymphatics, nerves.

Front 59

Arteriosclerosis

Back 59

1 thicken intima, incite thrombosis

2 mostly in elastic artery, b/c thin wall and high blood pressure

Front 60

Muscular - intima

Back 60

1 gap and occluding junction between endothelium and loose CT

2 internal elastic lamina

Front 61

Muscular - media

Back 61

smooth m cell dominate

held by gap junction

control blood flow

Front 62

Elastic

Back 62

Elastic (conducting) arteries - aorta and major branches

Front 63

Arterioles and Metarterioles-General

Back 63

General information - <0.1 mm diameter
a. thickness of wall approximates diameter of lumen
b. regulation of blood pressure - account for about 1/2 the resistance and maintain relatively high hydrostatic pressure
c. delivers blood to capillary beds under low pressure

Front 64

Arterioles - intima

Back 64

a. endothelial cells linked by tight junctions and relatively impermeable
b. little or no subendothelial CT and internal elastic lamina very thin

Front 65

Arterioles- media

Back 65

a. 1-3 layers of smooth muscle that are responsive to sympathetic nerves and metabolic stimuli
b. metarterioles (also known as pre-capillary sphincters) lead into capillary beds
- here is where blood pressure is maintained
- opening and closing of lumen controls blood flow to capillary beds

Front 66

Arterioles - pathologies

Back 66

a. arterial hypertension caused by excessive contractile tone in the smooth muscle
b. hemorrhagic shock - normally loss of 500 ml blood will trigger a reactive contraction of arteriolar smooth muscle in order to maintain normal blood pressure, but a loss of blood volume in excess of 500 ml will make maintenance of normal pressure difficult or impossible without immediate replenishment of the fluid (e.g. blood, plasma, saline)
c. anaphylactic shock - decreased blood pressure due to arteriolar muscle paralysis

Front 67

Capillaries -General

Back 67

only endothelium, basement membrane, & an occasional pericyte
a. sites of blood-tissue exchanges
b. inner diameter 5-10 micro-m (1-2 endothelial cells in a cross-section)

Front 68

Continuous capillaries

Back 68

continuous endothelium & basement membrane
a. most common type of capillary
b. occluding junctions between endothelial cells

Front 69

Fenestrated capillaries

Back 69

Attenuated endothelium with continuous basement membrane
a. large fenestrae within endothelial cells, some covered with diaphragms
b. found in tissues where rapid interchange of gases and metabolites

Front 70

Discontinuous capillaries (sinusoids)

Back 70

- discontinuous basement membrane
a. larger diameter (30-40 Micro-m) than other types of capillary
b. large gaps between adjacent endothelial cells
c. flow of blood slower than in other types of capillary

Front 71

Venules-General

Back 71

a. endothelial cells joined loosely by cell junctions and have a continuous basement membrane
b. vessels continuous with a capillary network or an arteriole

Front 72

Pericytic venule

Back 72

also called post-capillary venule
a. 10-50 micro-m lumen diameter, tunica intima only
b. loose organization of cell junctions and gaps between endothelial cells
c. usually possesses one or more pericytes
d. the usual vessel involved in cellular movement into and out of the tissue space

Front 73

Muscular venule

Back 73

- usually associated with arterioles
a. endothelial cells joined by extensive tight and gap junctions
b. tunica media contains 1-3 smooth muscle cells
c. tunica adventitia relatively thick

Front 74

Small Veins

Back 74

0.2-1.0 mm diameter
1. Endothelial cells joined by extensive tight and gap junctions
2. Tunica media contains 2-4 layers of smooth muscle cells

3. Tunica adventitia the thickest layer

Front 75

Medium Veins

Back 75

1-10 mm diameter
1. Endothelial cells joined by extensive tight and gap junctions
2. Tunica intima may form semilunar valves - especially numerous in limbs, prevent gravitational backflow of blood
3. Tunica media contains 4-10 layers of smooth muscle cells
4. Tunica adventitia the thickest layer

Front 76

Large Veins-General

Back 76

a. taken together veins form an important blood reservoir, normally have 4-5 times the volume of blood in corresponding arteries
b. includes major named veins and their main tributaries
c. these vessels are larger than 10 mm
d. walls are thin, approximately 1/20 the lumen's diameter

Front 77

Large Veins-intima

Back 77

a. endothelium held tightly together with tight cell junctions
b. well-developed CT and thick
c. contains numerous longitudinally-oriented smooth muscle cells

Front 78

Large Veins- Media

Back 78

a. relatively thin layer compared to same layer in arteries
b. smooth muscle oriented circularly

Front 79

Large Veins- adventitia

Back 79

a. thickest layer of the wall
b. contains longitudinally-oriented smooth muscle in dense irregular CT
c. smooth muscle strengthens wall and prevents its over distension

Front 80

Endocardium

Back 80

1. Possesses a continuous endothelium with junctions and an underlying basement membrane

2. Subendothelial layer - contains small blood vessels and in specialized myocardial cells of the impulse-conducting system of the heart

3. Cardiac valves

4. Cardiac skeleton

Front 81

Cardiac valves

Back 81

folds of endocardium with a central flat sheet of dense CT - the latter continuous with the dense CT of the annuli fibrosi
a. semilunar valves - tricuspid valves in aortic and pulmonary trunks
b. atrio-ventricular valves - free borders connected to papillary muscles by chordae tendinae
c. valves may be damaged by bacterial endocarditis - causes perforations

Front 82

Cardiac skeleton

Back 82

formed of dense irregular CT
a. annuli fibrosi - rings surrounding each valve and continuous with CT of valve
b. fibrous trigones - two triangular zones between annuli fibrosi
c. septum membranaceum - forms the superior portion of the interventricular septum
d. skeleton may be damaged in cases of rheumatic fever

Front 83

Myocardium

Back 83

1. Myocardium proper

2. Impulse conducting system

Front 84

Myocardium proper

Back 84

a. thinnest in atria and thickest in the left ventricle
b. fibers insert on components of the cardiac skeleton
c. muscle of atria and ventricles completely separate
d. fibers may be injured by reduction in available oxygen (ischemia)
e. muscle can not regenerate after injury, so scar tissue develops

Front 85

Impulse-conducting system

Back 85

a. formed of specialized cardiac muscle cells that coordinate and regulate contractions of atrial and ventricular muscles
b. sino-atrial node in endocardium of right atrium near the superior vena cava
c. impulses pass from SA node fibers next to atrioventricular node in endocardium of right interatrial septum
d. fibers then pass to atrioventricular bundle located within the septum membranaceum

e. Purkinge fibers

Front 86

Purkinge fibers

Back 86

impulses spread between cardiac muscle cells (in the myocardium) via intercalated Purkinge fibers - specific name for fibers which lie in the endocardium, connecting the SA & AV nodes, AV bundle and from the bundle to the myocardium - they appear larger and more vacuolated than normal cardiac muscle cells and conduct impulses much faster as well form discs

Front 87

Epicardium

Back 87

formed of dense irregular CT with adipocytes and visceral pericardium

Front 88

Mechanical

Metabolic

Back 88

mechanical: blood pressure

metabolic: local needs

Front 89

Functions of immune system

Back 89

1. immune surveillance and defense

2. immune tolerance

3. absorb lipids

4. maintain fluid balance

Front 90

Humoral immunity

Back 90

In blood stream

Antibodies against Ag;

Produced by plasma cells derived from B lymphocytes

Front 91

Cellular (cell-mediated) immunity

Back 91

Immunocompetent cells contact, react against, and destroy antigen;

mediated by T lymphocytes

Front 92

features of lymphoid

Back 92

1. Regions where lymphocytes represent the chief cellular constituent
2. Basically lymphocytes with a reticular connective tissue framework
3. Specialized entirely for the defense of the body against foreign substances

Front 93

Reticular Cells

Back 93

a. make reticular fibers
b. phagocytic - act like fixed macrophages
c. of mesodermal origin
d. ensheathe reticular fibers
e. unique ones in thymus - of endodermal origin & do not form reticular fibers

Front 94

Reticular Fibers

Back 94

Type III collagen that stains black with silver stains

4 Functions:

a. support lymphoid elements
b. line lymphatic capillaries (sinuses)
c. filtration - trap Ag and immunocompetent cells
d. provide place for interaction, cell division & differentiation

Front 95

Reticular Cells

Back 95

a. make reticular fibers
b. phagocytic - act like fixed macrophages
c. mesodermal origin
d. ensheathe reticular fibers
e. unique ones in thymus - of endodermal origin & do not form reticular fibers

Front 96

Diffuse lymphatic tissue

Back 96

– found in lamina propria
1. Reticular fibers produced by fibroblasts
2. Lymphocytes in relatively low concentrations

Front 97

Lymphatic nodule

Back 97

1. A rounded collection of tightly packed small lymphocytes on reticular CT
2. Germinal center

3. B dependant

4. Cap of tightly packed small lymphocytes.

5. not encapsulated by CT

6. primary vs secondary?

Front 98

Germinal center

Back 98

1 oval pale staining area

2 contains dividing cells and macrophages

Front 99

Solitary nodules

Back 99

1. Are not encapsulated by dense CT but communicate freely with surrounding non-lymphoid loose connective tissue.
2. May be primary or secondary
3. High endothelial venules (HEV) are frequently in diffuse lymphatic tissue associated with nodules. These are venules having tall cuboidal endothelial cells which allow the escape of lymphocytes from the vessel into the tissue. They may be found in any of the MALT

Front 100

Peyer's patches of ileum

Back 100

Large lymphoid aggregations in the lamina propria on the ab- mesenteric side of the small intestine. They may bulge into the intestinal lumen

Front 101

Appendix

Back 101

a small lumen and very active large lymphoid nodules.

Front 102

Tonsil

Back 102

a. found in fauces - form the ring of Waldeyer
b. consist of paired palatine and lingual tonsils and single pharyngeal
c. all are covered by epithelium on oral and pharyngeal surfaces
d. reach maximum development in childhood
e. sites of lymphocyte production

Front 103

Palatine tonsils

Back 103

a. between glossopalatine and glossopharyngeal arches
b. covered by moist, stratified squamous epithelium which invaginates the
surface to form crypts
c. primary and secondary nodules along crypts
d. capsule and trabeculae form framework
e. post-capillary venules and efferent lymphatic channels in CT

Front 104

Pharyngeal Tonsil (Adenoids)

Back 104

a. unpaired and found on roof and posterior wall of nasopharynx
b. lined on surface with respiratory epithelium (i.e. pseudostratified ciliated columnar with goblet cells)
c. no crypts but folds
d. formed of both diffuse and nodular lymphoid tissue

Front 105

Lingual Tonsils

Back 105

a. paired, on root of tongue
b. each with a single opening and a deep crypt that is surrounded by nodules
c. lined by moist stratified squamous epithelium

Front 106

Lymphatic Channels-general

Back 106

1. A drainage system, the smallest vessels end blindly
2. Found throughout the body except in cartilage, bone, bone marrow, CNS, eye, internal ear, epidermis
3. Convey lymph from extracellular space to venous circulation
4. Flow of lymph caused by breathing and contraction of surrounding skeletal muscle
5. Way that antibodies and lymphocytes return to blood

Front 107

Lymph

Back 107

1. An ultrafiltrate of blood plasma derived from the extracellular space
2. Contains water, electrolytes, plasma proteins, antibodies
3. Lymphocytes and macrophages conveyed in fluid

Front 108

Lymphatic Capillaries

Back 108

1. Variable shape and diameter, lined by endothelial cells
2. Lack continuous basement membrane or is entirely lacking
3. Supported by underlying collagen fibers

Front 109

Lymphatic Vessels and Ducts

Back 109

1. Have large lumens, thick walls and some possess valves
2. Unidirectional flow dependent on valves and muscle contractions
3. Lymph nodes occur along path
4. Ducts possess tunica intima, media and adventitia
a. right lymphatic duct
b. thoracic duct - begins at cisterna chili
5. All lymph returns to the blood-vascular system

Front 110

Lymph Nodes-General

Back 110

1. Occur in series along lymphatic vessels, usually in groups
2. Highly organized lymphoid tissue
3. Function to filter lymph

Front 111

1. General Features

Back 111

a. lymphoid tissue traversed by lymph sinuses lined by reticular cells
b. lymphocytes, macrophages, plasma cells
c. cortex and medulla defined by lymph sinuses

Front 112

2. Stromal Elements

Back 112

a. node surrounded by capsule which are continuous with trabeculae
b. formed by reticular cells and fibers

Front 113

3. Lymph Sinuses

Back 113

a. receive afferent lymphatic vessels into marginal (subcapsular) sinus
b. cortical sinuses arise from marginal sinus and follow trabeculae
c. continue into medulla as medullary sinuses; then continuous with efferent lymph channels
d. lymphocytes exit the node via the efferent lymph channels

Front 114

4. Cortex

Back 114

a. dense masses of lymphatic tissue with trabeculae
b. has primary and secondary lymphoid nodules as well as diffuse lymphoid tissue
c. far cortical area - B cell dependent zone
d. deep cortical area - T cell dependent zone

Front 115

5. Medulla

Back 115

a. medullary cords - formed by a couple layers of small lymphocytes surrounding blood capillaries
b. vessels anastomose and branch
c. spaces between cords are the medullary sinuses

Front 116

6. Blood supply

Back 116

a. blood vessels enter and leave at hilus
b. run in medullary cords and trabeculae
c. post-capillary venules in deep cortex - where lymphocytes enter the node

Front 117

Functions

Back 117

a. filters lymph
b. immunological surveillance
c. lymphocyte production

As lymph flows through lymphatic vessels in route to the blood-vascular system, it is filtered by at least one lymph node

Front 118

Phagocytosis

Back 118

99% of the antigen carried by lymph removed by macrophages spanning the sinuses of lymph nodes.

Front 119

Immune response

Back 119

Only 1% of the antigen entering a lymph node passes through a lymphoid nodule. This antigen stimulates the formation of germinal centers and active cellular proliferation causing enlargement of the lymph node.
Long-lived lymphocytes present in the blood-vascular circulation re-enter the lymph node via HEV to stimulate an immune response.

Front 120

Tumor cell

Back 120

may be trapped in lymph nodes. These cells may then metastasize.

Front 121

Tonsil(3 groups)

Back 121

1.palatine

2. pharyngeal

3. lingual

Front 122

MALT

Back 122

mucosa associated lymphoid tissue

1 GI

2 respiratory

3 reproductive

Front 123

Spleen - general

Back 123

1. Lymphoid organ that functions to clear blood of particulates and dead cells
2. Unique structure allows blood to contact macrophages

Front 124

Functions of the Spleen

Back 124

1. Blood Filtration
a. removes endogenous or foreign material from blood
b. destroys aged RBCs, platelets, and leukocytes in pulp cords
2. Hemoglobin Degradation
a. macrophages of cords metabolize iron and store it as hemosiderin pigment
b. heme degraded to bilirubin and excreted in the bile
3. Immune Activities
a. produce antibodies and participate in immune reactions
b. architecture supports interactions needed for mounting an immune response
4. Hemopoiesis
a. site of RBC production in the fetus
b. produces lymphocytes and macrophages throughout life

Front 125

Spleen - general

Back 125

a. parenchyma composed of white pulp and red pulp
b. nodular and diffuse lymphoid tissue present
c. venous sinuses and splenic cords

Front 126

Spleen- Stromal

Back 126

a. entire organ surrounded by thin capsule that gives rise to trabeculae
b. reticular cells and fibers support parenchyma
c. hilus where blood vessels enter/leave organ
d. large caliber blood vessels run in trabeculae

Front 127

White Pulp (Malpighian corpuscles)

Back 127

a. periarterial lymphoid sheaths (PALS) - lymphoid tissue surrounding a central artery where they leave trabeculae
b. germinal centers often present - B cell dependent areas, antigens cause them to enlarge
c. T cell dependent areas surround germinal centers

Front 128

Red Pulp - network of venous sinuses and cords

Back 128

a. pulp cords (Cords of Billroth)
- spongy cellular mass of reticular CT and cells
- contain all cellular elements of the blood
- macrophages are actively phagocytic
b. marginal zone - adjacent to white pulp
- contains small sinuses
- receives major portion of incoming arterial blood

Front 129

Arteries of spleen

Back 129

a. central artery - usually eccentrically placed arteriole in white pulp
b. penicillar arteries - arterioles that pass into red pulp, usually possess 1-2 layers of surrounding lymphocytes
c. capillaries - 2-3 from each penicillar arteries, layer of phagocytic cells surrounding, how they terminate unclear

Front 130

Venous Sinuses and Veins

Back 130

a. sinuses permeate entire red pulp, lined by flattened reticular cells and have wide lumens
b. walls of sinuses supported by ribs of reticular fibers and fenestrated basal lamina - allows easy passage of cells
c. venous sinuses empty into veins of red pulp, these have endothelial cell with continuous basal lamina
d. splenic vein drains from hilus
e. lymphatics poorly developed, only efferent channels

Front 131

Open circulation

Back 131

a. capillaries open directly into red pulp cords & gradually filters to sinuses
b. why the theory may be true:
- RBCs in cords
- inject dye into splenic artery - goes into cords and later into sinuses
- inject dye into splenic vein - sinuses & cords fill easily; not arteries

Front 132

Closed circulation

Back 132

a. capillaries communicate directly with lumens of venous sinuses
b. why the theory may be true:
- not enough RBCs in cords
- dye experiments may rupture small blood vessels

Front 133

Primary lymphoid organ

Back 133

1 thymus(T cell)

2 bone marrow(B cell, and pre-T cell)

Front 134

Secondary lymphoid organs

Back 134

1 lymphatic nodules- filter tissue fluid

2 lymph nodes - filter lymph

3 spleen - filter blood

Front 135

Cell types of parenchyma

Back 135

1 B cell

2 T

3 immunoblast

4 Ag presenting cells

Front 136

B vs T cells

Back 136

B: 35%, plasma and memory

T: 65%, Helper, killer, memory

Front 137

Immunoblast

Back 137

1 a lymphocyte that has been activated by an antigen

2 form from B and T

Front 138

Antigen presenting cell

Back 138

- monocyte- macrophage(MM) derived

- process AG to lymphocytes

- include: marcophage, epidermal langerhans cells, dendritic cells, epithelial cell of thymus

Front 139

HEVs

Back 139

high endothelial venules

- associated w/ diffuse lymphatic tissue

- allow lymphocytes to escape from vessel