Histology Flashcards
what are tissues
aggregates or groups of cells organized to perform specific functions
what are the four basic tissues
epithelial
connective
muscular
nervous
what are the similarites between all tissue types
all contain cells
cells that make up tissues have similar functions
define epithelium
layer(s) of closely apposed cells with a minute amount of intercellular material resting on a basement membrane
general functions of covering epithelial tissues
cover/line body cavities, organs, surfaces, tubes
what the general characteristics and functions of covering epithelial tissues
as an interference surface (protection, absorbtion, filteration, secretion, excretion)
exhibits polarity (apical-superior and basal-inferior surfaces)
avascular but innervated
supported by connective tissue
ability to regenerate
cellular
two types of classification of covering epithelium
on the bases of layers
on the bases of type of cells
describe simple squamous epithelium
single layer of thin flattened cells
ovoid centrally placed nucleus
locations where simple squamous epithelium is present
endothelium (internal surfaces of heart, blood vessel, lymphatics)
mesothelium (lines the pericardial, pleural, peritoneal cavities
alveolar epithelium
describe simple cuboidal epithelium
single layer of cube shaped cells
spherical centrally placed nucleus
hexagonal or polygonal surface view
locations where simple cuboidal epithelium is present
distal tubules of nephron
surface epithelium of ovary
lining of certain ducts of glands
describe simple columnar epithelium
single layer of column shaped cells
basally located oval nucleus
describe a special property of simple columnar non- ciliated epithelium and its locations
may exhibit microvilli to increase SA
lines gall bladder, stomach, intestine
describe a special property of simple columnar ciliated epithelium and its locations
contains cilia (crew cut appearance)- a dark band extending across the cell at the bas e of cilia due to presence of basal body lines uterine tubes and uterus
describe pseudostratified columnar ciliated epithelium
single layer of cells resting on basement membrane
not all cells reach luminal surface
nuclei lie at different levels within the basal2/3 of the epithelium
location of pseudostratified columnar ciliated epithelium
eustachian tubes, nasal cavities, nasopharynx, paranasal sinuses
describe properties and functions of stratified squamous epithelium
main protective layer
number of layers is variable
polyhedral cells which gradually flatten towards the surface layers
shape o nucleus corresponds to shape of cells
no spaces between cells visible under L/M
they protect underlying tissues
special property and location stratified squamous keratinized epithelium
non nucleated
contain keratin protein
epidermis of skin
epithelial lining in parts of oral cavity
special property and location of stratified squamous non-keratinized epithelium
no keratin formation
lines parts of oral cavity, esophagus, oropharynx, lower part of anal canal, vagina
describe special property and location of stratified cuboidal epithelium
2-3 layers
basal layer low columnar
surface layer cuboidal
ducts of sweat glands
describe stratified columnar epithelium and its location
non ciliated
parts of ductus deferens and male urethra/ main ducts of salivary glands/ conjunctival epithelium
describe the properties of cilia
motile extension of cytoplasm covered by cell membrane
contain central pair of microtubules surrounded by central sheath with radial spokes extending from them
nine doublet comprising of A and B subunit surrounding the central sheath
dynein and nexin proteins connect the 9 doublets
A subunit is of 13 tubulin polymers while B subunit is of 10
5-10 micrometer in height 0.2 um in diameter
describe microvilli
fingerlike projections of cytoplasm covered in cell membrane seen n E/M
striated and brush like border
variable number of microvilli are present on each cell
actin core is anchored at the base by terminal web(myosin 2 and tropomyosin) and at the sides by fascin and fimbrin
apical portion is called apex villin
1-2um in height/ 0.1um in diameter
describe stereocilia
long microvilli 30um villin is absent erzin replaces myosin 1 alpha actinin is present thick stem is connected by cytoplasmic bridges
locations of stereocilia
epididymis of ear
taste buds
locations of microvilli
epithelial cells of small intestine
epithelial cells of proximal convoluted tubules of nephron
locations of cilia
respiratory epithelium
epithelial lining of uterine tubes and uterus
write a short note on flagellum
it is a long motile, whip like projection from a cell
only sperm cells have this
avg length is 55um
produces undulating movements that help in the locomotion of the whole cells
describe goblet cell
unicellular gland mucous producing dilated upper part stem like base present in intestinal and respiratory epithelium
epithelium primarily involved in production of specific substances which are extruded from these cells are called glands
glands are classified into
exocrine glands (release their products on the epithelial surface) endocrine ( releae their products into the blood)
exocrine glands are classified on the basis of
morphology
nature of secretory products
mode of secretion
describe the morphology of glands
glands are supported by stroma of connective tissue
connecitve tissue that encloses the gland is called capsule, septa divides the capsule into lobes
thinner lobules further divide it it into lobules
blood nerve and lymphatics run along the connective tissue septa
describe myoepithelial cells
epithelial cells having the ability to contract
secretory protions of some glands are surrounded by these cells
simple tubular gland description and examples
secretory portion is a straight tubule
crypts of lieberkuhns (in small intestine)
simple coiled tubular gland and its examples
secretory portion is coiled and long
sweat glands
simple branched tubular glands description and examples
secretory portion is divided into two or more branches with tubular ends
glands of stomach and endometerial glands of uterus
simple alveolar glands description and examples
rounded ac like secretory portion
mucuos glands of penile urethra
simple branched alveolar glands description and examples
secretory portion consists of branched saclike secretory parts
sebaceous glands
meibomian glands of eyelids
simple tubulo-alveolar glands description and examples
secretory portion consists of branched tubules with sac like alveoli at their distal ends
small salivary glands of oral cavity, mucosal glands of conducting parts of respiratory tract
compound branched tubular glands description and examples
secretory portion is branched in long tubules
submucosal glands of duodenum
compound alveolar glands description and example
several saclike units with small ducts converge at larger ducts
exocrine pancreas
compound tubulo-alveolar glands
ducts of both tubular and alveolar secretory units converge at larger ducts
salivary glands
what is the classification of glands on the basis of mode of secretion
merocrine
apocrine
holocrine
describe merocrine secretion and examples of it
most common form of secretion
secretory product is exocytosed without any loss of cytoplasm
salivary gland, lacimal gland and intestinal gland
describe apocrine secretion and its examples
apical part of cytoplasm is lost along with its secretion
sweat glands located in axilla and perianal area
ceruminous gland of external auditory canal
holocrine secretion description and examples
entire cells laden with secretory molecule disintegrates releasing the secretory products
sebaceous glands
describe mucous secretion and its examples
pink flat peripheral nucleus cellular outline clear lumen is distinct no basal basophilia mucin and glycoproteins released pyloric gland of stomach
serous secretion description and example
thin watery protein rich secretion bluish purple appearance nucleus round and basal cell outline not clear lumen indistinct basal basophilic
what is connective tissue
diverse group of cells embedded in a tissue specific extracellular matrix
what are the components of connective tissue
extracellular matrix
cells
ground substance
classification of connective tissue
embryonic CT
adult CT
classification of embryonic CT
mesenchym
mucous tissue
classification of adult CT
CT proper
cartilage
bon
describe mesencymal CT
primarily in embryo spindle shaped cells of uniform size with processes Gap junctions viscous ground substance few fine collagen fiber
describe mucous/mucoid CT
spindle shaped cells widely separated
gelatinous ground substance
wispy collagen fibers
adult pulp of tooth vitreous chamber of eye nucleus pulposus
classification of CT proper
loose CT
dense CT
classification of loose CT
areolar
reticular
name the cells of CT
fibroblast macrophages mast cells adipose cells plasma cells wandering cells
describe fibroblast according to their forms and their function
inactive: large / somewhat flattened/ cytoplasmic process/ nucleus is small and deeply basophilic. cytoplasm i slightly eosinophilic
active: cell is enlarged/ more cytoplasmic processes/ nucleus is larger/ cytoplasm is slightly basophilic
produce fibers and ground substance for growth and repair
describe macrophages and their functions
phagocytic cells derived from monocytes
irregular shape
short cytoplasmic processes
carry out phagocytosis
antigen presenting cells
describe mast cells and their functions
large cells with basophilic granules containing histamine and heparin
show metachromasia in toludine dyes
absent in CNS
describe fat cells and their functions
large oval cells with signet ring appearance
nucleus is flat and pressed against the cell membrane
specialized to store triglyceride vesicle(s)
in histologic preparation fat is dissolved
white adipose stores fat
brown adipose helps maintain body temperature in newborns
describe plasma cells and their functions
B- lymphocytes
large ovoid cells with basophilic cytoplasm due to extensive RER
nucleus is spherical and eccentric and chromatin is coarsely arranged in a regular manner against nuclear membrane giving the cell a cart-wheel or clock face pattern
it produces antibodies
what are type of fibers found in CT
collagen
reticular
elastic
describe the structure and classification of collagen fibers
made up of collagen proteins (more abundant protein)
formed from tropocollagen (300nm L and 1.5 nm D)
tropocollagen formed of three alpha chains wound together in a helical structure
there are 14 types of collagen (type 1-14)
collagen type 1-3 form fibers while others a non-fibrillar
significance of type I collagen
most abundant variety of collagen 2-10um in diameter striations every 64nm found in dermis/ bones/ fasciae/organ capsule eosinophilic appearance
significance of type 2 collagen
found in hyaline and elastic cartilage/ vitreous body of eye
significance of type 3 collagen
makes reticular fibers
significance of type 4 collagen
found in basement membrane
write a short note on elastic fibers
made up of elastin
elastic property is due to random coiling of molecules
yellow color-highly elastic fibers form loose networks
thinner branching
found in ligamentum flavum / lungs and large arteries
significance of type 5 collagen
found mainly inn fetal membranes and placenta
write a short note on reticular fibers
made of type 3 collagen
stain black with silver dyes
form thin networks in highly cellular organs
found in blood vessels/intestine/uterus/urinary bladder
agryophilic
what is ground substance
amorphous, transparent, semi-fluid gel like substance composed of GAGs, proteoglycans, glycoproteins in which cells and fibers are embedded
types of GAGs
hyaluronic acid: in loose supporting tissue
sulphate group attached: chondrotin 4, 6/ dermatan/ heparan/ heparin/ keratan
describe GAGs
long unbranched polysaccharide chains of seven different types of repeating disaccharide units
they are acidic due to hydroxyl, carboxyl, sulphate groups attached to them
write a short note on proteoglycans
proteins which bind to GAGs
write a short note of glycoproteins
proteins attached to polysaccharide
adhesve property
provides strength, ridgidity to ground substance
they form binding sites for cell adhesion molecules and ECM molecules
examples include: fibrillin, fibronectin ,laminin, enactin
what is the function of ground substance
mechanical and structural support fills spaces lubricant barrier to large molecules facilitates cell growth and information exchange
general characteristics and example of loose CT
cellular
viscous gel lke ground substance
thin mesh work of collagen and elastin fiber
highly vascular
superficial fascia beneath epithelium
biological packing material for tissues and organs
bind tissues and organs while allowing a considerable degree of mobility
forms stroma of most organs
describe reticular CT and its function
consists of reticular fibers and reticular cells
reticular cells are stellate shaped and have long cytoplasmic processes and are lodged within meshwork of reticular fibers
form the supporting framework of liver spleen, bone marrow and lymphoid organs
some reticular cell posses fibroblastic activity while others give rise to erythrocytes and leukocytes
what is adipose tissue
it is composed of adipocytes organized into lobules separated by fibrous septa
describe white adipose tissue
UnilocularAdipocytes (Signet Ring) Spherical Polyhedral shape, Nucleus Flat Condensed Lipid + Vimentin Organic Solvent Xylene Energy Storage /Insulation & Cushioning Of Vital Organs Lipomas & Liposarcomas
describe brown adipose tissue
smaller multilocular
nucleus round eccentric
general characteristics of dense connective tissue
dense packing of fibers
few cell
less ground substance
limited vascular supply
dense CT is classified into
regular
irregular
describe dense regular CT
densely packed fibers arranged parallel to each other
flattened elongated fibroblasts present between fibers
occur in the from of tendons, ligaments and aponeurosis
great tensile strength
describe dense irregular connective tissue
occurs in the form of sheets
thick bundles of fibers run in all directions
occur in areas subject to mechanical stress
some fibroblasts and macrophages are found between fibers
mostly collagen fibers are present but other fibers are also there
form dermis of skin/ capsules of organs(liver, lymph nodes/ perichondrium/ periosteum
state general characteristics of tendon
CORD LIKE WHITE INEXTENSIBLE CELLS IN ROWS // FIBER TENDON CELL ENDOTENDINEUM,EPITENDINEUM
describe general characteristics of cartilage
Cartilage is a resilient and smooth elastic tissue that can be found in joints and between bones, rib cage, intervertebral discs, ear, nose etc.
It is not as stiff as bone and much more flexible and elastic.
It is avascular
Cartilage is surrounded by fibrous membrane known as perichondrium which is similar to periosteum in structure and function
what is perichondrium and describe its layers
Dense layer of fibrous connective tissue covering the cartilage.
Outer fibrous layer. This dense membrane of connective tissue contains fibroblast
cells that produce collagen.
Inner chondrogenic layer. This layer contains fibroblast cells that produce
Chondroblasts and chondrocytes (cartilage cells)
which cartilages dont contain perichondrium
hyaline articular cartilage and fibrocartilage
describe chondroblasts
produce the matrix of the cartilage, gets trapped in ECM and become less active in producing ECM, therefore become chondrocytes
describe chondrocytes
Chondrocytes produce all the structural components of the cartilage such as
collagen, proteoglycans, glycosaminoglycan and helps in repair of matrix
what are the types of cartilages
Hyaline Cartilage
Elastic cartilage
Fibrocartilage
what are the characteristics of hyaline cartilage
It has thin fibres which are not seen in matrix.
Have bluish glassy appearance because of presence of Chondroitin
Sulphate.
It is quite flexible
Contains type ll collagen
Precursor of bone.
Example: Trachea, nose, larynx, ribs
what are the characteristics of elastic cartilage
It has numerous yellow elastic fibres. Colour is yellow and appearance is opaque. Contains type ll collagen Maintains the shape of the structure. Example: epiglottis, external ear
describe the general characteristics and name examples of fibrocartilage
Type of cartilage with numerous white fibres
Colour is glistening white and appearance is opaque
Only cartilage that contains Type l in addition to Type ll collagen
Example: Symphysis pubis, ligaments
what are the general properties of muscle cells
excitability
extensibility
elasticity
contraction
muscles are classified on the basis of ——-
striated
non striated
striated muscles include
skeletal muscle
cardiac muscle
visceral striatedd
describe the characteristics of skeletal muscle cell
mesenchymal in origin variable length (1mm-35cm) CT supplies nerves and blood vessels diameter varies (10-100um) cylindrical shape with rounded ends multinucleated/ ovoid shape/ peripherally located close to the sarcolemma cytoplasm filled with myofibrils running parallel to the cell axis striations seen
describe characteristics of skeletal muscle
muscle fibers grouped into bundles called fasciculi
muscle fiber is surrounded by endomysium
fasciculi separated form each other by perimysium
whole muscle is surrounded by dense sheath of supporting tissue called epimysium
describe the characteristics of myofibril
show alternating light and dark bands
composed of thin (actin 5-7nm diameter and 1.5 um length) and thick filaments (myosin 12-16nm diameter and 1.5um length)
myofibrils arranged parallel to the axis of myofibrils
M line is where mysoin filaments are centrally attached
Z line is where actin is centrally attached
myosin is attached to z line by titin
area between two Z lines is called sarcomere
Central area of sarcomere containing myoin filaments is called A band
while the area bisceted by Z line free of mysoin is called I band
the area where actin and mysoin dont overlap is called H band
describe the structure of actin filament
two actin chain round into a double helix
tropomysoin is wound around actin helix and rest in the double helix groves
tropomyosin masks actin binding sites
troponin complex has three subunits I, C, T
functions of troponin subunits
I subunit for interaction between actin and troponin
C subunit for binding of calcium
T subunit for binding to tropomyosin
describe the structure of mysoin filament
mysoin filaments are made up of myosin proteins
myosin protein has 2 heavy and two light pairs
heavy chain forms the head and the arm of myosin
light chain stablizes the head
describe key characteristics of sarcoplasmic reticulum and sarcolemma
t-tubules: sarcolemma form tubular extension of ECF which transverse into muscle fiber
between T-tubules is the sarcoplasmic reticulum which forms a cistern on each side of the T-tubule called terminal cisternae
the T-tubule along with the terminal cisternae is called triad
sarcoplasmic reticulum stares calcium ions
describe the blood supply to skeletal muscles
large blood vessels enter epimysium, divide ramify throughout the perimysium, branches from the perimysial arteries transverse along the long axis of muscle fibers (capillary beds from around the muscle fiber)
describe nerve supply to the skeletal muscle
innervated by large motor neurons
a single motor nerve fiber and muscle fibers supplied by it are referred to as motor unit
difference between slow and fast muscle fibers
slow/type1/ red muscle fiber:
perform aerobic work slow fibers abundance of myoglobin small in cross-section numerous mitochondria rich in blood supply resistant to fatigue slow contraction bulky muscle of thigh and leg
fast/white/type 2
anaerobic work fast fibers less myoglobin large in cross-section contain less mitochondria relatively poor blood supply fatigue rapidly rapid contraction triceps' and biceps
describe cardiac muscles
involuntary but striated
branched syncitium with intercalated discs
collagen tissue supports extremely rich capillary network
describe cardiac muscle fibers
striation similar to skeletal muscle
length is about 100um and 15um diameter
single or two nuclei preset centrally
sarcoplasm is more abundant
myofibril form a branched myofibrillar network separated by rows of cytoplasm containing rows of mitochondria and sarcoplasmic reticulum
fat, glycogen and lipofuscin pigment granules seen
describe the sarcoplasmic reticulum and T-tubules in cardiac muscle cells
T-tubules lie at the level of Z line
they have greater diameter
terminal cisternae are not formed so triad is not well formed
cisterns narrower with frequent anastomoses
describe the structure of intercalated discs
darkly staining areas (o.5-1um) thick
mark the boundary of cell
interdigitate with other cells
always concede with Z line
how does the structure of intercalated discs relate to the functioning of cardiac muscle cells
these discs bind the cells
transmit forces of contraction
provide area of low electrical resistance for rapid spread of action potential throughout the myocardium
which types of cell junctions are present in intercalated discs
fascia adherens
macula adherens (desmosomes)
gap (nexus) junctions
describe fascia adherens
actin filament at the end of terminal sarcomeres insert into fascia adherens and transmit contractile forces from cell to cell
describe macula adheren
are less frequent and provide anchorage for intermideate filaments of cytoplasm
describe gap junctions
are sites of low electrical resistance through which impulse pass rapidly from cell to cell
describe blood supply of cardiac muscles
more extensively supplied than skeletal muscles
an extremely rich network of capillaries surround the muscle
describe nerve supply of cardiac muscle
supplies by sympathetic and parasympathetic nerves of autonomic nervous system
describe the differences between skeletal muscles and cardiac muscles
SK: cylindrical shape well defined striations intercalated disc absent less mitochondria T tubules A I band terminal cisternae are dilated Triad present
CM: branched syncitium striations present single central nucleus intercalated disc present mitochondria are more T tubules are present near Z line terminal cisternae are absent Diad present
describe the structure of smooth muscle
donot show striations
found in blood vessels and viscera
describe smooth muscle cell
spindle shape
diameter 3-8um
length 15-500um
ovoid nucleus located in the central
usually arranged in sheets
densely packed parallel to each other
no myofibrils are present
myofilaments are longtitudinally arranged in cytoplasm and are not as orderly as in skeletal muscle
mechanism of contraction is the same as in skeltal muscle
instead of Z lines there are dense bodies contiaing alpha actin
organelles are mainly concentrated around nucleus
no T tubules but cavelae present in sarcolemma
describe nerve supply of smooth muscle cells
multiunitary tye: rich nerve supply- nearly all muscle cells nrecievnerve terminals
example muscles of iris, ductus differens , large arteries unitary : much fewer nerve terminals. adjacent smooth muscle fibers joined together by gap junctions so implses pass from one cell to the next
they are innervated by autonomic nervous system
describe blood supply to smooth muscle
not s richly supplied as striated muscle, which is why they appear pinkish white
nervous system is classified into
CNS (brain and spinal cord)
PNS (all other nerves)
nervous system is functionally divided into
somatic nervous system (voluntary functions)
autonomic nervous system (control involuntary functions)
describe the characteristics of nervous tissue
nerve cell (structural and functional unit of nervous system) and supporting cells (protect, nourish , maintain neuron)
describe the general characteristics of neuron
cell body (soma/perikaryon) processes (axon/dendrites)
describe the classifications of neurons
morphological
functional
according to length of axon
describe morphological classification of neurons
unipolar: only one process (axon) e.g. mesencephalic nucleus of 5 cranial nerve
bipolar: have two process on opposite sides(axon and dendrite) e.g. cochlear neurons , vestibular ganglia, retina and olfactory epithelium
pseudo-unipolar: one process that divides into two branches close to the cell body (axon-CNS/ dendron-periphery) e.g. dorsal root ganglia neurons
multipolar: single axon and multiple dendrites e.g. pyramidal cells of cerebral cortex, purkinji cells of cerebellar cortex, anterior horn cells of spinal cord
describe function classification of neurons
sensory neuron: receive sensory stimuli and pass them to CNS
motor neurons: controls the effector such as muscles and glands
interneurons: connect neurons to form complex functional circuits
describe the classification of neruon on the basis of length of axons
gogi type 1: long axon that leave the gray matter and passes to other regions of CNS via white matter or leaves the CNS to become a peripheral nerve e.g. Pyramidal cells of cerebral cortex, anterior horns of spinal cord
golgi type 2: short axons which donot leave the part of gray matter in which the cell body of neuron lies . these are numerous in cerebral and cerebellar cortex and most function as interneurons
describe cell body of neuron
contains nucleus
nucleus is surrounded by cytoplasm
soma may be 4um in diamter (granule cells of cerebellar cortex) or as large as 135um (anterior horn of spinal cord)
shape varies according to number, orientationn of its processes
globular in pseudo-unipolar
fusiform in bipolar
stellate in multipolar
describe nucleus of neuron
single large pale staining centrally placed nucleolus is prominent
describe cytoplasm of neuron
contains organelles
basophilic nissl granules (RER and ribosomes) present except in axon hillock
microfilaments and neurofilaments are present
describe the properties of axon
single process
uniform contour
variable length
cytoplasm of axon is axoplasm
plasma membrane of axon is axolemma
side branches are called collaterals
axon dividing into multiple branches is called telodendria
axon collaterals and terodendria from small knob at their ends called terminal buttons
terminal button form synapses
golgi and nissl substances are absent
contains mitochondria, neurofilaments, microtubules, SER
describe the properties of dendrites
afferent process
contain all soma contents except golgi body
present at the sites ofsynaptic contacts
describe mylelinated nerve fibers
nerve fibers covered with myelin sheath (composed of lipoprotein myelin)
it is formed by schwann cells in PNS and oligodendrocytes in CNS around he axon
contains nodes of Ranvier
internode is the area between two nodes
what is the fnction of myelin sheath
provides insulation to axon
speeds up the rate of conduction by allowing action potentials to jump from one node to the next (saltatory conduction )
describe non myelinated nerve fibers
not convered by myelin sheath
in PNS unmyelinated axons lie in clefts formed by invaginations of schewann cell plasma membrane by the axons
a single schewann cell cell lodges many axons
a ingle unmyelinated nerve axon passes through clefts of a number of schewann cells
in CNS unmyelinated axons are not ensheathed an run fee
describe the classifcation of synapses
axodendritic : between axon and dendrite
axosomatic : between axon and soma
axoaxonic : between two axons
dendrodendritic : between two dendrites
describe the structure of nerve
nerve fiberes are enclosed within endoneurium
pernurium enclosed fascicles of nerve fibers
endoneurium encircle the whole nerve and contain perineurium
describe neuroglia and their types
they are supporting cells of CNS
they are of two types
neuroglia proper and ependyma
describe the type of neuroglia proper
astrocytes
protoplasmic astrocytes
fibrous astrocytes
describe the properties of astrocytes
Largest, most numerous Long processes expanded pedicles Protoplsmic & fibrous Spherical central euchromatic nucleus Lightly stained cytoplasm Functions Supportive Cover bare areas (nodes of ranvier, synaptic clef Scar tissue Movement of metabolite & waste Maintain chemical environment Regulation of extracellular environment Blood brain barrier
describe protoplasmic astrocytes
grey matter
short thick processes with many branches
describe fibrous astrocytes
white matter
long slender smooth
fewer processes
scarring cells
describe oligodendrocytes
both gray and white matter
myelination of CNS
produces myelin for several cells
describe microglia
mesenchymal in origin
short banching processes
phagocyticcells
describe ependymal cells
simple cuboidal epithelial cells that line the cavities of brain and spinal cord
apical surfaces possess microvilli
they are fluid transporting cell
from junctional complexes
