Download CARDIOVASCULAR SYSTEM HISTOLOGY

Eye
The eye has three tunics :
1-The sclera and cornea form the outer fibrous tunic .
2- The middle vascular layer (or uvea) consists of the choroid
, ciliary body , and iris ;
3-The retina forms the inner tunic .
Fibrous Layer
This layer includes two major regions :
1)the posterior sclera
and
2)anterior cornea
joined at the limbus
Sclera (s) :
Components :
-Dense irregular connective tissue
-Function :
Supports eye shape
Protects delicate internal structures
The transparent cornea consists of :
1- anterior stratified squamous epithelium on
Bowman’s membrane
2- a thick avascular stroma ,
3- inner endothelium on Descemet’s membrane .
At the circumference of the cornea is the
limbus or corneoscleral junction (CSJ),
where the transparent corneal stroma
merges with the opaque, vascular sclera (S).
The epithelium of the limbus is slightly
thicker than the corneal epithelium,
containing stem cells for the latter, and is
continuous with the conjunctive (C)
covering the anterior sclera and lining the
eyelids.
The stroma of the limbus contains
the scleral venous sinus (SVS), or canal of
Schlemm, which receives aqueous humor
from an adjacent trabecular meshwork at the
surface of the anterior chamber (AC).
Vascular Layer
The eye’s more vascular middle layer, known as the uvea,
consists of three parts, from posterior to anterior:
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1) the choroid,
2) ciliary body,
3) iris
consists of the ciliary body and its anterior
extension, the iris (I). The thick ring of the
ciliary body includes loose connective tissue
containing melanocytes, smooth ciliary
muscle (CM), numerous extensions covered
by epithelium called the ciliary processes
(CP), and the ciliary zonule (CZ), a system of
fibrillin-rich fibers that attach to the capsule
of the lens (L) in the center of the ciliary
body. Pieces of one zonular fiber can be seen
(arrow). Projecting into the posterior
chamber (PC), the ciliary processes produce
aqueous humor that then flows into the
anterior chamber through the pupil. Behind
the ciliary zonule and lens, a thin,
transparent membrane (not shown)
surrounds the vitreous body and separates
the posterior chamber from the vitreous
chamber (VC). X12.5. H&E.
The iris regulates the amount of light to which the retina is exposed. (a) The low-power
micrograph shows a section of the central iris, near the pupil (P). The anterior surface,
exposed to aqueous humor in the anterior chamber (AC), has no epithelium and consists
only of a matted layer of interdigitating fibroblasts and melanocytes. Cells of the external
pigmented epithelium (PE) are very rich in melanin granules to protect the eye’s interior
from an excess of light. Cells of the other layer are myoepithelial, less heavily pigmented,
and comprise the dilator pupillae muscle (DPM) that extends along most of the iris. Near
the pupil, fascicles of smooth muscle make up the sphincter pupillae muscle (SPM). X140.
H&E. The underlying stroma (S) contains many melanocytes with varying amounts of
melanin.
The deep stroma also is richly vascularized (arrows).
The myoepithelial dilator pupillae muscle (DPM) is more easily seen here,
in relation to the sphincter pupillae muscle (SPM) and
posterior pigmented epithelium (PE). X100. PT.
The lens is a transparent, elastic tissue that
focuses light on the retina. Surrounding the
entire lens is a thick, homogenous external
lamina called the lens capsule (LC). The
anterior surface of the lens, beneath the
capsule, is covered by a simple columnar lens
epithelium (LE). Because of its origin as an
embryonic vesicle pinching off of surface
ectoderm, the basal ends of the lens epithelial
cells rest on the capsule and the apical regions
are directed into the lens interior. At the
equator of the lens, near the ciliary zonule, the
epithelial cells proliferate and give rise to cells
that align parallel to the epithelium and
become the lens fibers. Differentiating lens
fibers (DLF) still have their nuclei but are
greatly elongating and filling their cytoplasm
with proteins called crystallins. The mature
lens fibers (MLF) have lost their nuclei and
become densely packed to produce a unique
transparent structure. The lens is difficult to
process histologically and sections usually have
cracks or blebs among the lens fibers. X200.
H&E.
Retina
The retina, the innermost tunic of the eye, develops with
two fundamental sublayers from the inner and outer layers
of embryonic optic cup
The two distinct layers of the retina are the
pigmented epithelium and the photosensitive
neural layer, which are derived from the outer
and inner layers of the optic cup, respectively.
Shown here is the interface between the two
layers. The pigmented epithelium (PE) is of
simple cuboidal cells resting on Bruch’s
membrane inside the choroid (C).
Rod cells and cone cells are neurons with their
nuclei collected in the outer nuclear layer
(ONL) and with axons of one end forming
synapses in an area called the outer plexiform
layer (OPL) and modified dendrites at the
other end serving as photosensitive structures.
These structures have mitochondria-rich inner
segments (IS) and photosensitive outer
segments (OS) with stacks of folded
membranes where the visual pigments are
located.
The inner segments of the rod and cone cells
are attached to elongated glial cells called
Müller cells, which are modified astrocytes of
the retina. The junctional complexes
of these attachments can be seen in light
micrographsas the outer limiting layer (OLL).
The inner nuclear layer (INL), with the cell
‫■■ اطالع فقط‬
bodies of several types of bipolar neurons
which begin to integrate signals from the rod
and cone cells.
■■ The outer plexiform layer (OPL), containing
fibers and synapses of the bipolar neurons and
rod and cone cells.
■■ The outer nuclear layer (ONL), with the cell
bodies and nuclei of the photosensitive rod
and cone cells.
■■ The outer limiting layer (OLL), a line formed
by junctional complexes holding the rod and
cone cells to the intervening Müller cells.
■■ The rod and cone layer (RCL), which
contains the outer segments of these cells
where the photoreceptors are located.
■■ The non-neural pigmented layer (PL), which
has several supportive functions important for
the function and maintenance of the neural
retina. X150. H&E
(a) The eyelid is a pliable tissue with skin
(S) covering its external surface and
smooth conjunctiva (C) lining its inner
surface. At the outer rim of the eyelid are
a series of large
hair follicles (F) for the eyelashes.
Associated with these hair follicles are
small sebaceous glands and modified
apocrine sweat glands. Internally eyelids
contain fascicles of striated
muscle (M) comprising the orbicularis
oculi muscle and closer to the conjunctiva
a thick plate of fibroelastic connective
tissue called the tarsus (T). This tarsal
plate provides structural support for the
eyelid and surrounds a series of large
sebaceous glands, the tarsal glands (TG)
(aka Meibomian glands), with acini
secreting into long central ducts (D) that
empty at the free edge of the eyelids.
X12.5. H&E.
At higher magnification, only the inner aspect
of the eyelid is seen, and it shows that the
conjunctiva (C) is a mucous membrane
consisting of a stratified columnar epithelium
with small cells resembling goblet cells and
resting on a thin lamina propria (LP). Large
cells undergoing typical holocrine secretion
are shown in the tarsal gland acini (TG), and
the
fibrous connective tissue in the tarsus (T)
surrounding the acini. Sebum from these
glands is added to the tear film and helps
lubricate the ocular surface. X200. H&E.
Lacrimal glands secrete most components
of the tear film that moisturizes,
lubricates, and helps protect the eyes.
The glands have acini (A) composed of
secretory cells filled with small, lightstaining granules and myoepithelial cells
(M).
Connective tissue surrounding the acini
contains blood vessels (V) of the
microvasculature
and
intraand
interlobular
ducts (D) converging as excretory ducts
that empty into the superior conjunctival
fornix between the upper eyelid
and the eye. X400. H&E.