Download lens fi bres

Histology 2016-2017
Department of Anatomy &Histology:
Dr.Rajaa Ali
*************************************************************
Sence organs: Eye IV
1. Cornea .
2. Aqueous humour
It is present in anterior and posterior chambers (described with sclerocorneal
junction).
3. Vitreous body
It is the transparent gel present behind the lens.
It is made of water (99%), hyaluronic acid and collagen fi brils.
4. Lens
Lens is a transparent, fl exible biconvex disc, that alters its refractive power by
changing its curvature.
Its posterior surface is less curved than the anterior surface.
It lies behind the iris and its most convex anterior surface is in contact with the
pupillary margin of the iris.
It is held in position by the suspensory ligament of lens which attaches the equator
of the lens (circumferential border) with the ciliary processes.
Lens comprises three components, namely, capsule, anterior subcapsular epithelium
and lens substance (Fig.).
Capsule—is a homogeneous elastic basement membrane that envelops the entire
lens. It is made of type IV collagen and proteoglycans.
Anterior subcapsular epithelium—is a simple cuboidal epithelium covering only the
anterior surface of lens beneath the capsule. Towards the equator of lens, the
cuboidal cells become columnar and become lens fi bres.
Lens substance—is formed by lens fi bres that are laid concentrically around the
original central core by differentiation and elongation of anterior lens epithelial cells
at the equator. The young fi bres are nucleated, whereas the old fi bres lose their
nuclei.
fig. : lens
With increasing age, the lens loses its elasticity leading to diffi culty in focusing an
object at close range, a condition called presbyopia. This condition is corrected by
wearing glasses with convex lens.
The transparent lens may become opaque (cataract) with increasing age or in
diabetes mellitus. This leads to interference with clear vision. In this condition the
opaque lens is removed and replaced by a synthetic one.
Accessory Structures of Eye
Eyelid (fig. )
Eyelids are two movable folds placed in front of the eye to protect it from injury and
excessive light.
The upper eyelid is larger and more mobile than the lower. The external surface is
covered by thin skin and the internal surface is covered by mucous membrane called
conjunctiva. The free border (mucocutaneous junction) presents many short curved
hairs, the eyelashes, which are arranged in double or triple rows.
The sebaceous glands (glands of Zeis) open into hair follicles.
The ciliary glands of Moll (modifi ed sweat glands) open on the free margin.
Eyelid.
Presence of :
o thin skin lined by stratifi ed squamous keratinized epithelium (epidermis) on
one side;
o palpebral conjunctiva lined by stratifi ed columnar epithelium on the
o other side;
o tarsal plate containing Meibomian glands;
o C.S. of skeletal muscle (orbicularis oculi).
The eyelid has the following layers from superfi cial to deep:
1. Thin skin—covers the external surface and contains fi ne hair follicles.
2. Subcutaneous layer—is very lax and delicate and is devoid of fat.
3. Muscle layer—is formed by skeletal muscle of orbicularis oculi.
4. Loose connective tissue layer—lies deep to the muscle layer and is continuous
with the subaponeurotic layer of scalp.
Effusion of blood can track down into this layer causing black eye.
5. Tarsal plate and Tarsal glands—tarsal plate is a crescent-shaped lamina of
dense fi brous tissue giving support to each eyelid. The tarsal plates are
attached to the orbital margin by orbital septum. Tarsal plate contains 15–20
tarsal (Meibomian) glands arranged in a single row oriented vertically at right
angle to the free margin. The ducts of these glands open on the lid margin.
Tarsal glands are modifi ed sebaceous glands and secrete an oily substance
which prevents evaporation and overfl ow of tears.
6. Palpebral conjunctiva—is the mucous membrane that lines the inner surface
of eyelid and is refl ected on to the sclera at superior and inferior fornices as
ocular conjunctiva. The conjunctiva forms a closed sac when eyelids are
closed. This is called conjunctival sac. The conjunctiva is made of a lining
epithelium supported by vascular connective tissue. The epithelium of
palpebral conjunctiva is stratifi ed columnar epithelium with goblet cells,
whereas that of ocular conjunctiva is stratifi ed squamous epithelium.
Lacrimal Gland (fig.)
o Lacrimal gland is a compound tubuloacinar gland responsible for secretion of
tears.
o Tears contains antibacterial enzyme, lysozyme and electrolytes (similar to
plasma).
o The general architecture of the gland is similar to that of salivary glands.
o The secretory acini have distended lumen and are composed of low columnar
secretory cells of serous type. Well developedmyoepithelial cells surround the
acini and intralobular ducts.
o Lacrimal gland consists of two parts, a larger orbital part situated in the
lacrimal fossa of the orbit and a smaller, palpebral part situated in the upper
eyelid. Both parts are separated from and also become continuous with each
other around the lateral margin of aponeurosis of levator palpebrae superioris.
The gland is drained by 12–15 ducts which arise from the orbital part and pass
through the lacrimal part to drain into the superior conjunctival fornix.
Lacrimal Gland.
Presence of :
o serous acini having distended lumen;
o low columnar epithelial lining and myoepithelial cells.
Fig. :lacrimal gland