Eyeball Department of Histology and Embryology Li jinxin No

The following are emphasized §General structure of the eyeball §Structure of the cornea §Structure and function of the retina

Eyeball §The eyeball consists of two parts:coats of eyeball and its accessory structures. §Coats of eyeball is composed of three layers:fibrous tunic,vascular tunic and retina. §Accessory structures include the aqueous,the lens and the vitreous body.

1 Coats of eyeball cornea Optic nerve vitreous body lens Diagram of eye Sclera Choroid Retina Ciliary body Iris

1.1 Fibrous tunic Fibrous tunic consists of cornea and sclera Cornea The cornea is composed of five layers.

Cornea (a)The corneal epithelium It is a stratified,squamous non- keratinising epithelium. (b)The anterior limiting lamina (c)The corneal stroma It is composed of layers of collagen fibrils arranged in an orderly manner. (d) The posterior limiting lamina (e)The corneal endothelium It is a single layer of cuboidal cells.

The transparency of the cornea is due to a combination of factors: (a)The absence of blood vessels. (b)The non-pigmented epithelium. (c)The regular organisation of collagen fibrils in the corneal stroma. (d)The maintenance of the state of hydration of the ground substance.

1.1.2 Sclera The sclera consists of dense connetive tissue Corneal limbus Scleral spur Sclera venous sinus Trabecular meshwork

1.2 Vascular tunic (uvea) The vascular tunic contains blood vessels and pigments.From anterior to posterior,it divides into three parts: the iris,the ciliary body and the choroid.

1.2.1 Iris The iris is shaped like a circular disc and has a central opening termed the pupil.It divides the space into an anterior chamber and a posterior chamber.

The iris divides into three layers: (a)The anterior layer It is a single layer of fibroblasts and melanocytes. (b)The iris stroma It is composed of loose connective tissue containing blood vessels and pigments. (c)The iris epithelium It is composed of double-layered cells. Myoepithelial cells and pigment epithelium.

1.2.2 Ciliary body It is composed of three parts: (a)The ciliary muscle (b)The ciliary stroma (c)The ciliary epithelium It is a double-layerd cuboidal epithelium. The outer layer is piment epithelium.The inner layer is a single cuboidal unpigment epithelium. The unpigment epithelium can produce aqueous humour.

Iris and ciliary body Ciliary muscle Ciliary stroma Ciliary processes Iris Ciliary epithelium

1.2.2 Choroid It is composed mainly of loose connective tissue which is heavily vascularised and pigmented. Retina Choroid Sclear

1.3 Retina The retina contains four layers of cells. Pigment epithelium Visual cells Bipolar cells Ganglion cells Choroid Sclera

1.3.1 Pigment epithelium This is a single layer of polygonal cells.

Pigment epithelium The apical regions of the cells send out processes which project between the rod and cone processes. The apical cytoplasm contains large quantities of melanin granules and phagosomes. Functions:(a) a protective role in relation to the photoreceptive cells. (b) Phagocytosis. (c) Keeping vitamin A.

1.3.2 Visual cells These are photosensitive bipolar neurons. They can be divided into three parts:cell body,outer process and inner process. The outer processes divide into a outer segment(A) and a inner segment(B). cell body inner process outer process A A B B

Outer segment (A) is a photosensitive part.It consists of abundant membranous disc. Inner segment (B) is the central part of metabolism.It contains mitochodria, rough endoplasmic reticulum, smooth endoplasmic reticulum and free ribosomes. Inner process is axon,it forms synapses with the dendrites of the bipolar cells. A B

Rods Each rod has a cylindrical outer segment. Membranous disc separate from the plasmalemma.It is constantly renewed. The lamellae contain rhodopsin. In function,rods respond to illumination of low intensity.

Cones Cones are similar to rods in appearance but each cone has a conical segment.Most of the membranous disc are continuous with the covering plasma membrane and are not constantly renewed. Cones contain one of three photopigments. They are sensitive to red, green or blue light. In function,cones are sensitive to bright light and colours.

1.3.3 Bipolar cells Each bipolar cell has two processes:an axon and a dendrite. Other cells: Horizontal neurons, Amacrine neurons Muller’s cells.

1.3.4 Ganglion cells These are multipolar neurons.The dendrites synapse with bipolar cells and amacrine cells.The axons run towards the optic disc forming optic nerve.

内界膜 Nerve fibres Ganglion cell Muller’s cell Bipolar cell Photoreceptor cell Pigment epithelium Layers of retina

1.5 Yellow spot In the centre of this yellow spot is termed the central fovea, in which only cones are present.The fovea is the area of most clear vision.

1.6 Optic papilla There are no photoreceptors.It is known as the blind spot.

2 Accessory structures §Aqueous humour §Lens §Vitreous body

Exercise §Statement the structure of retina. §Comparison the structure between the rods and the cones.