Ocular Tissue Types

Ocular Tissue Types

• The eye encounters light rays, which then pass through the tear film, anterior chamber, and lens to reach the retina.
• The cornea and lens focus light on the retina, which lines the posterior 2/3rds of the globe.
• The choroid, located between the retina and sclera, contains the vascular (blood) supply to the outer retina.

The 4 Main Tissue Types

• Nervous Tissue:
  • Composed of neurons and glia (support cells)
  • Found in the brain, spinal cord, and nerves
  • In the eye, associated with most ocular structures (efferent and afferent pathways)
  • The retina is mostly composed of nervous tissue
• Muscle Tissue:
  • Composed of myocytes (muscle cells)
  • Three types: skeletal, smooth, and cardiac
  • In the eye, skeletal muscle found in extraocular muscles (control eye movements) and some eyelid muscles
  • Smooth muscle found in iris (controls pupil size) and ciliary body (controls lens shape)
• Connective Tissue:
  • Composed of ground substance, fibres, and cells
  • Binds, supports, protects, and insulates the body
  • In the eye, found in the corneal stroma, sclera, and choroid
• Epithelial Tissue:
  • Covers body surfaces, lines glands, and forms secretory portions of glands
  • Not discussed in detail in this text

Connective Tissue Overview

• Connective tissue is largely acellular, with a ground substance, fibres, and cells
• Ground substance: unstructured material that fills the space between cells and contains fibres
• Fibres: collagen, reticular, and elastic fibres
• Cells: fibroblasts, plasma cells, macrophages, and mast cells

Connective Tissue Types

• Specialist Connective Tissue:
  • Cartilage, blood, bone, etc.
  • In the eye, found in the orbit (7 bones)
• Connective Tissue Proper:
  • Two types: loose and dense
  • Loose: aereolar (packing material) and adipose (fat)
  • Dense: regular (tendons and ligaments) and irregular (skin and surrounding internal organs)

Loose Connective Tissue

• Aereolar Connective Tissue:
  • Soft and pliable
  • Traps fluid (causing bruising) and supports, binds, protects, and stores nutrients
  • Found in the stroma of the iris
• Adipose (Fat) Connective Tissue:
  • Like aereolar tissue, but with a greater nutrient-storing ability
  • Found in the orbital cavity, surrounding the eyeball, muscles, nerves, and blood vessels
  • Decreases in the elderly, potentially leading to enophthalmos (eye sinking into the orbit)

Dense Connective Tissue

• Regular Dense Connective Tissue:
  • Highly fibrous, with closely packed bundles of collagen fibres
  • Confers enormous tensile strength (resistance to pulling in a single direction)
  • Found in the corneal stroma, with collagen fibres arranged in ~250 sheets (lamellae) running parallel to the corneal surface
• Irregular Dense Connective Tissue:
  • Thicker collagen fibres, running in all directions
  • Confers strength in all directions
  • Found in the sclera, which maintains the overall shape of the eyeball### Connective Tissue
• Divided into proper and loose or dense types
• Proper connective tissue is divided into loose and dense types
• Loose connective tissue is divided into areolar and adipose types
• Dense connective tissue is divided into regular and irregular types

Epithelial Tissue

• Forms boundaries between different environments
• Epithelial cells are usually polyhedral (often hexagonal) in shape, with strong attachments between adjacent cells (tight junctions)
• Most epithelial tissue is highly regenerative, innervated (supplied by sensory and motor nerve fibers), and avascular (contains no blood vessels)

Covering Epithelia

• Line the free surfaces of the body
• Can be thought of as a bit like the membrane of a cell
• Functions of covering epithelia include protection, absorption, and sensation
• All epithelial sheets rest on a basement membrane, which reinforces the epithelia and helps it resist stretching and tearing
• The side of the epithelium that rests on the basement membrane is called the basal surface, and the other side is called the apical surface
• Some apical surfaces are smooth, but most are covered with microvilli (little "fingers" that increase surface area) or cilia (hairs that are important for intercellular signaling, movement, and sensation)

Classification of Epithelia

• Based on the shape of the cells and the number of layers
• Three primary types of shape: squamous, cuboidal, and columnar
• Two types of layers: simple (one layer) or stratified (multiple layers)
• Simple epithelia are useful when an exchange of substances is required
• Stratified epithelia are for protection, and are always found in areas subject to abrasion
• Stratified epithelia are usually squamous in humans
• Pseudo-stratified epithelia are simple columnar epithelia that look stratified
• Transitional epithelia are cells that are round in shape when the organ is relaxed, but flatten when distended

Advanced Topics

• Keratinized epithelia: the apical layers of skin are composed of dead squamous cells, filled with the protein keratin, making it dry, impervious to water, and an effective barrier against abrasions
• Melanin: pigmented cells in the basal layers of skin, and in the epithelia lining the inside of the eye, which absorbs stray light and improves image quality

Glandular Epithelia

• Form the various glands of the body, which produce and secrete specific products

• Types of glandular secretion: serous (watery), mucous (thick and sticky), and sebaceous (oily)

• Most glands produce only one type of secretion, but some are mixed

• Glands can be endocrine (secrete hormones into the interstitial fluid) or exocrine (secrete products onto the epithelial surface)### Endocrine and Exocrine Glands

• Most glands are multicellular, but some individual hormone-producing cells are scattered throughout the digestive tract lining and in the brain.

• Generally ductless, with varied secretions (amino acids, peptides, glycoproteins, steroids).

Epithelial Tissue: Glandular Epithelia

• Glandular epithelia can be classified into two main types: endocrine and exocrine glands.
• Exocrine glands have ducts that transport secretions to the surface, whereas endocrine glands do not have ducts and secrete hormones directly into the bloodstream.

Unicellular Exocrine Glands

• Most common example is the mucus-secreting goblet cell.
• Found in the epithelium of the trachea and digestive tube (to protect/lubricate).
• Also numerous in the conjunctiva of the eye, where they help form the mucous layer of the tear film.

Multicellular Exocrine Glands

• Consist of groups of secretory cells connected to a free surface by ducts (also composed of epithelial cells).
• Formed through invagination (inward growth) of an epithelial sheet.
• Can be classified into three types based on the number of ducts: simple (one duct), compound (multiple ducts), and tubuloalveolar (a combination of both).

Classification of Multicellular Exocrine Glands

• Based on the shape of their secretory units: tubular (secretory units form tubes), alveolar (secretory units form small hollow cavities), or tubuloalveolar (a combination of both).

Modes of Secretion

• Merocrine: product is released via exocytosis (vesicles fuse with the cellular membrane, releasing contents to the extracellular space).
• Holocrine: whole cell ruptures to release product.
• Apocrine: tip of cell is shed and cell repairs the damage.

Glandular Epithelia Highlights

• Glands can be classified in terms of:
  • Where they release their product (endocrine or exocrine).
  • How many cells they contain (unicellular or multicellular).
  • Method of secretion (merocrine, holocrine, or apocrine).
  • Type of secretion (serous, mucous, sebaceous).