Connective tissue papillae

The BM has an enormous surface area compared to the epithelium due to connective tissue papillae on which the effective diffusional path length may depend. 

Appendageal structures commonly found within the skin are the hairs, hair follicles, associated sebaceous glands, apocrine and eccrine sweat glands, and arrector pili muscles. Hairs are formed by epidermal invaginations. These keratinized structures traverse the dermis and may extend into the hypodermis. The free part of the hair above the surface of the skin is the hair shaft, and the part deep within the dermis is the hair root, which forms an expanded knob-like structure called the hair bulb. This is composed of a matrix of epithelial cells in different stages of differentiation. Hair is composed of three concentric epithelial cell layers the outermost thin cuticle, a densely packed keratinized cortex, and a central medulla of cuboidal cells. The hair follicle consists of four major components (1) internal root sheath (internal root sheath cuticle, granular layer, pale epithelial layer) (2) external root sheath (several layers similar to the epidermis) (3) dermal papilla (connective tissue) and (4) hair matrix (comparable to the stratum basale of the epidermis). 

The palpebral conjunctiva is composed of nonkera-tinized stratified epithelium that decreases in thickness as it proceeds from the eyelid margin. Many mucin-secreting goblet cells are located near the fornix. The epithelium overlies the substantia propria, which consists of delicate connective tissue and blood vessels. Most of the immune system cellular components reside in the substantia propria.The stroma contains lymphocytes, lymphoid follicles, neutrophils, plasma cells, and mast cells, all of which proliferate extensively in conjunctival inflammatory disease. This proliferation leads to the formation of papillae and follicles. 

Specifically, the giant papillae found in VKC consist of dense fibrous tissue (connective tissue hyperplasia) as well as eosinophils, mast cells, basophils, polymorphonuclear leukocytes, lymphocytes, and macrophages. Mucous discharge contains eosinophils. Trantas dots, which appear as elevated white opacities at the limbus, contain eosinophils and epithelial cells. 

The epidermis is a very simple tissue composed entirely of cells. Although dermal papillae do penetrate into it from below it is readily (or potentially) detachable from these connective tissue elements. 

Asada Y et al (2001) 5 alpha-reductase type 2 is constitutively expressed in the dermal papilla and connective tissue sheath of the hair follicle in vivo but not during culture in vitro. J Clin Endocrinol Metab 86(6) 2875-2880... 

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