As we enter into pollen season, I thought it would be helpful to review some plant biology and give a very broad overview about pollen and the allergens that are produced in the spring. This information will be presented over the next several weeks in my blog.
Pollen, a fine to course powder, contains the microgametophytes of seed plants which will develop into the the male gametes (sperm cells) of these plants. Each pollen grain consists of a few cells which are the vegetative cells and reproductive cells. The reproductive cells has two nuclei, the tube nucleus that forms the pollen tube and the generative nucleus that divides to form two sperm cells. Pollens vary in size from 5 – 200 um.
The wall of the pollen grain has two layers: the exine ( outer layer ) and intine (inner layer). The exine, composed mostly of sporopollenin, is further divided into two layers called the tectum and the foot layer (which is just above the intine). These two layers are separated by the columella, which provides some structural support to the exine. It is the exine that gives pollens grain their unique features and allow identification of the pollen type (grass, tree, weed). These features can include thinning, ridges and pores, which serve as an exit for the pollen contents and allow shrinking and swelling of the grain caused by changes in moisture content. The pollen grain can have furrows which are called colpi (singular: colpus) the orientation of which classify the pollen as colpate or sulcate. Additionally, the exine often bears spines or warts, or is variously sculptured, and the character of the markings is often of value for identifying the type of pollen
The intine is a cellulose rich cell wall
The structure of a pollen grain. Source: www.tutorvista.com
The transfer of pollen grain (male) to the female reproductive structure (pistil) is called pollination. Pollination, i.e. pollen dispersal, is accomplished in two ways: through the wind and by insects. Wind dispersed pollen is called anemophilous and insects dispersed pollen is called entomophilous. For allergy sufferers, anemophilous pollinated plants are the ones that cause seasonal allergy and these are typically trees, grass and weeds. These plants in general do not have flowers or they have very small flowers. Exposure to particular aeroallergen pollens depends on the plants growing in a particular area. In addition, pollen specific factors such as buoyant density, ease of dispersion and profusion effect local pollen concentrations.
Future blog posts will discuss the allergens in grass, weed and trees.