Pollen is a microscopic structure that represents the male reproductive cell of seed-producing plants, including both flowering plants and conifers. These grains, typically measuring between 10 and 200 micrometers in diameter, transport the plant’s genetic material across distances, often via air, water, or animals. Pollen is necessary for sexual reproduction, allowing plants to produce seeds and perpetuate their species. The pollen grain features a complex cellular structure built for survival in harsh external environments.
Anatomy of the Pollen Grain
The pollen grain’s architecture protects its contents with a robust, two-layered wall system. The outer layer, the exine, is composed of sporopollenin, one of the most chemically stable biopolymers known. This durable layer allows the grain to withstand desiccation, high temperatures, and degradation during travel. The exine often features species-specific surface patterns and strategically placed openings called apertures, which serve as predetermined points for germination.
Beneath the exine is the thinner, more flexible intine, a wall primarily made of cellulose and pectin. The intine provides structural support and plays a direct role in germination, expanding to form the pollen tube. Inside the walls, the grain contains two primary cells: the larger vegetative cell and the smaller generative cell. The vegetative cell, or tube cell, contains the nucleus that directs pollen tube growth and provides energy and nutrients for development.
The generative cell holds the plant’s genetic material and floats within the vegetative cell’s cytoplasm. In many flowering plants, the pollen grain is released in this two-celled stage. The generative cell matures after transfer, dividing later to produce the male gametes needed for fertilization.
Pollen’s Essential Function in Fertilization
Fertilization begins when a pollen grain is transferred from the anther to the receptive stigma of a compatible flower, a process known as pollination. Landing on the stigma triggers germination, stimulated by a secreted sugary fluid.
During germination, the intine layer expands through an aperture, creating the slender, elongated pollen tube. The vegetative cell nucleus (tube nucleus) directs the tube’s growth as it burrows down through the style, the stalk connecting the stigma to the ovary. As the tube grows, the generative cell migrates toward the ovule.
The generative cell then undergoes mitosis, dividing to form two non-motile sperm cells. The pollen tube’s growth is guided by chemical signals from the ovule. Once the tube reaches the ovule, it enters through the micropyle, where it bursts to release the two sperm cells, setting the stage for double fertilization.
The Immune System’s Response to Pollen
For humans, interaction with pollen often results in an immune system overreaction known as allergic rhinitis, or hay fever. This reaction occurs because the immune system identifies certain proteins released from the pollen grain as harmful invaders. When a susceptible person is first exposed to allergenic pollen, their immune system undergoes sensitization.
During sensitization, the immune system produces immunoglobulin E (IgE) antibodies designed to target the pollen proteins. These IgE antibodies attach to the surface of specialized immune cells called mast cells, located in tissues like the nasal passages and eyes. Upon subsequent exposure, when pollen proteins bind to these IgE-coated mast cells, the cells are triggered to degranulate.
Degranulation involves the rapid release of chemical mediators, most notably histamine, into the surrounding tissue. Histamine initiates an inflammatory response, which is the body’s attempt to expel the perceived threat. The release of histamine increases blood flow and vascular permeability, leading to the familiar symptoms of hay fever, such as sneezing, a runny nose, and itchy, watery eyes.