Final answer:
The pigment-producing cells responsible for the pigmentation in the posterior portion of the vascular tunic are called melanocytes, which produce the pigment melanin, helping to protect the skin from UV radiation.
Step-by-step explanation:
The posterior portion of the vascular tunic is pigmented due to the presence of pigment-producing cells called melanocytes. These cells are responsible for producing the pigment melanin, which helps protect the skin by absorbing harmful ultraviolet (UV) radiation. Found in the stratum basale of the epidermis, melanocytes generate melanin, which is then transferred to keratinocytes via an organelle called a melanosome.
The amount of melanin produced varies among individuals, influenced by genetics and exposure to UV light, and determines skin color. As melanin absorbs UV light, it prevents the light from causing damage deeper in the skin. Those with more melanin have better natural protection against UV radiation.