Final answer:
Sinusoid capillaries, distinguished by wide gaps and no basal lamina, are essential for the passage of large molecules and cells in specialized organs such as the liver and bone marrow.
Step-by-step explanation:
Capillaries in which the endothelial cells are separated by wide gaps with no basal lamina are called sinusoid capillaries (or sinusoids). These are the rarest type of capillary, characterized by extensive intercellular gaps and incomplete basement membranes in addition to intercellular clefts and fenestrations. The large intercellular gaps allow for the passage of the largest molecules, including plasma proteins and cells.
Sinusoids are found in specialized locations, such as the liver, spleen, bone marrow, lymph nodes, and various endocrine organs, including the pituitary and adrenal glands. Their structure enables them to support the passage of large molecules and cells, vital for the functions of these organs. For example, the liver sinusoids process materials from the digestive tract and spleen, and release plasma proteins into circulation, while bone marrow sinusoids allow for the entry of newly formed blood cells into the blood supply.