That part of a multicellular organism outside the cells proper, usually taken to be outside the plasma membranes, and occupied by fluid.

The basic structural and functional unit of all organisms. Includes the plasma membrane and any external encapsulating structures such as the cell wall and cell envelope.

All of the contents of a cell excluding the plasma membrane and nucleus, but including other subcellular structures.

A heterodimeric protein complex that contains a serine protease inhibitor and a protease; formation of the complex inhibits serine protease activity.

A small amount of cytoplasm surrounded by a cell membrane that is generally retained in spermatozoa after spermiogenesis, when the majority of the cytoplasm is phagocytosed by Sertoli cells to produce \residual bodies\. Initially, the droplet is located at the neck just behind the head of an elongated spermatid. During epididymal transit, the cytoplasmic droplet migrates caudally to the annulus at the end of the midpiece; the exact position and time varies by species. The cytoplasmic droplet consists of lipids, lipoproteins, RNAs, a variety of hydrolytic enzymes, receptors, ion channels, and Golgi-derived vesicles. The droplet may be involved in regulatory volume loss (RVD) at ejaculation, and in most species, though not in humans, the cytoplasmic droplet is lost at ejaculation. Note that the cytoplasmic droplet is distinct from \excessive residual cytoplasm\ that sometimes remains in epididymal spermatozoa, particularly when spermiogenesis has been disrupted.