theca

Theca is a term derived from the Greek thekē, meaning case or sheath. In biology and medicine, theca typically denotes a sheath, capsule, or covering around a body or structure. The use of the term varies by anatomical region and organism, but it generally refers to a protective or enclosing layer.

In anatomy, the thecal sac refers to a dural sheath that surrounds the spinal cord and cauda equina, forming a compartment that contains cerebrospinal fluid. The calvarial and vertebral coverings and the surrounding meningeal layers contribute to the appearance and function of the thecal space and its contents.

The most widely described application is in ovarian physiology. The theca folliculi surround the developing ovarian follicle and are divided into two layers: the theca interna and thetheca externa. The theca interna consists of steroidogenic cells that produce androgens under luteinizing hormone (LH) stimulation, which diffuse to adjacent granulosa cells. There, aromatase converts them to estrogens under follicle-stimulating hormone (FSH) influence. The theca externa is primarily a fibrous connective tissue capsule. Together, the thecal layers interact with granulosa cells to regulate estrogen production during follicular development.

In other contexts, the term theca is used in botany and mycology to denote a protective case or covering around reproductive structures, though usage varies by group.

See also: thecal structures; thecal sac; ovarian theca interna and theca externa.