Theca Cell: Structure, Function & Diseases

Theca cell is a type of connective tissue and is found in the ovarian follicle, where it plays important roles in follicle maturation. Under the influence of LH, the cells become thecalutein cells through luteinization, as they are in the corpus luteum. Theca cell tumors and granulosa theca cell tumors are the best known diseases of the tissue type and are among the hormone-producing tumors.

What is the theca cell?

Ovarian follicles are composed of an oocyte and the surrounding follicular epithelial cells, also known as granulosa cells. In addition, the unit contains the connective tissue layers theca interna and externa, collectively known as theca folliculi. Maturing ovarian follicles are composed of correspondingly different types of cells. One cell type of the ovarian follicle is the so-called theca cell, as it is present in the theca folliculi and plays a major role in the growth of the follicle. To be distinguished from the theca cell is the thecalutein cell. These cells are found exclusively in the corpus luteum and develop from theca cells of the ovarian follicle. Theca cells are thus the precursor of thecalutein cells. Luteinization in terms of lipid storage distinguishes mature thecalutein cells from conventional theca cells.

Anatomy and structure

Theca cells are a variant of connective tissue found only in the ovarian follicle. Histologically, motile and resident cells in extracellular collagen matrices or amorphous ground substance form the connective tissue. Extracellular matrices form a three-dimensional meshwork with proteoglycans within the interstitial spaces. The resistant cell-fiber scaffold makes connective tissue nearly resistant to tensile forces and the ground substance distributes compressive forces. Theca cells are differentiated connective tissue that wraps around the ovarian cortex ovarii in a hem-like fashion in the form of theca folliculi and envelops the ovarian follicle at the later stage of maturation. Unlike undifferentiated connective tissue, the specialized and differentiated theca cells are capable of storing and producing substances. For example, thecalutein cells contain stored lipids.

Function and tasks

Theca cells perform various functions during the maturation of the ovarian follicle. They support the growth and final maturation of female follicles by expressing membrane-bound LH receptors. These receptors provide a binding site for luteinizing hormone. The peptide is synthesized in the adenohypophysis and stimulates secretion as well as synthesis of estrogens in the female gonads. LH is a regulatory dominant factor in the second half of the female cycle. In the first half of the cycle, the hormone stimulates the synthesis of estrogens, with a steep increase in secretion towards the middle of the cycle. This LH surge triggers ovulation and stimulates the synthesis of the corpus luteum. With the binding of LH to the LH receptors within the theca cells, the synthesis of steroids is triggered. More specifically, complex formation causes the production of testosterone. The testosterone, under the influence of FSH, is in turn converted within the granulosa cells of the follicles to the estrogen variant estradiol. In addition, theca cells luteinize to thecalutein cells as found in the corpus luteum. Due to the influence of LH, hypertrophy takes place in the theca cells, which leads to the storage of lipids and turns the theca cells of the ovarian follicle into thecalutein cells of the corpus luteum. Basically, the formation of theca cells accompanies the development from primary follicle to secondary follicle. The stage of the tertiary follicle causes differentiation in the cells into functionally and histologically distinct cell layers. In this way, theca interna and theca externa of the ovarian follicle develop. The inner cell layer, theca interna, like the granulosa cells, is responsible for estrogen synthesis in the follicle. The theca externa consists of contractile cells that expel the oocyte from the mature follicle at ovulation.

Diseases

Ovarian tumors are among the hormone-producing tumors and can originate in different types of tissue in the ovary. In addition to granulosa cell tumors, theca cell tumors exist, for example. Mixed forms are called granulosa-theca cell tumors. The tumors from these tissue types produce estrogens and partly androgens and occur frequently in women between 50 and 60 years of age.The mixed form of granulosa cell and theca cell tumors is also called the luteinizing variant of ovarian tumors and is particularly seen in women between 20 and 30 years of age. The tissue type of the tumors allows for prognosis. Apparently, the probability of malignancy is related to the cell type. Granulosa cell tumors, for example, are malignant in up to more than 50 percent of all cases. Theca cell tumors, on the other hand, have a probability of only about twelve percent. Thus, in most cases, pure theca cell tumors are benign tumors of the ovary. According to different sources, the luteinized variant of granulosa theca cell tumors is benign in almost all cases, whereas the conventional granulosa theca cell tumor is malignant with a probability of up to 27 percent. The symptomatology of ovarian tumors arising from degenerated theca cells differs primarily with the age of the patients. Postmenopausal women often experience bleeding as the first symptom. Pre-pubertal girls often develop isosexual pubertas praecox. This means that their sexual characteristics reach full development before puberty. In some cases, the symptoms in this case also affect the skeleton. For the theca cell tumor and the granulosa theca cell variant, the formation of symptoms depends primarily on the hormones produced by the tumor and the extent of hormone production. In addition to estrogens and androgens, the tumors may also produce other hormones in individual cases, which are then detectable in increased concentrations and can throw the entire organism out of balance.