Theca Cells vs. Granulosa Cells: Key Differences & Functions

Ovarian follicles are the fundamental units of the ovary, each containing an oocyte, or egg, surrounded by specialized cells. The development of this oocyte is dependent on the coordinated actions of different cell types within the follicle. This cellular teamwork ensures that an egg can properly mature and that the necessary hormones are produced to regulate the menstrual cycle.

Theca Cells: The Outer Architects of the Follicle

Theca cells form the outer layer of the developing ovarian follicle, creating a structural framework and performing endocrine functions. These cells are organized into two distinct layers: the theca externa, a fibrous capsule of connective tissue, and the theca interna, the hormonally active layer. Theca interna cells possess receptors for Luteinizing Hormone (LH), a gonadotropin released from the pituitary gland.

When stimulated by LH, theca cells absorb cholesterol from the bloodstream and convert it into androgens, primarily androstenedione and testosterone. These androgens do not remain within the theca cells; instead, they serve as precursor molecules that are subsequently transferred to the adjacent cell layer for further processing. This production of androgens is the primary contribution of theca cells to follicular hormone synthesis.

Granulosa Cells: Nurturing the Future Egg

Granulosa cells form the inner layer of the follicle, immediately surrounding the oocyte. They are directly responsible for the well-being and maturation of the egg, providing it with nutrients through gap junctions. In the early stages of follicular development, these cells are responsive to Follicle-Stimulating Hormone (FSH), another gonadotropin from the pituitary. FSH stimulation promotes their proliferation, causing the follicle to grow in size.

A defining function of granulosa cells is the conversion of androgens into estrogens, using an enzyme called aromatase to transform androstenedione and testosterone from theca cells into estradiol. This process is driven by FSH. As the follicle matures, granulosa cells also develop LH receptors, allowing them to contribute to progesterone production, particularly just before ovulation. They also secrete other substances, such as inhibin, which helps regulate pituitary hormone release.

A Dynamic Duo: How Theca and Granulosa Cells Collaborate

The production of estrogen relies on the partnership between theca and granulosa cells, an interaction described by the “two-cell, two-gonadotropin” model. According to this model, neither cell type can produce estrogen independently. The process begins when LH stimulates theca cells to synthesize androgens from cholesterol. These androgens then diffuse to the granulosa cells, where FSH stimulates the aromatase enzyme to convert them into estradiol.

This division of labor is necessary because theca cells lack aromatase, while granulosa cells lack the enzymes to produce androgens from cholesterol in early follicular stages. This collaboration ensures a synchronized and efficient hormonal output.

When Balance is Lost: Theca and Granulosa Cells in Ovarian Conditions

The regulated interplay between theca and granulosa cells is a requirement for normal ovarian function, and disruptions can lead to various medical conditions. A prominent example is Polycystic Ovary Syndrome (PCOS), a common endocrine disorder affecting women of reproductive age. In many cases of PCOS, theca cells become overactive, producing an excess of androgens. This hyperandrogenism can disrupt follicular development, leading to the formation of many small, arrested follicles and contributing to irregular menstrual cycles.

This imbalance can also affect granulosa cell function. Altered hormonal signals in conditions like PCOS can impair the ability of granulosa cells to effectively convert androgens to estrogens, further disrupting the hormonal feedback loops that govern the reproductive system. Understanding the functional status of both cell types is therefore a part of diagnosing and managing certain forms of infertility and ovarian dysfunction. Therapeutic strategies for these conditions may involve targeting the hormonal pathways that influence theca and granulosa cell activity to restore a more balanced follicular environment.