Mouse Ovary Anatomy: Structure, Follicles, and Hormones

The mouse ovary plays a central role in reproduction, housing developing oocytes and producing hormones essential for fertility. Its structure changes throughout the estrous cycle as follicles mature and regress. Studying its anatomy provides insights into reproductive biology and medical research.

Location And External Structure

The mouse ovary is a paired organ located in the dorsal abdominal cavity near the kidneys. This positioning reflects its embryological development from the urogenital ridge. Suspended by the mesovarium, a segment of the broad ligament, the ovary connects to the reproductive tract through the oviduct. This ligament provides structural support and serves as a conduit for blood vessels, lymphatics, and nerves.

Encased in a thin tunica albuginea, the external surface appears smooth in prepubertal mice but becomes irregular with age due to follicular rupture and corpus luteum formation. The tunica albuginea provides mechanical protection while allowing for the dynamic changes of folliculogenesis. Overlying this layer is a single layer of cuboidal epithelium, often called the germinal epithelium, which does not directly contribute to germ cell production but plays a role in ovarian surface remodeling after ovulation.

The mouse ovary is relatively small, measuring about 1–2 mm in diameter. Despite its size, it exhibits functional complexity. In sexually mature females, developing follicles or corpora lutea create visible protrusions on the surface, offering a visual indicator of ovarian activity useful in experimental settings.

Ovarian Layers (Cortex And Medulla)

The mouse ovary consists of two distinct regions: the cortex and the medulla. The cortex, the outermost portion, is densely populated with follicles at various developmental stages. This region contains a specialized stromal matrix that provides structural integrity and supports follicular growth and ovulation. Primordial follicles, the earliest oocyte-containing structures, are located at the periphery, surrounded by a single layer of flattened granulosa cells. As follicles develop, they move inward, gaining additional granulosa and theca cell layers that contribute to hormone production. The cortical stroma includes fibroblast-like cells and extracellular matrix components that regulate follicular expansion.

Beneath the cortex, the medulla contains blood vessels, lymphatics, and nerve fibers within a loose connective tissue matrix. This vascularized region ensures efficient hormonal exchange and oxygenation, essential for follicular maturation and corpus luteum function. Autonomic nerve fibers in the medulla modulate ovarian responsiveness to hormonal fluctuations and physiological stressors. The medullary stroma also houses interstitial cells involved in androgen production, which serves as a precursor for estrogen synthesis in developing follicles.

Follicular Stages

Follicular development in the mouse ovary progresses from dormant primordial follicles to fully mature preovulatory follicles. This process, known as folliculogenesis, is regulated by cellular interactions and endocrine signals that guide oocyte selection and growth.

Each follicle begins as a primordial structure with a single layer of flattened granulosa cells surrounding the oocyte. These follicles remain quiescent until recruited into the growing pool, influenced by ovarian and systemic hormonal cues. The transition to a primary follicle marks the first morphological change, as granulosa cells become cuboidal.

Secondary follicles develop additional granulosa layers and a theca layer, which plays a key role in steroid hormone synthesis. The formation of a fluid-filled antrum distinguishes tertiary, or antral, follicles, marking a critical phase in maturation. The antral cavity contains growth factors, hormones, and nutrients essential for oocyte development. Only a subset of follicles continues developing, while others undergo atresia, a degenerative process that eliminates non-viable follicles.

Corpus Luteum

After ovulation, the ruptured follicle transforms into the corpus luteum, a temporary endocrine structure essential for regulating the estrous cycle. This process, called luteinization, is driven by a surge in luteinizing hormone (LH), which triggers cellular remodeling. Granulosa and theca cells differentiate into luteal cells, producing progesterone to maintain uterine receptivity. The corpus luteum is highly vascularized to facilitate hormone transport.

In mice, the luteal phase is short-lived unless pregnancy occurs. If fertilization does not happen, declining LH levels initiate luteolysis, leading to reduced progesterone synthesis and increased apoptosis of luteal cells. Prostaglandin F2α disrupts luteal vascularization, accelerating tissue breakdown. Macrophages clear the remnants, allowing for subsequent follicular cycles. If pregnancy occurs, embryonic signals extend corpus luteum function to sustain progesterone production for early gestation.

Blood Supply And Innervation

The vascular and neural networks of the mouse ovary support folliculogenesis, ovulation, and hormone secretion. These systems ensure adequate oxygen and nutrients while enabling rapid endocrine signaling.

The ovarian artery, branching from the aorta, supplies blood through the mesovarium into the medulla, forming a capillary network that supports both cortical and medullary regions. The corpus luteum has particularly high vascularization to sustain progesterone production. Venous drainage occurs via the ovarian vein, which exhibits asymmetry in rodents—the right ovarian vein drains into the caudal vena cava, while the left drains into the renal vein. This arrangement influences hormone distribution in circulation.

Autonomic nerve fibers enter through the mesovarium alongside blood vessels. Sympathetic fibers from the superior ovarian nerve regulate blood flow and interact with ovarian cells to influence follicular development and steroidogenesis. Parasympathetic innervation, though less prominent, contributes to local neurotransmitter signaling. Sensory nerves detect mechanical and hormonal changes, relaying signals to the central nervous system. This neural integration allows the ovary to respond to physiological stressors and external cues.

Key Hormonal Functions

The endocrine activity of the mouse ovary is governed by hormones that regulate follicular development, ovulation, and luteal function. These hormones also influence reproductive behavior and overall physiological homeostasis.

Estrogen, primarily synthesized by granulosa cells, promotes follicular growth and prepares the reproductive tract for fertilization. Rising estrogen levels trigger the preovulatory LH surge, leading to ovulation and corpus luteum formation. Progesterone, secreted by luteal cells, supports uterine receptivity and regulates feedback mechanisms in the hypothalamic-pituitary-ovarian axis. Androgens, produced by theca cells, serve as precursors for estrogen and contribute to follicular recruitment.

Anti-Müllerian hormone (AMH), produced by granulosa cells of preantral and small antral follicles, regulates follicle recruitment and serves as a marker of ovarian reserve. Its expression declines as follicles develop, allowing for dominant follicle selection. Inhibins and activins modulate follicle-stimulating hormone (FSH) activity, balancing follicular growth and atresia. The interplay between these hormonal signals ensures ovarian function remains responsive to internal and external factors.