Female Puberty and the Estrous Cycle 

On this page, all aspects of the estrous cycle of the queen is explained including follicle and ooctye development. We also talk about endocrine regulation for the cycle and seasonality. 

Estrous Cycle

The estrous cycle is the cycling of different predictable reproductive events that happens throughout most of a female's life. It has two phases: luteal phase and follicular phase. The follicular phase is when follicles are developing and then ovulate. This phase includes the proestrus, estrus, and interestrus phases. The luteal phase is when the corpus luteum is developing and functional. Due to the queen being an induced ovulator, the queen does not have a true luteal phase. This is because the corpus luteum does not form every cycle. Thus, the queen has an interestrus phase and not a metestrus phase (3).

Proestrus is a period when antral follicles are maturing and the female is preparing for mating (3).

Hormone & Follicular activity: high follicular growth, increasing estrogen levels, decreasing progesterone levels (1)

Physiological Changes: increased vaginal epithelium (2) and antral follicles are maturing (3)

Behavior: attract but not receptive to toms, increased rubbing and "friendly" behavior (1)

Duration: usually less then 24 hours (2)

Estrus is a period when mating occurs and when ovulation occurs if induced (3).

*If copulation happens, ovulation happens within this period and then move onto diestrus (2)

*If copulation does not happen, then ovulation does not happen, and the queen moves on to interestrus (2)

Hormone & Follicular activity: highest estrogen level then decreases, low progesterone levels (1), increased vaginal cytology, follicles reach maturity (2), and possibly ovulation (3)

Physiological Changes: vulva slightly enlarged, increased blood flow, and slight discharge (may not be noticed due to feline grooming habits) (1)

Behavior: vocalization and receptive to toms, lordosis (a mating posture when the back is arched with forelimbs lowered but hind limbs extended) with possible treading of hind legs (1)

Duration: 5-7 days (2)

Interestrus is a transition period between estrus phases when ovulation does not occur (3).

*Occurs between estrus and the next proestrus phase and is a period when queens are non-receptive to toms (2)

* Proestrus, estrus, and interestrus can repeat many times (2)

Hormone & Follicular activity: dominate follicles undergo atresia (the death or degeneration of follicles)(2), basal progesterone and estrogen levels (1)

Physiological Changes: decreased vaginal cytology (2)

Behavior: non-receptive to toms (1)

Duration: 8-9 days (2)

Diestrus is a period when the corpus luteum is developing and becomes functional (3). 

*If ovulation occurs, diestrus will continue after estrus (2)

Hormone & Follicular activity: one or more corpora lutea, increase in progesterone levels until high, low estrogen levels (2)

Physiological Changes: changes in vaginal cytology (2) and the corpus luteum(s) is developing and becomes functional (3)

Behavior: non-receptive to toms (1)

Duration: about 35 days (1)

Anestrus is a period of non-cycling (3).

*Mostly seen in free-roaming queens during winter months (short days) (1)

Hormone & Follicular activity: cycle is ceases (2), basal progesterone and estrogen levels (1)

Physiological Changes: normal (1)

Behavior: normal (1)

Duration: about October through February (2)

• More about different types of anestrus are discussed below

Endocrinology of the Estrous Cycle

Oocyte Development

Oocyte development occurs in four phases. The first phase occurs prenatally and is characterized by the breakdown of the sex cords and enhanced mitotic division of primordial germ cells within the fetal ovaries, establishing the follicular reservoir for the queen’s entire life (2). The primordial germ cells enter meiosis I but are arrested in prophase I as primary oocytes (primordial follicles) (3). This process begins at days 40-50 of fetal development and is typically completed by eight days post-partum (1). 

This second phase of oocyte maturation is called the dictyotene (also known as nuclear or prophase arrest). The primary oocytes remain in dictyate arrest until the female cat reaches puberty (4). After birth, the oocytes are encapsulated and surrounded by a layer of squamous pre-granulosa cells, forming the primordial follicles (1). 

Around the time of puberty, the third phase of oocyte maturation begins. According to Senger (2012), the volume of the cytoplasm of some oocytes increases by up to 50 times. The formation of gap junctions between the oocyte and the surrounding layer of granulosa cells allows for the diffusion of materials required for the growth of the oocyte. This period of primary oocyte growth is also characterized by the formation of a layer of mucopolysaccharide material around the cytoplasm of the oocyte, called the zona pellucida.

The final phase occurs once the oocyte has reached a critical minimum volume. Physiological changes within the cell occur and the gap junctions break down (4). The luteinizing hormone (LH) surges that occur after the onset of puberty stimulate the completion of meiosis I, resulting in the formation of the secondary oocyte (haploid) and first polar body (3). The secondary oocyte progresses to meiosis II and again is arrested in metaphase II until fertilization occurs (2). 

Follicle Development

As described in the section above, primary oocytes are enveloped by a single layer of squamous pre-granulosa cells, forming primordial follicles (1). Primordial follicles appear around one month after birth and remain quiescent in the ovaries until the onset of puberty (1). 

Around the onset of puberty, a complex cascade of activator signaling pathways between the oocyte and surrounding granulosa cells triggers the activation of some primordial follicles (5). The process of primordial follicles maturing into primary follicles begins with the granulosa cells surrounding the oocyte becoming cuboidal and the oocyte itself resuming growth, as outlined in the section above (1). Additionally, the zona pellucida forms around the cytoplasm of the oocyte, and a basement membrane separates the granulosal layer from the ovarian stroma (1,5).

Primary follicles then transition into secondary follicles. This process is characterized by the proliferation of granulosa cells, forming two or more layers, and the appearance of theca cells around the periphery of the follicle (1). The granulosa cells of the secondary follicles acquire FSH receptors. The next stage of follicular development is the antral or Graafian follicle, which features a fluid-filled cavity known as the antrum. The oocyte is now surrounded by cumulus cells originating from undifferentiated granulosa cells and 2-3 layers of thecal cells (1,6). 

Under the influence of a surge in luteinizing hormone (LH), the antral follicle undergoes final maturation, leading to the rupture of the follicle and the release of the mature oocyte (ovulation) (4). During this stage, the primary oocyte progresses from prophase I to metaphase II of meiosis and becomes arrested again until fertilization, now termed a secondary oocyte (5).

Follicle development includes the stages of recruitment, selection, and dominance. Initially, a group of primordial follicles starts growing (recruitment), but only a few are chosen (selection) based on hormonal signals, primarily follicle-stimulating hormone (FSH). Among these, several follicles become dominant, outcompeting others due to their sensitivity to elevated luteinizing hormone (LH) pulse frequency and inhibin levels. These dominant follicles are crucial for ovulation, releasing only the most viable oocytes.