Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Reproductive cycle
Introduction
Every healthy human female experiences reproductive cycles that organize their reproductive system for pregnancy. The reproductive cycles are under the dynamic influence of integrated action of hormones from hypothalamus, anterior pituitary, and gonadal ovarian steroids. Ovarian cycle is directly regulated and synchronized by the anterior pituitary follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Uterine cycle is controlled by a direct reaction to ovarian steroid hormones (estrogen and progesterone). The definitive rheostat of the pituitary gonadotropins (FSH and LH) is situated centrally in the hypothalamus, which is highly responsive to plasma concentration of ovarian steroid hormones. The gonadotropin synthesis and secretion from anterior pituitary are augmented and regulated by the release of gonadotropin-releasing hormone (GnRH) from hypothalamus into hypothalamic–hypophyseal–portal circulation.
Puberty in females
An overview of menarche
In girls, puberty is considered as the development of the genital organs, secondary sexual characteristics, and the appearance of menarche. Thus, the main physiological mechanisms that determine puberty in girls include ovarian growth and maturation with increased synthesis and secretion of sex steroid hormones with folliculogenesis and ovulation. These changes are responsible for the occurrence of menarche in girls. Menarche is the memorable beginning of a woman’s reproductive life. It is commonly accepted that the cyclical hormonal variabilities that control the reproductive cycle have an essential natural impact on a woman’s physical and mental health. This cycle duration is from 25 to 35 days, usually of 28 and 26 days of length in women’s reproductive life. The usual age group at menarche is 9–14 years and the mean is 12–13 years. Body mass index and lifestyle may influence the age at menarche. Menarche is associated with an anovulatory cycle and ideally appears 2–2.5 years after the initial stages of mammary gland development. In the 1st year postmenarche, the menstrual cycles are typically irregular and anovulatory ranges from 21 to 45 days. Menstrual cycles can remain continuously irregular until the 3–5th year postmenarche. The predominance of primordial and preantral follicles occurs before puberty; tiny follicles can grow through this period of development. These small follicles are gonadotropin-independent. The volume of ovary increases by the commencement of adolescence, attains increase in bulk or size rapidly after menarche especially between 13 and 16 years, and remains constant or decreased marginally subsequently. Polycystic ovarian morphological changes are noticed in healthy young girls; this morphology is not associated with reduced ovulatory rate, hyperandrogenism, or metabolic disorders. In the initial phase of postmenarche, ovarian morphological changes on transabdominal ultrasound scan show multicystic ovaries and significantly raised ovarian volume that differs from ovarian morphological features observed in elder women.
The menstrual cycle
In the normal ovulatory menstrual cycle, four hormones FSH, LH, estrogen, and progesterone, with the central pulsatile release of GnRH, regulate the normal reproductive activities in women like continuous cycles of follicular growth and development, ovulation, and endometrial preparation for the implantation of the blastocyst ( Fig. 3.1 ). Active high secretion of FSH through the luteal–follicular switch leads to the staffing of a troop of follicles and the appearance and progression of a dominant follicle called mature Graafian follicle. From the ovary, estradiol and inhibin release are the main inhibitors of ongoing excitatory release of FSH, while high estradiol with other prospective elements is significant for the LH surge, centrally regulated at anterior pituitary in women. Corpus luteum releases progesterone and estradiol to prepare the uterine endometrium for implantation, and its decease permits FSH to become high with the start of a next cycle.
The phases of menstrual cycle with endocrine control
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here