21.3 Ovarian and Uterine Cycles

Question 1

1º follicle

Answer 1

A primary follicle (1° follicle) is an early stage of ovarian follicle development

Question 2

1º oocyte

Answer 2

A primary oocyte (1° oocyte) is an immature egg cell arrested in prophase I of meiosis since fetal development. It remains dormant until puberty, when it resumes meiosis during the menstrual cycle, potentially maturing into a secondary oocyte for ovulation.

Question 3

2º follicle

Answer 3

A secondary follicle (2° follicle) is a developing ovarian follicle that forms from a primary follicle. It is characterized by multiple layers of granulosa cells and the appearance of a fluid-filled cavity (antrum). The follicle continues growing and secreting estrogen, eventually maturing into a tertiary (Graafian) follicle, which is ready for ovulation.

Question 4

2º oocyte

Answer 4

A secondary oocyte (2° oocyte) is a mature egg cell that has completed meiosis I and is arrested in metaphase II of meiosis until fertilization. It is released during ovulation and, if fertilized by sperm, will complete meiosis II, forming a mature ovum and a polar body. If fertilization does not occur, the secondary oocyte degenerates.

Question 5

corpus luteum

Answer 5

The corpus luteum is a temporary endocrine structure formed from the ruptured follicle after ovulation. It secretes progesterone and estrogen to maintain the uterine lining for potential pregnancy. If fertilization occurs, it is sustained by hCG. If not, it degenerates into the corpus albicans, leading to a drop in hormones and the start of menstruation.

Question 6

estrogen

Answer 6

Estrogen is a primary female sex hormone produced mainly by the ovaries and, in smaller amounts, by the adrenal glands and placenta during pregnancy. It regulates the menstrual cycle, supports follicle development, and helps maintain secondary sexual characteristics like breast development. Estrogen also plays a role in bone health, cardiovascular function, and mood regulation. Its levels fluctuate throughout the ovarian cycle, peaking before ovulation.

Question 7

fertilization

Answer 7

Fertilization is the process where a sperm cell fuses with a secondary oocyte to form a zygote. It typically occurs in the ampulla of the fallopian tube. Upon sperm entry, the oocyte completes meiosis II, and the genetic material from both gametes combines to create a diploid zygote. This marks the beginning of embryonic development.

Question 8

follicular phase

Answer 8

The follicular phase is the first phase of the ovarian cycle (Days 1–14), beginning with menstruation and ending with ovulation. FSH stimulates follicle growth, and the dominant follicle secretes estrogen, which thickens the endometrium. Rising estrogen levels trigger an LH surge, leading to ovulation around day 14.

Question 9

HCG

Answer 9

hCG (human chorionic gonadotropin) is a hormone produced by the placenta during pregnancy. It supports the corpus luteum, prompting it to continue secreting progesterone to maintain the uterine lining. hCG is the hormone detected in pregnancy tests and typically increases in early pregnancy. Its levels peak around the end of the first trimester and then decrease.

Question 10

implantation

Answer 10

Implantation is the process where a fertilized egg (zygote) embeds itself into the endometrium (lining of the uterus) around 6–10 days after fertilization. The embryo releases hCG to signal the body to maintain the corpus luteum and continue producing hormones like progesterone, ensuring the uterine lining stays thick and supportive for the developing embryo. Successful implantation is critical for establishing a pregnancy.

Question 11

luteal phase

Answer 11

The luteal phase is the second half of the ovarian cycle (Days 15–28), following ovulation. After the egg is released, the ruptured follicle transforms into the corpus luteum, which secretes progesterone and estrogen. These hormones prepare the endometrium for possible implantation of a fertilized egg. If pregnancy doesn’t occur, the corpus luteum degenerates, hormone levels drop, and the menstruation process begins.

Question 12

menopause

Answer 12

Menopause is the natural biological process marking the end of a woman’s menstrual cycles and reproductive years, typically occurring between ages 45–55. It is defined as having gone 12 consecutive months without a period. During menopause, the ovaries stop releasing eggs and produce less estrogen and progesterone, leading to symptoms like hot flashes, mood changes, night sweats, and vaginal dryness. These hormonal changes also increase the risk of bone loss and cardiovascular issues.

Question 13

menstruation

Answer 13

Menopause is the natural biological process marking the end of a woman’s menstrual cycles and reproductive years, typically occurring between ages 45–55. It is defined as having gone 12 consecutive months without a period. During menopause, the ovaries stop releasing eggs and produce less estrogen and progesterone, leading to symptoms like hot flashes, mood changes, night sweats, and vaginal dryness. These hormonal changes also increase the risk of bone loss and cardiovascular issues.

Question 14

placenta

Answer 14

The placenta is a temporary organ that forms during pregnancy, connecting the developing fetus to the uterine wall. It allows for the exchange of nutrients, oxygen, and waste between the mother and fetus. The placenta also produces hormones like hCG, progesterone, and estrogen, which help maintain pregnancy and support fetal development. It acts as a barrier, protecting the fetus from some harmful substances, although not all. The placenta is expelled after childbirth in the third stage of labor.

Question 15

polar body

Answer 15

A polar body is a small, non-functional cell produced during oogenesis (egg development) as a byproduct of meiosis. When a primary oocyte undergoes meiosis I, it divides unevenly, resulting in one large secondary oocyte and a smaller polar body, which contains very little cytoplasm. After the secondary oocyte undergoes meiosis II (if fertilized), another polar body may be formed. Polar bodies eventually degenerate and do not contribute to reproduction.

Question 16

progesterone

Answer 16

Progesterone is a key hormone in the female reproductive system, primarily produced by the corpus luteum after ovulation and, during pregnancy, by the placenta. It plays a critical role in preparing the endometrium for the implantation of a fertilized egg and maintaining a pregnancy. Progesterone also helps to regulate the menstrual cycle and suppresses further ovulation during pregnancy. If pregnancy does not occur, progesterone levels drop, leading to menstruation.

Question 17

proliferative phase

Answer 17

The proliferative phase is the phase of the menstrual cycle that occurs after menstruation and before ovulation. It typically lasts from day 6 to day 14. During this phase, rising levels of estrogen stimulate the endometrium (uterine lining) to thicken and regenerate in preparation for a potential pregnancy. The cervical mucus also becomes more fertile, helping sperm travel more easily. The proliferative phase ends with ovulation, triggered by a surge in LH and FSH.

Question 18

secretory phase

Answer 18

The secretory phase is the part of the menstrual cycle that follows ovulation and lasts from day 15 to day 28. During this phase, the corpus luteum secretes progesterone, which further thickens the endometrial lining and prepares it for potential implantation of a fertilized egg. Estrogen levels also remain elevated. If pregnancy does not occur, the corpus luteum degenerates, causing a drop in progesterone, leading to the start of menstruation and the shedding of the uterine lining.

Question 19

vesicular follicle

Answer 19

A vesicular follicle (also known as a Graafian follicle) is a mature ovarian follicle that has developed from a secondary follicle. It is characterized by the presence of a fluid-filled antrum (cavity) surrounding the oocyte. The vesicular follicle is ready for ovulation and typically contains a fully developed secondary oocyte. When ovulation occurs, the vesicular follicle ruptures, releasing the egg into the fallopian tube.