The corpus luteum and the luteal phase of the ovarian cycle

2021-06-09 03:03 PM

In normal women, the corpus luteum grows to about 1.5 cm in diameter 7 to 8 days after ovulation. The corpus luteum then begins to atrophy and eventually loses its secretory function as well as the pale-yellow colour of fat about 12 days after ovulation.

Within a few hours after the ovum is released from the follicle, the remnants of the granulosa cells and the endothelial cells rapidly become corpus luteum cells.

They double or more in size and are filled with fat that gives them a pale-yellow colour. This process is called luteinization, and this whole mass of cells is called the corpus luteum, shown in the figure.

Figure. Stages of follicular development in the ovary, and corpus luteum formation.

Blood supply vessels are also abundantly developed into the corpus luteum.

The granulosa cells in the corpus luteum develop a fine endoplasmic reticulum, secreting large amounts of progesterone and estrogenic (progesterone more than estrogenic). Intrinsic cells secrete mainly male hormones such as testosterone and androstenedione rather than female hormones. However, most of these hormones are also converted into female hormones by the granulosa cells.

In normal women, the corpus luteum grows to about 1.5 cm in diameter 7 to 8 days after ovulation. The corpus luteum then begins to atrophy and eventually loses its secretory function as well as the pale-yellow colour of fat about 12 days after ovulation. Becoming corpus luteum degenerates. A few weeks later, it is replaced by connective tissue and after a month or so it disappears.

The luteal-stimulating function of LH

The transformation of granulosa cells and mantle cells in the corpus luteum cell wall depends largely on the amount of LH secreted from the anterior pituitary. In fact, this ability gives LH the name "luteal-stimulating hormone". The process of luteinization is also based on the ovulation of the follicle. An atypical hormone located in the follicular fluid, called a luteal-inhibitor factor, seems to keep luteinization under control until after ovulation has occurred.

The secretion of the corpus luteum is another function of LH .

The corpus luteum is a powerful secretory gland that secretes large amounts of progesterone and oestrogen. Once LH (mostly secreted during follicular proliferation) has acted on granulosa cells and endothelial cells to take place, luteal cells appear to have been established. and will undergo stages (1) proliferation, (2) growth, (3) secretion, and finally (4) degeneration. It all happened in about 12 days.

When there is another hormone with similar properties to LH, that is a placental hormone (hCG), secreted from the placenta, can act on the corpus luteum to prolong its existence, usually occurring during at least the first 2 to 4 months of pregnancy.

Atrophy of the corpus luteum and the start of the next ovarian cycle

Oestrogen and progesterone (to a lesser extent), secreted by the corpus luteum during the luteal phase of the ovarian cycle, exert strong feedback on the anterior pituitary to maintain FSH secretion and LH is always low.

In addition, the corpus luteum cells secrete small amounts of the hormone inhibin, like the inhibin secreted by Sertoli cells in the male testes, which inhibits the secretion of the anterior pituitary gland, especially FSH secretion. As a result, blood levels of FSH and LH remain low, and the disappearance of these two hormones causes complete degeneration of the corpus luteum, a process called luteal degeneration.

Normal degeneration occurs precisely around the 12th day of the corpus luteum, around the 26th day of the normal menstrual cycle, 2 days before the onset of menstruation. At that point, FSH secretion stops abruptly, and progesterone and inhibin lose their inhibitory effects on the anterior pituitary, allowing the anterior pituitary to increase secretion of FSH and LH again. FSH and LH initiate the development of new follicles, starting a new ovarian cycle. Very little secretion of the hormone’s progesterone and oestrogen at this time leads to menstrual bleeding in the uterus, which will be discussed later.

About every 28 days, gonadal hormones from the anterior pituitary gland stimulate about 8-12 new follicles to begin developing in the ovaries. One of these follicles eventually becomes a "mature follicle" and sheds on day 14 of the cycle. Mainly oestrogen is secreted during follicular development.

After ovulation, the secretory cells of the deciduous follicle turn into the corpus luteum, which secretes large amounts of two important female hormones - progesterone and oestrogen. After 2 weeks, the corpus luteum degenerates, resulting in a sharp drop in the ovarian hormones oestrogen and progesterone, and menstruation begins. It then starts a new ovarian cycle.