 FOLDER 23.2 Clinical Correlation: In Vitro Fertilization aaba b FIGURE F23.2.1 • Early developmental stages of the human embryo. a. This image, obtained with an interference contrast microscope equipped with Nomarski optics, shows a human fertilized oocyte with two pronuclei. The zygote develops after alignment and dissolution of nuclear membranes of both female and male pronuclei. The resulting cell will contain a diploid complement of 46 chromosomes. 400. b. This image shows a 48-hour-old human embryo growing in a special growth medium. At this stage, the embryo consists of four cells. In IVF procedures, it is at this stage that the embryo is usually transferred into the uterine cavity. 400. (Courtesy of Dr. Peter Fehr.) gonadotropin-induced neural apoptosis inhibitory protein (NAIP), which inhibits and delays apoptotic changes in the granulosa cell. NAIP gene expression is present in all stages of the growing follicle but absent in follicles undergoing atresia. A high level of gonadotropins inhibits apoptosis in ovarian follicles by increasing the expression of NAIP in the ovaries.
The oocyte undergoes typical changes associated with degeneration and autolysis, and the remnants are phagocytosed by invading macrophages. The zona pellucida, which is resistant to the autolytic changes occurring in the cells associated with it, becomes folded and collapses as it is slowly broken down within the cavity of the follicle. Macrophages in the connective tissue are involved in the phagocytosis of the zona pellucida and the remnants of the degenerating cells. The basement membrane between the follicle cells from the theca interna may separate from the follicle cells and increase in thickness, forming a wavy hyaline layer called the glassy membrane. This structure is characteristic of follicles in late stages of atresia.
Enlargement of the cells of the theca interna occurs in some atretic follicles. These cells are similar to theca lutein cells and become organized into radially arranged strands separated by connective tissue. A rich capillary network develops in the connective tissue. These atretic follicles, which resemble an old corpus luteum, are called corpora lutea atretica.
The interstitial gland arises from the theca interna of the atretic follicle.
As atretic follicles continue to degenerate, a scar with hyaline streaks develops in the center of the cell mass, giving it the appearance of a small corpus albicans. This structure eventually disappears as the ovarian stroma invades the degenerating follicle. In the ovaries of a number of mammals, the strands of luteal cells do not degenerate immediately but become broken up and scattered in the stroma. These cords of cells contribute to the interstitial gland of the ovary and produce steroid hormones. The development of the interstitial gland is most extensive in animal species that have large litters.
In the human ovary, there are relatively few interstitial cells. They occur in the largest numbers in the first year of life and during the early phases of puberty, corresponding to times of increased follicular atresia. At menarche, involution of the interstitial cells occurs; therefore, few are present during the reproductive life span and menopause. It has been suggested that in humans the interstitial cells are an important source of the estrogens that influence growth and development of the secondary sex organs during the early phases of puberty. In other species, the interstitial cells have been shown to produce progesterone.