Fertilization, Weeks 1 And 2 Of Development

The amniotic cavity is the fluid-filled space that surrounds and protects the developing embryo. It appears on the eighth day of development as a slit-like space between the trophoblast (outer layer of cells) and the embryoblast (inner cell mass). The other options listed do not accurately describe the formation of the amniotic cavity. The extraembryonic mesoderm is a layer of cells that surrounds the amniotic cavity, but it does not directly contribute to its formation. The exocoelomic membrane, connecting stalk, and chorion are all structures associated with the development of the placenta, but they are not directly involved in the formation of the amniotic cavity.

The secondary oocyte completes the second meiotic division at fertilization. This means that when fertilization occurs, the secondary oocyte undergoes the second meiotic division, resulting in the formation of a mature egg and a polar body. This is a crucial step in the process of fertilization as it ensures that the egg is ready to fuse with the sperm and form a zygote. Once fertilization occurs, the egg is now capable of developing into an embryo.

During fertilization, the sperm releases acrosomal enzymes which break down the zone pellucida. The zone pellucida is a glycoprotein layer surrounding the oocyte. The acrosomal enzymes help the sperm penetrate through this layer and reach the oocyte for fertilization to occur. This process allows the sperm to fuse with the oocyte and release its genetic material, leading to the formation of a zygote.

A primary follicle is the initial stage of follicle development in the ovary. It contains a primary oocyte, which is an immature egg, and is surrounded by a single layer of cuboidal follicular cells. This stage does not have a large antral cavity, which is present in later stages of follicle development. The theca interna and theca externa are not present in the primary follicle.

Progesterone is the correct answer because it is the hormone responsible for preparing the endometrium (lining of the uterus) for implantation of a fertilized egg. It enhances glandular secretion in the endometrium, making it more receptive to implantation. FSH and LH are involved in the regulation of the menstrual cycle and the development of eggs, while estrogen plays a role in the growth and thickening of the endometrium. Testosterone is primarily a male sex hormone and does not have a direct role in the reproductive cycle of females.

The oocytes are cultured until they reach the normal second stage of meiotic arrest, which indicates that they are in metaphase. Therefore, fertilization is carried out at the metaphase stage of oocyte development.

During the 2nd week of development, the syncytiotrophoblast layer surrounds the lacunae spaces. This layer is responsible for the formation of the placenta and plays a crucial role in nutrient and gas exchange between the mother and the developing embryo. The syncytiotrophoblast layer also helps in implantation of the embryo into the uterine wall.

During fertilization, when a sperm with 23 chromosomes (including one Y chromosome) fertilizes an oocyte (which also has 23 chromosomes), it results in a male embryo. This is because the presence of the Y chromosome determines the development of male characteristics. Therefore, the correct answer is 23, Y.

During the 1st and 2nd week of development, the syncytiotrophoblast degrades the endometrium of the uterus. This is an important step in the implantation process. The syncytiotrophoblast is a layer of cells that forms on the outer surface of the blastocyst. It secretes enzymes that help to break down the endometrial tissue, allowing the blastocyst to burrow deeper into the uterine lining and establish a connection with the maternal blood supply. This degradation of the endometrium is necessary for successful implantation and subsequent development of the embryo.

The secondary oocyte is arrested 3 hours before ovulation. This is because the secondary oocyte, which is released during ovulation, needs time to complete its maturation process before it can be fertilized. The arrest allows for the necessary changes to occur within the oocyte, such as the completion of meiosis, before it can be released from the ovary. Once ovulation occurs, the secondary oocyte is then able to be fertilized by a sperm cell.

LH, or luteinizing hormone, stimulates the theca interna during the reproductive cycle. The theca interna is a layer of cells surrounding the ovarian follicle, and it plays a crucial role in the production of estrogen. LH triggers the theca interna to produce androgens, which are then converted into estrogen by the granulosa cells. This estrogen production is important for the maturation of the follicle and the preparation of the uterus for potential pregnancy. Therefore, LH is the correct answer as it directly stimulates the theca interna.

The ultrasound examination of an anovulatory patient with PCOS reveals multiple enlarged but immature follicles in her ovaries. This indicates that the oocytes within these follicles have not yet matured fully. The first meiotic prophase is the stage in which oocytes undergo DNA replication and begin to divide, but they have not yet completed the division into two cells. Therefore, the oocytes in this patient's ovaries have advanced to the first meiotic prophase stage of meiosis.

An undescended testis of a prepubertal male is removed because it is at a higher risk of developing testicular cancer. The histologic analysis reveals the stage of development of the germ line cells in this testis. The germ line cells are the cells that give rise to sperm. In this case, the cells have advanced as far as the primordial germ cell stage. Primordial germ cells are the earliest stage of development of germ line cells, and they are the precursors to all other stages of sperm development. Therefore, the correct answer is primordial germ cell.

A secondary follicle is surrounded by two layers of cells called the theca interna and the theca externa. These layers provide support and nourishment to the developing follicle. The theca interna is responsible for producing androgens, which are converted into estrogen by the granulosa cells within the follicle. The theca externa is a connective tissue layer that provides structural support. Together, these layers play a crucial role in the growth and maturation of the secondary follicle.

During prophase I of meiosis I, the amount of DNA is 4n and the number of chromosomes is 46. This is because during meiosis I, the DNA replicates once in the S phase of interphase, resulting in the formation of sister chromatids. These sister chromatids pair up during prophase I to form homologous chromosomes, and each homologous pair consists of two sister chromatids, hence the 4n DNA. The number of chromosomes remains the same as the diploid number, which is 46 in humans.

The question states that the woman displays none of the characteristics of Down syndrome, which suggests that the nondisjunction leading to the presence of two copies of chromosome 21 in some polar bodies did not occur in the primary oocyte in her mother's germline. It also suggests that the nondisjunction did not occur in the primary oocyte in her own germ line, as this would result in the presence of two copies of chromosome 21 in all polar bodies. Therefore, the most likely explanation is that the nondisjunction occurred in a blastomere in her own germ line, leading to the presence of two copies of chromosome 21 in some polar bodies and no copies in others.