Fertilization and Implantation

Dr. Raymond Colello EMBRYOLOGY: Fertilization and Implantation Recommended Reading: Larsen, pp. 1-51 Langman, pp. 3-52 Moore, pp. 4-37 OBJECTIVES: Following the lecture the student should be able to: 1.) Describe the chronology of events taking place during the first 8 weeks of embryonic development. 2.) Describe the origin and fate of the primordial germ cells. 3.) Compare and contrast spermatogenesis and oogenesis. 4.) Describe the periods of susceptibility to teratogens. 5.) Describe how hormonal regulation effects endometrium development and ovarian activity during a menstrual cycle. 6.) Define the functions of the corpus luteum, zona pellucida, acrosomal enzymes, trophoblast cells and the chorionic villi. 7.) Define spermatogonia, primary spermatocyte, secondary spermatocyte, oogonia, primary oocyte, secondary oocyte and ovum. II.) Gametogenesis and Fertilization A.) General Embryological Terms in Gametogenesis and Fertilization 1.) Oocyte: female sex cell produced in the ovaries 2.) Sperm: male sex cell produced in the testes 3.) Gametes: male or female sex cells 4.) Zygote: cell formed from the union of the oocyte and sperm 5.) Fertilization: process by which male and female gametes fuse 6.) Cleavage: mitotic cell division of the zygote 7.) Morula: cleavage of zygote to 32 or more cells 8.) Trophoblast: outer cell mass of morula giving rise to the placenta 9.) Blastocyst: the morula forming a fluid-filled central cavity The development of a human begins with the formation and differentiation of the male and female sex cells or gametes, which will fuse during fertilization to initiate the embryonic development of a new individual. In humans, the cell line that gives rise to the gametes are called the primordial germs cells which originate on the yolk sac at and 4 weeks of embryonic development and migrate to the posterior body wall or presumptive gonadal region (fig.1). Fig. 1: The primordial germ cells differentiate in the endodermal layer of the yolk sac at 4 to 6 weeks of development and migrate to the dorsal body wall (A). Between the 6 and 12weeks, the primordial germ cells induce formation of the genital ridges (B). (taken from Larsen, "Human Embryology", 1993) Here they proliferate to form compact strands of tissue called the primitive sex cords, which swell to become the genital ridges, or the primordial gonads. Within the developing gonads, the primordial germ cells differentiate into precursor cells called spermatogonia in the male and oogonia in the female, both of which are diploid (contain a complement of 23 pairs of chromosomes or a total of 46 chromosomes). During gametogenesis, these cells undergo meiosis, so that the gametes produced are haploid, containing only 23 chromosomes (fig. 2). Fig. 2: Nuclear maturation of germ cells in meiosis in the male and female. In the male, the primordial germ cells remain dormant until puberty, when they differentiate into spermatogonia and commence mitosis. Throughout adulthood, the spermatogonia produce primary spermatocytes, which undergo meiosis and spermatogenesis. In the female, the primordial germ cells differentiate into oogonia, which undergoes mitosis and then commence meiosis as primary oocytes during fetal life. The primary oocytes remain arrested in prophase I until stimulated to resume meiosis during the menstrual cycle. (taken from Larsen, "Human Embryology", 1993) The timing of gametogenesis differs in the two sexes. In males, the primordial germ cells remain dormant until puberty, when the seminiferous tubules mature and the germs cells differentiate into spermatogonia. These undergo meiosis, reducing their chromosome numbers by half, and mature into spermatozoa. In females, the primordial germ cells go through a series of mitotic division early in fetal development, differentiate into oogonia, and then all begin meiosis by the fifth embryonic month. B.) Spermatogenesis and Oogenesis In males, increased secretion of testosterone by the testis at puberty stimulates the growth of the testis and the maturation of the seminiferous tubules. This, in turn, induces spermatogenesis whereby the dormant primordial germ cells divide several times and then differentiate into spermatogonia (fig.3). In females, the total number of primary oocytes that individual will ever possess is produced in the ovaries by 5 months of fetal life. By the 3 rd month of development, some oogonia give rise to primary oocytes that enter prophase of the 1st meiotic division. At birth, all the oogonia have been transformed to dormant primary oocytes that are surrounded by a single layer of follicle cells, thus forming the primary follicle. These cells will then lay dormant until after puberty when they resume development in response to hormonal signals, which initiate the menstrual cycle. The total number of primary oocytes at birth is roughly 1 million, with these numbers declining to 400,000 at the time of puberty. Fewer than 500 will be released during the reproductive years of a woman (fig. 4). Fig. 3: (A) A schematic section through the seminiferous tubule wall. The spermatogonium just under the outer surface of the tubular wall (basal side) undergoes mitosis to daughter cells, which may either continue to divide by mitosis or may commence meiosis as primary spermatocytes. The differentiating cells translocate to the tubular lumen. (B) From puberty sperm are constantly produced in the seminiferous tubules and passed through the tubules and into the epididymis, were they are stored (taken from Larsen, "Human Embryology", 1993). Fig. 4: Depiction of folliculogenesis and ovulation in the ovary (taken from Larsen, "Human Embryology", 1993). C.) Ovulation At puberty, the female begins to undergo regular monthly cycles, called menstrual cycles, which are controlled by the release of hormones from the hypothalamus and anterior pituitary gland. The release of gonadotrophin-releasing hormone by the hypothalamus stimulates cells of the pituitary gland to release the gonadotrophins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which stimulate and control cyclic changes in the ovary (fig.5), leading to ovulation. In fact, the resumption of meiosis and ovulation are stimulated by an ovulatory surge of FSH and LH. Fig. 5: (A) Schematic drawing of changes taking place in the uterine mucosa (endometrium) during a regular menstrual cycle in which fertilization fails to occur. Note the corresponding changes in the ovary (taken from Larsen, "Human Embryology", 1993). (B) The ovaries are covered by a layer of cells. The cells which are destined to become ova (eggs) pass into the substance of the ovaries, where they are surrounded by a follicle membrane. Each month a single follicle matures, bursts in one ovary's surface and is released. If fertilized, the corpus luteum- which develops at the site of the egg's follicle-grows and secretes hormones that maintain pregnancy (taken from "Atlas of Anatomy", Marshall Cavendish books, 1991). The thickening of the endometrium (womb lining) is caused by hormones, which are released at the end of menstruation. During this time a follicle is maturing in the ovary as a result of FSH release and begins to produce estrogen, which acts to control the cycle of uterine endometrium. After the egg is released, through the stimulation of LH hormone, a mass of cells (the corpus luteum) forms in the empty follicle. These cells secrete the hormone, progesterone, which causes the endometrium to thicken further. If pregnancy occurs, the corpus luteum persists and continues to maintain the womb lining. If it does not, the corpus luteum shrinks and hormone release drops. As a result, the womb lining is shed-this being known as the menstrual period (Fig. 6). Fig. 6: Ovarian, endometrial and hormonal events of the menstrual cycle. C.) Fertilization Fertilization takes place in the ampullary region of the uterine tube, the region of the tube located close to the ovary (fig. 7). Spermatozoa, however, are not capable of fertilizing an egg until they have undergone a process called capacitation, whereby the glycoprotein coat and the seminal proteins are removed from the sperm's acrosome (head of the sperm). This change allows the acrosome to release enzymes required to penetrate the glycoprotein shell of the oocyte, the zona pellucida. Concurrently, the oocyte is released from the ovary during ovulation and "swept" into the opening of the uterine tube by the finger-like fimbriae of the tube lying adjacent to the ovary (fig.7). The fusion of the spermatozoan cell membrane with the oocyte membrane causes the oocyte to resume meiosis (fig. 8). Once penetration of the zona pellucida has occurred, the cell membranes of the sperm and oocyte can fuse, forming zygote. This represents the zero point of embryonic development. D.) Cleavage The zygote now undergoes a series of mitotic divisions, resulting in a rapid increase in the number of cells, now called blastomeres (Fig. 9). This process of cleavage normally occurs as the zygote passes through the tube and toward the uterus (fig.7C). Cleavage, however, subdivides the zygote without increasing its size. By 3 days post-fertilization, the embryo consists of 6-12 cells, and by 4 days, 16-32 cells. By the 32-cell stage, the embryo is now termed a morula. This structure gives rise to both the embryo proper and the placenta and related structures. The cells that will make these structures begin to segregate during cleavage so that some blastomeres lie in the center of the morula (called the inner cell mass) and some Fig. 7: Cleavage and transport down the oviduct. (taken from”Atlas of anatomy”,Marshall Cavendish books, 1991). Fig. 8: (A) Fertilization. Penetration of a spermatozoa into the oocyte. (taken from Larsen, "Human Embryology", 1993). lie in the periphery (called the outer cell mass). The inner cell mass gives rise to the embryo proper and is therefore called the embryoblast while the outer cell mass gives rise to the placenta and is therefore called the trophoblast. By the 4-day post-fertilization, the morula develops a fluid-filled cavity and is transformed into a blastocyst.

Load more