First Week of Development: Ovulation to Implantation PDF

 Chapter 3 First Week of Development: Ovulation to Implantation 31 Primary oocyte Granulosa Zona pellucida Antrum cel...

 Chapter 3 First Week of Development: Ovulation to Implantation 31 Primary oocyte Granulosa Zona pellucida Antrum cells Theca externa Theca interna A Primordial follicle B Growing follicle C Vesicular follicle Figure 3.2 A. Primordial follicle. B. Growing follicle. C. Vesicular follicle. Every day from the pool of primordial follicles A, some begin to develop into growing follicles B, and this growth is independent of FSH.Then, as the cycle progresses, FSH secretion recruits growing follicles to begin development into vesicular (antral) follicles. C. During the last few days of maturation of vesicular follicles, estrogens, produced by follicular and thecal cells, stimulate increased production of LH by the pituitary gland (Fig. 3.1), and this hormone causes the follicle to enter the mature vesicular (graafian) stage, to complete meiosis I, and to enter meiosis II, where it is arrested in metaphase approximately 3 hours before ovulation. follicle. Coincident with final development of breaks free (ovulation) and floats out of the the vesicular follicle, there is an abrupt increase ovary (Fig. 3.3). Some of the cumulus oopho- in LH that causes the primary oocyte to com- rus cells then rearrange themselves around the plete meiosis I and the follicle to enter the zona pellucida to form the corona radiata preovulatory mature vesicular stage. Meiosis II (Figs. 3.2B to 3.6). is also initiated, but the oocyte is arrested in metaphase approximately 3 hours before ovula- Corpus Luteum tion. In the meantime, the surface of the ovary After ovulation, granulosa cells remaining in begins to bulge locally, and at the apex, an avas- the wall of the ruptured follicle, together with cular spot, the stigma, appears. The high con- cells from the theca interna, are vascularized centration of LH increases collagenase activity, by surrounding vessels. Under the influence of resulting in digestion of collagen fibers sur- LH, these cells develop a yellowish pigment and rounding the follicle. Prostaglandin levels also change into lutein cells, which form the corpus increase in response to the LH surge and cause luteum and secrete estrogens and progesterone local muscular contractions in the ovarian wall. (Fig. 3.3C). Progesterone, together with some Those contractions extrude the oocyte, which estrogen, causes the uterine mucosa to enter the together with its surrounding granulosa cells progestational or secretory stage in prepara- from the region of the cumulus oophorus tion for implantation of the embryo. Antrum Granulosa cells Luteal cells Ovarian stroma Theca interna Theca externa Blood vessels 1st polar body Oocyte in Cumulus 2nd meiotic oophorus Fibrin division cells A Mature vesicular follicle B Ovulation C Corpus luteum Figure 3.3 A. Mature vesicular follicle bulging at the ovarian surface. B. Ovulation. The oocyte, in metaphase of meiosis II, is discharged from the ovary together with a large number of cumulus oophorus cells. Follicular cells remaining inside the collapsed follicle differentiate into lutean cells. C. Corpus luteum. Note the large size of the corpus luteum, caused by hyper- trophy and accumulation of lipid in granulosa and theca interna cells. The remaining cavity of the follicle is filled with fibrin. 32 Part 1 General Embryology Figure 3.4 Relation of fimbriae and ovary. Fimbriae collect the oocyte and sweep it into the uterine tube. Oocyte Transport syncytiotrophoblast of the developing embryo. Shortly before ovulation, fimbriae of the uter- The corpus luteum continues to grow and forms ine tube sweep over the surface of the ovary, and the corpus luteum of pregnancy (corpus the tube itself begins to contract rhythmically. It luteum graviditatis). By the end of the third is thought that the oocyte, surrounded by some month, this structure may be one third to one granulosa cells (Figs. 3.3B and 3.4), is carried into half of the total size of the ovary.Yellowish luteal the tube by these sweeping movements of the cells continue to secrete progesterone until the fimbriae and by motion of cilia on the epithelial end of the fourth month; thereafter, they regress lining. Once in the tube, cumulus cells withdraw slowly as secretion of progesterone by the tro- their cytoplasmic processes from the zona pellu- phoblastic component of the placenta becomes cida and lose contact with the oocyte. adequate for maintenance of pregnancy. Removal Once the oocyte is in the uterine tube, it is of the corpus luteum of pregnancy before the propelled by peristaltic muscular contractions of fourth month usually leads to abortion. the tube and by cilia in the tubal mucosa with the rate of transport regulated by the endocrine FERTILIZATION status during and after ovulation. In humans, the fertilized oocyte reaches the uterine lumen in Fertilization, the process by which male and approximately 3 to 4 days. female gametes fuse, occurs in the ampullary region of the uterine tube. This is the wid- Corpus Albicans est part of the tube and is close to the ovary If fertilization does not occur, the corpus luteum (Fig. 3.4). Spermatozoa may remain viable in the reaches maximum development approximately female reproductive tract for several days. 9 days after ovulation. It can easily be recognized as Only 1% of sperm deposited in the vagina a yellowish projection on the surface of the ovary. enter the cervix, where they may survive for Subsequently, the corpus luteum shrinks because many hours. Movement of sperm from the of degeneration of lutean cells (luteolysis) and cervix to the uterine tube occurs by muscular forms a mass of fibrotic scar tissue, the corpus contractions of the uterus and uterine tube and albicans. Simultaneously, progesterone produc- very little by their own propulsion. The trip tion decreases, precipitating menstrual bleeding. from cervix to oviduct can occur as rapidly as If the oocyte is fertilized, degeneration of the 30 minutes or as slow as 6 days. After reach- corpus luteum is prevented by human chori- ing the isthmus, sperm become less motile and onic gonadotropin, a hormone secreted by the cease their migration. At ovulation, sperm again  Chapter 3 First Week of Development: Ovulation to Implantation 33 become motile, perhaps because of chemoat- ampulla is not an advantage, since capacitation has tractants produced by cumulus cells surrounding not yet occurred and such sperm are not capable the egg, and swim to the ampulla, where fertil- of fertilizing the egg. Much of this conditioning ization usually occurs. Spermatozoa are not able during capacitation occurs in the uterine tube and to fertilize the oocyte immediately upon arrival involves epithelial interactions between the sperm in the female genital tract but must undergo and the mucosal surface of the tube. During this (1) capacitation and (2) the acrosome reac- time, a glycoprotein coat and seminal plasma pro- tion to acquire this capability. teins are removed from the plasma membrane that Capacitation is a period of conditioning in overlies the acrosomal region of the spermato- the female reproductive tract that in the human zoa. Only capacitated sperm can pass through the lasts approximately 7 hours. Thus, speeding to the corona cells and undergo the acrosome reaction. A Corona radiata cells Phase 1 Phase 2 Polor body in division Acrosome Inner acrosomal membrane dissolves Sperm nucleus Secondary oocyte in Plasma 2nd meiotic division membrane Fusion oocyte and sperm cell membranes B Phase 3 Figure 3.5 A. Scanning electron micrograph of sperm binding to the zona pellucida. B. The three phases of oocyte pen- etration. In phase 1, spermatozoa pass through the corona radiata barrier; in phase 2, one or more spermatozoa penetrate the zona pellucida; in phase 3, one spermatozoon penetrates the oocyte membrane while losing its own plasma membrane. Inset shows normal spermatocyte with acrosomal head cap. 34 Part 1 General Embryology Figure 3.6 A. Oocyte immediately after ovulation, showing the spindle of the second meiotic division. B. A spermatozoon has penetrated the oocyte, which has finished its second meiotic division. Chromosomes of the oocyte are arranged in a vesicular nucleus, the female pronucleus. Heads of several sperm are stuck in the zona pellucida. C. Male and female pronuclei. D,E. Chromosomes become arranged on the spindle, split longitudinally, and move to opposite poles. F. Two-cell stage. The acrosome reaction, which occurs after enzymes (acrosin) allows sperm to penetrate the binding to the zona pellucida, is induced by zona zona, thereby coming in contact with the plasma proteins.This reaction culminates in the release of membrane of the oocyte (Fig. 3.5). Permeability of enzymes needed to penetrate the zona pellucida, the zona pellucida changes when the head of the including acrosin- and trypsin-like substances sperm comes in contact with the oocyte surface. (Fig. 3.5). This contact results in release of lysosomal enzymes The phases of fertilization include from cortical granules lining the plasma mem- brane of the oocyte. In turn, these enzymes alter Phase 1, penetration of the corona radiata properties of the zona pellucida (zona reaction) Phase 2, penetration of the zona pellucida Phase 3, fusion of the oocyte and sperm cell to prevent sperm penetration and inactivate spe- cies-specific receptor sites for spermatozoa on the membranes zona surface. Other spermatozoa have been found Phase 1: Penetration of the Corona embedded in the zona pellucida, but only one Radiata seems to be able to penetrate the oocyte (Fig. 3.6). Of the 200 to 300 million spermatozoa normally deposited in the female genital tract, only 300 Phase 3: Fusion of the Oocyte and to 500 reach the site of fertilization. Only one Sperm Cell Membranes of these fertilizes the egg. It is thought that the The initial adhesion of sperm to the oocyte is others aid the fertilizing sperm in penetrating the mediated in part by the interaction of integrins barriers protecting the female gamete. Capacitated on the oocyte and their ligands, disintegrins, on sperm pass freely through corona cells (Fig. 3.5). sperm. After adhesion, the plasma membranes of the sperm and egg fuse (Fig. 3.5). Because the Phase 2: Penetration of the Zona plasma membrane covering the acrosomal head Pellucida cap disappears during the acrosome reaction, The zona is a glycoprotein shell surrounding the actual fusion is accomplished between the oocyte egg that facilitates and maintains sperm binding membrane and the membrane that covers the and induces the acrosome reaction. Both binding posterior region of the sperm head (Fig. 3.5). In and the acrosome reaction are mediated by the the human, both the head and the tail of the sper- ligand ZP3, a zona protein. Release of acrosomal matozoon enter the cytoplasm of the oocyte, but A B  Chapter 3 First Week of Development: Ovulation to Implantation 37 Figure 3.8 Development of the zygote from the two-cell stage to the late morula stage. The two-cell stage is reached approximately 30 hours after fertilization; the four-cell stage is reached at approximately 40 hours; the 12- to 16-cell stage is reached at approximately 3 days; and the late morula stage is reached at approximately 4 days. During this period, blasto- meres are surrounded by the zona pellucida, which disappears at the end of the fourth day. The main results of fertilization are as follows: blastomeres (Fig. 3.8). Until the eight-cell stage, they form a loosely arranged clump (Fig. 3.9A). Restoration of the diploid number of After the third cleavage, however, blastomeres chromosomes, half from the father and half maximize their contact with each other, forming from the mother. Hence, the zygote contains a compact ball of cells held together by tight junc- a new combination of chromosomes different tions (Fig. 3.9B).This process, compaction, segre- from both parents. Determination of the sex of the new gates inner cells, which communicate extensively by gap junctions, from outer cells. Approximately individual. An X-carrying sperm produces a 3 days after fertilization, cells of the compacted female (XX) embryo, and a Y-carrying sperm embryo divide again to form a 16-cell morula produces a male (XY) embryo. Therefore, the (mulberry). Inner cells of the morula constitute chromosomal sex of the embryo is determined the inner cell mass, and surrounding cells com- at fertilization. Initiation of cleavage. Without fertilization, pose the outer cell mass. The inner cell mass gives rise to tissues of the embryo proper, and the oocyte usually degenerates 24 hours after the outer cell mass forms the trophoblast, which ovulation. later contributes to the placenta. CLEAVAGE BLASTOCYST FORMATION Once the zygote has reached the two-cell stage, it About the time the morula enters the uterine undergoes a series of mitotic divisions, increasing cavity, fluid begins to penetrate through the the numbers of cells. These cells, which become zona pellucida into the intercellular spaces of the smaller with each cleavage division, are known as inner cell mass. Gradually, the intercellular spaces A B Figure 3.9 Scanning electron micrographs of A uncompacted and B compacted eight-cell mouse embryos. In the uncompacted state, outlines of each blastomere are distinct, whereas after compaction, cell–cell contacts are maximized, and cellular outlines are indistinct. 38 Part 1 General Embryology A B C Figure 3.10 A. Section of a 107-cell human blastocyst showing inner cell mass and trophoblast cells. B. Schematic representation of a human blastocyst recovered from the uterine cavity at approximately 4.5 days. Blue, inner cell mass or embryoblast; green, trophoblast. C. Schematic representation of a blastocyst at the sixth day of development showing trophoblast cells at the embryonic pole of the blastocyst penetrating the uterine mucosa. The human blastocyst begins to penetrate the uterine mucosa by the sixth day of development. become confluent, and finally, a single cavity, the involved in interactions between leukocytes and blastocele, forms (Fig. 3.10A,B). At this time, endothelial cells that allow leukocyte “capture” the embryo is a blastocyst. Cells of the inner cell from flowing blood. A similar mechanism is now mass, now called the embryoblast, are at one pole, proposed for “capture” of the blastocyst from and those of the outer cell mass, or trophoblast, the uterine cavity by the uterine epithelium. flatten and form the epithelial wall of the blasto- Following capture by selectins, further attach- cyst (Fig. 3.10A,B). The zona pellucida has dis- ment and invasion by the trophoblast involve appeared, allowing implantation to begin. In the integrins, expressed by the trophoblast and the human, trophoblastic cells over the embryoblast extracellular matrix molecules laminin and fibro- pole begin to penetrate between the epithelial nectin. Integrin receptors for laminin promote cells of the uterine mucosa on about the sixth day attachment, while those for fibronectin stimulate (Fig. 3.10C). New studies suggest that L selec- migration. These molecules also interact along tin on trophoblast cells and its carbohydrate signal transduction pathways to regulate tro- receptors on the uterine epithelium mediate phoblast differentiation, so that implantation is initial attachment of the blastocyst to the uterus. the result of mutual trophoblastic and endome- Selectins are carbohydrate-binding proteins trial action. Hence, by the end of the first week 40 Part 1 General Embryology Figure 3.11 Events during the first week of human development. 1, oocyte immediately after ovulation; 2, fertilization, approximately 12 to 24 hours after ovulation; 3, stage of the male and female pronuclei; 4, spindle of the first mitotic divi- sion; 5, two-cell stage (approximately 30 hours of age); 6, morula containing 12 to 16 blastomeres (approximately 3 days of age); 7, advanced morula stage reaching the uterine lumen (approximately 4 days of age); 8, early blastocyst stage (approxi- mately 4.5 days of age; the zona pellucida has disappeared); and 9, early phase of implantation (blastocyst approximately 6 days of age). The ovary shows stages of transformation between a primary follicle and a preovulatory follicle as well as a corpus luteum. The uterine endometrium is shown in the progestational stage. The proliferative phase begins at the end of the occur, the endometrium assists in implantation menstrual phase, is under the influence of estro- and contributes to formation of the placenta. gen, and parallels growth of the ovarian follicles. Later in gestation, the placenta assumes the role The secretory phase begins approximately 2 to of hormone production, and the corpus luteum 3 days after ovulation in response to progesterone degenerates. produced by the corpus luteum. If fertilization At the time of implantation, the mucosa of the does not occur, shedding of the endometrium uterus is in the secretory phase (Fig. 3.12), during (compact and spongy layers) marks the begin- which time uterine glands and arteries become ning of the menstrual phase. If fertilization does coiled and the tissue becomes succulent. As a Maturation of follicle Ovulation Corpus luteum Corpus luteum of pregnancy Implanted embryo Implantation begins Gland Compact layer Spongy layer Basal layer 0 4 14 28 Menstrual phase Follicular or Progestational or Gravid phase proliferative phase secretory phase Figure 3.12 Changes in the uterine mucosa correlated with those in the ovary. Implantation of the blastocyst has caused development of a large corpus luteum of pregnancy. Secretory activity of the endometrium increases gradually as a result of large amounts of progesterone produced by the corpus luteum of pregnancy.  Chapter 3 First Week of Development: Ovulation to Implantation 41 Figure 3.13 Changes in the uterine mucosa (endometrium) and corresponding changes in the ovary during a regular menstrual cycle without fertilization. result, three distinct layers can be recognized in Summary the endometrium: a superficial compact layer, an intermediate spongy layer, and a thin basal With each ovarian cycle, a number of primary layer (Fig. 3.12). Normally, the human blastocyst follicles begin to grow, but usually only one implants in the endometrium along the anterior reaches full maturity, and only one oocyte is or posterior wall of the body of the uterus, where discharged at ovulation. At ovulation, the oocyte it becomes embedded between the openings of is in metaphase of the second meiotic division the glands (Fig. 3.12). and is surrounded by the zona pellucida and some If the oocyte is not fertilized, venules and granulosa cells (Fig. 3.4). Sweeping action of tubal sinusoidal spaces gradually become packed with fimbriae carries the oocyte into the uterine tube. blood cells, and an extensive diapedesis of blood Before spermatozoa can fertilize the oocyte, into the tissue is seen. When the menstrual they must undergo phase begins, blood escapes from superficial arteries, and small pieces of stroma and glands 1 Capacitation, during which time a break away. During the following 3 or 4 days, glycoprotein coat and seminal plasma proteins the compact and spongy layers are expelled from are removed from the spermatozoon head the uterus, and the basal layer is the only part 2 The acrosome reaction, during which of the endometrium that is retained (Fig. 3.13). acrosin- and trypsin-like substances are released This layer, which is supplied by its own arteries, to penetrate the zona pellucida the basal arteries, functions as the regenerative layer in the rebuilding of glands and arteries in During fertilization, the spermatozoon must the proliferative phase (Fig. 3.13). penetrate 42 Part 1 General Embryology 1 The corona radiata enters the uterus on the third or fourth day after 2 The zona pellucida fertilization, a cavity begins to appear, and the blastocyst forms. The inner cell mass, which 3 The oocyte cell membrane (Fig. 3.5) is formed at the time of compaction and will As soon as the spermatocyte has entered the develop into the embryo proper, is at one pole oocyte, of the blastocyst. The outer cell mass, which surrounds the inner cells and the blastocyst cavity, 1 The oocyte finishes its second meiotic divi- will form the trophoblast. sion and forms the female pronucleus The uterus at the time of implantation is in 2 The zona pellucida becomes impenetrable to the secretory phase, and the blastocyst implants other spermatozoa in the endometrium along the anterior or pos- terior wall (Fig. 3.12). If fertilization does not 3 The head of the sperm separates from the tail, occur, then the menstrual phase begins, and the swells, and forms the male pronucleus (Figs. 3.6 spongy and compact endometrial layers are shed. and 3.7) The basal layer remains to regenerate the other After both pronuclei have replicated their layers during the next cycle (Fig. 3.13). DNA, paternal and maternal chromosomes intermingle, split longitudinally, and go through a mitotic division, giving rise to the two-cell stage. Problems to Solve The results of fertilization are 1. What is the role of the corpus luteum, and 1 Restoration of the diploid number of what is its origin? chromosomes 2. What are the three phases of fertilization, 2 Determination of chromosomal sex and what reactions occur once fusion of the 3 Initiation of cleavage sperm and oocyte membranes takes place? 3. What are the primary causes of infertility in Cleavage is a series of mitotic divisions that men and women? results in an increase in cells, blastomeres, which become smaller with each division. After three 4. A woman has had several bouts of pelvic divisions, blastomeres undergo compaction inflammatory disease and now wants to have to become a tightly grouped ball of cells with children; however, she has been having diffi- inner and outer layers. Compacted blastomeres culty becoming pregnant. What is likely to be divide to form a 16-cell morula. As the morula the problem, and what would you suggest?