Embryology: 1st week of development
The first week of embryonic development is filled with an eclectic arrangement of physical and biochemical changes. Each step is a part of a cascade of events that must be intricately coordinated in order to produce a healthy baby at the end of the thirty eight to forty week period. However, the events of the first week of gestation are highly dependent on prior events that create the ideal environment for fertilization and implantation to occur. This article aims at briefly reviewing the processes preceding fertilization. There will be detailed evaluation of the ovulation cycle, copulation, fertilization and the first week of development.
- Formation of gametes
- The menstrual cycle
- Fertilization of the oocyte
- Cleavage and migration of the zygote
- Early implantation of the blastocyst
- Clinical aspects
Formation of gametes
Meiosis is a modified form of cellular division that results in the production of genetically unique haploid (containing 23 chromosomes) progeny from a mature diploid cell (contains 46 chromosomes). There are several differences between the typical form of cellular replication known as mitosis and the mechanism by which gametes (sex cells) are formed. The overall nomenclature of the stages of meiosis is similar to that of mitosis.
The differences are such that:
- meiosis is divided into two stages - meiosis I and II
- prophase I has five subdivisions - leptotene, zygotene, pachytene, diplotene, and diakinesis
- prophase I also contains a stage during which there is exchange of genetic information between homologous pairs of chromosomes
- meiosis II does not have an interphase as there is very little time between the completion of meiosis I and the commencement of meiosis II
There are staunch differences between spermatogenesis (the development of male gametes) and oogenesis (development of female gametes). Spermatogenesis commences at the onset of puberty with the aid of hormones from the hypothalamic – pituitary – gonadal axis. The chief hormone that stimulates spermatogenesis is testosterone. It is produced by Leydig cells within the testes and acts on the sertoli cells within the seminiferous tubules (also in the testes).
They eventually promote the transformation of spermatogonia to primary spermatocytes, which then form secondary spermatocytes and finally spermatids. Four haploid spermatids are the derived from each spermatogonium. The spermatids are round and must undergo further maturation to become spermatozoa in order to be effective in reproduction. Therefore, the cells enter the spermiogenesis phase; where the cells elongate, develop an acrosome and grown a tail. The finished, mature spermatocyte (i.e. spermatozoa) is subsequently stored within the epididymis until they are needed.
Oogenesis on the other hand begins during intrauterine life. Oogonia begin to enlarge and proliferate as they become primary oocytes. They also enter the reduction replication process but are arrested in early prophase I. The primary oocytes are also enclosed within a thin layer of flat cells known as granulosa (follicular) cells. These cells are responsible for stalling the meiotic process until the onset of puberty. The primary oocyte and granulosa layer are collectively referred to as the primordial follicle.
At birth, there are roughly 2 million primordial follicles within the ovaries of the female infant. About 98% of these follicles will be reabsorbed during childhood; and of the remaining 2%, only about 400 primordial follicles will mature over the reproductive lifespan of the individual (i.e. from menarche to menopause). Also note that the use of chemical contraceptive methods (pills, patches, injections) significantly reduces the amount of follicles that mature.
Both the surrounding follicular cells and enclosed oocyte continue to develop. The follicular cells transition from squamous to cuboidal then columnar cells; following which they become stratified around the growing oocyte forming primary follicle . The surrounding cells are subsequently joined by the bi-layered theca folliculi. The hypothalamic – pituitary – gonadal axis also influences the maturation of the follicle and oocyte.
The primary oocyte only completes meiosis I after ovulation has occurred (i.e. the oocyte has been extruded from the follicle. The progeny of this division is a small, redundant first polar body and a significantly larger secondary oocyte. However, it is arrested in metaphase II until fertilization takes place. Prior to this evolution, the follicle develops a fluid filled antrum and becomes a secondary follicle.
The menstrual cycle
The hypothalamic – pituitary – gonadal axis not only promotes oocyte maturation, but it also acts on the endometrial lining of the uterus in order to prepare it for possible implantation. This is a cyclical process that occurs each month and is characterised by proliferation and shedding of the endometrial lining. The overall process is called the menstrual or endometrial cycle.
Day one of the cycle is marked by shedding of the inner lining of the uterus and usually lasts for about 5 to 7 days. This occurs as a result of a reduction in progesterone in cases where fertilization does not occur. Leading up to this point in time, the corpus luteum (structure that remains after a tertiary [Graafian] follicle ruptures), which is responsible for secreting progesterone, degenerates. Consequently, friable vessels and eroded endometrial lining bleeds and dead endometrium slough off. This is referred to as the menstrual phase.
Subsequently there is growth of the selected ovarian follicle and proliferation of the endometrial lining. This portion – also called the proliferative phase – usually lasts around 9 days and corresponds to an increase in estrogen secretion. Estrogen promotes repair of the superficial endometrium as well as an increase in the size and number of glands in the lining. It also acts on the spiral arteries that perfuse the endometrium, resulting in lengthening of these vessels.
At the end of the proliferative phase, the Graafian follicle ruptures under the influence of luteinizing hormone, releasing the secondary oocyte. Recall that oocytes are immobile and consequently rely on the fimbriae of the fallopian tubes to sweep it into the ampulla. Several things occur during this period:
- The walls of the Graafian follicle involute and form the corpus luteum.
- Increased progesterone secreted from the corpus luteum results in engorgement of the epithelial glands of the endometrial lining.
- The secondary oocyte enters the ampulla of the fallopian tube, where it awaits fertilization. Whether or not fertilization occurs, the oocyte moves toward the uterine cavity via peristaltic forces of the tube.
This stage is called the luteal phase and it usually lasts about 13 days. Failure of fertilization to occur ushers in the ischemic phase. The unfertilized oocyte continues via peristaltic activity towards the uterine cavity where it degenerates and is reabsorbed. There is contraction of the spiral arteries and reduction in glandular secretions as the supply of supporting hormones decline. Extended period of spiral artery constriction results in venous stasis and ischemic necrosis of the endometrium.
Copulation or the act of sexual intercourse facilitates the deposition of seminal fluid from the penis into the vaginal vault. The process is quite multifaceted, involving psychosocial, biochemical and physiological components. Strictly from a biological perspective, arousal of males requires parasympathetic involvement in order to attain an erection. This results in vasodilation and filling of the bulbous cavernous with blood. Arousal in females also results in clitoral engorgement, but more importantly the glands of the vaginal vault secrete lubricants that facilitate penetrance.
Following insertion of the penis into the vaginal orifice, a series of pelvic thrusting culminates in male (and possibly female) ejaculation. Ejaculation occurs in two phases. In the emission phase the semen migrates to the prostatic urethra via the ejaculatory ducts. In the ejaculatory phase, the vesical sphincter closes at the neck of the bladder (to prevent diversion of semen into the bladder), the bulbospongiosus and urethral muscles contract, resulting in expulsion of semen from the external urethral os.
The peristaltic contractions of the ductus deferens are regulated by the sympathetic nervous system. To aid in remembering this, recall that parasympathetic system allows the male to point (erection) and the sympathetic system allows the male to shoot (ejaculate). Roughly 400 million spermatocytes are expressed in the fornix of the vaginal and around the external os of the uterus.
Motility of the sperm is affected by the environmental pH. Therefore the cells move slower in the acidic vagina, but increase in speed in the more alkali uterus. When the woman is ovulating, her cervical plug is less viscous, which makes it easier for spermatocytes to propel through the cervical os with their powerful tails. The enzyme vesiculase (product of the seminal glands) which was added to semen induces coagulation of some of the ejaculate. This process leads to the formation of a vaginal plug that can also reduce the possibility of backflow from the uterus to the vagina.
As the semen enters the uterus, prostaglandins that were added to the seminal fluid stimulate uterine contractions. This helps the spermatocytes to reach the fallopian tubes a lot faster, where they can meet a waiting secondary oocyte. Only about 200 sperm actually makes it to the ampulla of the fallopian tube.
Fertilization of the oocyte
The sperm that make it into the uterus undergo further processing in order to successfully fertilize an oocyte. Glands of the uterus and fallopian tubes release activating factors that remove the seminal protein and glycoprotein coating of the spermatocyte. These structures usually coat the outer surface of the acrosome and would otherwise prevent the acrosome reaction from occurring. The resultant spermatocytes are more active than they previously were; but they are still structurally identical to their previous state. Oocytes secrete chemotactic agents that attract the spermatocytes to their location.
The entire process takes about 24 hours to complete. It begins with the release of hyaluronic acid from the acrosome and production of tubal mucosal enzymes. The enzymes scatter the corona radiata (follicular cells) that surrounds the secondary oocyte, thus allowing the sperm to propel forward.
The spermatocyte then encounters the zona pellucida deep to the corona radiata. The acrosome will attach to zona pellucida sperm binding protein 3 (ZP3). This surface glycoprotein on the zona pellucida of the secondary oocyte, along with calcium ions, sperm plasma membrane, progesterone and prostaglandins are intricately involved in the impending acrosome reaction. The resulting channel that is formed releases acrosin, esterases and neuraminidase enzymes that lyse the zona pellucida. The spermatocyte can then follow the progressing pathway. Acrosin also stimulates the zona reaction, which renders the zona pellucida impermeable to successive spermatocytes.
Fusion of the spermatocyte and oocyte plasma membrane results in degradation of both plasma membranes at the point of contact. At this point, the secondary oocyte completes meiosis II and forms a mature oocyte. The second polar body that is generated from this division is extruded (along with the first).
The chromosomes of the mature oocyte then decondense and form a pronucleus. The nucleus of the spermatocyte also begins to enlarge within the cytoplasm of the mature oocyte and also forms a pronucleus. The resulting oocyte now has two haploid pronuclei within its membrane. At this stage it is referred to as an ootid. As the pronuclei fuse and the chromosomes mix, a single celled diploid zygote with a unique arrangement of chromosomes is formed. Chromosomal gender is dependent on the genotype of the fertilizing spermatocyte and is determined at the point of fertilization. If the spermatocyte was 23, Y then the genetic sex is male; while a 23, X spermatocyte would yield a genetic female.
Cleavage and migration of the zygote
The resulting zygote undergoes a series of mitotic division, which commences about 30 hours after fertilization has occurred. With each division, the progeny (known as blastomeres) become smaller. The blastomeres are retained within the zona pellucida. Peristaltic activity continues to move the dividing ball of cells from the fallopian tube into the uterine cavity. When there are more than 8 blastomeres within the zona pellucida, the cells undergo compaction, where each cell realigns with adjacent cells to maximize the space available. It also prepares the cells to separate into the inner and outer cell masses. Division continues and on the 3rd day after fertilization, there are 12 to 32 blastomeres clustered together. The resulting ball of cells is referred to as a morula.
The morula now arrives in the uterine cavity on the 4th day post fertilization. It begins to accumulate fluid from the uterine cavity via the fenestrated zona pellucida. The resulting fluid filled space separates the blastomeres into a thin outer trophoblast and a cluster of blastomeres. The trophoblast provides nourishment for the developing blastocyst and the inner cells – which will subsequently be referred to as the embryoblast – gives rise to the embryo. This overall stage is known as blastogenesis; and the product of conception is referred to as the blastocyst.
Early implantation of the blastocyst
As the blastocyst floats in the uterine cavity, there is degradation of the zona pellucida to facilitate rapid growth and expansion of the blastocyst. The embryo absorbs nutrients from the uterine secretions. On day 6 the trophoblasts differentiates into the cytotrophoblast and syncytiotrophoblast as the blastocyst attaches to the endometrium at the embryonic pole.
The syncytiotrophoblast (which is the outer layer of the blastocyst) extends papillary projections into the endometrium and releases enzymes that breaks down the maternal tissue. This allows further implantation of the blastocyst into the endometrium. Additionally, the embryo can derive more nutrients from the maternal cells as it continues to dig deeper into the endometrium. By the 7th day, the superficially implanted blastocyst has a primary endoderm (hypoblast) that resides on the blastocystic surface of the embryoblast.
The events of the first week of pregnancy often goes unnoticed. The expectant mother will continue her regular routines, unaware of the intricately orchestrated events that are underway. On rare occasions, some females may notice implantation spotting towards the end of the first week. This phenomenon occurs as the blastocyst begins to burrow into the endometrial lining.
The enzymes secreted from the syncytiotrophoblast layer of the blastocyst causes breakdown of the capillary beds at the implantation site. As a result, the capillaries are converted into sinuses into which the blood will drain. The nutrients subsequently diffuses across the trophoblast membrane to nourish the blastocyst. The debris generated from the process is shed via the vaginal os, similar (but lighter than) to the natural menstrual flow. However, this may be misinterpreted as the onset of their monthly menses.
The subtle events present an even more exhilarating surprise if the conceptus is actually a multiple pregnancy (i.e. twins, triplets, etc). Multiple pregnancies either result from more than one oocytes being released at the time of ovulation and each being fertilized by an individual sperm (dizygotic) or a single oocyte that was fertilized divides into two separate zygotes (monozygotic).
Monozygotic twins are also referred to as identical twins, as their genetic composition contains very little disparity. This phenomenon arises after the zygote is formed and undergoes its first cell division. At the two cell stage the cells partition into separate zygotes, each of which will continue to undergo typical development. The resulting embryo may either have one inner cell mass and two embryonic discs, or two inner cell masses and two embryonic discs. Both these embryos will share a chorion (i.e. monochorionic). However, the conceptus with two inner cell masses will have their own amniotic sacs (diamniotic), while the conceptus with one inner cell mass will share an amniotic sac (monoamniotic).
The occurrence of dizygotic twins depends on polyovulation; where the expectant mother would have released at least two oocytes during a single ovulatory period and both would have been successfully fertilized. The series of events (cell division, implantation, etc) would occur for each zygote as it usually does in a singleton pregnancy. All dizygotic twins are dichorionic, and will have their own amniotic sac as well.
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