Embryology: 1st week of development

Meiosis

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

Spermatogenesis

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

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.

Menstrual phase

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.

Proliferative 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.

Luteal phase

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.

Arousal

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.

Ejaculation

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.

Sperm motility

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.