Overview – Embryology

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Gametes and their predecessor cells (the primordial germ cells) are differentiated from other somatic cells very early, and emigrate from the ectoderm (third week) via the extraembryonic endoderm (fifth week) into the primordium of the future gonads, the gonadal ridge. There, through an interaction with the coelomic epithelial cells, the primordium for the testis evolves in the seventh week, if a Y chromosome is present, or the primordium for the ovary in the eighth week, if it is absent .

The development of the testis occurs under the influence of testosterone among other factors. This is produced by the Leydig's interstitial cells that stem from the mesenchyma of the gonadal ridge, in an initial stage of activity (beginning of the 7th week). A second surge of secretory activity of the same cells starts at puberty. This leads to the maturation of the gonadal epithelium and to the growth and lumen formation in the tubuli seminiferi contorti. Spermatogenesis, which takes place from puberty onwards leads to a 64 day-long cycle in which the spermatogonia develop into sperm cells. At the beginning of spermatogenesis three steps of mitosis up to primary spermatocytes type I occur before meiosis commences. The first meiosis lasts 24 days, of which the prophase, with its four typical histological phases, takes the longest time. The secondary spermatocytes are engendered in the first meiosis and they immediately continue with the second meiosis, which is very brief because neither a synthesis of DNA nor a new grouping of the chromosomes takes place. The results of the second meiosis are the haploid spermatids. Within 24 days they differentiate themselves to become sperm cells that are then released into the lumen of the tubuli. Sperm cell production happens within innumerable temporally and spatially separated spermatogenesis waves that are spread throughout the whole lengths of the tubuli, that are wound up in each other in a spiral fashion. The sperm cell production is subject to large variations with an average value of around 100 million / day.

Oogenesis begins in roughly the 7th week (stage 20). The secondary germinal cords that have grown into the ovarian cortex decompose into individual groups of cells. A lively proliferation result, whereby the oogonia, similar to the spermatogonia, remain connected with each other via cellular bridges, permitting a synchronization of the mitosis and the subsequent meiosis steps (prophase). As soon as these oogonia have commenced with meiosis, they are named primary oocytes (12th week). All oocytes are arrested in the first meiosis at the end of the prophase. This interphase is termed dictyotene and can last until adulthood. The primary oocytes loose themselves from their cellular binding; they become surrounded by flat, somatic cells (follicle or granulosa cells) and are now called primordial follicles. In the 20th week nearly 7 million germ cells are formed and the whole cortex consists of these primordial follicles. After birth, only around 2 million are present and after puberty only around 250,000 remain in each ovary.

For a successful fertilization a mature spermatozoon, with a haploid set of chromosomes, and a mature oocyte, in which the second meiosis is arrested in the metaphase, are required. The union of the two gametes normally occurs in the ampullar part of the fallopian tube. The process in which he spermatozoon penetrates the oocyte is often designated as impregnation. This causes the oocyte to complete its second meiosis and also leads to the formation of a haploid set of chromosomes. The whole fertilization, up to the formation of the zygote (condition in which the united sets of chromosomes of the two pronuclei are in the metaphase of the first mitotic division) lasts roughly 24 hours. The goal is the replication of the diploid set of chromosomes and the determination of the chromosomal gender.

The mature spermatozoon consists of:

The sperm cells go through a series of activation procedures during their ascent through the cervix, uterine cavity and tube, which is known as capacitation. Thereby certain macromolecules, which originate in the seminal fluid, are eliminated from the surface of the sperm cells. Further, the sperm cells are made more mobile and prepared for the acrosome reaction.

After a successful impregnation of the oocyte by a spermatozoon a cortical reaction of the oocyte is triggered, which leads to a polyspermy block. Further, the completion of the second meiosis (meiosis II) thereby is effected. This procedure leads to the expulsion of the second polar body. The haploid nucleic material of the spermatozoon and of the oocyte swells and forms the paternal and maternal pronuclei. They approach each other slowly and roughly 22 hours after impregnation the zygote arises in that the two sets of chromosomes move to a equatorial position within the spindle. The first cell division takes place with the mitotic spindle, which comes from the proximal centrosome of the spermatozoon. With the creation of two daughter cells the fertilization is complete. The preimplantation phase has thereby begun.

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Overview - Embryology