Learning aims
- Origin and migration of the germ cells
- Male gonadal primordium
- Female gonadal primordium
- Spermatogenesis
- Oogenesis
What you should already know
- General cytology
- Mitosis
- Meiosis
Delving deeper
- Why do the gametes have to be haploid?
- How is haploidy achieved?
- When are gametes produced?
- How can the enormous difference in size between female and male gametes be explained?
- Wherein lies the importance of the germline?
Introduction
Mature germ cells are also termed gametes. There are male gametes, the spermatozoa (sperm cells), and female gametes, the oocytes (egg cells). Through the fusion of the gametes during the fertilization a zygote is created, the first cell of a new individual. In order for it to have the normal number of 46 chromosomes, each of the gametes has half, i.e., 23 chromosomes. The reduction of the number of chromosomes and the recombination of the genetic material are processes, both of which occur during meiosis. Meiosis is a special form of cell division that only takes place in gametogenesis.
The male gametes are produced in large numbers in the testicles from puberty onwards and for the rest of the man's life. Several million of them are present in a typical ejaculate.
- Head of a sperm cell
- Tail of a sperm cell
- Epithelium of the epididymal duct
In this picture a large number of spermatozoa are seen during their passage through the ductus epididymis.
The female gametes - oocytes - are already generated and stored during the embryonic and fetal periods and a certain number of them (1 - 2 million) are present in both ovaries when a baby girl is born. The number of the oocytes in the ovaries is thought to constantly diminish right up to menopause. During the fertile period in the life of a woman, from menarche until menopause, roughly 400 oocytes (approximately 13 periods x 30 years) are ovulated. Hormonal regulation, operating in cycles, is responsible for the maturing and expulsion of the oocytes from the follicles.
- Cytoplasma of the oocyte (ooplasma)
- Cell nucleus
- Nucleolus
- Pellucid zone
- Corona radiata cells
- Cytoplasmic processes of a corona radiata cell
The picture shows an oocyte (diameter ca. 0.1 mm) from a human tertiary follicle.
In these two images (Figs 1. and 2.), the enormous difference in size between human oocytes and spermatozoa becomes evident: In the oocyte nucleolus numerous sperm cell heads would have room. The oocyte is surrounded by a thick glycoprotein layer (pellucid zone) through which cytoplasmic processes of the corona radiata cells pass, providing conduits for oocyte nourishment to take place.
Mature germ cells and their direct antecedents are special cells. They derive from primordial germ cells that first emigrate into the umbilical vesicle (stage 11) and return after a period of time (stage 14) for further development.
These can be identified already in the second week of embryonic development, at a moment in time in which the gender of the embryo has indeed been established chromosomally, but the associated features of the respective sex are not yet present.
Following an extended journey the primordial germ cells arrive at the initially primitive and sexually undifferentiated gonadal primordium in the sixth week. It is only through this "immigration" of the primordial germ cells that the gonadal primordium can further - and later gender - specifically develop.