Cell Biology
C.S. Sureka, C. Armpilia in Radiation Biology for Medical Physicists, 2017
In plants and animals, there are two major types of cells: germ cells and somatic cells. Germ cells are involved in reproduction, for example, egg cells in female and sperm cells in male. All other cells in the body are somatic cells. Each human somatic cell contains two complete sets of chromosomes (one from each parent). This number is known as the diploid (2n) number. For example, in humans, somatic cells contain 46 chromosomes organized into 23 pairs. But germ cells have unpaired chromosomes and are known as the haploid (n) number. In humans, this number is 23 unpaired chromosomes. Out of 23 pairs of chromosomes in human cells, there are two types: 22 pairs are autosomes and 1 pair is allosome (sex chromosome). Sex chromosomes in females are homozygous (XX) and in males are heterozygous (XY).
Gametogenesis
Frank J. Dye in Human Life Before Birth, 2019
The cells of the body may be classified into two categories: somatic cells and germ cells. Somatic cells are not directly involved in reproduction, and include muscle cells, skin cells, and bone cells. By contrast, germ cells are directly involved in reproduction. The subset of germ cells directly involved in fertilization are called gametes or sex cells and include spermatozoa and ova (or sperm and eggs). The formation of gametes is gametogenesis and comes in two varieties: spermatogenesis in men and oogenesis in women. Gametogenesis is the only process in the body involving meiosis (all other cell divisions are mitotic) and is confined to the gonads. However, during development, the germ cells arise outside the embryonic body and subsequently migrate into the developing gonads.
Epigenetic Reprogramming of Mammalian Primordial Germ Cells
Cristina Camprubí, Joan Blanco in Epigenetics and Assisted Reproduction, 2018
Primordial germ cells (PGCs) are the founder population of the germline, from which the gametes (oocytes and spermatozoa) arise. Hence, they are responsible for the transmission of genetic information from one generation to the next in order to perpetuate the species. PGCs are specified in the early post-implantation embryo and this process is characterized by three extraordinary features: (i) the place of origin of the germ cells is distant from where they mature to gametes, which entails a tightly regulated migration process to reach the gonadal ridges; (ii) the germ cells go through complex genome-wide transcriptional dynamics and epigenetic reprogramming with the purpose to form oogonia in the ovary or prospermatogonia in the testis; (iii) the germ cells undergo meiosis, a process that generates genetic variability in the mature (haploid) gametes. Any defects during germ cell formation and/or migration will compromise the persistence of the specie or limit its evolution.
Connexins in the development and physiology of stem cells
Published in Tissue Barriers, 2021
Anaclet Ngezahayo, Frederike A. Ruhe
In developing organisms, pluripotency is closed after the formation of the three germ cell layers during gastrulation, which correlates with the implantation of the embryo in the maternal endometrium. Further development is relayed to stem cells in the germ layers that produce cells solely of germ cell lineage to form different tissues and organs. These stem cells in germ layers can be considered as adult stem cells. In culture, EPSCs and iPSCs do not resume gastrulation. Cells can be primed to differentiate into specific germ layer cells as mentioned above for the primitive endoderm in embryoid bodies.121 Thereafter, the cells develop into adult somatic cells corresponding to the germ cell layer. EPSCs and iPSCs are therefore a convenient model to analyze the role of Cxs and GJIC in the development of all cell types. For the nervous system, which represents the ectodermal germ layer differently, Cxs are expressed in NSCs as discussed above.
The existence and potential of germline stem cells in the adult mammalian ovary
Published in Climacteric, 2019
The mammalian ovary is a highly dynamic organ that undergoes many structural and functional changes as it fulfills its two major roles of producing female gametes and the synthesis of sex steroids. In the human ovary, germ cells (oocytes) are formed during fetal life and they are enclosed within somatic cells (granulosa cells) to form primordial follicles. The primordial follicles consist of an oocyte, arrested at the diplotene (dictyate) stage of prophase I of meiosis, surrounded by a few flattened somatic cells (granulosa cells). For many years it has been assumed that there is a limited period during which oocytes can be formed and that the adult ovary has no capacity for germ cell renewal, and therefore primordial follicles represent a pool of oocytes that must last the woman throughout her reproductive lifespan (Figure 1).
Initial germ cell to somatic cell ratio impacts the efficiency of SSC expansion in vitro
Published in Systems Biology in Reproductive Medicine, 2018
Itai Gat, Leila Maghen, Melissa Filice, Shlomit Kenigsberg, Brandon Wyse, Khaled Zohni, Peter Saraz, Andrée Gauthier Fisher, Clifford Librach
Spermatogenesis depends on cell-cell interactions between somatic and germ cell populations. The complex interplay within the testicular niche includes growth factors provided by Sertoli and interstitial cells in addition to vascular network stimuli targeted to SSCs [Franca et al. 2016]. Unique junctional complexes between SSCs and Sertoli cells are formed over the basement membrane through active cytoskeleton that regulates the blood-testis barrier, enabling metabolic support and immunological compartmentalization of germ cells [Smith et al. 2012]. SSC expansion in vitro is crucial for further clinical fertility preservation applications such as auto-transplantation or in vitro spermatogenesis (followed by IVF-ICSI) to become a reality. While SSCs can be greatly expanded in culture and able to restore fertility upon transplantation in rodents and large animals [Valli et al. 2014b], similar results are not yet achievable in human, and long term SSC proliferation using human spermatogonial stem cells remains challenging. The requirements for optimal SSC culture conditions have been suggested to differ for rodents and human SSCs, and this requires further studies [Medrano et al. 2016]. Nonetheless, the initial somatic to germ cell ratio established during primary SSC culture has been shown to impact the efficiency of early human SSC culture [Gat et al. 2017].