Germ Cells, Stem Cells And Somatic Cells, Testicular And Ovarian Development Notes

What are the distinctions between germ cells, stem cells and somatic cells? Compare and contrast testicular and ovarian development, structure and function.

Germ cells, stem cells and somatic cells differ in fundamental ways, although there are several types of each. Stem cells are identifiable by three key characteristics, those of multipotency/pluripotency (or indeed, totipotency), indefinite self-renewal, and the production of progeny which are committed to differentiate to different cell types. Self-renewal means that stem cells divide to produce daughter stem cells (as well as those daughter cells which will form differentiating somatic cell precursors) for the lifespan of the organism. Multipotency means that stem cells can give rise to at least 3 different types of differentiated daughter cells, pluripotency and totipotency respectively more types and all types in the final body; these are the cells found in the zygote which can give rise to cell types not found in the final embryo, such as trophoblasts which form the amniotic sac and part of the placenta. Stem cells divide to produce more stem cells as well as committed progenitor cells which will differentiate into different cell types depending upon the type of stem and progenitor cells; these new cells with replenish groups of somatic cells which may have been lost due to damage or age. Stem cells therefore give rise to somatic cells, for example haematopoietic stem cells in bone marrow give rise to differentiated red blood cells via progenitor cells and reticulocytes to replace red blood cells broken down by the liver after around three months. Germ cells can be viewed as a specialised subset of stem cells; these are responsible for spermatogenesis and oogenesis as they produce haploid daughter cells which differentiate into gametes and can be found in adults in the reproductive organs. Germ cells are crucial to the process of creating new combinations of alleles as, uniquely, they divide by meiosis, randomly segregating parental chromosomes and resulting in haploid gametes which can combine to create genetically different offspring. Somatic cells, however, are differentiated, have limited lifespans and are not able to self-renew. Somatic cells constitute all the differentiated cell types in the body, from fibroblasts to osteocytes to chromaffin cells, and once formed they rarely differentiate again. They are the end progeny of stem cells and not involved in reproduction or generation of new cells, but carry out the normal functions of their tissue. Somatic cells are also involved in nurturing populations of stem cells, for example the somatic support cells in male and female gonads, Sertoli cells and ovarian follicle cells.

The primitive gonads, precursors of both testes and ovaries, develop from the interactions between both germ cells and somatic cells. Adult germ cells are derived from primordial germ cells, PGCs, which may be differentiated by germ plasm, a cytoplasmic component of cells shown in drosophila to induce germ cell formation by transplantation into eggs of a different phenotype, resulting in some phenotypic offspring from the donor egg². PGCs are known via cell tracing studies to originate in the epiblast cells of the yolk sac and undergo a lengthy migration via the endodermal gut tube to the presumptive gonad region of intermediate mesoderm inside the coelomic cavity near the body’s dorsal wall. Here they induce the formation of the genital, or gonadal, ridges on the medial sides of the mesonephroi by stimulating coelomic epithelium cells to proliferate and differentiate to somatic support cells¹. Expression of the WT1 gene and the SF1 gene it activates³ is vital for the formation of the gonadal ridge; SF1 knockout mice display renal and gonadal agenesis and persistent müllerian ducts and sex reversal in males⁴. Similarly, humans with mutations in the SF1 gene have identified as 46,XY-karyotype individuals with female external genitalia and persistent müllerian ducts⁴. The somatic support cells invest the germ cells in the 6^th week, a critical event as the germ cells will degenerate without their support. At the same time, the müllerian ducts begin to form lateral to the mesonephric ducts which, along with the mesonephroi, are part of the primitive embryonic kidney.

These are the last stages which are common to the development of both male and female embryos, or ‘ambisexual’. Around the start of 7^th week, testis-determining factor, the SRY gene from the Y chromosome, becomes active in males and stimulates the indifferent, bipotential gonads to form testes rather than the ‘basic’ female development path. This, the primary sex determination, is initiated by the SRY protein which in turn initiates a cascade of other signals. Firstly, the somatic support cells become Sertoli cells and secrete anti-müllerian hormone, which is responsible for the degeneration of the müllerian ducts in males. The somatic pre-Sertoli cells also recruit other somatic cells from the mesonephric mesenchyme in 9^th week that become Leydig cells and start to secrete testosterone. Testosterone is then able to initiate differentiation of many male reproductive features such as the epididymis, vas deferens and seminal vesicles (from the Wolffian duct) in the embryo, and further male reproductive development at puberty. It is also the precursor of dihydrotestosterone which initiates development of the prostate gland and external genital features such as the penis and scrotum¹. (It should not be forgotten that these hormones also act on the developing brain to produce ‘male’ psychological characteristics.) Testosterone is also responsible for upkeep of the Wolffian duct in male embryos, as a crystal of testosterone implanted next to a rabbit foetus’ ovary induces the formation of a Wolffian duct. However, a whole testis grafted next to a foetal ovary produced not only Wolffian duct formation but also müllerian duct degeneration, proving...

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