Homeostasis and soma-germ line interaction

Submitted to Submitted byDepartment of Zoology Bhavya VashishtKurukshetra University M.Sc. (F) ZoologyKurukshetra Roll No. 4 Sem III

Contents• Introduction• Weismann's Theory of the Continuity of the germplasm• Determination between germ line and soma and their

interaction– In Unicellular organisms– In Insects– In C. elegans– In Mammals

i. Germline and soma interaction during early mouse developmentii. Interaction between Sertoli cells and germ cells in the testis • Conclusion

Introduction• Homeostasis is a process that maintains the stability of the

human body's internal environment in response to changes in external conditions. The concept was described by Claude Bernard in 1865 and the word was coined by Walter Bradford Cannon in 1926

• Weismann recognized that animals are made up of: Somaplasm (body cells) , which contain Germplasm (gamete-producing cells) • The cell line derived from somatic cells is called the soma cell

line and the cell line derived from germ cells is called the germ line

Weismann theory of continuity of germplasm

At each generation, the embryo that develops from the zygote not only sets aside some germplasm for the next generation but also produces the cells that will develop into the body, the soma, of the organism. In Weismann's view, the somaplasm simply provides the housing for the germplasm, seeing to it that the germplasm is protected, nourished, and conveyed to the germplasm of the opposite sex to create the next generation.

Determination between Germline and soma cell line

In unicellular organisms

In microorganisms, all life's functions are embodied in a single cell. However, some unicellular organisms, like the ciliated protozoan have a complete genome in their micronuclei, which are passed on to the next generation, as well as genes in a macronucleus, which is not.

In insectsAt the 4th mitotic division in the gall midge (an insect) egg, 2 of the 16 nuclei become pinched off in a small amount of cytoplasm at one end of the egg. At the 5th mitosis, these two nuclei divide normally, producing daughter cells with the full complement of chromosomes (2n = 40) of the species.But this is not so for the other nuclei. When each of these nuclei reaches anaphase, only 8 of their 40 chromosomes (dyads) separate and move to opposite ends of the spindle. The remaining 32 chromosomes stay at the metaphase plate and eventually disintegrate.

In Caenorhabditis elegansAt each of the first four mitotic divisions, one daughter cell (red) loses this totipotentiality and all her daughters will be restricted to forming the somatic tissues of the animal. The second daughter cell — shown in blue in the figure (P1, P2, P3, and P4) retains its totipotentiality. At the 5th mitotic division, both daughter cells remain totipotent and go on, in due course, to form the sperm and eggs. The different fates of the germline (blue) and somatic cells (red) are controlled by certain proteins that are retained in the germline but destroyed in the somatic cells.

In Mammals

Ectoderm cells of future extra embryonic membrane secrete

cytokines

Signals cells in developing mesoderm

Allantois or amnion

Primordial germ cells

Germline and soma interaction during early mouse development

• Primordial germ cells have first been identified in the mouse during gastrulation, about 7.25 days post coitum, as a compact clump of cells located in extra embryonic position. One side of the clump is bounded by yolk sac endoderm, the other by extra embryonic mesoderm.

• Evidence that the progenitors of primordial germ cells come from embryonic ectoderm (epiblast) is that injection of a fluorescent lineage marker into single epiblast cells 6-6.5 days post coitum, followed by analysis of serial sections after 1-2 days cultured in vitro, has established that primordial germ cells and all the rest of the extra embryonic mesoderm, are derived from cells located in the proximal region of the epiblast.

Germ cell migration During the period that germ cells translocate from their initial extra-embryonic

location into their definitive location within the urinogenital ridge, there is evidence for somatic influences exerted on the germ cells.

For example-

The extra cellular matrix glycoprotein fibronectin appears to play an important role in Xenopus germ cell migration. Migrating mouse germ cells explanted in vitro adhered preferentially to fibronectin rather than to laminin or collagen type I. As they start to leave the hindgut their adhesion to fibronectin decreases.

In Genital Ridge• The migrating germ cells reach the site of the future gonad 10-11 days post

coitum. During the same period, somatic cells are massing in the same region, to form the genital ridge. These cells are derived in part from the pre existing mesenchyme, but mostly from the degenerating mesonephric tubules and also probably from the coelomic epoithelium. The first somatic cells to differentiate are from the supporting cell lineage, which in the male gives rise to Sertoli cells.

• Until the time mouse germ cells enter genital ridges, their appearance and behavior seems identical whether they are in a female or a male embryo.

Interaction between germ cells and Sertoli cells in Testes

The Sertoli cells and the germ cells communicate with each other by following methods

1. Sertoli cell-germ cell contact2. Gap junctions between Sertoli cells and Germ cells3. Proteins secreted by Sertoli cells

Diagram showing Germ cells and Sertoli cells

Sertoli cell Germ cell contact• Sertoli cells and germ cells are in close physical proximity within the seminiferous

tubules of the testis. Direct Sertoli cell- germ cell contact could certainly influence cell function.

• For example, when pachytene spermatocytes were co- cultured with Sertoli cells, an increased secretion of androgen binding protein (ABP) induced by follicle stimulating hormone (FSH) was observed. This increased secretion was not seen when the two cell types were separated by a layer of agar.

Diagram showing Sertoli cell Germ Cell Contact

Gap Junctions between Sertoli cells and Germ cells

• The Sertoli cells and spermatogonic cells communicate with the help of desmosomes. When these are open, connexons ( protein subunit of gap junctions) form channels which are 1.6 to 20 nm in diameter. Intercellular secondary messengers could therefore pass between cells.

Diagram showing gap junctions

Proteins secreted by Sertoli cells

• Sertoli cells secrete numerous proteins which can be subdivided into following groups -

1. Transporters2. Proteases and protease inhibitors3. Basement membrane proteins4. Hormone and growth factors

Transporters• Transferrin is an iron transport protein secreted by Sertoli cells

and may be required for germ cell division and subsequent development.

• Ceruloplasmin is a copper transport protein which also acts as feroxidase in converting Fe+2 to Fe+3 and may be essential to loading of iron onto transferrin by Sertoli cells.

• Androgen binding protein (ABP) and insulin like growth factor-3 (IGF-3) which may also have roles in transport

Protease and Protease Inhibitors Proteases (involved in remodeling of seminiferous epithelium.)1. plasminogen activator2. metalloproteases 3. type IV collagenase.

Protease inhibitors (regulate the function of these proteases)4. α2 – macroglobulin 5. cystatin

Basement membrane Proteins of seminiferous tubules

1. Type IV collagen2. Lamanin3. proteoglycans

• These may have a direct effect on spermatogonia that are in intimate contact with basement membrane.

• Lamanin shares sequence similarity with epidermal growth factor and can stimulate proliferation of gonocytes isolated from neonatal testes and co cultured with Sertoli cells.

Hormones and Growth Factors Transforming growth factor-β (TGF-β) secreted by

sertoli cells could have an effect on developing spermatogonia, TGF-β may act to suppress growth of spermatogonia prior to puberty.

Somatomedin C can bind to pachytene spermatocytes in vitro suggesting it may have some influence on meiosis.

Injecting inhibin into mice caused reduction in number of spermatogonia in testes.

Conclusion

• There are different mechanisms for determination between the soma cell line and germ line in different group of animals. The interaction between the soma and the germ line plays a key role in the development of an organism.

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