Date of Award:

5-1-1971

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Biology

Department name when degree awarded

Zoology

Committee Chair(s)

Hugh P. Stanley

Committee

Hugh P. Stanley

Committee

Datus M. Hammond

Committee

Paul B. Carter

Abstract

An ultrastructural study of cellular differentiation in embryonic and mature testes of Xenopus laevis was made. Primordial germ cells were identified in the dorsal mesentery at a stage prior to gonad formation. The primordial germ cells of Xenopus possess all the characteristics common to primordial germ cells of other species, but in addition possess a highly lobate nucleus. This nuclear configuration suggests that primordial germ cells in Xenopus are metabolically quite active cells. Cells possessing the lobate nucleus and similar in size and morphology to primordial germ cells in the dorsal mesentery and early gonad can be found in all developmental stages and in the sexually mature testis. It is therefore concluded that there is a continuation of the primordial germ cell stage into the sexually mature testis. Identification of spermatogonia in embryonic stages was aided by the presence of an area of medium density in the nuclei and a dense, finely granular material directly associated with the mitochondria in germ cells and spermatogonia. Primary spermatocytes, identified by the presence of synaptinemal complexes, also contain similar granular material in association with the mitochondria. This material is similar in structure and association to the chromatoid body of mammals but differs in the stages of spermatogenesis in which it is found. A new and interesting structure is found in primary spermatocytes. It is a membrane structure in the form of flattened vesicles which persist to the spermatid stage and are apparently cast off with the cytoplasm in the form of expanded vesicles. The formation of a flagellum is the first evidence of spermatid differentiation. The acrosomal vesicle forms in close proximity to the flagellar base and it soon attaches to the nucleus. Subsequently the nucleus and attached acrosomal vesicle apparently rotate about 180 degrees. It appears, therefore, that the polarity of the spermatids as a whole is first established by the outgrowth of the flagellum, whereas nuclear polarity is made evident at the time of acrosomal adhesion. When the acrosome first attaches to the nucleus it indents the nucleus. Later as spermatid elongation begins there is a reversal in the indentation, and the acrosome folds over the anterior end of the nucleus. The reversal of indentation is interpreted as evidence of changes in fluid pressure within the two structures. The spermatid contains a few microtubules which may function in the initiation of nuclear elongation or in the movement of cytoplasm from anterior to posterior regions. These microtubules have moved away from the area of the nucleus before nuclear elongation has progressed very far. On the other hand, follicle cell microtubules appear at about the time spermatid nuclear elongation occurs and are oriented parallel to the axis of elongation forming a sheath closely surrounding each germ cell. Their time of appearance and orientation with respect to the spermatid suggests that the follicle cell microtubules may participate in the elongation processes of the spermatid. In the midpiece of intermediate spermatids through mature spermatozoa there is a conspicuous fibrous element interpreted as the homologue of ciliary rootlets associated with basal bodies in cilia. Ultrastructural observations on sterile testes of embryos treated with ultra-violet light before first cleavage indicated that there is little interaction between primordial germ cells and somatic cells prior to metamorphosis that is responsible for specific morphological differentiation of the somatic cells.

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