Comparative Spiralian Oogenesis—Structural Aspects: An Overview

Considerable variety exists in ovarian structure and cellular interaction in spiralians. During their development the eggs of Dwpatra cuprea , are associated with nurse cells; there are no follicle cells in this species. The nurse cells have prominent nuclei and connect to oocytes via cytoplasmic bridges through which ribosomes, mitochondria and other inclusions pass. More commonly, follicle cells surround a portion of, or entire, oocytes in many species of spiralians. Usually, as in Ilyanassa , they have a well developed compliment of organelles. The structure and distribution of organelles within follicle cells implies that they are functionally active, but precisely in what manner during oogenesis is poorly understood. Other cell types, such as Leydig and interstitial cells also seem to play a role in oogenesis. Within the oocyte, a host of components including yolk, lipid, mitochondria, ribosomes, membranous cisternae, cortical granules, etc. are accumulated. Autosynthetic yolk formation is prevalent among spiralians. Surface differentiation includes microvillar development. This may be uniform in some eggs or restricted to certain regions ( e.g. , the animal hemisphere) in other oocytes. Oocyte-follicle cell interactions change during oogenesis. The topographical association of the oocyte with other ovarian cells influences subsequent animal-vegetal polarity and other ooplasmic differences. Examples of ooplasmic localizations are discussed. Conventional EM has revealed no unusual cortical structure in many oocytes although occasionally microtubules and microfilaments are present.