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Glossary - Z




Z1, Z4

Somatic gonad precursor cells (S)

Blast cells which act as stem cells for the somatic gonad.

See Somatic primordium

Z2, Z3

Germinal cells (S) Germline precursors (S)
Primordial germ cells (S)

Blast cells which act as stem cells for the germline sperm and oocytes.

See Primordial germ cell

Z band

Z line (S)
D band (S)
D zone (S)

A linear array of Z discs in a well ordered, striated muscle (as in many vertebrate muscles), or a staggered array of dense bodies in an obliquely striated muscle (as in C. elegans). In a vertebrate muscle, this zone runs perpendicular to the muscle sarcomeres, whereas in the nematode, due to the large stagger of the dense bodies, the band runs virtually parallel to the muscle (only diverging by 7-12 degrees).

See A band
See I band
See Myofilament lattice

Z disc

Z disk (S)
Z body (S)
Dense body (S)

A discrete dark-staining attachment site (equivalent to the dense body in nematode muscle) at which actin filaments are anchored at either end of a single sarcomere. Where the Z discs are perfectly flat, the orderly array at the vertebrate Z line can look very linear, but in many cases the Z-discs are concave, or slightly out of alignment, so that the Z line takes on a zigzag or step-like appearance. The corresponding dense bodies in the nematode muscle are spaced far apart with an extreme stagger.

Z line  

A well described transverse structure of striated muscles of higher animals, which is replaced in obliquely striated muscle in C. elegans by an oblique series of Z bands. Z lines represent the terminal structure of the sarcomere.

See Z band
Zona pellucida Oolemma (S)
Egg membrane (S)

This term (from Latin meaning transparent zone) refers to the coating of the early oocyte, including its plasma membrane. It can derive from several sources:

1) secretions of the oocyte
secretions of the gonadal sheath cells
3) secretions contributed by the uterus after fertilization. 

Direct observation by TEM in C. elegans shows at least three distinct stages in its development, but do not clearly delineate their sources. Within the gonad sheath, the primary oocyte’s plasma membrane is i) covered by a basal lamina, within the spermatheca it is ii) briefly covered by a fertilization membrane, and in the uterus the fertilization membrane is quickly replaced by iii) the eggshell, which is overlain by a vitelline layer, and underlain by a lipid layer. The earlier stages (basal lamina and fertilization membrane) may be considered developmental stages of the zona pellucida, but the eggshell probably shares no components and ought to be considered a separate entity.

See Fertilization membrane
See Lipid layer
See Vitelline layer


Archetypal body pattern for metazoan animals.


Egg (S)

The diploid cell formed from the union of two haploid gametes.

Zygote formation  

The process through which an oocyte and a spermatozoon combine to form a fertilized embryo, which includes the fertilization event, creation of a fertilization membrane, eggshell formation, reformation and migration of the pronuclei, formation of a pseudocleavage constriction, pronuclear rotation and fusion, disappearance of the procleavage constriction, constriction of the cytoplasm, first mitosis and formation of the first cleavage membrane (Schierenberg, 1978; Gönczy and Rose, 2005; Oegema and Hyman, 2006).


A step in meiotic prophase progression, preceding pachytene. Germline cells in the “transition zone” of the distal gonad arm enter this stage coordinately.

Edited for the web by Laura A. Herndon. Last revision: October 4, 2013. This section should be cited as: Herndon, L.A. and Hall, D.H. 2010. Glossary Z. In WormAtlas.  doi:10.3908/wormatlas.6.26