Spermatid organelles

Spermiogenesis

The nuclear envelope

The nuclear envelope is only similar to that of classical cells in the youngest spermatids.
It is made of two membranes separated by a space #60 nm and is decorated with nuclear pores located on its entire surface. In the round spermatids, the nuclear pores make rapid contacts with the chromatoid body (nucleo-cytoplasmic exchanges ?).
Before the release of the manchette, the inter-membrane space of the nuclear envelope is linked to the endoplasmic reticulum, forming a nucleo-cytoplasmic exchange apparatus. The endoplasmic reticulum is lost afterwards.
As soon as the acrosomal vesicles are secreted, a few nm wide zone is differentiated : the two membranes of the nuclear envelope are closer (distance # 5 nm) and they lose their nuclear pores. The perinuclear substance first appears as a diffuse substance, made up of sulfur-containing proteins, linked to the thinnest parts of the nuclear envelope. The acrosomal vesicles join it.
This differentiated zone spreads progressively backwards.
By the end of spermiogenesis, the nuclear envelope is totally differentiated with the exception of the portion called "redundant nuclear envelope" containing pores, and surrounding the "basal knobs" (most often two of them, located at the posterior part of the nucleus).

The perinuclear substance

The perinuclear substance spreads progressively over the surface of the differentiating nuclear envelope : the two membranes are closer and lose their nuclear pores. In round spermatids, the differentiated nuclear envelope is generally decorated by a thin layer of heterochromatin, often associated with one nucleolus. The spreading of the perinuclear substance precedes that of the acrosome on the nuclear surface. The perinuclear substance cannot be considered as an ordinary cytoplasm remnant located between the acrosome and the nucleus. In the recently hypophysectomized animal, the perinuclear substance spreads normally, but is unable to fix the acrosome. It contains sulfur rich proteins and globular actin. By the end of spermiogenesis, the apical part of the perinuclear substance is differentiated as a perforatorium, containing dehydrated protein crystalloids. In few species, the perinuclear substance also contains pseudo crystalline cords or lamella, the striated bodies encircling the nucleus. Photo, JL Courtens

Perinuclear substance and perforatorium in a late spermatid of the rabbit.

The manchette

Immediately before nuclear elongation, and following the attachment of the flagellum to the nucleus, microtubules, issued from the proximal centriole attach to the nuclear envelope in the portion not covered with the acrosome. They make a sort of skirt, limited, as a belt, by the (peri) nuclear ring, attached to the plasma membrane. The microtubules are linked together by a network of fibers, and are linked to the nuclear envelope by intermediate fibers. The experimental isolation of nuclei shows that they are also linked to the chromatin by fine fibers passing through the nuclear envelope. The manchette plays a morphogenetic role on nuclei. In fact, when a new manchette reforms after being temporarily depolymerized by colcemide, its shape is often odd, and the nuclei are also modified. The manchette is internally (and also externally, in the rat) lined by a network of endoplasmic reticulum linked to the nuclear envelope. The three organelles are the constituents of the nucleocytoplasmic exchange apparatus implied in the transition of spermatid specific nucleoproteins. Nuclear disorders in protein composition are obtained after the destruction of the manchette by colcemide. The nuclear ring slips backwards at the end of nuclear elongation, driving the backward movement of the manchette. This movement is followed by the depolymerisation of microtubules the disorganization and loss of the endoplasmic reticulum the appearance of the postacrosomal lamina Photo, JL Courtens

The postacrosomal lamina

The postacrosomal lamina is a proteic differentiation of the plasma membrane that it lines up internally in the portions of the nucleus not covered by the acrosome.

It is built up when the (peri) nuclear ring and associated manchette slips backwards.

It is linked to the nuclear envelope, by the perinuclear substance and is limited by the acrosome, and the postnuclear ring. The roles of the postacrosomal lamina are probably important, because, this portion of the plasma membrane is the first to be recognised by the oocyte at fertilization.

The postnuclear ring

The post nuclear ring appears before nuclear elongation. It's shape is a torus being located both in and out of the nucleus. It marks the limit between the differentiated / not differentiated zones of the nuclear envelope. The external torus is attached to the plasma membrane. By its morphological organization, the postnuclear ring tightly separates the anterior and posterior compartments of the spermatid. It can be considered as a motor for cell differentiation, because when slipping backwards at the end of nuclear elongation, it drives the movement of the nuclear ring and of the associated manchette. Following this movement, the perinuclear substance spreads posteriorly and the postacrosomal lamina is synthesized. Photo JL Courtens

The chromatoid body

The chromatoid body is the male equivalent to the "nuages" found in female germ cells and ova. This germ cell specific organelle is polymorph and is present in primary spermatocytes as the inter-mitochondrial cement linking two to three mitochondria. In spermatids, it is isolated, forming a proteic mass surrounded (rat) or not by glycoprotein vesicles. In the young round spermatids it is located close to the Golgi apparatus and the centrioles, and it moves backwards with them prior to nuclear elongation. When it approaches the implantation fossa of the flagellum (step 8), it becomes associated with the annulus, a differentiated portion of the plasma membrane limiting the cytoplasmic channel of the flagellum. In late spermiogenesis, the annulus slips towards a more distal position, allowing the mitochondria to affix to the flagellum, forming a typical helix. The chromatoid body disappears during this migration. In its distal position, the annulus is called the "Jensen ring", separating the main and mid pieces of the flagellum. The chromatoid body contains very basic proteins and long-lived (at least 15 days) messenger RNA. Nucleolar proteins and proteins specific of the interchromatin granules are also present. A part of the splicing machinery for processing of the pre-messenger RNA seems to be stored in this place. The chromatoid body is sometimes called the cytoplasmic nucleolus. It's most probable role is related to the storage of mRNA to be used by the haploid cells. The annulus is in a proximal position in spermatids step 13 (left and right) and distal in step 14 (center/left). Photos JLCourtens

The Centrioles

photo JL Courtens

The two centrioles, proximal (on the nuclear side) and distal (on the flagellar side) are located in the center of a cytoplasmic area devoid of organelles, but containing microtubules organizers implicated in the formation of the mitotic apparatus. Each of them is constituted by a cylinder made of 9 triplets (association of 3 microtubules). The distal centriole is the origin of cilia and flagella. In the spermatids, a flagellum is present at step 1. According to the species, it is either temporarily intra-cytoplasmic or soon projecting outside the cell. The centrioles are located first close to the acrosome, and migrate in a distal position before being associated with the nucleus at step 7, defining an antero/posterior cell axis. The attachment site is differentiated as a nuclear implantation fossa for the flagellum made of a substance similar to the perinuclear substance, and also associated with the nuclear envelope.

The neck of a spermatid photo JL Courtens

Immediately before the nuclear elongation, the length of the proximal centriole increases, forming the centriolar adjunct probably involved in the synthesis of the manchette microtubules. This transitory adjunct disappears with the manchette.

The articular piece of the neck is constituted by a several striated columns joining the base of the nucleus, and linked to the dense fibers surrounding the axoneme. In many mammals, the neck is specially rich in Zn.

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