Spermatogenesis

Spermatogenesis progresses through three phases sper-matocytogenesis, meiosis, and spermiogenesis.[1'2] Sper-matocytogenesis is the mitotic divisions of spermatogonia present in the basal compartment at the onset of spermatogenesis. Spermatogonia proliferate and differentiate, becoming primary spermatocytes. Primary sperma-tocytes are then transported to the adluminal compartment during transient dissolution of Sertoli cell junctions. Spermatocytes proceed through first meiosis, during which four chromatids of each paired and replicated homologous chromosome form a tetrad. Chromatids of adjacent chromosome pairs often fuse at discrete points so that portions of chromatids are exchanged, contributing to the genetic variation between sperm. Secondary sperma-tocytes, the daughter cells of first meiosis, undergo second meiosis rapidly to form haploid round spermatids. No further DNA replication or cell division occurs in cells that have completed meiosis.

A round spermatid is genetically competent to initiate embryonic development if microinjected into a mature egg, a method of in vitro fertilization used in some cases of human infertility. However, dramatic morphological changes occur as the cytoplasm and organelles of the spermatid are extensively reorganized and modified. Key among these changes in domestic animal sperm are nuclear condensation and formation of the acrosome, mitochondrial helix, and flagellum.[2] With these changes, the spermatid gradually elongates to form a sperm.

Nuclear condensation is caused by addition of prot-amines to the nuclear chromatin with formation of disulfide bonds that condense the chromatin, prevent any further transcription, and impart the characteristic shape of the sperm head (typically paddle-shaped in farm animals). Formation of a flagellum, characterized by the 9 + 2 arrangement of microtubules typical of cilia and flagella in other cells, is directed by the distal centriole. The sperm flagellum or tail is also characterized by nine

SERTOLI CELLS MYOID CELLS

Fig. 1 Organization of the mammalian testis and excurrent ducts. (From Ref. 6, used with permission.)

SERTOLI CELLS MYOID CELLS

Fig. 1 Organization of the mammalian testis and excurrent ducts. (From Ref. 6, used with permission.)

coarse outer fibers that help determine the flagellar beat pattern. Vesicles elaborated by the Golgi apparatus become associated with the nucleus and ultimately form the acrosome, a lysosome-like structure that conforms to the anterior sperm head. The acrosome contains enzymes that aid in sperm penetration of the egg during fertilization. Mitochondria align end-to-end in a helical arrangement surrounding the base of the flagellum and form the sperm midpiece. Most of the spermatid cytoplasm is removed as a residual body during release of the sperm from the seminiferous tubule, referred to as spermiation.

ENDOCRINE AND PARACRINE REGULATION OF SPERMATOGENESIS

Basic aspects of neuroendocrine regulation of the hypothalamic pituitary gonadal axis are well established in the male. Gonadotropin releasing hormone (GnRH) from the hypothalamus stimulates release of the gonadotropins LH and FSH from the anterior pituitary gland. LH stimulates testosterone release from Leydig cells, whereas FSH stimulates Sertoli cell proliferation during development and various Sertoli cell functions.[2,3] Germ cells lack receptors for testosterone and FSH, suggesting that these hormones regulate spermatogenesis indirectly through their actions on Sertoli cells. Precise roles for these hormones in spermatogenesis remain under investigation. FSH appears to stimulate spermatogonial proliferation, while testosterone promotes spermatid association with Sertoli cells during spermiogenesis.[4] Both hormones promote germ cell survival by inhibiting apoptosis (programmed cell death) and are necessary for quantitatively normal spermatogenesis.

KINETICS OF SPERMATOGENESIS

Quantitative and qualitative aspects of spermatogenesis have been described for many mammalian species.[1,2] Briefly, a differentiating group of spermatogonia in the basal compartment begins spermatocytogenesis within a short section of the seminiferous tubule. These cells progress along with more advanced generations of germ cells (spermatocytes and spermatids) within the same tubule section through a series of well-characterized cellular associations called stages.[1,2,5] Progression is from the basal to the adluminal compartment. Stages reappear in a cyclic pattern within a tubule section as new generations of spermatogonia enter the process. Stages also appear sequentially along the tubule length, forming a wave as the spermatogenic cycle is slightly advanced in adjacent tubule sections. The wave thus provides a mechanism for providing continuous sperm release without overfilling or occluding tubules.

Fig. 2 Cytology of the bull testis. (From Ref. 2, used with permission.)

The exquisite timing of the cycle is intrinsically controlled and always the same within a species. Coordination of spermatogenic events is partly a function of intimate Sertoli germ cell association and the persistence of cyto-plasmic bridges between germ cells within each generation. Within a tubule section, several cycles are required for a group of spermatogonia to complete spermatogenesis and emerge as sperm, a lengthy process that takes 39 days in the boar to 61 days in the bull. The precise timing and long duration of spermatogenesis are important considerations in identifying causes for reduced semen quality and fertility.

SPERM TRANSPORT AND SEMEN

From the seminiferous tubule, sperm enter the rete testis, a network of collecting channels in the central core of farm animal testes (Fig. 1). From the rete, sperm move into a series of efferent ducts that exit the testis and converge with the initial segment of the single, highly convoluted epididymal duct. Anatomically, the epididymis is a discrete organ attached to the side of the testis and characterized by caput (head), corpus (body), and cauda (tail) regions. Changes to sperm during epididymal transit, collectively referred to as sperm maturation, are required for the acquisition of sperm motility and fertilizing ability.

Sperm transport is a function of hydrodynamic flow of male tract secretions, contraction of smooth muscle surrounding the excurrent ducts, and ciliary beat in the efferent ducts. Sperm transport from the testis through the epididymis is intrinsically regulated and requires many days.

During ejaculation, sperm are transported from the epididymis to the urethra by contractions of the vas deferens and combine with accessory sex gland secretions.

Accessory glands in farm animals include the seminal vesicles (which secrete the most fluid), the prostate, and the Cowper's glands. Seminal fluid provides a vehicle for sperm transport during ejaculation. However, numerous macromolecules are novel to or enriched in seminal fluid and have been ascribed various roles in sperm transport and fertilization.1-6-1

Semen, the combination of sperm and seminal fluids, can be harvested by intervention with an artificial vagina as the male mounts another animal or dummy mount, by controlled electrical stimulation of the pelvic genitalia, and other methods. Ejaculate volume averages approximately 1, 5, 100, and 200 ml in the ram, bull, stallion, and boar, respectively, with corresponding average sperm concentrations of approximately two billion, one billion, 100 million, and 200 million sperm per ml.[6] Sperm collected from farm animals is used for artificial insemination and in vitro fertilization and can be cryopreserved. In fact, the vast majority of dairy cattle in the United States are bred with frozen semen.

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