Proceedings of the Australian Reproduction Veterinarians Seminar, Surfers Paradise, 2016 Norman, S - Spermatogenesis and Normal Sperm Morphology – A Review

Spermatogenesis and Normal Sperm Morphology – A Review

Assoc. Prof Scott Norman BVSc, PhD, GCEd, DipACT, MANZCVS Registered Specialist in Veterinary Reproduction

Assoc. Professor in Theriogenology School of Animal and Veterinary Sciences

Locked Bag 588 Wagga Wagga, NSW, 2678

Australia

Understanding the significance of sperm morphology requires a basic understanding of sperm cytology and spermatogenesis. An overview of sperm production and sperm structure is presented in Figures 1 to 3.

Figure 1 - An overview of sperm production and delivery. From Senger 1999

Proceedings of the Australian Reproduction Veterinarians Seminar, Surfers Paradise, 2016 Norman, S - Spermatogenesis and Normal Sperm Morphology – A Review

Figure 2 - The sperm head The acrosome contains enzymes and is necessary for the sperm to enter the zona pelucida of the ovum.

Figure 3 - The sperm mid-piece and tail. From Senger 1999.

Proceedings of the Australian Reproduction Veterinarians Seminar, Surfers Paradise, 2016 Norman, S - Spermatogenesis and Normal Sperm Morphology – A Review

To demonstrate how more in-depth assessment can assist with identifying more subtle abnormalities, examples of electron-microscopy microstructure are shown in Figures 4 and 5.

Figure 4 - Electron micrographic appearance of normal spermatozoa

Figure 5 - Electron micrographic appearance of abnormal spermatozoa. Note the abnormal plasma membrane and uneven density of nuclear material.

Proceedings of the Australian Reproduction Veterinarians Seminar, Surfers Paradise, 2016 Norman, S - Spermatogenesis and Normal Sperm Morphology – A Review

Comparative Sperm Production Capacity Bull Boar Ram Stallion Dog

Sperm X 106 /g testis/day 16 27 25 20 15

Testis Wt (g) 350 360 275 200 30

Total sperm produced/day (billion)

11 19 14 8 0.5

Duration of spermatogenesis (days)

61 34 49 57 62

The Process of Spermatogenesis Spermatogenesis is the process of producing spermatozoa from stem cells which are known as Type A spermatogonia. There are three phases – proliferation (mitosis) phase; the meiotic phase; and the differentiation (= spermiogenesis) phase. Phases one and two combined are called spermatocytogenesis (as they culminate in the production of a primary spermatocytes), contrasting with spermiogenesis of Phase 3, which culminates with the formation of a spermatozoon. Spermatogenic Terms

• Stages – these represent the cell associations (as seen histologically within a section through the depth of the seminiferous epithelium) that progress past a given point within the seminiferous epithelium. It is useful to use the analogy of “semesters” in a vet degree - ie five “cell associations” throughout the depth of the vet degree represented by year one to year 5.

• Cycles – this is the time taken for a stage to repeat itself within a point of the seminiferous epithelium. It is useful to use the analogy of “years” in a vet degree.

• Spermatocytogenesis – The processes leading up to the production of primary spermatocytes

• Spermiogenesis – The processes leading up to the production (differentiation) of spermatozoafrom primary spermatocytes.

• Daily Sperm Production (DSP) – The daily sperm production capacity of the two testes.

• Daily Sperm Output (DSO) – The actual daily output that can be achieved. It approaches, but usually does not match, daily sperm production.

• Extragonadal Reserves – The number of sperm within the tubular tract. Usually including and distal to the tail of the epididymis.

• Epididymal Transit Time – The time taken for final sperm maturation as it transits from the caput to the tail of the epididymis. Only sperm within the tail of the epididymis are capable of fertilisation.

The time from differentiation of Type A spermatogonia to release of the resulting sperm in dogs is 62 days, so abnormalities in sperm may be seen much later than the insult that caused them.

Proceedings of the Australian Reproduction Veterinarians Seminar, Surfers Paradise, 2016 Norman, S - Spermatogenesis and Normal Sperm Morphology – A Review

Phase 1 (Proliferation, Mitosis) The generation of spermatogonia committed to becoming more advanced cell types, plus maintenance of stem-cell numbers. Spermatogonia are primitive cell types = Stem cells. Through mitosis there is a progression from Type A1 spermatogonia to A2, A3, A4, Type I, and Type B Most are committed to becoming spermatocytes, but some revert back to Type A spermatogonia. This latter process maintains stem-cell (Type A1) numbers. Phase 2 (Meiosis) This process produces haploid spermatids. The number of chromosomes is reduced to haploid via meiosis of Type B spermatogonia. This process generates primary and then secondary spermatocytes. The crossing over of DNA material from one chromosome to a homologous chromosome when chromatids separate during meiotic prophase ensures that each secondary spermatocyte and subsequent spermatid is genetically unique. Phase 3 (Differentiation) This process occurs “Adluminal”. This means that they are occurring on the luminal side of the Sertoli-cell tight junctions. Therefore, if major problems occur within the testis, round spermatids are usually the most primitive cell-type that will be seen – since they are able to be sloughed into the lumen of the seminiferous tubule. During normal maturation, round spermatids differentiate into viable spermatozoa. Essential is the development of a flagellum for movement and the mitochondrial helix for energy production. Morphological defects induced by heat commonly occur at the end of mitosis and first meiosis (ie primary and secondary spermatocytes) Freidman, R., Scott, M., Heath, S.E., Hughes, J.P., Daels, P.F., and Tran, T.Q., The effects of increased testicular temperature on spermatogenesis in the stallion. J. Reprod. Fert. Suppl., vol.4:127-134. 1991 It is widely accepted that many morphological abnormalities begin within the testes because of defective differentiation, or in the epididymides because of faulty maturation and/or epididymal transit (Barth and Oko, 1989).

Proceedings of the Australian Reproduction Veterinarians Seminar, Surfers Paradise, 2016 Norman, S - Spermatogenesis and Normal Sperm Morphology – A Review

Figure 6 – Despite the American terminology, this is a great analogy to use to help understand the concepts of spermatogenic stages and spermatogenic cycles. Devised

by Prof. Phil Senger (2005)

Proceedings of the Australian Reproduction Veterinarians Seminar, Surfers Paradise, 2016 Norman, S - Spermatogenesis and Normal Sperm Morphology – A Review

Figure 7 - The Spermatogenic Cycle. Note the 8 Stages (Semesters) and the 13.5 day

length of the spermatogenic "year". (Senger 2005)