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