Page 298 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

280 Veterinary Histology of Domestic Mammals and Birds

is hormonally regulated. The process by which germ cells smaller, denser nucleus in which the chromatin is more
VetBooks.ir (spermatogonia) are transformed into spermatozoa is clumped. B-spermatogonia are not in contact with the
described as spermatogenesis.
basement membrane. These pear-shaped cells have a het-
erochromatic nucleus with usually only one nucleolus.
SPERMATOGENESIS At the end of telophase, spermatogonia undergo incom-
The term spermatogenesis describes the processes of plete division, leaving cytoplasmic connections between
multiplication and differentiation that occur in the semi- daughter cells. These bridges facilitate exchange of sub-
niferous tubules to produce male gametes. These cells stances between cells and aid in coordinating subsequent
then undergo further maturation within the epididymis. steps in the process of cell division. The cytoplasmic con-
Spermatogenesis is divided into two stages: nections are only lost towards the end of spermiogenesis,
shortly before delivery of spermatozoa into the tubular
· spermatocytogenesis and lumen.
· spermiogenesis. Primary spermatocyte (spermatocytus prima-
rius): Having arisen from the mitotic division of
Spermatocytogenesis begins with the division of the male B -spermatogonia, primary spermatocytes (Figures 13.4
2
spermatogonial stem cells, the spermatogonia, into two to 13.6) pass through all stages of meiotic prophase:
daughter cells. One of these remains as a new stem cell. leptotene, zygotene, pachytene, diplotene and diakine-
The other undergoes further mitotic divisions, ultimately sis. Exchange of paternal and maternal genetic material
giving rise to primary spermatocytes. These cells go occurs during this process. This initial phase of meiosis I
through a first maturation (meiotic) division to produce lasts around 20 days, with the subsequent phases (meta-
secondary spermatocytes. Immediately after their forma- phase, anaphase and telophase) progressing more quickly.
tion, secondary spermatocytes undergo a second meiotic Consequently, primary spermatocytes are observed at all
division, resulting in the formation of four haploid sperma- stages of the seminiferous epithelial cycle. During meiosis
tids (see Chapter 1, ‘The cell’, ‘Meiosis’). I (first maturation division), the spermatocytes migrate
During subsequent spermiogenesis, spermatids differ- towards the tubular lumen and the volume of the cells
entiate into sperm cells (spermatozoa) without further gradually increases.
cell division. Primary spermatocytes are characterised by nuclei con-
taining chromatin strands of varying density, a large Golgi
sPeRmatocytogenesis apparatus, abundant mitochondria and rough ER. They
The number of mitotic divisions of the spermatogonia (and are diploid, the number of chromosomes varying with spe-
the duration of spermatogenesis) is species-dependent. The cies (dog 78, cat 38, pig 38, sheep 54, horse, ox and goat 60).
following sequence is considered to occur in the bull: one The amount of DNA is doubled through the formation of
A -spermatogonium divides into two A -spermatogonia. chromatid pairs. Meiosis I (reductional division) gives rise
1 2
One of the A -spermatogonia divides to give two further to two secondary spermatocytes, each containing half the
2
A -spermatogonia, that initially remain dormant as new original complement of chromosomes (haploid) and half
1
stem cells. The other A -spermatogonium divides to form the amount of DNA.
2
two A -spermatogonia. One of these degenerates while Secondary spermatocyte (spermatocytus secundar-
3
the other divides mitotically to give rise to I-, B - and ius): After a brief interkinetic period, a second maturation
1
B -spermatogonia from which primary spermatocytes division occurs in which secondary spermatocytes are
2
are formed. At this stage, multiplication of the newly transformed into spermatids. This equatorial division is
formed A -spermatogonia is initiated. Multiplication of not preceded by doubling of the DNA, thus the resulting
1
spermatogonia follows a similar schema in other domestic cells are haploid and contain half the amount of DNA of
species, with variations in the number and nomenclature the secondary spermatocyte. As secondary spermatocytes
of the individual divisions. divide almost immediately, they are rarely encountered in
Spermatogonium: Spermatogonia vary in their histological sections.
structure and in their location within the wall of the semi- Secondary spermatocytes lie near the tubular lumen.
niferous tubule. They are around 16 μm in diameter and have a spherical,
A-spermatogonia (diameter 13 μm) are closely asso- weakly staining nucleus.
ciated with the tubular basement membrane and are
present at all stages of the seminiferous epithelial cycle sPeRmiogenesis
(Figures 13.4 and 13.5). These stem cells are spherical During spermiogenesis, spermatids undergo conspicuous
and have an ellipsoid, euchromatic nucleus that may con- changes through which they are transformed into sper-
tain several nucleoli. The cytoplasm is rich in organelles. matozoa (Figures 13.4 to 13.7). These modifications occur
I-spermatogonia (intermediate spermatogonia) have a gradually and mainly involve the nucleus, Golgi appara-

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