350
UNIT 3
Organ Systems
Spermatids
Spermatids
Spermatids
Spermatogonia
Spermatogonia
Spermatogonia
Primary
Primary
spermatocyte
spermatocyte
Primary
spermatocyte
Interstitial cell
Interstitial cell
of Leydig
of Leydig
Interstitial cell
of Leydig
Intermediate
Intermediate
Intermediate
Early
Early
Early
Sertoli
Sertoli
cells
cells
Sertoli
cells
Fibroblast
Fibroblast
Fibroblast
Late
Late
Late
Interstitial connective
Interstitial connective
tissue
tissue
Interstitial connective
tissue
Myoid
Myoid
cell
cell
Myoid
cell
A
Late spermatids
Early spermatids
Primary spermatocytes
Sertoli cell nuclei
Sertoli cells
Sertoli cells
Basement
membrane
Sertoli cells
Sertoli cells
Spermatogonium
B
Figure 18-4A.
Cells in the seminiferous tubules.
H&E,
3
458
There are two types of
cells in the seminiferous tubules
:
spermatogenic (germ) cells
and
supporting (Sertoli) cells
. The
spermatogenic cells
consist of
1.
Spermatogonia
: These cells have round or oval nuclei and
are located near the basement membrane. They can be sub-
divided into
type A
and
type B
cells.
Type A cells
are stem
cells that divide slowly and give rise to
type B cells
.
2.
Spermatocytes
: These derivatives of spermatogonia B cells
undergo meiosis. They move toward the lumen and can be
divided into
primary
(
± rst meiotic division
) and
secondary
(second meiotic division) spermatocytes
.
Primary sperma-
tocytes
in prophase are most commonly seen in sections.
Their large nuclei contain strands of condensed chromo-
somes.
Secondary spermatocytes
complete the second mei-
otic division very quickly, so they are rarely seen.
3.
Spermatids
: These cells have small interphase nuclei that
range from spherical to thin and elongated. They can be
classiF
ed as
early
,
intermediate
, or
late
spermatids
, based
mainly on the appearance of the nucleus.
Figure 18-4B.
Seminiferous epithelium.
EM,
3
4,600
This low magniF cation view shows almost the full thickness of the
seminiferous epithelium. Sertoli cells
are the nongerm cells that
organize the epithelium and divide it into two compartments,
basal
and
adluminal
(±ig. 18-12A). The only cell type in the
basal com-
partment
is the
spermatogonium
, which, like the Sertoli cells, contacts the basement membrane. This patch of epithelium appears to
be in stage one of the six stages (cell associations) of seminiferous epithelium. The
adluminal compartment
contains three cohorts of
cells, each at a different stage of
spermatogenesis
. The least advanced cells are
primary spermatocytes
in prophase of meiosis I. The
chromosomes of these diploid cells have begun condensing, and homologous pairs have aligned into synaptonemal complexes.
Early
spermatids
, which appear as small, undifferentiated cells, predominate in the middle and superF cial regions of the epithelium. The third
and most advanced cell type here is the
late spermatid
. The heads of a few can be seen surrounded by the cytoplasm of a Sertoli cell.
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