376
UNIT 3
Organ Systems
Figure 19-5A.
Primary follicles, ovary.
H&E,
3
202; inset
3
438
Primary follicles
develop from primordial follicles. Each
primary
follicle consists of a
primary oocyte
and
cuboidal follicle cells
.
These follicle cells increase in height (from squamous cells to
cuboidal cells), and their cellular layers gradually increase as the
follicle continues to grow. At this stage, follicle cells are called
granulosa cells
, because their cytoplasm begins to have a granu-
lar appearance. The primary follicles can be classiF
ed into
unil-
aminar primary follicles
and
multilaminar primary follicles
. The
unilaminar primary follicle
has a single layer of cuboidal granu-
losa cells with a smaller oocyte. The
multilaminar primary fol-
licle
has several layers of cuboidal granulosa cells surrounding a
relatively large oocyte. As the oocyte increases its size, the
zona
pellucida
emerges as an amorphous layer between the surface
of the oocytes and the surrounding granulosa cells (±ig. 19-5B).
Situated outside of the basement membrane of the granulosa
cells are stromal cells that fl
atten and develop into a sheath that
surrounds the follicle; this layer is called the
theca folliculi
.
Cuboidal follicle cells
Cuboidal follicle cells
of primary
of primary
(unilaminar) follicle
(unilaminar) follicle
Cuboidal follicle cells
of primary
(unilaminar) follicle
Primordial
Primordial
follicle
follicle
Primordial
follicle
Germinal
Germinal
epithelium
epithelium
Germinal
epithelium
Zona
Zona
pellucida
pellucida
Zona
pellucida
Oocyte
Oocyte
Oocyte
A
Primary
oocyte
Granulosa
cells
Theca
folliculi
Primordial
follicle
Zona pellucida
Granulosa cells
Microvilli
Theca folliculi
Cytoplasm of oocyte
Cytoplasm of oocyte
Cytoplasm of oocyte
B
Figure 19-5B.
Growing (primary) follicle.
EM,
3
3,200; inset (
color
) H&E,
3
152
The
oocyte
in the center of this
growing follicle
has been sectioned off center so that the nucleus is not shown. Although the oocyte has
begun to grow, it is still arrested in prophase of meiosis I. Note the membrane-bound vesicles in the cytoplasm of the oocyte; these will
participate in the cortical granule reaction if the oocyte becomes fertilized. The follicle cells that surrounded the oocyte have prolifer-
ated and transformed into
granulosa cells
. At this stage, the granulosa comprises about two layers of cuboidal cells. The inner granu-
losa cells no longer have smooth close contact with the surface of the oocyte because a layer of amorphous extracellular material, the
zona pellucida
, has developed. As the granulosa cells continue to proliferate, several layers of cells will accumulate, and ultimately,
a fl
uid-F
lled space, the
antrum
, will develop. Changes are also underway in the stroma adjacent to the growing follicle. The stromal
cells (
±
broblasts
) have become concentrated and fl attened against the basal lamina of the granulosa. These
theca folliculi cells
will
develop properties of steroid hormone–synthesizing cells if development of the follicle continues.
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