1) Spermatogenesis and oogenesis both involve meiosis to produce haploid gametes from diploid germ cells. 2) In spermatogenesis, spermatogonia undergo mitosis and differentiate into spermatocytes, then spermatids through meiosis. Spermiogenesis transforms spermatids into mature sperm. 3) In oogenesis, oogonia become primary oocytes that arrest in prophase I until after puberty. A few complete meiosis I and II if fertilized, becoming ovulated ova.

1. SELINA SRAVANTHI
SAROJINI NAIDU VANITA PHARMACY MAHA VIDYALAYA,
TARNAKA, HYDERABAD, TELANAGANA

3. Primordial Germ
Cells
Spermatogonia
10 Spermatocytes
20 Spermatocytes
Spermatids
Sperm Cells

5.  Sperm are produced within the seminiferous
tubules.
 Interspersed within the tubules are large
cells which are the sustentacular cells
(sertoli's cells), which support and nourish
the other cells.
 Spermatogenesis takes 65–75 days

6.  Early in embryonic development, primordial germ
cells enter the testes and differentiate into
spermatogonia
 It begins with the spermatogonia, which contain the
diploid (2n) number of chromosomes.
 Spermatogonia - Stem cells - when they undergo
mitosis, some spermatogonia remain near the
basement membrane of the seminiferous tubule in an
undifferentiated state to serve as a reservoir of cells
for future cell division and subsequent sperm
production.
 The rest of the spermatogonia lose contact with the
basement membrane, squeeze through the tight
junctions of the blood–testis barrier, undergo
developmental changes, and differentiate into
primary spermatocytes.
 Primary spermatocytes, like spermatogonia, are
diploid (2n); that is, they have 46 chromosomes.

7.  Each primary spermatocyte replicates its DNA and then meiosis
begins.
 The two cells formed by meiosis I are called secondary
spermatocytes.
 Each secondary spermatocyte has haploid number (n) i.e 23
chromosomes.
 Each chromosome within a secondary spermatocyte, however, is
made up of two chromatids (two copies of the DNA) still attached
by a centromere.
 No replication of DNA occurs in the secondary spermatocytes.
 In meiosis II, the chromosomes line up in single file along the
metaphase plate, and the two chromatids of each chromosome
separate.
 The four haploid cells resulting from meiosis II are called
spermatids. A single primary spermatocyte therefore produces
four spermatids via two rounds of cell division (meiosis I and
meiosis II).

9.  A unique process occurs during spermatogenesis.
As spermatogenic cells proliferate, they fail to
complete cytoplasmic separation (cytokinesis).
 The cells remain in contact via cytoplasmic
bridges through their entire development.
 This pattern of development- synchronized
production of sperm in any given area of
seminiferous tubule.
 It may also have survival value in that half of the
sperm contain an X chromosome and half contain
a Y chromosome.
 The larger X chromosome may carry genes
needed for spermatogenesis that are lacking on
the smaller Y chromosome.

10.  The final stage of spermatogenesis, spermiogenesis, is the
development of haploid spermatids into sperm.
 No cell division occurs in spermiogenesis; each spermatid
becomes a single sperm cell.
 During this process, spherical spermatids transform into
elongated, slender sperm.
 An acrosome forms atop the nucleus, which condenses and
elongates, a flagellum develops, and mitochondria multiply.
 Sertoli cells dispose of the excess cytoplasm that
sloughs off.
 Finally, sperm are released from their connections to
Sertoli cells, an event known as spermiation.
 Sperm then enter the lumen of the seminiferous tubule.
 Fluid secreted by Sertoli cells pushes sperm along their
way, toward the ducts of the testes.
 At this point, sperm are not yet able to swim.

12.  Each day about 300 million sperm
complete the process of
spermatogenesis.
 Head contains nucleus with 23 highly
condensed chromosomes
 Acrosome- a caplike vesicle filled with
enzymes (hyaluronidase and
proteases)- help a sperm to penetrate
a secondary oocyte to bring about
fertilization.
 Neck - behind the head - contains
centrioles -form the microtubules that
comprise the remainder of the tail
 Middle piece- contains mitochondria
arranged in a spiral – provide the
energy (ATP) for locomotion of sperm

13.  The formation of gametes in the ovaries is
termed OOGENESIS
 In contrast to spermatogenesis, which begins in
males at puberty, oogenesis begins in females
before they are even born.
 Oogenesis occurs in essentially the same
manner as spermatogenesis,
 Meiosis takes place and the resulting germ
cells undergo maturation.

15.  During early fetal development, primordial (primitive) germ
cells migrate from the yolk sac to the ovaries- differentiate
within the ovaries into OOGONIA
 Oogonia are diploid (2n) stem cells that divide mitotically to
produce millions of germ cells.
 Even before birth, most of these germ cells degenerate in a
process known as atresia
 A few develop- larger cells called primary oocytes that enter
prophase of meiosis I during fetal development but do not
complete that phase until after puberty.
 During this arrested stage of development, each primary
oocyte is surrounded by a single layer of flat follicular cells,
and the entire structure is called a primordial follicle
 The ovarian cortex surrounding the primordial follicles
consists of collagen fibers and fibroblast-like stromal cells.
 The remainder of the primary oocytes undergo atresia.

17.  Each month after puberty until menopause,
gonadotropins (FSH and LH)- stimulate the
development of several primordial follicles,
 A few primordial follicles start to grow,
developing into primary follicles
 Each primary follicle consists of a primary
oocyte - surrounded by several layers of cells
called granulosa cells. The outermost
granulosa cells rest on a basement membrane.
 As the primary follicle grows- forms a clear
glycoprotein layer- zona pellucida between the
primary oocyte and the granulosa cells.
 In addition, stromal cells surrounding the
basement membrane begin to form an
organized layer called the theca folliculi.

19.  On maturation - primary follicle develops into a
secondary follicle.
 Secondary follicle - the theca differentiates into two
layers:
› Theca interna - highly vascularized- secrete
estrogens
› Theca externa - outer layer of stromal cells and
collagen fibers.
 Granulosa cells - secrete follicular fluid - builds up in a
cavity - antrum in the center of the secondary follicle.
 The innermost layer of granulosa cells - firmly attached
to the zona pellucida - corona radiata

22.  The secondary follicle - becomes larger, turning into
a mature (graafian) follicle
 Just before ovulation, the diploid primary oocyte
completes meiosis I, producing two haploid (n) cells
of unequal size—each with 23 chromosomes
› The smaller cell produced by meiosis I, called the
first polar body, is essentially a packet of
discarded nuclear material.
› The larger cell, known as the secondary oocyte,
receives most of the cytoplasm.
 Once a secondary oocyte is formed, it begins
meiosis II but then stops in metaphase.
 The mature (graafian) follicle soon ruptures and
releases its secondary oocyte, a process known as
OVULATION.

23.  At ovulation, the secondary oocyte is expelled into the
pelvic cavity then into the uterine tube together with the first
polar body and corona radiata.
 If fertilization does not occur, the cells degenerate.
 If sperm are present- penetrates the secondary oocyte -
meiosis II resumes.
 The secondary oocyte splits into two haploid cells, of
unequal size- larger cell ovum, or mature egg & the
smaller one is the second polar body.
 The nuclei of the sperm cell and the ovum then unite,
forming a diploid zygote.
 First polar body undergoes another division to produce
two polar bodies.
 The primary oocyte ultimately gives rise to three haploid
polar bodies - all degenerate, and a single haploid ovum.
Thus, one primary oocyte gives rise to a single gamete
(an ovum).