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Lecture One
Gametogenesis: Conversion of Germ
Cells Into Male and Female Gametes
Primordial Germ Cells – The endoderm of the yolk sac is lined on the outside by wellvascularized extraembryonic mesoderm. During the third week, primordial germ cells,
which arise in the extraembryonic mesoderm near the base of the allantois, become
recognizable in the lining of the yolk sac. Soon these cells migrate into the wall of the
gut and the dorsal mesentery as they make their way to the gonads, where they
differentiate into oogonia or spermatogonia. Migration occurs during the fourth through
fifth weeks. During their migration to and in the indifferent gonads, they multiply by
mitosis. During the second through fifth month of pregnancy the primordial germ cells,
now properly called oogonia, undergo intense mitosis in the embryonic ovary. Shortly
thereafter, many oogonia undergo atresia (degeneration). As the oogonia enter the first
meiotic division late in the fetal period, they are called primary oocytes. Primary
oocytes enter the diplotene stage of prophase I in the early months after birth. For
males, primordial germ cells continue to divide by mitosis throughout life or differentiate
into spermatogonia just before puberty.
Chromosomes – In the human female, there are normally 22 pairs of autosomes and 1
pair of X chromosomes. In the male, 22 pairs of autosomes, plus one X, and one Y sex
chromosome. The result is a diploid (2n) number of 46 chromosomes. The letter "c"
represents the amount of DNA present or simply c = the c in chromatid. At fertilization,
for each of the 23 chromosomes (1n, 1c) donated from the egg there is a homologous
(related and hopefully normal) chromosome donated from a sperm, and vice versa.
When chromosomes condense during mitosis, each half of the pair is called a chromatid.
So, after duplication of DNA, and just when anaphase begins, there are 92 (2n, 4c)
chromatids in the dividing cell. This is still diploid, but with twice the DNA. After
anaphase, each daughter cell is back to the 46 chromosomes (2n, 2c).
During meiosis I there is a 2n, 4c stage: this is also diploid with twice the DNA. After
meiosis, each oocyte and sperm has a haploid (1n, 1c) number of chromosomes, which is
a set of 23 chromosomes (22 autosomes +1 sex). This is doubled to the diploid number
(2n, 2c) upon fertilization.
Mitosis – A cell cycle consists of a G1 phase where there is restoration of cytoplasm
after mitosis and repair of any damaged DNA before the cell goes into the S phase.
During the S phase there is duplication of DNA and centrioles. Next, there is a G2 phase
where there is an accumulation of energy and synthesis of tubulin for assembly into
microtubules to be used in the subsequent event, which is mitosis. Mitosis begins with
prophase, where there is condensation of the duplicated chromosomes, the centrioles
migrate to opposite poles of the cell, and the nuclear membrane begins to disappear.
Next, the chromosomes consisting of two chromatids joined together at a centromere,
line up on an equatorial plane. This is metaphase where the microtubules form a mitotic
spindle associated with the centromeres and centrioles. There is no nuclear membrane
here. In anaphase the chromatids separate and migrate of opposite poles of the cell. The
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last part of mitosis is telophase, which is characterized by the reappearance of nucleoli,
chromatin, and the nuclear envelope. If there is no possibility of the cell going into
mitosis (muscle, nerve) it goes into the G0 phase.
Meiosis – Meiosis is a reduction of the typical 46 chromosomes to half that number in the
ova and sperm, the gametes. In contrast to mitosis, the 46 chromosomes that have been
duplicated in the S phase do not line up along the equatorial plane, but instead, the pairs
of chromatids associate with its similarly numbered chromosome to form a homologous
pair or tetrads.
The prophase of meiosis I (reductional division) is divided into leptotene,
zygotene, pachytene, diplotene, and diakenesis stages. In the primary oocyte, the first
three stages of prophase occur promptly, but shortly after birth the process is held in the
diplotene stage until ovulation. The diplotene stage is where the homologous pairs
exchange genetic material by crossing over at areas called, chiasma. At puberty and just
before ovulation (egg breaks free from ovary), the process continues through diakenesis,
metaphase I, anaphase I, and telophase I. The homologous pairs do not separate at the
centromere, but instead, give rise to two unequal progeny: a secondary oocyte and a
polar body. The secondary oocyte will proceed to meiosis II (equational division) but
it will stop at metaphase II until a sperm fertilizes the egg. When fertilization does occur,
a zygote is formed by the union of egg and sperm pronuclei and two more polar bodies
for a total of three.
Spermatogenesis begins in the seminiferous tubules of the male after the onset of
puberty. Up to that time, spermatogonia maintain their population through mitosis of
type A spermatogonia. After puberty, some type A give rise to type B spermatogonia
that differentiate into primary spermatocytes and enter prophase I of meiosis but are not
held at the diplotene stage, as are the oocytes. They do stay in the diplotene stage about
22 of the 64 days it takes to get from spermatogonia to mature sperm. Primary
spermatocytes give rise to secondary spermatocytes that will go through meiosis II to
make spermatids, and finally, spermatozoa.
Oogenesis – When primordial germ cells have arrived at the indifferent gonad they
differentiate into oogonia and acquire flat epithelial cells from the surface of the gonad,
follicular cells, that later will become granulosa cells. Most oogonia undergo mitosis
with the normal amount of DNA 2n, 2c. n is the number of chromosomes and c is the
amount of DNA in a single set [n] before DNA replication has occurred. Many oogonia
and primary oocytes die or become atretic
Others will precede to meiosis I and stay in the diplotene stage instead of going to
metaphase. These are primary oocytes and together with the follicular cells are called,
primordial follicles.
The primary oocyte has a chromosomal compliment of 2n, 4c, since it has duplicated
DNA in the first meiosis, but still only one set of DNA from each one of each of the two
parents (1n +1n = 2n). The follicular cells secrete oocyte maturation inhibitor
preventing primary oocytes from passing through the diplotene stage. After birth, the
follicular cells are more cuboidal and may be a couple layers thick and with the primary
oocyte it is called is a primary follicle. The follicular cells are now called granulosa
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cells. Early development of the follicle occurs without the significant influence of
hormones, but as puberty approaches, continued follicular maturation requires the action
of the pituitary gonadotrophic hormone follicle-stimulating hormone (FSH) on the
granulosa cells, which by this time developed FSH receptors on their surfaces.
Each month 15 – 20 follicles begin to form primary (preantral), secondary (antral or
Graafian), and tertiary (preovulatory). Of course, only one primary oocyte will reach
maturity.
The primary follicle has a zona pellucida between the oocyte and the granulosa cells.
This is a glycoprotein that is secreted by both the granulosa cells and the oocyte. As the
primary follicle matures, the follicular cells' secretions begin to separate these granulosa
cells and an antrum is formed, hence, producing a secondary follicle. There is still a
primary oocyte at this stage. While this is occurring, the stroma of the ovary tissue
directly around the granulosa cells differentiates into theca folliculi. These cells will
organize into an outer theca externa and a theca interna. The theca externa is
somewhat inert, but the theca interna has receptors for luteinizing hormone (LH). The
theca interna cells produce androgens (testosterone). The testosterone passes out of the
thecal cells and into the granulosa cells, which under the influence of FSH, produce
aromatase. Aromatase converts testosterone into estrogens (mainly 17β-estradiol).
For the secondary follicle, when its antrum gets large the primary oocyte sets on a
mound of granulosa cells, the cumulus oophorus. Why only one follicle matures is not
completely understood. One reason is that the dominant follicle becomes independent of
FSH and will secrete large amounts of inhibin that suppresses FSH secretion from the
pituitary. The other FSH-dependent follicles become atretic.
The tertiary (Graafian) follicle protrudes at the surface of the ovary (stigma) and
sharp spike in LH from the pituitary leads to completion of meiosis I and ovulation. The
completion of meiosis I now forms a secondary oocyte [1n, 2c], that is, half the number
of chromosomes but they are double the DNA, still connected by a centromere. A polar
body is produced and lies between the zona pellucida and egg. Meiosis II begins and
arrested in metaphase II. About three hours will pass before ovulation. After ovulation,
the Graafian follicle becomes a corpus luteum. Unless fertilization occurs 24 hours after
ovulation, the egg will die. The layer granulosa cells still adherent to the zona pellucida
is called the corona radiata.
Spermatogenesis (spermatogonia to spermatozoa) – Shortly before puberty the sex cords
acquire lumens, becoming seminiferous tubules and at puberty primordial germ cells are
transformed into type A spermatogonia [2n, 2c]. Type A spermatogonia replenish
themselves by mitosis, but also give rise to type B spermatogonia, which become
primary spermatocytes [2n, 4c]. Primary spermatocytes pass through the blood-testes
barrier (made by Sertoli cells) and complete meiosis I to become secondary
spermatocytes [1n, 2c]. Secondary spermatocytes complete meiosis II, becoming
spermatids [1n, 1c], which shed excess cytoplasm, the residual body, and differentiate
into spermatozoa. Spermatogenesis is divided into two parts. (1) spermatocytogenesis –
spermatogonia up to spermatids, and (2) spermiogenesis – spermatids to mature sperm.
Metamorphosis of spermatids to spermatozoa includes: formation of and acrosome
(an enzyme-filled structure important for fertilization) and a flagellum with mitochondria
around its proximal part. The sperm cell has a (1) head with the acrosome and nucleus,
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(2) midpiece with the centrioles and proximal flagellum with the mitochondria, and (3)
the tail.
Sertoli cells that form the blood - testes barrier have tight junctions and form an
immunological barrier between forming sperm cells and the rest of the body, including
spermatogonia. They form two compartments (1) basal compartment – has free access
to materials in blood, and (2) adluminal compartment – inside the blood testes barrier.
Sertoli cells have receptors for FSH and are stimulated by that hormone to make
androgen-binding hormone, which binds testosterone (made by Leydig cells). The
testosterone has a strong influence on the course of spermatogenesis. Sertoli cells can
produce inhibin, as granulosa cells do for the ovary, to inhibit the production of FSH by
the pituitary. These cells are supporting cells derived from surface epithelium, as are
follicular cells in the ovary.
Leydig cells are between the sex cords of the testes; hence, they are called interstitial
cells of Leydig. They produce testosterone starting at the eighth week of gestation and
have receptors for LH. These cells are homologous to the thecal cells of the ovary.
NOTE: Depending on which author or text is read, Graafian follicles may be either
secondary or tertiary follicles.
Chromosomal abnormalities
Humans normally have 46 chromosomes, 23 each from the male (46, XY) or female (46,
XX)
Of the 23 chromosomes there are 22 autosomes and a sex chromosome (X or Y).
The X can come from either a male or female, but the Y is contributed by the male.
An embryo cannot survive without at least one X chromosome, but it can without the Y
chromosome.
The Y chromosome (has the Sry gene for the testes-determining factor) only
differentiates the fetus into a phenotypic male. In some mice, if there is a Y chromosome
and the Sry gene is missing, the mice will phenotypically be female.
The X chromosome has genes that are necessary for normal cell development and
metabolism.
Primordial germ cells are not necessary for differentiation of the male phenotype or
development of testes. However, for the female, normal primordial germ cells are
necessary for normal development of the ovaries. In Turner's syndrome (45, XO),
primordial gem cells degenerate shortly after reaching the gonads, resulting in formation
of a streaked gonad.
The X or Y chromosomes are in the somatic cells, as well as primordial cells, of the
embryo and the adult human.
Terms
haploid – 23 chromosomes = n
diploid – 46 chromosomes = 2n
triploid – doesn't happen in humans, but it does in some other species such as plants
euploid – exact multiples of n (diploid and triploid are euploid)
aneuploid – chromosome number that is not exactly n (trisomy and monosomy are
aneuploid)
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nondisjunction
mosaicism
translocations
deletions
Down's syndrome - (47, XX) or (47, XY) trisomy 21
Klinefelter's syndrome - (47, XXY or 48, XXXY)
Turner syndrome – partial (46, X + defective X) or complete loss of X chromosome
(45, X); 43% of Turner syndrome are mosaics
XYY – supermale
cri-du-chat syndrome – example of a partial deletion on short arm of chromosome 5
Angelman syndrome (happy puppet syndrome) – an example of genomic imprinting
where a deletion occurs in the long arm of chromosome 15 that is maternally derived.
Prader-Willi syndrome - an example of genomic imprinting where a deletion occurs in
the long arm of chromosome 15 that is paternally derived.
fragile-X syndrome – part of the X chromosome has a weak area that is fragile and
subject to breakage due to an abnormal expansion of the CCG triplet repeat
Questions for Chapter One
1. Which of the following migrate from the yolk sac?
a. primary oocytes
b. oogonia
c. primordial germ cells
d. spermatogonia
2. Most _______ undergo atresia during embryonic development.
a. oogonia
b. spermatogonia
c. spermatocytes
d. graffian follicles
3. About which of the following time periods do oogonia begin the first stage of
meiosis?
a. At puberty.
b. Late in the fetal period.
c. At the time of ovulation
d. After fertilization.
4. The stage at which chiasma are formed in the tetrads, for either primary oocytes or
primary
spermatocytes, is which of the following?
a. diplotene stage of prophase I
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b. metaphase of meiosis I
c. diplotene stage of meiosis II
c. prophase of meiosis II
5. Which of the following has the chromosomal characteristic 2n, 4c?
a. primordial germ cell
b. primary oocyte
c. spermatid
d. secondary spermatocyte
6. Granulosa cells of a follicle originate from which of the following?
a. Mesoderm of the gonad.
b. Surface epithelial cells of the ovary.
c. Yolk sac endoderm.
d. Stroma of the ovary.
7. Granulosa cells and _________ cells have receptors for follicle stimulating hormone.
a. theca interna
b. Leydig
c. theca externa
d. Sertoli
8. As granulosa cells proliferate and an antrum is formed, the primary oocyte sits on a
mound of
those and this mound is called the __________.
a. theca externa
b. corona radiata
c. cumulus oophorus
d. stigma
9. Which of the following is not a part of a spermatozoon?
a. residual body
b. head
c. midpiece
d. flagellum
10. Blood-testes barriers are formed by _________.
a. Leydig cells making androgen-binding protein
b. Leydig cells with LH receptors
c. Sertoli cells making androgen-binding protein
d. Sertoli cells making testosterone