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Unformatted text preview: Key concepts of gametogenesis
Generic term for the genesis of haploid gametes out of diploid primordial germ cells
Collective designation for male and female germ cells (oocytes = mature egg cells and
spermatozoa); they arise through oogenesis and spermatogenesis and, following the
meiotic reduction division, contain only a single (= haploid) chromosome set. Through
the fertilization the diploid zygote is generated from the male and female gametes.
Comprises the non- somatic cells of an individual that are determining for the
transmission of the genetic material.
comprises the entire development from oogonium to a mature oocyte.
Descendants of the primordial germ cells in the female gonadal primordium.
The outer surface consists of a single-layered, cuboidal epithelium and it contains the
The outer surface consists of a single-layered, flat epithelium and it encircle the
Primordial germ cells
are generated in the wall of the vitelline sac (yolk sac) towards the end of the 3rd week.
They make their way into the gonadal primordium until the end of the 6th week.
The earliest discernible indication of an organ or structure during embryonic
The surface epithelium has several layers; it harbors just the oocyte.
comprises the entire development from spermatogonia up to spermatozoa. It can be
subdivided into spermatocytogenesis (from spermatogonia up to spermatides) and
spermiogenesis (spermatohistogenesis). Spermatocytogenesis Commentary about meiosis
In the metaphase of the first meiosis, the tetrads (i.e., the two homologous
chromosomes that have divided into two chromatids [= (2n, 4c], respectively) are
visible. In the first meiosis the chromosomes are distributed to the daughter cells
(reduction). Thereby each daughter cell receives one of the two homologous
chromosomes (1n, 2c), while the two chromatids from the respective chromosomes
remain bound together.
Note: in the usual mitosis the two chromatids of each chromosome end up in various
In the second meiosis (representing a division) the centromeres divide, whereby the
two chromatids are separated. Thereby each daughter cell contains a set of 23
chromatids (now termed chromosomes again). The cells are now haploid and are
called spermatids. In the prophase of the first meiotic division various stages can be distinguished:
leptotene, zygotene, pachytene, diplotene, diakinesis.
In the Meiosis of the oocyte an in-between stage is added: the dictyotene, the socalled interphase.
In the leptotene (= condensing) stage the DNA forms itself into chromosomes that
already consist of 2 daughter chromosomes (= chromatids), due to the doubled DNA
In the zygotene stage the homologous (i.e., maternal and paternal) chromosomes
become paired by a synaptonemal complex as bivalents Every bivalent inserts with
both ends on the inner surface of the nuclear membrane. Due to their close proximity
they can be mistaken for a haploid set of chromosomes!
In the pachytene (= exchange of genetic material) stage, which lasts the longest
time, the chromosomes shorten and thicken themselves by substantial knotting of the
DNA and the two chromatids of each chromosome draw away from each other along
their lengths. They remain connected together only by the centromere, so that the
typical scissor or pliers form of the chromosomes becomes visible. One recognizes the
4 chromatids of a bivalent (tetrad formation) only now. In this stage the formation of
the so-called chiasmata, the "crossing over", occurs.
In the diplotene stage the synaptonemal complexes loosen and partial separation of
each pair of sister chromatids from their homologous counterparts occurs. The
chromatids are still held together at the centromeres and the sites of crossing over.
The dictyotene stage is the resting phase of the oocyte. It lasts from birth up to
sometime between the twelfth and fiftieth year of age, depending on the moment of the
atresia of the follicle or of the ovulation. The chromosomes are decondensed during
In the diakinesis a further condensation of the chromosomes and, simultaneously, a
further separation of the chromatids takes place that, however, still hang together at the
chiasmata. Nucleolus and the nuclear membrane dissolve. Key concepts having to do with fertilization
The release of various lytic (dissolving) enzymes from the acrosomal cap of the
spermatozoon. The enzymes help the sperm cell make its way through the corona
radiata and the pellucid zone.
Technical term for the changes that must take place to a freshly ejaculated
spermatozoon so it gets closer to being capable of fertilizing an oocyte.
When the follicle tears, the oocyte is washed out together with cumulus cells that
surround it. These granulosa cells that form the cumulus oophorus in the mature follicle
look like a wreath and are thus called the corona radiata.
The sexual act with the aim of producing conception/pregnancy.
The ejaculate is made up of spermatozoa (10%) and the seminal plasma fluid (90%).
Union of the male and female germ cells to become a new organism. The first signs of
fertilization (pronucleus) that can be observed in assisted fertilization occur roughly
16-20 hours after the impregnation.
the watery-yellow fluid (2-6 ml) that pours out of the follicle into fallopian tube at the
time of ovulation.
An oocyte in which the spermatozoon has entered (impregnation), but the fusion of the
pronucleus (which normally takes between 18 - 26 hours) has not yet taken place.
Impregnation (human medicine) / insemination (veterinary medicine):
2. Addition of spermatozoa to the oocyte during assisted fertilization.
Artificial introduction of spermatozoa into the inner female genital tract. Meiosis:
A special form of cell division, occurring in sex cells, by means of which each daughter
nucleus receives half the number of chromosomes characteristic of the somatic cells of
the species. Oocyte:
Synonym for egg cell.
Primary oocyte ==> oocyte before the end of the first meiosis.
Secondary oocyte ==> oocyte after the end of the first meiosis (driven by the rapid rise
in LH [luteinizing hormone] and FSH [follicle stimulating hormone] concentrations)
A membrane that envelops the oocyte. The cumulus cells are anchored there.
Pronucleus (synonym for haploid nucleus):
The paternal or maternal nuclear material that is formed in the impregnated oocyte.
When both come together, they arrange themselves into a prophase nucleus.
Collective term for all the glandular secretions (from the ampulla of the deferens duct,
seminal gland, prostate, bulbourethral gland, urethral gland) which are mixed together
during the ejaculation process.
Spermatozoon, spermatozoa / sperm cell, sperm cells are synonyms
The first cell of the embryo. It only exists for a short time (a few hours). From the very
beginning it is "born" with a prophase nucleus and its "job" is to divide as soon as
possible. Key concepts of the preimplantation phase
The first cell of the embryo. It exists from the metaphase of the first mitosis to the twocell stage. It thus exists only for a short time.
Mitotic division, beginning with the zygote into ever smaller daughter cells
(blastomeres) until the morula stage
(Diminutive of the Latin "morum", mulberry) solid, spherical collections of 16 - 30
blastomeres; result of the cleavage on the 4th day after the fertilization. Develops
further to blastocyst.
Cells resulting from the cleavage of the zygote; they divide without growing and by
each division they become smaller, the plasma/nucleus-relationship shifts in the
direction of more nucleus.
The cell collection that develops from the morula on roughly the 5th day after the
fertilization, forming the blastocyst cavity (blastula) and in which an outer cellular layer
(trophoblast) and an inner cell mass (embryoblast) can be distinguished.
Thickening of the cell packing. The prerequisite is the setting up of cell contacts and a
reduction of the cellular volume.
Cellular outer wall of the blastocyst that at the nidation in the endometrium
differentiates further into two layers: the syncytiotrophoblast and the cytotrophoblast.
Consisting of only few cells, the small, inner part of a blastocyst from which the
embryonic disk develops following a successful implantation.
Abembryonic pole and embryonic pole of the blastocyst:
The location within the blastocyst where the small cell collection, consisting of a few
cells, is found is termed the embryonic pole. The opposite pole is then called the
abembryonic pole. Hatching from the pellucid zone occurs at the abembryonic pole
whereby implantation in the endometrium happens at the embryonic pole end. Hatching:
Emergence of the blastocyst out of the protective pellucid zone at the end of the 5th
Protective, tough covering of unchangeable size that surrounds the oocyte and the cells
of the embryo up till the moment of hatching. The various forms of hormonal contraception
Estrogen and progesterone, combined or sequentially in pill form
Progesterone alone in the form of pills
Progesterone alone in the form of an implant
Progesterone given off by an intra-uterine system
How the hormonal method of contraception functions
• They lower FSH secretion so that normal follicle maturation does not occur and
no LH peak happens. This effect is mainly due to estrogen, but progesterone
can also inhibit ovulation when ingested in high dosages.
They transform the uterine endometrium so it becomes pseudoatrophic, thereby
making an implantation practically impossible, should an ovulation and
fertilization take place.
They modify the cervix mucous so that it becomes less permeable for sperm
cells (this effect is mainly due to the progesterone).
Most pills are monophasic, i.e., they always contain the same amounts of estrogen
and progesterone for the whole cycle. Nevertheless, bi- and triphasic pills also exist
that attempt to imitate the cyclic physiological changes. The ideal pill yields the lowest
amounts of estrogen and progesterone but, at the same time, is an efficient
contraceptive with minimal side effects.
The main side effects
• Thrombo-embolisms: primarily due to the alteration of coagulation factors that
are induced by the estrogen. Risk factors are age (for those older than 35),
arterial hypertonia and tobacco abuse
Myocardial infarction and cerebral insult, especially with smokers that are older
than 35. The arterial disease is primarily due to the effects of progesterone. This
lowers the HDL-2 value (high density lipoprotein) that has a heart protective
Hypertonia: blood volume and angiotensin secretion are elevated.
Some studies have suggested that patients who are older than 25 and take
progesterone for a long time have an increased risk of breast cancer. Oral
contraceptives taken for an extended period of time are also supposed to
increase the risk for cervical carcinomas.
Less prominent side effects
• Estrogen: Bleeding, feeling bad, aching breasts, headaches
Estrogen + progesterone: Weight gain, absence of menstruation bleeding
Progesterone: Acne, depression, lowering of libido ...
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This note was uploaded on 11/21/2011 for the course ANAT 1502 taught by Professor Johnh.romfh during the Spring '10 term at Life Chiropractic College West.
- Spring '10