Why does spermatogenesis continue into old age
Embryology: spermatogenesis and oogenesis
Table of Contents
Image: “Spermatogenesis” by Phil Schatz. License: CC BY 4.0
Development of the germ cells
At the beginning of embryonic development, the primordial germ cells lie in the yolk sac wall. From the fourth week they migrate via the hindgut into the differentiating gonads or gonads. Depending on whether these develop into the ovary or testicle, one speaks from now on of oogonia or spermatogonia.
Especially on the sex cells or Gametes is that she is one haploid chromosome set have. They only contain 23 chromosomes with mixed maternal and paternal parts. This makes sense with regard to a later fertilization of the egg cells by the sperm, so that they then one together diploid set of chromosomes can form.
Meiosis is responsible for halving the set of chromosomes. First be in a Reduction division the 46 chromosomes are divided into two cells with 23 chromosomes each, so that the DNA content is now 1n2c, which means half the chromosome set but with twice the number of chromosomes. In the next Equation division no DNA replication takes place, so that only the sister chromatids are separated and the new DNA content can be given as 1n1c.
Oogenesis - maturation of female germ cells
Oogenesis begins in the fetal period but is then stopped. It does not continue until the age of 12-15 through puberty.
12th - 16th week of pregnancy: prenatal maturation of female germ cells
During this time the primary oocytes or 1st order oocytes. Instead, the oogonia multiply by dividing mitotically. In addition, they grow a little bit. But the prophase of meiosis does not end. The cells remain in the Diploma of the prophase and still have a diploid set of chromosomes. Their resting state is also called because of the net-like appearance of the cell nucleus Dictyotene.
The primary oocytes are from Follicular epithelial cells, which arise from the disintegrating germinal cords, surround. This creates a flat, single-layer envelope around the oocytes. This complex is called Primordial follicle designated. About seven million eggs are created by the sixth month. By the time they are born, however, around five million of them perish. No new primary oocytes are formed postnatally!
Postnatal maturation of female germ cells
Even before puberty, some follicles mature into tertiary follicles, which are then broken down. This process is called Follicular atresia designated. At the beginning about 200,000 primordial follicles are still arrested in the dictyotene. Several primordial follicles now mature each month Primary follicles with still single-layer, but now cubic hulls.
Only a few of these primary follicles continue to develop. They become so-called Secondary follicles with a multilayered covering of follicular epithelial cells. These form a substance made up of glycoproteins known as Zona pellucida surrounds the egg cell. The oocyte continues to grow and moves into an eccentric position. Cavities are formed that lead to a large follicular cavity, the Antrum, that with Liquor folliculi is filled. Thus a tertiary follicle has matured.
In the tertiary follicle, the oocyte, which protrudes into the follicular cavity surrounded by the zona pellucida, forms the Cumulus oophorus or egg mound. The zona pellucida of the egg cell is directly adjacent to a layer of follicular epithelial cells, which are called Corona radiata is called because it is jagged. The tissue of the ovary differentiates into one that is rich in blood vessels and produces androgens Theca interna as well as a follicle stabilizing, connective tissue Theca externa.
Under the influence of the follicle stimulating hormone (FSH) Only one of the tertiary follicles matures about seven days before ovulation each month Graaf's follicle approach. This is 1.5 to 2.5 centimeters in diameter and on the outside of the ovary as stigma to recognize. This Graaf's follicle is the egg cell that will eventually “jump”.
About twelve hours before ovulation, the first meiosis of meiosis ends in Graaf’s follicle. This is a firstPolar bodies tied off with a set of chromosomes. The other set of chromosomes remains in the now as secondary oocyte or 2nd order oocyte designated egg cell. This receives the majority of the cytoplasm and the cell organelles, so that the polar body is much smaller and quickly degenerates. The nucleus of the egg cell is still diploid.
On the 14th day of the female cycle, ovulation or ovulation. The egg cell "jumps" together with the zona pellucida and corona radiata, so it leaves the ovary and becomes of the Fimbriae of the ovary. At the same time, the second division of meiosis begins. This is also stopped again, this time in the metaphase. It will not continue until the egg is fertilized.
If the egg cell is fertilized, the second meiosis continues. Again becomes a second polar body pinched off, which again hardly receives any cytoplasm. This remains in the fertilized oocyte, the ovum. Now there is a haploid chromosome set in the cell nucleus, which can unite with the chromosome set of the sperm.
Since the egg cell only takes the zona pellucida and the corona radiata with it during ovulation, the rest of the follicle remains in the ovary. It collapses and fills with blood from the theca interna. Therefore, at this point in time, it is called Corpus rubrum designated.
Under the influence of the Luteinizing Hormone (LH) of the lutein cells, this changes into a yellowish one Corpus luteum. The yellowish color can be traced back to the lipids stored in the lutein cells. The corpus luteum produces progesterone and something estrogen, which leads to the changes in the Endometrium, i.e. the structure of the uterine lining in preparation for a possible Nidation the egg cell, leads.
When the egg cell is fertilized, the corpus luteum produces as Corpus luteum graviditatis hormones continue up to the 20th week of pregnancy. It is maintained by the hormone hCG (human chorionic gonadotropin). This is also used for diagnostics in the context of pregnancy tests. Its function is then taken over by the placenta and the corpus luteum graviditatis degenerates.
If there is no fertilization, it happens Corpus luteum menstruationis. This already degenerates after about ten days. In both cases, a whitish scar tissue remains Corpus albicans back. This process continues as an ovarian cycle until the final absence of menstruation, the menopause, gone.
Spermatogenesis - maturation of male germ cells
In contrast to oogenesis, it is possible for male germ cells not only to apply prenatally, but also to replicate postnatally.
Prenatal maturation of male germ cells
Prenatally, the male germ cells are removed from the testicles after they have immigrated Sertoli support cells stopped in their maturation process. They remain in the testes as largely undifferentiated cells, the tubules of which are not yet canalized. Only with the onset of puberty do the seminiferous tubules develop and the maturation process of the spermatogonia into mature sperm begins.
Postnatal maturation of male germ cells
With the onset of puberty, the spermatogonia multiply strongly. A distinction is made between two types: the first type always provides new spermatogonia, the second type begins to mature. In a first step, this second type emerges primary spermatocytes or 1st order spermatocyteswhen the spermatogonia enter the prophase of meiosis. They have a diploid set of chromosomes 2n2c. This phase lasts for about three weeks.
The long prophase is followed by the further phases of the first meiosis, at the end of which two spermatocytes are formed. These are then saved as secondarySpermatocytes or 2nd order spermatocytes designated. They go through the second meiosis, at the end of which the division into four haploids Spermatids he follows. With each division, the size of the cells halves, so that the spermatids make up about a quarter of the primary spermatocytes.
The subsequent process is called Spermiogenesis designated. It is the last step in spermatogenesis and makes the spermatids functional Sperm. Are the secondary spermatocytes still connected by cytoplasmic bridges because both the first and the second meiotic division have a incomplete diakinesis now contain four separate ones mature sperm.
Initially, organelles and excess cytoplasm are in Residual bodies constricted and the long kinozilie or the Sperm tail out. This process begins in the seminiferous tubules and continues on its way to the epididymis. This is where the final maturation takes place, during which a functional sperm with head, neck and tail is created.
The sperm head contains the nucleus and is dated Acrosome covered. It is followed by a short neck that has that proximal centriol which plays a role in the fertilization of the egg cell. This is followed by the tail, which is divided into three parts:
- Middle piece
- Main piece
- End piece
The middle piece contains the mitochondria. It provides the energy for the movement of the flagella via adenosine triphosphate (ATP). The flagellum consists of the main and end pieces of the tail. In it are Microtubuleswho are the typical 9 × 2 + 2 structure have, so are arranged in pairs. The finished sperm are stored in the Ductus epididymidis saved. The entire process of ripening took around 80 days.
However, sperm must still undergo a final maturation: After ejaculation there is a so-called seven-hour period Conditioning or capacitation instead, in which the membrane components change, but not the morphology of the sperm. This usually takes place in the tubes and is absolutely necessary for fertilization, as it is not possible without conditioning Acrosome reaction can come.
Pathological processes during spermatogenesis and oogenesis are mainly to be found on the chromosomal level. An example of disturbed gametogenesis is the Nondisjunction, an incorrect distribution of chromosomes in the gametes. This can occur during both the first and second meiosis and results in gametes with too many and too few chromosomes. Only a few misdistributions are compatible with postnatal life.
A typical example of meiotic nondisjunction is the Trisomy 21 or that Down syndrom, which is rarely caused by translocation and in which chromosome 21 is present in triplicate. The clinical picture is characterized, among other things, by the characteristic face, short stature and the four-finger furrow. Trisomies 13 and 18 are also viable trisomies that can result from nondisjunction.
While there is a numerical aberration of the autosomes in the above-mentioned trisomies, there are also syndromes that can be traced back to a numerical aberration of the gonosomes. This includes that Turner Syndrome with a chromosome set 45, X0 and that Klinefelter Syndrome with at least 47, XXY.
If the nondisjunction takes place in one of the blastomeres after fertilization, it is possible that only a few cells contain this chrosomomal abnormality and thus a mosaic is created. This mosaic formation is also possible with the trisomies described above, as well as the Klinefelter and Turner syndromes. Here, clinical changes are less pronounced than if all cells carry this change in them.
Above all, the mother's age plays a role with regard to chromosomal defects. Since the oocytes in a late pregnancy can be in prophase for more than 40 years and are exposed to environmental influences the entire time, the risk of chromosomal defects increases dramatically from the age of 35.
Image: “X-ray image of the legs of a 10-year-old boy with achondroplasia” from Kinderradiologie Olgahospital Klinikum Stuttgart. License: CC BY-SA 3.0
But the paternal age also plays a role. With older paternal age, there are no chromosomal maldistributions, but higher rates of new mutations. This is attributed to the ongoing DNA replication in the sperm. The DNA of a 40-year-old man has already been replicated 610 times. The higher the replication rate, the greater the risk of a replication error, i.e. a new mutation. Relationships can, for example, be used for the Achondroplasiaor that Marfan Snydrome Find.
Popular exam questions related to spermatogenesis and oogenesis
The solutions can be found below the references.
1. What is right?
- Primordial follicles mature into secondary follicles.
- Secondary oocytes arise prenatally.
- Secondary oocytes remain in the diploma of prophase.
- Primary oocytes begin the first meiotic division prenatally.
- After ovulation, the first meiotic division begins.
2. Mature sperm are produced during spermiogenesis. You…
- ... are characterized by incomplete diakinesis.
- ... do not require any further maturation or change to fertilize the egg.
- ... consist of a main, middle and end piece.
- ... finally mature in the seminiferous tubules.
- ... have a head that is covered by the acrosome.
3. What is the first structure in the yolk sac wall?
- Primordial germ cells
ALLEX Everything for exam B, 1st edition - Thieme
Dual anatomy series, 2nd edition - Thieme Verlag
Engel W, Laccone F, Sancken U: The paternal age from a human genetic point of view. Journal of Reproductive Medicine and Endocrinology 2004; 1 (4), 263-267
Human chorionic gonadotropin via DocCheck Flexikon
Moore, K.L. inter alia: Embryology, 6th edition - Urban & Fischer
Schünke, Michael et al .: Prometheus learning atlas of anatomy, 3rd edition - Thieme
Schulze, Susanne: Embryology mediscript short textbook, 2nd edition - Urban & Fischer
Solutions to the exam questions: 1D, 2E, 3D
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