ANAT2341 Lab 1 - Gametogenesis

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ANAT2341 Lab 1: Introduction | Gametogenesis | Oogenesis | Spermatogenesis | Fertilization | Sex Determination | Online Assessment | ANAT2341 Lab 1 - Quiz

Meiosis

Mitosis and meiosis.jpg

Comparison of Mitosis and Meiosis

Meiosis Germ cell division (haploid)

  • Reductive division generates the haploid gametes (egg, sperm)
  • Each genetically distinct from the parent
  • Genetic recombination (prophase 1)
    • Exchanges portions of chromosomes maternal/paternal homologous pairs
  • Independent assortment of paternal chromosomes (meiosis 1)

Homologous chromosomes pairing unique to meiosis

  • Each chromosome duplicated and exists as attached sister chromatids before pairing occurs
  • Genetic Recombination shown by chromosomes part red and part black
    • chromosome pairing in meiosis involves crossing-over between homologous chromosomes

Meiosis I and II

  • Meiosis I - separates the pairs of homologous chromosomes, reduces the cell from diploid to haploid.
  • Meiosis II - separates each chromosome into two chromatids (chromosome behavior in meiosis II is like that of mitosis).

Figure 14.32. Comparison of meiosis and mitosis

Prophase I - The homologous chromosomes pair and exchange DNA to form recombinant chromosomes. Prophase I is divided into five phases:

  1. Leptotene - chromosomes start to condense.
  2. Zygotene - homologous chromosomes become closely associated (synapsis) to form pairs of chromosomes consisting of four chromatids (tetrads).
  3. Pachytene - crossing over between pairs of homologous chromosomes to form chiasmata (form between two nonsister chromatids at points where they have crossed over)
  4. Diplotene - homologous chromosomes begin to separate but remain attached by chiasmata.
  5. Diakinesis - homologous chromosomes continue to separate, and chiasmata move to the ends of the chromosomes.

Prometaphase I - Spindle apparatus formed, and chromosomes attached to spindle fibres by kinetochores.

Metaphase I - Homologous pairs of chromosomes (bivalents) arranged as a double row along the metaphase plate. The arrangement of the paired chromosomes with respect to the poles of the spindle apparatus is random along the metaphase plate. (This is a source of genetic variation through random assortment, as the paternal and maternal chromosomes in a homologous pair are similar but not identical. The number of possible arrangements is 2n, where n is the number of chromosomes in a haploid set. Human beings have 23 different chromosomes, so the number of possible combinations is 223, which is over 8 million.)

Anaphase I - The homologous chromosomes in each bivalent are separated and move to the opposite poles of the cell.

Telophase I - The chromosomes become diffuse and the nuclear membrane reforms.

Cytokinesis I - Cellular cytoplasmic division to form two new cells, followed by Meiosis II.

Prophase II - Chromosomes begin to condense, nuclear membrane breaks down and spindle forms.

Metaphase II - Spindle fibres attach to chromosomes, chromosomes align in cell centre.

Anaphase II - Chromosomes separate and move to the opposite poles of the cell.

Telophase II - Chromosomes reach spindle pole ends and the nuclear membrane reforms.

Cytokinesis - Cellular cytoplasmic division to form new cells.

Comparison of Meiosis/Mitosis

McGraw-Hill Animation comparing Mitosis and Meiosis

  • After DNA replication 2 nuclear (and cell) divisions required to produce haploid gametes
  • Each diploid cell in meiosis produces 4 haploid cells (sperm) 1 haploid cell (egg)
  • Each diploid cell mitosis produces 2 diploid cells


Female Gametogenesis

Chromosome structure

In females, the total number of eggs ever to be produced are present in the newborn female.

  1. All eggs are arrested at an early stage of the first meiotic division as a primary oocyte (primordial follicle). Following purberty, during each menstrual cycle, pituitary gonadotrophin stimulates completion of meiosis 1 the day before ovulation.
  2. In meiosis 1, a diploid cell becomes 2 haploid (23 chromosomes) daughter cells, each chromosome has two chromatids. One cell becomes the secondary oocyte the other cell forms the first polar body.
  3. The secondary oocyte then commences meiosis 2 which arrests at metaphase and will not continue without fertilization.
  4. At fertilization meiosis 2 completes, forming a second polar body. Note that the first polar body may also undergo this process forming a third polar body.

Female gametogenesis

Female Abnormalities

Trisomy 21 female karyotype

Meiotic non-disjunction resulting in aneuploidy, most are embryonic lethal and not seen. The potential for genetic abnormalities increase with maternal age.

  • Sex chromosome aneuploidy
    • monosomy X - Turner's Syndrome
    • trisomy X - Triple-X syndrome
    • 47 XXY - Klinefelter's Syndrome

Male Gametogenesis

In males, sperm continues to be generated throughout life from a stem cell population in the testis. Spermatozoa maturation involves two processes meiosis and spermiogenesis Male gametogenesis.jpg

The above figure compares meiosis to the female (the polar bodies have been removed and labelling updated).

Differences in Mammalian Meioses

Female Oogenesis Male Spermatogenesis
Meiosis initiated once in a finite population of cells continuously in mitotically dividing stem cell population
Gametes produced 1 / meiosis 4 / meiosis
Meiosis completed delayed for months or years completed in days or weeks
Meiosis Arrest arrest at 1st meiotic prophase no arrest differentiation proceed continuously
Chromosome Equivalence All chromosomes exhibit equivalent transcription and recombination during meiotic prophase Sex chromosomes excluded from recombination and transcription during first meiotic prophase
Gamete Differentiation occurs while diploid (in first meiotic prophase) occurs while haploid (after meiosis ends)

Additional Information

The information below is not part of today's Practical.

Genetics

Inheritance Pattern images: Genetic Abnormalities | autosomal dominant | autosomal recessive | X-linked dominant (affected father) | X-Linked dominant (affected mother) | X-Linked recessive (affected father) | X-Linked recessive (carrier mother) | mitochondrial inheritance | Codominant inheritance | Genogram symbols | Genetics

Terms

  • autosomal inheritance - some hereditary diseases are described as autosomal which means that the disease is due to a DNA error in one of the 22 pairs that are not sex chromosomes. Both boys and girls can then inherit this error. If the error is in a sex chromosome, the inheritance is said to be sex-linked.
  • gene - a sequence of DNA that encodes an individual protein.
  • genome - the complete genetic information in the form of DNA available to a specific species.
  • sperm - See spermatozoa. The male haploid reproductive cell, often used generically (and incorrectly) to describe these cells and the fluid of the ejaculate. Term is a shortened form of scientifically correct term spermatazoa.
  • sperm annulus - (Jensen's ring; Latin, annulus = ring) A region of the mammalian sperm flagellum connecting the midpiece and the principal piece. The annulus is a septin-based structure formed from SEPT1, 4, 6, 7 and 12. Septins are polymerizing GTPases that can act as a scaffold forming hetero-oligomeric filaments required for cytokinesis and other cell cycle roles.
  • spermatid - Intermediate cell in spermatozoa development, within the testis seminiferous tubule they lie in the luminal cell layer to the secondary spermatocyte. These small cells are haploid and in spermiogenesis change their cellular structure and shape to form spermatozoa.
(More? Spermatozoa Development | Testis Development | Fertilization | Lecture - Cell Division/Fertilization)
  • spermatogenesis - (Greek, genesis = origin, creation, generation) The term used to describe the process of diploid spermatagonia division and differentiation to form haploid spermatazoa within the testis (male gonad). The process includes the following cellular changes: meiosis, reoorganization of DNA, reduction in DNA content, reorganization of cellular organelles, morphological changes (cell shape). The final process of change in cell shape is also called spermiogenesis.
  • spermiogenesis - (Greek, genesis = origin, creation, generation) The maturation process of the already haploid spermatids into the mature spermatozoa shape and organization. This process involves reorganization of cellular organelles (endoplasmic reticulum, Golgi apparatus, mitochondria), cytoskeletal changes (microtubule organization) and morphological changes (cell shape, acrosome and tail formation). The process of maturation of the spermatids into spermatozoa: chromatin condenses, nucleus becomes smaller, the Golgi apparatus is modified to form the acrosome, microtubules are reorganised to form the tail, mitochondria are relocated to the initial segment of the tail and the majority of cell cytoplasm is discarded.
  • spermatogonia - These cells form in the embryo from the primordial germ cell and are located in the seminiferous tubule adjacent to the basal membrane. The cells can either divide and separate to renew the stem cell population, or they divide and stay together as a pair (Apr spermatogonia) connected by an intercellular cytoplasmic bridge to begin to differentiate and eventually form spermatazoa.
  • spermatozoa - (spermatozoon, singular term) The male haploid gamete cell produced by meiosis in the testis (male gonad) seminiferous tubule. In humans, produced from puberty onwards and develop from the diploid stem cell the spermatogonia. The developmental meiosis is called spermatogenesis and the final morphologiccal (shape) change is called spermeiogenesis. The mature human spermatozoon formed from the spermatid has a head, neck and tail and is about 60 µm long. At ejaculation these cells undergo capacitation are activated and become motile.
  • spermatozoa head - Following spermiogenesis, the first region of the spermatozoa containing the haploid nucleus and acrosome. In humans, it is a flattened structure (5 µm long by 3 µm wide) with the posterior part of nuclear membrane forming the basal plate region. The human spermatozoa is about 60 µm long, actively motile and divided into 3 main regions (head, neck and tail).
  • spermatozoa neck - Following spermiogenesis, the second region of the spermatozoa attached to basal plate, transverse oriented centriole, contains nine segmented columns of fibrous material, continue as outer dense fibres in tail. In humans, it forms a short structure (1 µm). The human spermatozoa is about 60 µm long, actively motile and divided into 3 main regions (head, neck and tail).
  • spermatozoa tail - Following spermiogenesis, the third region of the spermatozoa that has a (head, neck and tail). The tail is also divided into 3 structural regions a middle piece, a principal piece and an end piece. In humans: the middle piece (5 µm long) is formed by axonema and dense fibres surrounded by mitochondria; the principal piece (45 µm long) fibrous sheath interconnected by regularly spaced circumferential hoops; the final end piece (5 µm long) has an axonema surrounded by small amount of cytoplasm and plasma membrane.
  • spermatogonial stem cells - (SSCs) The spermatagonia cells located beside the seminiferous tubule basal membrane that either divide and separate to renew the stem cell population, or they divide and stay together as a pair (Apr spermatogonia) connected by an intercellular cytoplasmic bridge to differentiate and eventually form spermatazoa.
  • sperm protein 56 - A component of the spermatozoa acrosomal matrix released to the sperm surface during capacitation.


ANAT2341 Lab 1: Introduction | Gametogenesis | Oogenesis | Spermatogenesis | Fertilization | Sex Determination | Online Assessment | ANAT2341 Lab 1 - Quiz

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Cite this page: Hill, M.A. (2019, October 23) Embryology ANAT2341 Lab 1 - Gametogenesis. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/ANAT2341_Lab_1_-_Gametogenesis

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© Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G