2010 BGD Practical 3 - Gametogenesis

Practical 3: Oogenesis and Ovulation | Gametogenesis | Fertilization | Early Cell Division | Week 1 | Implantation | Week 2 | Extraembryonic Spaces | Gastrulation | Notochord | Week 3 | Quiz

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Introduction

Gametes are formed in the gonads; male testis and female ovary by the mechanism of meiotic cell division. The generation of gametes, and their regulation, is substantially different in males and females. After puberty, gonads also synthesis and secrete sex hormones into the bloodstream required for development of the secondary sex characteristics.

Female Gametogenesis

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 Abnormalities

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

• Autosomal chromosome aneuploidy
  • trisomy 21 - Down syndrome
  • trisomy 18 - Edwards syndrome
  • trisomy 13 - Patau syndrome

• 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

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  Historic testis drawing

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  Adult Seminiferous tubule showing spermatozoa developmental stages

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  Seminiferous tubule cross-section and supporting cells

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  Human spermatozoa

Human Spermatozoa Development

• Spermatogenesis process of spermatagonia mature into spermatazoa (sperm).
• Continuously throughout life occurs in the seminiferous tubules in the male gonad- testis (plural testes).
• At puberty spermatagonia activate and proliferate (mitosis).
• about 48 days from entering meiosis until morphologically mature spermatozoa
• about 64 days to complete spermatogenesis, depending reproduction time of spermatogonia
• follicle stimulating hormone (FSH) - stimulates the spermatogenic epithelium
• luteinizing-hormone (LH) - stimulates testosterone production by Leydig cells

Mature human spermatozoa 60 µm long, actively motile divided into 3 main regions (head, neck and tail) head - (flattened, 5 µm long by 3 µm wide) the nucleus and acrosome. Posterior part of nuclear membrane forms the basal plate. neck - (1 µm) attached to basal plate, transverse oriented centriole, contains nine segmented columns of fibrous material, continue as outer dense fibres in tail. tail - 3 parts a middle piece, principal piece and end piece middle piece - (5 µm long) axonema and dense fibres surrounded by mitochondria principal piece - (45 µm long) fibrous sheath interconnected by regularly spaced circumferential hoops end piece - (5 µm long) axonema surrounded by small amount of cytoplasm and plasma membrane

Links: Spermatozoa Development | MBoC - Sperm | MBoC - Highly simplified drawing of a cross-section of a seminiferous tubule in a mammalian testis | MBoC - Cytoplasmic bridges in developing sperm cells and their precursors

Puberty

• In humans at puberty, hormonal and morphological changes occur within the gonad and other systems (secondary sex characteristics).
• Within the testis the immature Sertoli cells cease to proliferate and differentiate.
• Spermatogonium proliferate and spermatogenesis begins.
• It takes about 70 days for cells to mature from the diploid spermatogonium to a primary spermatocyte.
• This maturation occurs in waves along the seminiferous tubules.

Ejeculate

• release of spermatozoa and accessory gland secretions from the male genital tract (3.5 ml)
• 200-600 million sperm, by volume less than 10 % spermatozoa
• Accessory Gland secretions - 60 % seminal vesicle, 30 % prostate and 10 % bulbourethral

Male Abnormalities

Azoospermia - Non-obstructive azoospermia (NOA) and Obstructive azoospermia (OA)

• Oligospermia - (Low Sperm Count) less than 20 million sperm after 72 hour abstinence from sex
• Azoospermia - (Absent Sperm) blockage of duct network
• Immotile Cilia Syndrome - lack of sperm motility

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)

Practical 3: Oogenesis and Ovulation | Gametogenesis | Fertilization | Early Cell Division | Week 1 | Implantation | Week 2 | Extraembryonic Spaces | Gastrulation | Notochord | Week 3 | Quiz

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Glossary Links

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2010 BGD: Lecture 1 | Lecture 2 | Practical 3 | Practical 6 | Practical 12

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Cite this page: Hill, M.A. (2024, April 19) Embryology 2010 BGD Practical 3 - Gametogenesis. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/2010_BGD_Practical_3_-_Gametogenesis

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G