2010 BGD Practical 3 - Gametogenesis

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.

[../Notes/images/week1/image_001l.gif See large image with labels]

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 the first polar body.

[../Notes/images/week1/image_002l.gif See large image with labels]

3. The secondary oocyte then commences meiosis 2 which arrests at metaphase and will not continue without fertilization.

Ovulation releases this oocyte from the ovary.

[../Notes/images/week1/image_003l.gif See large image with labels]

Male Gametogenesis

In males, sperm continues to be generated throughout life from a stem cell population in the testis.

Image: Figure 20-22. Highly simplified drawing of a cross-section of a seminiferous tubule in a mammalian testis

Text: Sperm Are Produced Continuously in Many Mammals

Image: Figure 20-23. Cytoplasmic bridges in developing sperm cells and their precursors.

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).

primary spermatocyte -> secondary spermatocyte-> spermatid->sperm

Maturation involves meiosis and spermeogenesis

Ejeculate: 200-600 million sperm, 3.5 ml ; By volume < 10 % sperm (Accessory Glands, 60 % seminal vesicle, 10 % bulbourethral, 30 % prostate)

Abnormalities: 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

Meiosis initiated once in a finite population of cells

1 gamete produced / meiosis

Completion of meiosis delayed for months or years

Meiosis arrested at 1st meiotic prophase and reinitiated in a smaller population of cells

Differentiation of gamete occurs while diploid (in first meiotic prophase)

All chromosomes exhibit equivalent transcription and recombination during meiotic prophase

Male Spermatogenesis

Meiosis initiated continuously in a mitotically dividing stem cell population

4 gametes produced / meiosis

Meiosis completed in days or weeks

Meiosis and differentiation proceed continuously without cell cycle arrest

Differentiation of gamete occurs while haploid (after meiosis ends)

Sex chromosomes excluded from recombination and transcription during first meiotic prophase

Next

Link to next page in this Practical 3 - [BGDlabfertilization3.htm Fertilization]

Terms

• antral follicle - (secondary follicle) the stage following preantral (primary) in the decription of the sequence ovarian follicle development (primordial, preantral, antral).
• antrum - (L. a cave), cavity; a nearly-closed cavity or bulge. In the ovary this refers to the follicular fluid-filled space within the follicle.
• atretic follicle - An ovarian follicle that fails to mature and degenerates. Also called "atresia" refering to the process of degeneration of the ovarian follicle. This process can occur at any stage of follicle development (folliculogenesis).
• clomiphene citrate - drug taken orally to promote the process of follicle/egg maturation.
• corona radiata - Layer of follicle cells of cumulus oophorus remaining attached to zona pellucida of oocyte after ovulation. Also called granulosa cells.
• corpus albicans - (L. corpus = body, L. albicans = whitish); a degenerating corpus luteum in ovary.
• corpus luteum - (L. corpus = body, L. luteum = yellow) The remains of ovarian follicle after ovulation that acts as an endocrine organ supporting pregnancy and preventing menstruation (loss of the endometrial lining). de Graaf first observed it in the ovary of a cow as a yellow structure.
• cortical - (L. corticalis) at the outside (like the bark of a tree), usually combined with medulla meaning the core.
• cumulus oophorus - (L. cumulus = a little mound G. oon = egg + phorus = bearing); part of the wall of an ovarian follicle surrounding and carrying the ovum (oocyte).
• follicle - (L. folliculus = little bag,dim. of L. follis). A structure which develops in the ovary and contains a developing egg (oocyte).
• follicular fluid - the fluid found in the antrum of a secondary follicle. Secreted by cells in the wall of the follicle. This fluid is released along with the oocyte at ovulation.
• germinal epithelium - cellular component covering surface of ovary, it is continuous with mesothelium covering mesovarium. Note that it is a historical misnomer, as it is not the actual site of germ cell formation.
• Graafian follicle - named after Regnier de Graaf (1641-1673), an historic Dutch physician embryologist who studied pregnancy using rabbits.
• granulosa cells - the supporting cells that surround the developing egg within the follicle thecal layers.
• mesovarium - mesentry of the ovary formed from a fold of the broad ligament that attaches the ovary
• medullary - (L. medius = in the middle) relating to the medulla; pith, marrow, inner portion of an organ. Usually combined with cortex (cortical) meaning the outer layer.
• ovulation - release of the oocyte from the mature follicle. In humans generally a single oocyte is released from a cohort of several maturing follicles.
• preantral follicle - (primary) the stage following primordial in the decription of the sequence ovarian follicle development (primordial, preantral, antral).
• primary follicle - (preantral) the stage following primordial in the decription of the sequence ovarian follicle development (primordial, preantral, antral).
• primordial follicle - the first stage in the decription of the sequence ovarian follicle development (primordial, preantral, antral). Present in the ovary from birth, located in the stroma of the ovary cortex beneath the tunica albuginea. The primordial follicle is the oocyte and the surrounding follicular cells.
• primordial germ cell - oocyte present in the primordial follicle ovary from birth, located in the stroma of the ovary cortex beneath the tunica albuginea. The primordial follicle is the oocyte and the surrounding follicular cells.
• secondary follicles - (antral follicle) the stage following preantral (primary) in the decription of the sequence ovarian follicle development (primordial, preantral, antral).
• stromal cells - in the ovary, cells surrounding the developing follicle that form a connective tissue sheath (theca folliculi). This layer then differentiates into 2 layers (theca interna, theca externa). This region is richly vascularized and involved in hormone secretion.
• superovulation therapy - a fertility drug treatement (oral clomiphene citrate and/or injectable FSH with or without LH) aimed at stimulating development/release of more than one follicle during a single menstrual cycle.
• theca folliculi - stromal cells in the ovary, cells surrounding the developing follicle that form a connective tissue sheath. This layer then differentiates into 2 layers (theca interna, theca externa). This region is vascularized and involved in hormone secretion.
• theca externa - stromal cells forming the outer layer of the theca folliculi surrounding the developing follicle. Consisting of connective tissue cells, smooth muscle and collagen fibers.
• theca interna - stromal cells forming the inner layer of the theca folliculi surrounding the developing follicle. This vascularized layer of cells respond to LH (leutenizing hormone) synthesizing and secreting androgens which are processed into estrogen.
• tunica albuginea - dense connective tissue layer lying between germinal epithelium and cortical region of ovary.
• uterus - site of embryo implantation and development. Uterine wall has 3 major layers: endometrium, myometrium, and perimetrium. Endometrium can be further divided into the functional layer (shed/lost during menstruation) and basal layer (not lost during menstruation).
• zona pellucida - extracellular layer lying directly around the oocyte underneath follicular cells. Has an important role in egg development, fertilization and blastocyst development. This thick extracellular matrix consists of glcosaminoglycans and 3 glycoproteins (ZP1, ZP2, ZP3).

References

NCBI Bookshelf - Endocrinology: An Integrated Approach. Nussey, S.S. and Whitehead, S.A. Oxford, UK: BIOS Scientific Publishers, Ltd; 2001. The ovary - folliculogenesis and oogenesis | Spermatogenesis

Mitosis and Meiosis: MBoC Fig 20-6

Oogenesis: MBoC ch20f16.gif

Spermatogenesis: Dev Biol Ch19f18.jpg

Seminiferous Tubule: Dev Biol ch19f17.jpg