GametogenesisGametogenesis

Embryology course 2010

• Arrangement of the male gonads

• Arrangement of the female gonads

• Spermatogenesis

• Oogenesis

SpermatogenesisSpermatogenesis

Structure of the germinal epithelium

Developmental stages of spermatogenesis

• A-spermatogonium

• B-spermatogonium

• Primary spermatocyte (= spermatocyte order I)

• Secondary spermatocyte (= spermatocyte order II)

• Spermatid

• Sperm cell (= spermatozoon)

The temporal course of spermatogenesis

Mitosis of the spermatogonia

16 days Up to the primary spermatocytes

First meiosis 24 days For the division of the primary spermatocytes to form secondary spermatocytes

Second meiosis A few hours For engendering the spermatids

Spermiogenesis 24 days Up to the completed sperm cells

Total ~64 days    

Spermatocytogenesis

• spermatogonia: • homonymous division • heteronymous division

• primary spermatocytes (I): enter into the first meiosis

• secondary spermatocytes go directly into the second meiosis

• spermatids

Local course of spermatogenesis - the spermatogenesis wave

Structure of the sperm cell

• Nuclear condensation: thickening and reduction of the nuclear size, condensation of the nuclear contents into the smallest space.

• Acrosome formation: Forming a cap (acrosome) containing enzymes that play an important role in the penetration through the pellucid zone of the oocyte.

• Flagellum formation: generation of the sperm cell tail.

• Cytoplasma reduction: elimination of all unnecessary cytoplasm.

• The neck contains the two centrioles (proximal and distal) among other things.

• The mid piece consists of a sheath of ring-shaped mitochondria grouped around the axoneme to provide the energy for the flagellar movement.

• The principle piece has a sheath of ring fibers around the axoneme.

• The tail consists of only the 9+2 structure of the axoneme

Leydig's interstitial cells and hormonal regulation

OogenesisOogenesis

GroupGroup type of type of eggegg

type of cleavage

blastula blastula cavity

AmphioxusAmphioxus,,mammalsmammals

isolecithal,isolecithal,OligolecithaOligolecithaor alecithalor alecithal

holoblastic (or complete):

initial cleavage planes extend through

egg)

sphere with single layer

wall

large, central sphere

amphibians,amphibians,lampreyslampreys mesolecithal mesolecithal holoblastic

sphere with layered wall

small sphere

reptilesreptiles telolecithaltelolecithal

meroblastic (incomplete)

: initial cleavages

planes limited to

animal pole, the region

opposite the yolk)

cell disk on surface of

yolk, bilaminar

flat space between

epiblast and hypoblas

developmental sequence

• Primordial germ cell - oogonium - primary oocyte - primary oocyte in the dictyotene

• BirthThe continuation of the development / maturation of the oocyte begins again only a few days before ovulation

• The developmental sequence of a follicle goes through various follicle stages:

• Primordial follicle - primary follicle - secondary follicle - tertiary follicle (graafian follicle)

• Since a follicle can die at any moment in its development (= atresia), not all reach the tertiary follicle stage.

The follicle stages from primordial follicle to tertiary follicle

Temporal course of the number of germ cells / follicles

• Phase A:Primordial germ cells grow, proliferate 6th to 8th week.

Phase B:Spurt of growth: cellular clones of the oogonia are formed, whereby the cells remain connected with each other through cellular bridges; 9th to the 22nd week.

Phase C:The oogonia become primary oocytes that enter the prophase of the first meiosis; 12th to the 25th week.

Phase D:The primary oocytes become arrested in the prophase: the primordial follicles are created; 16th to the 29th week.

Phase E:At around the 14th week a quantitatively increased decline in the number of germ cells commences as well as atresia in all of the follicle stages.

Spermatogenesis Oogenesis

Number of gametes

Principle: continuous production. Although from puberty to old age sperm cells are constantly being engendered, the production is subject to extreme fluctuations regarding both quantity and quality.

Principle: Using up the oocytes generated before birth. Continual decrease of the oocytes, beginning with the fetal period. Exhaustion of the supply at menopause.

Meiotic output

Four functioning, small (head 4 m), motile spermatozoids at the end of the meiosis

One large, immotile oocyte (diameter 120 m) and three shriveled polar bodies are left at the end of the meiosis

Fetal period

No meiotic divisions Entering into meiosis (arrested in the dictyotene stage)

No germ cell production Production of the entire supply of germ cells

The ovarian cycle

• the ovarian cycle (follicle maturation) that peaks in the ovulation and the subsequent luteinization of the granulose cells

• cyclic alterations of the endometrium that prepare the uterine mucosa so fertilized oocytes can "nest" there. In the absence of implantation, the mucosa will be eliminated (menstrual bleeding)

• The control circuit of the hormonal cycle has two essential control elements: – The pulsatile liberation of GnRH, as well as

FSH and LH – The long-loop feedback-effect of estrogen

and progesterone on the hypothalamic-hypophysial-system (these two hormones are synthesized in the [ready to rupture] follicle and so originate in the ovary, thus the name "long loop").

• Follicle phase: recruitment of a so-called follicle cohort and, within this, the selection of the mature follicle. This phase ends with ovulation. Estradiol is the steering hormone. Normally, it lasts 14 days, but this can vary considerably!

• Luteal phase: progesteron production by the "yellow body" (= corpus luteum) and lasts 14 days (relatively constant).