1.1 - Reproduction, Gametes

BIOL 216

Oogenesis

Egg development

Mitotic divisions complete before birth

Production of mature gametes stops ~age 50

Spermatogenesis

Sperm development in males

Continuous

All 4 haploids become viable, mature sperm cells eventually

Gametogenesis

Development of gametes (haploids), either sperm OR egg, from diploid cells called PGCs

Main difference between primordial germ cells & spermatogonia?

PGCs are the general term for the precursors of gametes, while spermatogonia is the specific precursor for spermatogenesis

PGCs can become EITHER spermatogonia or oogonia

Primordial Germ Cells (PGCs)

Primary undifferentiated stem cell type that will differentiate towards gametes - either spermatozoa or oocytes

Precursor to gamete development

Primary Oocyte

Oogenesis Stage 1

Initial egg stage

Secondary Oocyte

Oogenesis Stage 2

Fertilizable egg stage

Large cell produced by the first meiotic division

Polar Bodies

Oogenesis

Contains little cytoplasm

Byproducts of oocyte (2nd/Ovum) formation

Tends to die off before meiosis 2 AKA doesn’t become a gamete

Spermatogonium

Sperm precursor

Develops from PGCs

Primary Spermatocyte

Spermatogenesis Stage 1

Initiating sperm cell

Secondary Spermatocyte

Spermatogenesis Stage 2

Intermediate sperm cell

Spermatids

Spermatogenesis Stage 3

Developing sperm cells

4 haploid cells produced by meiotic division of spermatogonia

Spermatozoa

Spermatogenesis Stage 4

Mature sperm cells

Fertilization

Fusion of haploid sperm nucleus & haploid egg nucleus

Triggers creation of a zygote

Totipotent

Potential of a cell to become into any cell type within the organism

Egg coat

Outer protective coat of egg cells

Jelly-like shell

Cortical granules

Specialized Golgi structures , borders the inner-area of the egg

Protects the egg against polyspermy

Polyspermy

Fertilization of one egg cell with multiple sperm cells

Acrosomal vesicle

Specialized Golgi structure @ tip of sperm head

Helps mature sperm bind to & penetrate the egg coat

Acrosome - vesicle with enzymes that facilitate egg penetration

Yolk

Egg’s energy supply

P granules

Type of germ granules in C. elegans

Complexes of RNA and proteins - contributes to polarization of embryo → which side of organism will produce germline (gametic/hormonic) cells

Moves to posterior end (area that becomes germline cells) of zygote BEFORE the 1st cleavage

Found in the gonad in adult germ cells

Gonad

The organs which produce gametes & hormones in organisms

Female = ovaries

Male = testes

Implication

The fate of the cells that will become germ cells is FIXED early in development

Bone Morphogenetic Protein (BMP) signaling

Triggers specification of primordial germ cells in mice

Essential for embryonic development/formation of organs

Activates transcription factors → promotes expression of germ cells

Blimp1

Transcription factor BMP signaling activates in mice

Initiates PGC fate

Prdm14

Transcription factor BMP signaling activates in mice

Maintains pluripotent & epigenetic reprogramming in PGCs

Allows for flexibility before committing to either spermato-/oogenesis

Epiblast

Layer of cells in the early embryo

Gives rise to all tissues in the embryo

Extra-embryonic ectoderm

Tissue outside embryo

Contributes to surrounding support structures for embryo AKA placenta

Gastrulation

Early embryonic process

Creates 3 germ layers: ectoderm, mesoderm, endoderm

dpc

“days post coitum”

AKA days after sex

Sry genes

Activate Sox9 genes → triggers development of testes INSTEAD of ovaries

(Ovary-development is DEFAULT)

Sox9 genes

Activated via Sry genes → triggers development of testes INSTEAD of ovaries (actually is the gene triggering it)

Suppresses ovary development

(Ovary-development is DEFAULT)

Wolffian duct

Male internal genitalia

Present in undifferentiated-sex-organisms

Will degrade during feminization

Mullerian duct

Female internal genitalia

Present in undifferentiated-sex-organisms

Will degrade during masculinization

Bipotential gonads

Can become either male OR female sex organs depending on differentiation

Female: ovaries

Male: testes

When do germ cells commit to spermatogenesis?

12.5 dpc

When do germ cells commit to oogenesis?

13.5 dpc

Oogonia

Mitotically active cells the produce primary oocytes

Pre-meiosis

Ovum

Haploid cell produced by meiosis that becomes a functional gamete

Spermatogonia

Mitotically active cells in the gonads

Gives rise to spermatocytes

Stages where oogenesis is arrested

Prophase 1 & Metaphase 2

Hormone in females that stimulates follicle growth/maturation

FSH

Ovulation

Breaking open of the follicle to release the secondary oocyte

Corpus Luteum

Temporary structure in ovary that forms after secondary oocyte is released during ovulation

Surrounds ruptured follicle

Produces progesterone → thickens uterus lining for pregnancy

Degenerates if fertilization DOESN’T occur → menstruation

Maintains early pregnancy until placenta takes over

How many follicles mature per month?

1 follicle

FSH

“follicle stimulating hormone”

Released by pituitary gland (triggered by hypothalamus though)

Inhibited by low levels of estradiol

Stimulates testosterone/estrogen production in gonads

+ LH triggers puberty

LH

“luteinizing hormone”

Released by pituitary gland (triggered by hypothalamus though)

Inhibited by low levels of estradiol

Stimulates testosterone/estrogen production in gonads

Surge in Females: triggers ovulation (~day 14)

+ FSH triggers puberty

GnRH

Hormone released by hypothalamus into the blood

TriggerED by estradiol - inhibited by combo of estradiol & progesterone

TriggerS release of LH & FSH by pituitary gland

What day does LH tend to spike in females, triggering ovulation?

~day 14/15

Period of follicular growth/maturation in females

~day 0-5 (growth)

~day 5-10/14 (maturation)

Period of corpus leuteum development & degeneration

~day 14/15-20 (development)

~day 20-25/28 (degeneration)

Estradiol

Hormone secreted by growing follicle (& corpus luteum, slightly)

Helps build up uterine lining → low levels = menstruation

Increases in amount as follicle continues to grow

Peak in amount → LH surge → ovulation

Progesterone

Hormone secreted by corpus luteum

Helps maintain uterine lining for potential pregnancy

Peaks during luteal phase (after ovulation)

Period of menstrual flow phase + hormone activity

~days 0-5

Active bleeding/menstruation

Increasing: LH, FSH, Estradiol

Low: Progesterone

Period of proliferative phase + hormone activity

~days 5-14/15

Rebuilding of uterine lining

Increasing: LH, FSH, Estradiol**

Low: Progesterone

Estradiol peaks @ day 12-ish → triggers LH spike → ovulation triggered

Period of secretory phase + hormone activity

~days 14/15-28

Uterine lining thickens for potential pregnancy

Increasing: Progesterone

Decreasing: LH, FSH, Estradiol

Progesterone peaks @ day 19-ish IF no fertilization → decreases slowly

When does the fertilized egg cleave for the first time?

~24 hours post-coitum

When does implantation occur in zygote formation?

~7dpc

Trophoblast

Outer layer of blastocyst

Helps form the placenta

Aids in exchange of gas & waste between child and mother

Blastocyst

~day 5-6 of a fertilized egg

Rapidly dividing ball of cells

How long it takes for maturation of typical sperm in human males

~7 weeks

Testes

Male genital

Developed in abdomen but descends into the scrotum

Seminiferous tubules

Network of tubules located in testes

Location of sperm maturation, direction moves inwards to center of tubules

Forms from Sertoli & Leydig cells

Sertoli cells

Forms the walls of seminiferous tubules

Supports germ cells in males with nutrients

Secretes Inhibin hormone

Leydig cells

Adjacent to seminiferous tubules in testes

Produces androgens AKA testosterone

Seminal vesicles

Pair of glands that connects the vas deferent with the urethra → forms ejaculatory duct

Secretes many components of semen

Prostate gland

Secretes prostate fluid AKA a component of semen

Cytoplasm bridges

Connects not fully-mature spermatids (all 4 haploids)

Allows for the exchange of components

At what stage do developing sperm cells become haploid?

At the end of Meiosis 1 → secondary spermatocytes are haploid

Why are many redundancies built into the female reproductive cycle?

To increase resilience & decrease the chance of internal failure

Inhibin

A peptide hormone secreted by Sertoli cells (males) OR follicular cells (females)

Inhibits secretion of FSH from anterior pituitary (gland in pituitary gland) → stops creation of sperm cells (males)

What hormones within the spermatogenesis process contribute to negative feedback loops?

Inhibin → anterior pituitary

Testosterone → anterior pituitary & hypothalamus

What hormone binds to receptors on Leydig cells?

Luteinizing hormone (LH)

What hormone binds to receptors on Sertoli cells?

Follicle Stimulating Hormone (FSH)

Where do androgens released from Leydig cells bind to stimulate spermatogenesis?

Nucleus of Sertoli cells

Where do Luteinizing hormones (LH) bind in females?

Theca Cells (ovarian follicles)

Produces androgens AKA testosterone → triggers ovulation during the LH surge

Where do Follicle Stimulating Hormones (FSH) bind in females?

Granulose Cells

Convert androgens AKA testosterone to estrogen

Produces progesterone

What hormone is shared as a factor in negative feedback loops within males & females?

Inhibin