reproductive system

sperm & egg anisogamy

• sperm are just cheap gametes

  • spray & pray

• eggs are expensive gametes

  • eggs all in one basket

• these are two strategies that succeed because of the other’s existence

basal gonads

• anisogamy can co-occur without gonad specialization

• sponges have no gonads but can still undergo sexual reproduction

• sperm and egg are both produced by the same tissue types

bipotential gonads

• can differentiate into either an ovary or testis

• can only be one, not both

HPG axis

the hypothalamus (anterior) pituitary gonadal axis

• controls the release of sex hormones and is very conserved

first step of the HPG axis

gonadotropin-releasing hormone (GnRH) rises in sexual maturity

second step of the HPG axis

when GnRH increase

• this activates the anterior pituitary to secrete:

  • luteinizing hormone

  • follicle stimulating hormones

third step of the HPG axis

once the AntPit secretes its hormones, the gonads are stimulated to produce testosterone or estradiol progesterone

layers of the ovary

• cortical region (secondary sex cords)

• germs cells (becomes eggs)

• medullary region (primary sex cords)

if ‘female’ signals are produces elsewhere, the cortex of the early gonad develops and pre-egg cells call primordial germ cells collect there

when progesterone drops in the final stage of the luteal phase, it triggers:

• spawning (non-amniotes)

• depositing of an egg (reptiles)

• menstruation (mammals)

“ok, on to prepping again”

FSH and LH rise during menstruation because they…

• they were being inhibited by progesterone

• this stimulates the preparation of another round of eggs in the follicular phase

estrogen levels rise and…

flip from being inhibitory (when at low levels) to stimulatory (at high levels) to FSH and LH production

LH surge causes the follicle to…

rupture, releasing the egg (ovulation) and marking the beginning of the luteal phase

the progesterone produced by the corpus luteum inhibits LH and FSH

this progesterone:

• maintains + thickens uterus in placental mammals to allow implantation

• halts vitellogenesis and stimulates shell deposition in egg layers

“hey we just released an egg, lets not make another one for a hot second”

vitellogenesis

• the liver responds to rising estradiol by producing vitellogen, the fat that helps develop the yolk

• progesterone is a signal that the yolk-building phase is done

spontaneous ovulation

occurs on a clock-driven cycle for most mammals and birds (~29-32 days in humans)

induced ovulation

requires some specific event, generally the presence of a male or actual copulation

layers of the teste

• tunica albuginea

• seminiferous tubules (primary sex cords)

• germs cells

• rete testis

under the influence of ‘male’ signals, the medulla of the gonad develops into cords of blind tubes and primordial germ cells migrate here

spermatogenesis

• those same primordial germ cells become spermatogonia and undergo nearly constant mitosis

• one daughter cell stays in the teste as a spermatogonia and the other daughter cell starts meiosis as a spermatocyte (an ‘infinite’ supply)

• four sperm are produced per meiosis

weisman barrier

• august weismann

• the concept that hereditary information moves only from germline cells to somatic cells, never in reverse

• ‘a copy of a copy of a copy of a copy’

genetic sex determination

• what most mammals do

• for most groups, SOX9 presence is the driver of masculinization

• SOX9 absence drives feminization

• SRY (sex determining region of the y chromosome) only turns on SOX9

• XY and ZW systems is common

environmental sex determination

• sociality is part of the environment

fertilization methods

• can be external or internal

• basal trait is external and there is a gradual move to internal in more derived groups

• male structures are more heterogenous than female across groups

female shell glands

• in groups with hard-walled eggs, the shell glands deposit calcium matrix around

• this protects against

  • damage

  • predation

  • water loss

cloaca

one exit point for feces/ urine/ gametes and is present in fish, amphibians, some reptiles, and monotremes

• gametes can be passed externally or internally

layers of the uterus

• endometrium

  • stratum basalis

  • stratum functionalis (how the embryo gets fed during pregnancy)

• myometrium (smooth muscle- used for contractions)

• perimetrium (membrane surrounding the whole thing

ovipary

egg laying

• fish

• birds

• reptiles

ovoviviparity

internal egg retention until hatching

• great white and makos

viviparity

live birth

• mammals