11.4 Gametogenesis

Gametogenesis

The process by which diploid precursor cells undergo meiotic division to become haploid gametes

Males gametogenesis

Spermatogenesis - produce spermatozoa (sperm)

Female gametogenesis

Oogenesis - produce ova (eggs)

Where does gametogenesis occur and what steps does it involve?

Occurs in gonads

* Multiple mitotic divisions and cell growth of precursor germ cells
* Two meiotic divisions (meiosis I and II) to produce haploid daughter cells
* Differentiation of the haploid daughter cells to produce functional gametes

Process of spermatogenesis

* Spermatogenesis describes the producton of spermatozoa (sperm) in the seminiferous tubules of the **testes**
* The process begins at puberty when the germline epithelium of the seminiferous tubules divides by mitosis
* These cells (*spermatogonia*) then undergo a period of cell growth, becoming *spermatocytes*
* The spermatocytes undergo two meiotic divisions to form four haploid daughter cells (*spermatids*)
* The spermatids then undertake a process of differentiation in order to become functional sperm cells (*spermatozoa*)

Oogenesis

* Oogenesis describes the production of female gametes (ova) within the **ovaries** (and, to a lesser extent, the oviduct)
* The process begins during foetal development, when a large number of primordial cells are formed by mitosis
* These cells (%%*oogonia*%%) undergo cell growth until they are large enough to undergo meiosis (becoming *primary oocytes*)
* The primary oocytes begin meiosis but are arrested in prophase I when granulosa cells surround them to form follicles.

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* The primary oocytes remain arrested in prophase I until puberty, when a girl begins her menstrual cycle
* Each month, hormones (FSH) will trigger the continued division of some of the primary oocytes
* These cells will complete the first meiotic division to form two cells of unequal size
* One cell retains the entirety of the cytoplasm to form a *secondary oocyte*, while the other cell forms a polar body
* The polar body remains trapped within the follicle until it eventually degenerates
* The secondary oocyte begins the second meiotic division but is arrested in metaphase II
* The secondary oocyte is released from the ovary (ovulation) and enters into the oviduct (or fallopian tube)
* The follicular cells surrounding the oocyte form a corona radiata and function to nourish the secondary oocyte
* If the oocyte is fertilised by a sperm, chemical changes will trigger the completion of meiosis II and the formation of another polar body (the first polar body *may* also undergo a second division to form a third polar body)
* Once meiosis II is complete the mature egg forms an *ovum*, before fusing its nucleus with the sperm nucleus to form a zygote (if fertilized)
* If not fertilized then it will remain an ovum and degenerate after a while.

Differences between spermatogenesis and oogenesis

1. Number of cells produced
* Spermatogenesis → four functional gametes produced
* Oogenesis → cells don’t divide equally and only one functional gamete is formed (plus 2 - 3 polar bodies)
2. Size of cells produced
* Spermatogenesis → cell size is equal and equal amounts of cytoplasm
* Oogenesis → one cell with all cytoplasm
3. Timing of process
* Spermatogenesis → production of gametes is continues which starts in puberty and continues until death
* Oogenesis
* begins before birth with formation of a fixed number of oocytes
* continues during puberty
* ends during menopause

Seminiferous tububle

* The testes are composed of seminiferous tubules which produce sperm
* Each tubule is surrounded by a basement membrane which is lined by germline epithelium
* The germline epithelium will divide by mitosis to make spermatogonia (which divide by meiosis to make spermatids)
* The spermatids differentiate into functional spermatozoa, which are then released into the lumen of the tubule
* These developing spermatozoa are nourished by Sertoli cells, which reside in the tubule lining
* Outside of the tubules are blood capillaries and interstitial cells (Leydig cells) which produce testosterone

Ovary + types of follicles

* The ovary contains follicles in various stages of development
* These follicles will develop over the course of a menstrual cycle and hence will not always be apparent upon inspection
* Primordial follicles contain egg cells that have been arrested in prophase I (primary oocytes)
* Some of these follicles will develop each month into primary follicles and then secondary follicles
* Each cycle, one follicle will become a dominant Graafian follicle and rupture to release the secondary oocyte
* The ruptured follicle will then develop into a short-lived corpus luteum, which secretes key ovarian hormones
* Eventually the corpus luteum will degenerate to form a corpus albicans

Sperm

* Three parts, head, mid-piece and tail
* Head:
* Haploid nucleus → contains paternal DNA
* Acrosome cap = contains hydrolytic enzymes that help to penetrate the jelly coat of the egg
* Centrioles - Needed by a zygote to divide
* Mid-piece - contains high numbers of mitochondria which provide the energy (ATP) needed for the tail to move
* Tail - composed of microtubule structure called the axoneme, which bends to facilitate movement

Egg

* Two layers
* Zona pellucida - a glycoprotein matrix which acts as a barrier to sperm entry
* Corona radiata - external layer of follicular cells which provide support and nourishment to the egg cell
* Within egg → numerous cortical granules which release their content upon fertilisation to prevent polyspermy
* No nucleus in egg until after fertilization has occurred

External fertilization

* involves fusion of gametes outside of body of parent
* common in aquatic animals where water acts as a medium via which the gametes can travel
* susceptible to environmental influences, such as predators and pH changes
* consequently, species that reproduce this way usually release large quantities of gametes to compensate for losses
* Process of releasing gametes into water = spawning

Internal fertilization

* involves fusion of gametes inside body of parent
* requires a method by which the gamete of one parent can be introduced inside the body of another
* Terrestrial animals typically use internal fertilization so as to prevent exposure and desiccation of gametes or embryos
* Internal fertilization offers more protection to the gametes and embryos, but at a potential survival cost to the parent 

Three processes in fertilization in humans

1. Capacitation
2. Acrosome reaction
3. Cortical reaction

1. Capacitation (human fertilization)

* Occurs after ejaculation
* Chemicals released by the uterus dissolve the sperm’s cholesterol coat
* Improves sperm motility (larger chance of reaching egg)
* Also destabilizes acrosome cap, which is necessary for the acrosome reaction to occur upon egg and sperm contact

2. Acrosome reaction (human fertilization)

* when sperm reaches egg, acrosome reaction allows sperm to break through surrounding jelly coat
* Sperm pushes through the follicular cells of the corona radiata and binds to the zona pellucida
* The acrosome vesicle fuses with the jelly coat and releases digestive enzymes which soften the glycoprotein matrix
* Sperm pushes its way through softened jelly coat and binds to exposed docking proteins on egg membrane
* Membrane of egg and sperm fuse and sperm nucleus (and centriole) enters the egg

3. Cortical reaction (human fertilization)

* occurs once a sperm has successfully penetrated an egg in order to prevent polyspermy
* Cortical granules within the egg’s cytoplasm release enzymes (via exocytosis) into the zona pellucida (jelly coat)
* These enzymes destroy sperm binding sites and also thicken and harden the glycoprotein matrix of the jelly coat
* This prevents other sperm from being able to penetrate the egg (polyspermy), ensuring the zygote formed is diploid 

What prompts completion of meiosis II after fusion of egg and sperm

influx of Ca^2+

What happens with zygote after fertilization

Will undergo several mitotic divisions to form a solid ball of cells called a morula

* as the morula continues to divide, it undergoes differentiation and cavitation to form a blastocyst.

Three sections of blastocyst

* An *inner cell mass* (that will develop into the embryo)
* A surrounding outer layer called the *trophoblast* (this will develop into the placenta)
* A fluid filled cavity called the *blastocoele* 

implantation of blastocyst

* The blastocyst breaks the jelly coat that was surrounding it and preventing its attachment to the endometrium
* Digestive enzymes are released which degrade the endometrial lining, while autocrine hormones released from the blastocyst trigger its implantation into the uterine wall

after implantation of blastocyst

* The growing embryo will gain oxygen and nutrients from the endometrial tissue fluid, ensuring its continued development
* The entire process (from fertilization to implantation) takes roughly 6 – 8 days

secretion of human chorionic gonadotropin (hCG)

* After implantation of blastocyst it secretes hCG,
* promotes maintenance of the corpus luteum within the ovary and prevents its degeneration
* As a consequence of this, the corpus luteum survives and continues to produce both oestrogen and progesterone

Oestrogen function

inhibits FSH and LH production by the pituitary gland, preventing the release of more eggs from the ovaries

Progesterone function

functions to maintain the endometrium (which is nourishing the embryo) and thicken the cervix

levels of hCG

* maintained for roughly 8 – 10 weeks while the placenta is being developed
* After this time, the placenta becomes responsible for progesterone secretion and nourishing the embryo
* At this point the corpus luteum is no longer required and begins to degenerate as hCG levels drop

Function placenta

* life support for foetus
* It facilitates the exchange of materials between the mother and foetus
* It secretes hormones to maintain the pregnancy after the corpus luteum has degenerated

structure placenta

* Disc-shaped structure
* Formed from development of trophoblast upon implantation and eventually invades the uterine wall
* Maternal blood moves via open ended arterioles into intervillous spaces within the placenta called lacunae
* Chorionic villi extend into these pools of blood and mediate the exchange of materials between the foetus and the mother
* Exchanged material is transported from the villi to the foetus via an umbilical cord, which connects the foetus to the placenta
* Upon birth, the placenta is expelled from the uterus with the infant – it is then separated from the infant by severing the umbilical cord (the point of separation becomes the belly button)

Material exchange across placenta

* Chorionic villi extend into the intervillous space (lacuna) and exchange materials between the mother and foetus
* Chorionic villi are lined by microvilli = larger surface area
* Foetal capillaries within the chorionic villi lie close to the surface to minimise diffusion distance from blood in the lacunae
* Materials such as oxygen, nutrients, vitamins, antibodies and water will diffuse from the lacunae into foetal capillaries
* Foetal waste (such as carbon dioxide, urea and hormones) will diffuse from the lacunae into the maternal blood vessels

Hormonal role in pregnancy

* The placenta takes over the hormonal role of the ovaries (at \~12 weeks) and begins producing estrogen and progesterone
* Estrogen stimulates the growth of uterine muscles (myometrium) and the development of the mammary glands
* Progesterone maintains the endometrium, as well as reducing uterine contractions and potential maternal immune responses
* Both estrogen and progesterone levels drop near the time of birth

Process of childbirth name and how does it occur

Parturition - via positive feedback under hormonal control

How is birth initiated?

* In the case of childbirth, fetal growth eventually causes stretching of the uterine walls, which is detected by stretch receptors
* This triggers the release of hormones (oxytocin) that induce uterine muscles to contract, further reducing space in the womb
* This causes more stretching and hence more contraction until the origin stimulus (the foetus) is removed (i.e. birth)

Hormonal control during birth

* After 9 months baby is big causing strain on both mother and infant
* This induces releases of chemicals which trigger a rise in estrogen, estriol in particular
* Estriol prepares the smooth muscle of the uterus for hormonal stimulation by increasing its sensitivity to oxytocin
* Estriol inhibits progesterone which was preventing uterine contractions from occuring while foetus developed
* Now that the uterus is primed for childbirth, brain triggers release of oxytocin from posterior pituitary gland
* Oxytocin stimulates the uterine muscles to contract, initiating the birthing process (and inhibits progesterone)
* The foetus responds to this uterine contraction by releasing prostaglandins, which triggers further uterine contractions
* As the uterine contractions trigger the release of chemicals that cause further contractions, a positive feedback loop ensues
* Contractions will stop when labour is complete and the baby is birthed (no more stretching of the uterine wall)

gestation period

time taken for a foetus to develop – beginning with fertilization and ending with birth

* The duration of the gestation period will differ markedly between different species of animal

Two main factors that contribute to the length of the gestation period

* Animal size / mass – larger animals tend to have longer gestation periods (as they tend to produce larger offspring)
* The level of development at birth – more developed infants will typically require a longer gestation period

Atricial vs. precocial level of development

* Altricial mammals give birth to relatively helpless, undeveloped offspring that need extended rearing
* Precocial mammals give birth to more developed offspring that are mobile and independent and require minimal rearing
* Generally, altricial mammals (e.g. marsupials and rodents) require shorter gestation periods than precocial mammals (e.g. ungulates such as cows, pigs and rhinoceroses)