Oogenesis

definition of oogenesis

• Oogenesis is the formation of ova in the ovaries

• Oogenesis begins before a female is born

definition of oogonia

• Millions of egg mother cells or oogonia, develop in the ovaries of a female prenatally

• In humans they start to develop between weeks 4 and 8 and are present in the foetus between weeks 5 and 30.

• Oogonia are diploid (2n) cells

development of oogonia

• Oogonia undergo mitosis before the female is born

• Oogonia develop into primary oocytes, still diploid (2n)

primary oocytes

• Primary oocytes can then begin meiosis I, but stop at prophase I

• At birth, there are hundreds of thousands of primary oocytes per ovary, still in the first prophase contained in primordial follicles

primordial follicles

• surrounded by a single layer of flattened ovarian follicular epithelial cells. (These cells are also known as granulosa cells).

primordial follicle during puberty

• under the influence of hormones, it is stimulated and becomes a primary follicle

• The oocyte enlarges, and the follicular cells divide. A follicle that has two layers of follicular cells is called a primary follicle.

primary oocyte: under going meiosis

• primary oocyte completes the first stage of meiosis producing two haploid cells (n)

• Both receive half the chromosomes but are unequal in size

• One gets most of the cytoplasm (the secondary oocyte) while the other gets little (first polar body)

secondary oocyte

• The secondary oocyte immediately starts the second division of meiosis but stops at metaphase II

• At this stage, ovulation occurs – the follicle ruptures, releasing the secondary oocyte and  a polar body

• This first polar body may also undergo a second  meiotic division to produce two polar bodies

function of the polar bodies

• to discard excess chromosomes and chromatids in the process of making one haploid egg 

• The three polar bodies produced die and disintegrate, taking the excess DNA with them.

secondary ooycte: transportation for fertilisation

• The secondary oocyte travels down fallopian tube for fertilisation

• If not fertilised, eliminated through menstruation

if the oocyte is fertilised by sperm…

• chemical changes will trigger the completion of meiosis II and the formation of another polar body and a immature egg called an ootid (n)

• The time spent as an ootid is  only minutes

• The ootid matures into an ovum which then fusing its nucleus with the sperm nucleus to form a zygote (2n)

difference between secondary oocyte and ovum

secondary oocyte	ovum
resulting oocyte when meiosis 1 is completed and gives rise to ootid on completion of meiosis 2	ovum is the mature female gamete in mammals, which gives rise to the embryo
prod from a primary oocyte - meiosis 1	prod when ootid matures
each chr. has 2 chromatids	each chromosome consists of a single chromatid
devs into ovum by undergoing meiosis 2	devs into zygoote after fertilisation
less differentiated female gamete	the differentiated female gamete

draw the process of meitotic events and process of follicle development in the ovary that take place