When is the secondary oocyte formed

After ovulation the oocyte is arrested in metaphase of meiosis II until fertilization. At fertilization , the secondary oocyte completes meiosis II to form a mature oocyte 23,1N and a second polar body.

In males, gametogenesis begins at puberty and continues into advanced age. Primordial germ cells 46,2N migrate into the testes at week 4 of development and remain dormant.

At puberty, primordial germ cells differentiate into type A spermatogonia 46,2N. Type A spermatogonia divide by mitosis to form either more type A spermatogonia to maintain the supply or type B spermatogonia. Under the influence of estrogen released during the first half of the menstrual cycle , three changes take place in the uterine tubes to facilitate its capture of the egg:.

The uterine tubes move closer to the ovaries physical approximation. The fimbriae on the ends of the tubes beat more rapidly increased fluid current. The number of ciliated cells in the epithelium of the fimbriae increase increase in ciliation. Sperm are deposited in the upper vagina and must overcome several obstacles to reach an egg in the ampulla of one of the uterine tubes. The egg itself is viable for only about 24 hours. Table 1 - Obstacles to Sperm Transport. The alkaline seminal fluid temporarily neutralizes the normal acidity pH 4.

The composition of cervical mucus changes during menstrual cycle. Sperm can most easily penetrate the thinner E-mucus that predominates during the last few days before ovulation, as opposed to the thicker G-mucus. Two modes of transport:. Slow — the rest of the sperm swim their way up the last part of the cervical tube, are stored in cervical crypts folds of the cervix , and are slowly released into the uterus over days.

Table 2 - Karyotypes of Germ Cells and Gametes. Primordial germ cell. After ovulation, the fringelike end of the fallopian tube sweeps the oocyte inside of the tube, where it begins its journey to the uterus. If the secondary oocyte is fertilized by a sperm as it is passing through the fallopian tube, it completes meiosis and forms a mature egg and another polar body.

The polar bodies break down and disappear. If the secondary oocyte is not fertilized, it passes into the uterus as an immature egg and soon disintegrates.

What's amazing about these cells? Oogenesis The process of producing eggs in the ovary is called oogenesis. Maturation of a Follicle Beginning in puberty, each month one of the follicles and its primary oocyte starts to mature also see Figure below.

Ovulation and Fertilization After 12—14 days, when the follicle is mature, it bursts open, releasing the secondary oocyte from the ovary. Summary Immature eggs form in the ovaries before birth. Each month, starting in puberty, one egg matures and is released from the ovary. Release of an egg is called ovulation. Review When does a female begin to produce her eggs? What is a polar body? Describe ovulation. Explain your answer.

Primordial germ cells multiply during fetal development. At birth, the ovary contains around primordial follicles which contain primary oocytes. These primary oocytes do not undergo further mitotic division, and they remain arrested in the prophase stage of meiotic division I , until sexual maturity see Meiosis , in the topic cell.

Compare this to male gametogenesis. At sexual maturity, two hormones, produced by the pituitary gland: follicle stimulating hormone FSH and lutenising hormone LH cause these primordial follicles to develop. In each ovarian cycle, about 20 primordial follicles are activated to begin maturation. When activated, the first meiotic division is completed. When this happens, the primary follicle has matured into a secondary follicle.

The second division then starts, and a Graafian follicle is formed. This contains a secondary oocyte. This second division is not completed, unless the ovum is fertilised.

This diagram shows how primary oocytes in the primordial follicles are diploid , and are starting their first meiotic division. The pairs of homologous chromosomes one maternal, one paternal pair up on the spindle, and genetic material can be swapped over.