Covers how female gametes form in the ovaries, from oogonia arising from primordial germ cells in the embryo through to the mature ovulated oocyte. It takes the stages of follicle development in order, the arrest points of meiosis before birth and at metaphase II, and the changes in cytoplasmic organelles during maturation. It also explains the zona pellucida, cumulus cells and the cortical reaction, and includes four mnemonics for recalling follicle stages, meiotic arrest, surrounding structures and organelle rearrangement.
Topic 34 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 2 sections, about 1,189 words (8 min of reading), with 10 flashcards.
In this note
Oogenesis. Mature mammalian ova.
MNEMONICS
Oogenesis. Mature mammalian ova.
Oogenesis is the biological process through which female gametes, or eggs, are formed.
This process occurs primarily within the ovaries.
Unlike spermatogenesis, which results in four equal gametes, oogenesis involves unequal divisions of the cytoplasm during meiosis.
As a result, only one large functional ovum is produced from each primary oocyte, along with two or three smaller polar bodies that typically degenerate.
Oogenesis begins before birth and follows a prolonged and interrupted timeline. It starts in the fetal stage and continues, in phases, until menopause.
From puberty, during each menstrual cycle, typically one oocyte resumes maturation and may be ovulated.
Origin and Early Development of Oogonia
Oogonia are the earliest recognizable female germ cells.
These germ cells originate from primordial germ cells known as gonocytes, which first appear in the epiblast during early embryonic development.
They migrate away from the embryonic body to the yolk sac, which provides a protective environment where they avoid premature differentiation.
Around the fourth to sixth week of gestation, these cells migrate from the yolk sac back into the embryo, moving towards the developing gonadal ridges near the mesonephros, a primitive kidney structure.
Once in the gonadal ridges, the gonocytes proliferate and differentiate into oogonia.
By the tenth to twelfth week of human embryogenesis, oogonia enter the first meiotic division and become primary oocytes.
Somatic Cell Support and Follicle Formation
As the oogonia settle into the ovary, they become surrounded by somatic cells that will develop into supportive follicular structures.
The somatic cells directly in contact with oocytes differentiate into granulosa cells. These cells are analogous to Sertoli cells in males and play a crucial role in nurturing and regulating oocyte development.
Granulosa cells form the first layer around the oocyte, establishing what is called a primordial follicle.
Additional somatic cells located outside the granulosa layer differentiate into theca cells, which are similar in function to Leydig cells in the testis.
Theca cells contribute to steroid hormone production and respond to luteinizing hormone after puberty.
A fully developed ovarian follicle includes the oocyte, surrounding granulosa cells, theca cells, and the extracellular structures between them.
Follicle Development Stages
1. Primordial Follicle:
Contains a dormant primary oocyte arrested in the first meiotic division.
Surrounded by a single flat layer of granulosa cells.
2. Primary Follicle:
The oocyte begins to grow in size.
Granulosa cells transition to a cuboidal shape but remain in a single layer.
Continue reading free
The whole note, all 2 sections, is free with an AceMed account, with its 10 flashcards. Premium adds flashcard recall, highlights and Ask AI.
