Oogenesis and Follicular Development AP Biology > Reproduction and Development > Reproduction and Development OOCYTE AND FOLLICULAR DEVELOPMENT Ovarian follicles: Functional units of the ovary • Oocytes Female sex cells - The ovary houses millions of oocytes. However, only a very small percentage of these will reach maturity; most will undergo atresia (aka, degeneration). • Follicular cells Support the oocyte and produce hormones that regulate ovarian and uterine events (the events of the ovarian and uterine cycles are discussed in detail, elsewhere). Developmental processes: Embryologic origins • Oocytes arise from primordial germ cells (undifferentiated stem cells) that migrate to the gonadal ridge early in fetal development (approximately 3-6 weeks gestation). • Germ cells proliferate via mitosis. • Once they reach the gonadal ridge, they become oogonia, which reside in clusters (aka nests). • The majority of oogonia undergo atresia (aka, degeneration). • The minority enter meiosis I. Primordial follicle • After meiosis I is initiated, the primoridial follicle forms around the oocyte. • Comprises a single layer of flat granulosa cells, which interact to guide follicular maturation. • Basement membrane (aka, lamina) surrounds the follicle. • After the primordial follicle forms, meiosis I arrests in prophase (dictyone phase). Puberty • Following puberty, follicles are cyclically "recruited" for further development; at any given time after this, more than 90% of follicles present in the ovary are arrested in the primordial stage. 1 / 3 • Despite meiotic arrest, the oocyte and follicle may continue to grow. Primary follicle • Primary oocyte, which has grown larger. • Zona pellucida is thick a-cellular coat that covers the oocyte; the zona pellucida enables fertilization. - It displays sperm receptors and facilitates the acrosome reaction; after fertilization, the zona pellucida prevents additional sperm from joining with the oocyte. • Granulosa cells proliferate via mitosis and change from flat to cuboidal, which reflects their greater activity. Secondary follicle • Primary oocyte achieves meiotic and developmental competence (it is capable of completing meiosis and preparing for implantation). • Cuboid granulosa cell layers have multiplied from 1 to now 6-9 layers (typically) by the end of the secondary follicular stage. • Thecal cells, which arise from the ovarian interstitium, begin to accumulate around the basement membrane of the secondary follicle. • Of the secondary follicles, some will be "recruited" by follicle-stimulating hormone (FSH) to become tertiary follicles. Early Tertiary Follicle • Granulosa cells are separated by an antrum, which is a fluid-filled cavity within the follicle; notice that it "pushes" the oocyte to one side of the follicle. • Thecal cells differentiate into two layers: - The theca externa, aka, external thecal cells, form the outermost layer of the tertiary follicle; this layer will form the follicular capsule. - The theca interna, aka, internal thecal cells, form the inner layer; these cells will have key roles in hormonal communication and vascularization of the follicle. Pre-ovulatory surge in LH initiates oocyte development resumption and Meiosis I completion; oocyte enters meiosis II, then arrests in metaphase; a Polar body forms. Late tertiary stage (aka, Graafian, pre-ovulatory stage): • Cumulos oophorus is the collection of granulosa cells that support the secondary oocyte and polar body. • Corona radiata is a subset of the cumulus oophorus that directly surrounds the zona pellucida. Of the late tertiary follicles, only one, the so-called "dominant follicle," is ovulated. Ovulation • Ovulated secondary oocyte takes the corona radiata with it. 2 / 3 • Ruptured follicle transitions physiologically and morphologically to become the corpus luteum, which acts as a temporary endocrine gland. • If the oocyte is not fertilized, it will be discharged with the menstrual fluid: - In this case, the corpus luteum will involute and become the corpus albicans ("white body"); it will no longer produce hormones. • If fertilization does occur, the secondary oocyte will resume and complete meiosis II, and a another polar body will be formed. • If the fertilized oocyte successfully implants, the corpus luteum will persist until the placenta can perform its hormonal duties. Powered by TCPDF (www.tcpdf.org) 3 / 3.