25.2
Oogénesis
En las mujeres humanas, la ovogénesis produce un óvulo u óvulo maduro por cada célula precursora que entra en la meiosis. Este proceso se diferencia e…
- [Instructor] En los seres humanos, los ovocitos maduros
se generan mediante el proceso de Ovogénesis.
El Ovogénesis comienza durante el desarrollo embrionario
en los ovarios en crecimiento de las hembras.
Aquí las celular diploides precursoras
llamadas individualmente ovogonias, entran en mitosis,
y se dividen para formar ovocitos primarios.
Cada uno está rodeado por células estromáticas.
Esos ovocitos comienzan la primera etapa de la meiosis.
Meiosis I, que sucede hasta que los cromosomas maternos
y paternos intercambian información.
En este momento la meiosis se detiene
debido a los factores inhibidores derivados de
las células estromáticas.
El proceso solo reinicio en la pubertad.
Aquí las señales de los ciclos menstruales causan que
el ovocito primario se agrande y entre en Meiosis I,
produciendo dos células haploides de igual tamaño.
La célula rica en citoplasma más grande
se convierte en ovocito secundario, mientras que
el primer corpúsculo polar más pequeño se disuelve.
El ovocito secundario entra por poco tiempo
en la segunda fase de la meiosis, meiosis II.
Pero el proceso se detiene nuevamente.
La ovulación desprende esta célula
y aquellas que la rodean del ovario.
Y el complejo migra a la trompa de falopio
donde ocurre la fertilización si hay esperma presente.
La meiosis se reanuda luego de la fertilización
y termina cuando el ovocito secundario se divide
formando un corpúsculo polar secundario de menor tamaño
que se disipa de manera similar, y un óvulo más grande.
El último es el óvulo haploide
en el cual los núcleos paternales y maternales
son temporalmente visibles.
Por lo tanto la ovogénesis solo produce un óvulo
por cada ovocito primario
que completo el proceso con éxito.
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Q1: When does oogenesis begin in human females?
Oogenesis commences during embryonic development in the forming female ovaries. Diploid precursor cells called oogonia enter mitosis and divide to form primary oocytes, each surrounded by supporting cells. These primary oocytes then begin meiosis I, which halts until puberty when menstrual cycle signals restart the process.
Q2: Why does oogenesis produce unequal cell divisions?
Oogenesis produces asymmetric divisions to ensure the mature egg receives sufficient cytoplasm and nutrients for early development. During meiosis I, a large secondary oocyte containing most cytoplasm forms alongside a smaller first polar body that dissolves. This unequal distribution repeats in meiosis II, guaranteeing the ovum has adequate resources to support embryonic growth after fertilization.
Q3: What happens to meiosis during oogenesis after ovulation?
After ovulation, the secondary oocyte enters the fallopian tube where fertilization may occur. Meiosis II remains arrested until sperm fertilizes the egg. Upon fertilization, meiosis II resumes and completes, producing a second polar body that dissolves and a mature haploid ovum with distinct paternal and maternal nuclei.
Q4: How does maternal age affect oogenesis and chromosome segregation?
Increased maternal age elevates the risk of chromosome segregation errors during meiosis I or II, potentially causing aneuploidy—an abnormal chromosome number. Proposed explanations include degradation of the meiotic spindle apparatus over time or accumulation of abnormal cells in ovaries. Women 35 and older are typically offered prenatal testing to detect chromosomal abnormalities.
Q5: How does endometriosis impact oocyte quality?
Endometriosis, where uterine lining accumulates outside the uterus, may compromise oocyte quality. Oocytes from women with endometriosis show defects in the meiotic spindle apparatus or reduced fertilization rates during in vitro fertilization. Scientists hypothesize poor oocyte quality results from increased immune-associated proteins or altered hormone levels in affected patients.
Q6: What environmental factors can negatively affect oogenesis?
Environmental exposures including tobacco use, hormone supplementation, and bisphenol-A from plastics have been suggested to adversely affect oogenesis and meiosis. These factors may increase the risk of aneuploidy and compromise oocyte quality. Research continues to clarify the mechanisms by which environmental stressors influence the oogenesis process.
Q7: Why does oogenesis yield only one mature egg per primary oocyte?
Oogenesis produces a single mature ovum because meiotic divisions are asymmetric. Meiosis I generates a large secondary oocyte and a small first polar body; meiosis II produces a large ovum and a second polar body. The smaller polar bodies dissolve, ensuring only one egg cell with sufficient cytoplasm and nutrients completes development after cleavage and blastulation.