Reproduction Flashcards
Gametogenesis
The production of sperm and eggs, takes places through the process of meiosis. During meiosis, two cell divisions separate the paired chromosomes in the nucleus and then separate the chromatids that were made during an early stage of cell’s life cycle. Meiosis produces haploid cells with half of each pair of chromosomes normally found in diploid cells.
Oogenesis
occurs in the most outermost layers of the ovaries. As with sperm production, it starts with a germ cell, called oogonium, but this call undergoes mitosis to increase in number, eventually resulting in up to about one to two million cells in the embryo. Results in 1 large ovum.
- Oogenesis: production of egg cells (ova)
- Oogonia (stem cells) multiply by mitosis and store nutrients.
o Primary oocytes
o Develop in primordial follicles (sack of cells that surrounds ovum)
o At birth, female presumed to have lifetime supply of primary oocytes
o Primary oocytes are arrested in the beginning stages of meiosis
Meiosis is different for oocytes. Primary oocyte undergoes meiosis
Meiosis I resume and completes during evolution.
Meiosis II resumes and completes after fertilization. Only one ovum produced.
Spermatogenesis
occurs in the wall of the seminiferous tubules, with stem cells in the periphery of the tube and the spermatozoa at the lumen of the tube. Has diploid ells and result in 4 sperm cells.
Yolk amount across species
- Alecithal eggs – no yolk (placental mammals)
- Microlecithal eggs – small amounts (marsupials)
- Mesolecithal eggs – intermediate amounts (amphibians, fish)
- Macrolecithal eggs – large amounts (insects, reptiles, birds, some fish)
In egg laying animals, more yolk = more development time in egg
Vitellogenesis
deposition of lipids and proteins (vitellogenin). Vitellogenin is a marker of environmental estrogens when seen in males.
Oviparity
Fish/amphibians: produce gelatinous eggs, water needed throughout development, fertilization occurs after egg laying (internal or external fertilization)
Reptiles/birds/monotremes: Produce eggs with calcium carbonate layer, fertilization occurs before eggshell formed (internal fertilization)
Amniotic egg: embryo produces 4 extraembryonic membranes
Chorion: the outermost membrane surrounding and embryo of a reptile, bird, or mammal. In mammals it contributes to the forming of the placenta.
Amnion: a membrane forming a fluid-filled cavity (the sac) that encloses embryo ( bag of waters)
Allantois: the fetal membrane lying below the chorion in m any vertebrates, formed as an outgrowth of the embryos gut. In birds it grows to surround the embryo, in eutherian mammals it forms part of placenta.
Yolk sac: sac attached to an embryo, formed by cells of the hypoblast adjacent to the embryonic disk.
Placenta
provides supply of nutrients, oxygen, and hormones to fetus.
Ovaries (female gonads)
produce female gametes (ova).
Oviduct
contains ciliated fimbriae that creates current to move oocyte into uterine tube
Uterus (placental mammals)
The endometrium is the innermost layer of the uterine wall. Myometrium is the middle layer of the uterine wall. Endometrium is composed of columnar epithelium. Myometrium is composed of muscle layer which developed by the uterine myocytes.
Follicle recruitment
after puberty, multiple primordial follicles develop (cohorts). At each stage, many follicles die (atresia). Takes ~1 year to primordial follicle to reach graafian follicle stage. Birth: ~700, 000 potential eggs. Only ~ 500 eggs ovulated.
The ovarian cycle
follicle growth takes ~1 year for follicle to grow into graafian follicle that will ovulate egg. Humans- monthly (~28) series of events associated with maturation of one oocyte.
Three phases
- Follicular phase: period of vesicular follicle growth (days 1–14)
- Ovulation: Release of oocyte from the follicle occurs between phases
- Luteal phase: period of corpus luteum activity (days 14–28); time during which fertilization needs to occur
Endocrine control over oogenesis
the hypothalamus secretes gonadotropin releasing hormone (GnRH). GnRH stimulates the secretion of luteinizing hormone (LH) and follicle stimulating hormone (FSH) from the anterior pituitary gland. FSH and LH stimulate different cells of the follicle. Thecal cells and granulosa cells.
