Structure of Ovaries

Transcription

1 Structure of Ovaries

2 What Are Oocyte Accessory Cells? Follicle cells somatic connected by desmosomes, gap junctions, interdigitating microvilli for anchorage and exchange function in yolk transport, egg coat synthesis, etc. Nurse cells invertebrates only sisters of oocyte by incomplete divisions polyploid and synthetically active

3 Two Types of Ovary Panoistic no nurse cells follicular epithelium Meroistic both nurse and follicle cells Present different strategies for oogenesis

4 Ovary in Drosophila

5 Drosophila Ovariole

6 Nurse Cells and Oocyte in D.m. Note fusome and arrow pointing to oringinal cell

7 How is Oocyte Distinguished from Nurse Cells? Nest is syncitium of 16-cells all from one primary germ cell by mitosis 2 enter meiosis synaptonemal complex 1 loses SC and one becomes egg

8 How is Oocyte Distinguished from Nurse Cells? fes: female sterile mutant has nest of 14 cells, none with 4 bridges, some complete division, loss of synchrony, sterility fusome indicates which will be the oocyte grows away from one of spindle poles fused ER vesicles plug bridges all bridges point towards one future oocyte it may contain determinants or direct traffic (storage)

9 Transport in Meroistic Ovaries Oocyte condensed chromosomes-inactive Nurse cells polyploid, polytenic (~1000x haploid) and very active RNA synthesis Transport RNA large things pass at end (mitochondria, ribosomes, etc) Microtubules through ring canals which are lined with microfilaments Ring canals yellow Microtubules red

10 RNA and Information Storage

11 Is RNA Transported? The House Fly Pulse chase with radioactive uridine Autoradiography RNA from nurse cell nuclei to oocyte cytoplasm

12 Why Meroistic? Hypothesis nurse cells allow rapid accumulation of large amounts of RNA, etc. in oocyte prediction: length of oogenesis is much less than in panoistics 10 d in fly, ~1 year in frog

13 Then How Do Panoistics Do It? Even with a long time, filling up a large egg with stores when only 4C nucleus is active is a problem Adaptations lampbrush chromosomes multiple nucleoli

14 Portion of a Lampbrush Chromosome

15 What Are Lampbrush Chromosomes? Prominent unusual chromosomal structures not seen in meroistics occur in prophase I GV: early diplotene ~ 20,000 loops/gv each m long contain ~ 5% of total DNA at any one time sites of intense RNA synthesis RNA persists in mature oocyte and used later

16 Questions for Our Hypothesis How much total RNA in oocyte is mrna? How much information can it specify? Can it all be synthesized by a single GV nucleus in the time available for oogenesis?

17 Answers I Only ~1% total RNA is mrna (>95% is rrna) So it might be an even bigger problem to synthesize rrna

18 Answers II Total mrna is 0.9% of total (single copy) DNA information can encode ~10,000 different proteins two classes of mrna I (25%): very abundant, ~300 different mrnas each present in 20,000,000 copies/ egg II (75%): abundant, ~10,000 different mrnas each present in 2,000,000 copies

19 Answers III How long to make? Maximal transcription rate for average gene is ~ 30 mrnas/min/gene would take 10 days to make class II would take 100 days to make class I so there is time enough

20 But How Do Panoistics Solve Massive rrna Problem? Massive storage in large egg cells for protein synthesis following fertilization In meroistics made by polyploid nurse cells Strategy in panoistics is to amplify the genes for rrna = rdna

21 How is rdna Amplified? rdna is already repeated a few hundred copies/haploid Ribosomal genes in tandem

22 How is rdna Amplified? Differential replication or amplification of these genes multiple nucleoli floating free of chromosomes ~ 4000X increase in # rdna genes/genome ~ 1200 extrachromosomal nucleoli DNA circles with only rdna rrna factories rdna ~ 75% of total DNA in GV at some stages

23 Advanced Reading Not required Synthesizing Ribosomes h=ribosomes