Fertilization, cleavage Homeobox genes

1

GnRH (LHRH)

LH FSH E P every month 10-20 follicles enter the cycle - only 1 ovulates! -

FSH - 200-300 million spermatozoa 300-500 reach the site of fertilization - 1-3 mm/minute (cca 1-3h)

*

6-7 months!! 2

Ovary

Graafian-follicle Thick mucus

uterine tube

Stigma

Cervix glands

Fimbriae

Spermatozoa

Oocyte

Cumulus

thin, clear, abundant mucus, alcaline pH

Spermatozoa

Spermium

During ovulation

Cervix glands 3

Fertilization Spatium perivitellinum

- capacitation - activation in female genital tract, glycoprotein and protein coat removed (7h) - passes through corona radiata cells - acrosome reaction - species specific receptors for spermatozoa trypsin-like enzymes, acrosin - crosses the zona pellucida - fusion of the sperm cell membrane and membrana vitellina, the head and fibrous structures of tail enter the oocyte - cortical and zona reaction prevents polyspermia - Ca++ wave 2. meiotic division resumes - perivitellin space - spermatozoon brings centriole and essential enzymes metabolic activation, transcription of preformed RNA → Embryogenesis starts 4

-the 2nd meiotic division finishes: definitive oocyte and 2nd polar body - male and female pronucleus, DNA replication (but the pronuclei do not unite) - mitotic spindle, (2x23 chromosomes, each with 2 chromatids - 1st division – 2-cell stage

The main results of fertilization 1. diploid chr. No. restored 2. determination of sex 3. initiation of cleavage

5

spermatozoa after the acrosome reaction

6

1st week: • morula • blastocyst • implantation begins •2nd week :

• bilaminar germ disk • implantation finished

•3rd week: • gastrulation • trilaminar germ disk

7

Cleavage

blastomeres 2 cells: 30 h

4 cells: 40 h

compaction: 8-cell stage (E-cadherin)

polarization: apical cytocortex inner basolateral surface

inner cell mass: embryo outer cell mass: trophoblast (placenta!) 12-16 cells: 3rd day 8

fluid penetrates

late morula: 4th day

blastocyst: 5th-6th day polar trophoblast embryoblast blastocyst cavity mural trophoblast zona pellucida 9

10

Homeobox Genes: 0.1% of the genom, short conservative regulator genes /180 bp/ participate in a great diversity of developmental processes (60-80% similarity between species). Code proteins: homeodomain protein segments (60 amino-acids, even greater the similarity), helix-turn-helix conformation: connects to DNA like a clothes pin. This “molecular clothes pin” switches on or off the gene.

HelixturnHelix

Two groups: - Those homeobox genes that are arranged in linear groups (sequences), we call the HOX-genes (1st class homeobox genes) → determine body axes. - Other members of the homeobox gene family exert organ-, tissue- and cellspecific effects on developmental processes through their regulator genes, their action depending on time and location. (PAX, EMX, OTX, MSX, stb.→ paired, empty spiracles, orthodenticle és muscle segment homeobox gének).

11

HOX gének - 1984 invertebrates – max. 13 HOX genes vertebrates – 3-8 copies (clusters) of max. 13/chromosome HOX genes mammals: 39 HOX genes, in 4 clusters (a,b,c,d,parallel series)

Bithorax

Antennapedia

- defines the body pattern along the A-P axis according to the same pattern/mechanism, but creates diversity

b-13

fruit fly

12

HOX GENES: Mutation in Michigan Family

Defects: thumbs and toes Women: uterine abnormalites & sterility DNA examination: Mutation present in HOXa13 Dickman, 1997. Science 245: 1568

13

Drosophila

Mouse

mouse embryo

Human

14

Hoxd Gene Expression in the Forelimb

Mouse experiment: HOXa3 – voice box, thyroid, parathyroid glangs, thymus HOX d3 – atlantooccipital joint Analogous HOX genes can replace the proper function on a different chromosome

15