DEVELOPMENT AND MALDEVELOPMENTGAMETOGENESIS pptx

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GAMETOGENESIS

The production of ova and sperm occurs via the process of meio￾sis (whereas somatic cells undergo division via mitosis). Oogene￾sis produces ova, and spermatogenesis produces sperm. One sper￾matogonium results in four sperm, and one oogonium results in one

ovum and two polar bodies. Meiosis is a reduction division nor￾mally allowing each gamete to contain 23 chromosomes (haploid).

Thus, when fertilization occurs and the two haploid gametes unite,

the resulting zygote contains 46 chromosomes (diploid) under nor￾mal circumstances.

Two meiotic divisions occur, and each contains several stages.

FIRST MEIOTIC DIVISION

A. Prophase I has five stages.

1. Leptotene, wherein the chromatin condenses into individ￾ual elongated threadlike structures.

2. Zygotene, the migration of single threadlike chromosomes

toward the nuclear equatorial plate where homologous chro￾mosomes pair to form bivalents that exchange segments at

several points (synapses).

3. Pachytene, where chromosomes contract and thicken, then

split longitudinally into two chromatids attached at the cen￾tromere.

4. Diplotene is marked by crossing-over of the nonidentical

chromatid constituents of homologous chromosomes at

bridges or chiasms. The male sex chromatids (X and Y chro￾matids), however, do not cross over.

5. Diakinesis, the last stage, which occurs when the bi￾valents contract, chiasms move toward the ends of the

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DEVELOPMENT AND

MALDEVELOPMENT

CHAPTER

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BENSON & PERNOLL’S

56 HANDBOOK OF OBSTETRICS AND GYNECOLOGY

chromosome, homologs pull apart, and the nuclear mem￾brane disappears.

B. During metaphase I, the very short and thick bivalents are

aligned along the equatorial plate of the cell spindle forms.

C. In anaphase I, the centromeres divide so that the homologous

chromatids (rather than the identical sister chromatids) are

drawn to opposite poles of the spindle.

D. Telophase I is marked by spindle breakage, division of cellu￾lar cytoplasm, and formation of a nuclear membrane. The cell

cytoplasm is equally divided in the male but is unequally

distributed in the female. In the latter, most of the cytoplasm

goes to the secondary oocyte so that basically only nuclear

material becomes the first polar body, which subsequently

disintegrates.

SECOND MEIOTIC DIVISION

A. Metaphase II reveals new spindle forms, and the chromosomes

align along the equatorial plate.

B. In anaphase II, the chromatids pull apart to opposite poles of

the spindle, with complete division of the centromere.

C. Telophase II entails the division of the spindle and cell cyto￾plasm (again equally in the male and unequally in the female),

forming one ovum and the second polar body.

Secondary oocyte development arrests at metaphase II until pen￾etration by a sperm. Then meiosis is completed, and the polar body

is discarded.

FETAL MALDEVELOPMENT

TERMINOLOGY

A chromosome is the paired basic structure containing the genes in

a linear arrangement. Humans have 23 pairs of chromosomes (46

total), of which 22 pairs are autosomes and 1 pair (either XX or

XY) determine the individual’s sex. A locus is a gene’s specific site

on a chromosome. A gene is a sequence of chromosomal nucleotides

that forms the production code for specific proteins, that is, unit of

genetic information. Alleles are different genes that occupy the same

position on homologous chromosomes and potentially affect a

similar function. Heterozygous refers to dissimilar members of a

gene pair, and homozygous refers to similar members of a gene