Despite announcements and obituaries, news of the death of the gene has been greatly exaggerated, or so says the gene as it struggles to survive and find a safe haven from which to steer its course through development and evolution. In this short piece, I consider recent claims that the gene is dead. I conclude that the gene is alive and well, living and functioning in the cell, which is both its natural home and a fundamental unit of evo-devo.

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PubMedDate not supplied

Cellular interactions during cartilage and bone development.

Both interactions between like cells, as between chondrogenic cells in a developing cartilaginous rudiment, and between unlike cells, as in epithelial-mesenchymal interactions, are dealt with in this review. Such interactions may involve direct apposition of cell membranes or may be mediated via interaction with peri- or extracellular matrices. An ontogenetic approach is taken in which cellular interactions involved in five processes of the development of cartilage and bone are discussed, the five being (1) origin of the cells, (2) migration of the cells within the embryo, (3) localization of the cells at their final embryonic site, (4) differentiation, and (5) morphogenesis. Some emphasis is placed on interactions affecting neural crest-derived cells both before and during their migration and on interactions, especially epithelial-mesenchymal interactions, that precede cytodifferentiation of chondroblasts or osteoblasts. Whether epithelial or mesenchymal specificity is required for such interactions to occur is discussed with reference to the otic vesicle-otic mesenchyme interaction that leads to differentiation and morphogenesis of the cartilaginous otic capsule.

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PubMedDate not supplied

Detailed staging of inbred C57BL/6 mice between Theiler's [1972] stages 18 and 21 (11-13 days of gestation) based on craniofacial development.

A detailed staging table of inbred C57BL/6 embryonic mice was developed to facilitate a study of the stage-by-stage cellular and molecular mechanisms underlying cranial skeletal development and elucidation of the developmental mechanisms potentially involved in evolutionary changes in cranial skeletal morphology exhibited by different inbred strains of mice. Mice were mated for only 2 hr and embryos were recovered every 2 or 4 hr between 11 and 13 days of gestation. Theiler's [1972] stages 18 through 21 were divided into substages and divisions based on the development of five external structures--the frontonasal area, eyes, auditory meatus, mandibular and hyoid auricular hillocks/pinna, and vibrissae--and three internal histological structures--eyes, tongue, and vibrissae. Each substage and division was designated with a decimal point: e.g., substage 20.1 and division 20.11. Embryos were staged using the staging table and the relationship of the substages and divisions with days of gestation was examined. The results showed considerable intra- and inter-litter variation in stages of embryos, suggesting that days of gestation are not a good indicator for staging embryos. Our staging table offers a more reliable and precise method to standardize embryonic development. Regression analyses of substages on days of gestation showed that the duration of stages increased from stages 18 to 21. Estimated durations were 3.5 hr for stages 18 and 19, 8.8 hr for stage 20, and 38.8 hr for stage 21. Our staging table also provides baseline information on development of the frontonasal area (muzzle) and vibrissae and development and the transformation of the auricular hillocks into the pinna. The developmental sequence of mystacial and labial vibrissae indicated highly regulated differentiation and morphogenesis of vibrissal development at stages 20 and 21. Three hyoid auricular hillocks transiently became four hillocks at stage 19.1 before transforming to the pinna during stage 21. The second and third hyoid auricular hillocks were the major contributor to the pinna before stage 21.2, whereas mandibular auricular hillocks contributed to the pinna from stage 21.32 onward. The staging table has already served to demonstrate stage-specific skeletogenesis of the first arch cartilages in inbred C57BL/6 mice and to reveal differences in the onset of timing of skeletogenesis among inbred C57BL/6, C3H/He and CBA/J mice.

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PubMedDate not supplied

Stage-specific onset of condensation and matrix deposition for Meckel's and other first arch cartilages in inbred C57BL/6 mice.

Condensation is a multistep process, involving cell recruitment, cell-to-cell contact by cell adhesions, and concomitant changes in cell shape. Condensation of prechondrogenic cells down-regulates and/or inactivates cell proliferation and enhances the activities of cartilage-specific genes. Timing of onset and duration of condensation are thus important regulatory processes mediating cellular and molecular events during chondrogenesis. The present study was undertaken to examine timing of onset and duration of condensation and onset of matrix formation for first arch cartilages in inbred C57BL/6 mice. Because timing can only be reliably assessed in very precisely staged embryos, mice were mated for only 2 hr, pregnancy was determined by weight increase, and embryos were assigned to substages and divisions of Theiler's [1972] stages on the basis of external development of the frontonasal area, eyes, vibrissae, mandibular and hyoid auricular hillocks (pinna) and some internal structures [Miyake et al., 1996]. Condensation and matrix formation were determined using PNA lectin histochemistry, type II collagen immunohistochemistry, Mallory's trichrome, Hall and Brunt's quadruple and toluidine blue stains on serially sectioned embryos, and 3-D reconstruction. A single, continuous, first arch chondrogenic condensation was identified. It consisted of three components: 1) a rostral component for the symphysis; 2) a core component for the major portion of Meckel's cartilage; and 3) a caudal component for the caudo-lateral area of Meckel's cartilage and the two ear cartilages, the malleus and incus. The caudal component was further divided into rostral and caudal subcomponents which gave rise to the malleus and incus, respectively. Cellular arrangement differed among the three components, probably reflecting major axes of matrix growth. The condensation was only weakly recognized by peanut agglutinin lectin (PNA), unless sections were pretreated with neuraminidase, showing the masking effects of sialic acid. The core and caudal components of the condensation appeared at stage 20.12 and ended at stage 21.14 and 21.32, respectively. The symphysial component began at stage 20.2 and ended at stage 21.32. Deposition of sulfated proteoglycans began at stage 21.14 in the core component of the condensation--preceding slightly deposition of type II collagen--and at stage 21.32 in the symphysial and caudal components.

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PubMedDate not supplied

Variability of embryonic development among three inbred strains of mice.

We examined the relationships between litter size, embryonic growth, days of gestation, onset and duration of morphological stages and development of the first arch skeleton in three inbred strains of mice--C57BL/6, CBA/J and C3H/He. Detailed embryonic staging was based on craniofacial development between 11 and 18 days of gestation. Considerable intra- and interlitter variation of morphological stages of embryonic development exists in all three inbred strains. The relationship of morphological stages to days of gestation reveals that each stage has a different duration, being shortest at Theiler's stage 18 and longest at stage 21 in all three inbred strains. Embryos of CBA/J mice tend to reach each stage later than do embryos of the other two strains, i.e., morphological development is slowest in CBA/J. The greatest length, a measurement of embryonic growth, increases at a constant rate during gestation in all three strains. In C57BL/6 and CBA/J, more embryos tend to be implanted in the right horn of the uterus than in the left, whereas in C3H/He an even number of embryos tends to be implanted in both horns. Timing of the development of Meckel's cartilage differs between the three inbred strains: both condensation and onset of matrix deposition begin one stage earlier in C57BL/6 than in CBA/J and C3H/He. On the other hand, alkaline phosphatase, one of the earliest markers for bone development, is expressed at the same time in all three inbred strains. Differences in timing of skeletal development between the strains may be attributed in part to the genealogical closeness OF CBA/J and C3H/He mice.

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