Mammary-Specific Inactivation of the pRb Pathway Induces Extensive Abnormalities

<p>Histologic comparisons of nontransgenic (A–D), mosaic (F₀ line 2 [E–H]), and transgenic (F₁, line 3 [I–L]) lactating mammary glands reveals that T₁₂₁ expression results in increased proliferation and apoptosis. Hemotoxylin and eosin staining shows acini of the normal lactating gland are composed of a single layer of secretory epithelial cells (A) with milk-filled lumen. Consistent with atrophy, transgenic animals have a lower density of acini demonstrated by the presence of lipid-filled adipocytes (asterisk in [K]). Acini composed of T₁₂₁-expressing cells are atypical. Many are collapsed and composed of tall columnar epithelia of large hyperchromatic cells with papillary tufting (arrows in [I]). Transgene-expressing cells have large pleomorphic nuclei (open arrows in [G]) as compared to nuclei of nonexpressing cells (arrows in [G]). Staining for T₁₂₁ expression (blue in [B]–[J]) indicates the line 2 F₀ animal is mosaic, showing localized expression (F), whereas the transgene expresses throughout the gland of an F₁ line 3 animal (J). Increased proliferation assayed by PCNA staining (red) is also localized in the mosaic founder (G), but found throughout the F₁ transgenic gland (K). Similarly, TUNEL staining (brown) demonstrates increased apoptosis in transgenic animals (H and L); moreover, the regionalized apoptosis in the mosaic gland (H) strongly suggests that transgene expression and not precocious involution is the cause. All samples are from primiparous females on lactation day 1.