Testicular Development Chapter 14 Testicular Development © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 14.1 Major developmental landmarks and relative circulating hormone levels in the mouse during fetal, neonatal, and pubertal growth. This schematic shows the development of the mouse testis from initial differentiation around 11 dpc until puberty. Major changes in the testis (or relevant developmental events in the whole animal) are indicated by arrows. Relative levels of AMH, FSH, LH, and testosterone (T) are shown. Source: Modified from Ref. 3. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 14. 2 Modeling of testicular cord development in the mouse FIGURE 14.2 Modeling of testicular cord development in the mouse. This figure is reproduced in color in the color plate section. This figure is taken from Ref. 9 and shows lateral (B, F, J, N), coelomic (C, G, K, O), and mesonephric (D, H, L, P) views of models generated from testis cord outlines prepared from whole mount testis. The left panel in each row (A, E, I, M) is a representative image from the data set used to compile the models. The results show that cords initially form as a network of irregular cell clusters that are subsequently remodeled to form regular parallel loops, joined by a flattened plexus at the mesonephric side. Branched, fused, and internalized cords are commonly observed. Parts A–D, E–H, I–L and M–P show models of testicular development at 12.5, 13.5, 14.5 and 15.5 dpc, respectively. Scale bar, 100 μm. Source: Modified from Ref. 9. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 14. 3 Histology of the fetal testis FIGURE 14.3 Histology of the fetal testis. This figure is reproduced in color in the color plate section. (A) Fetal mouse: low magnification of a semithin section showing seminiferous tubules and interstitial tissue (I). (B) Fetal mouse: higher magnification of (A) showing gonocytes (G) in the central part of the sex cord with the Sertoli cells (S) around the periphery. The peritubular myoid cells (PMC) form a concentric layer around the cord, and Leydig cells (L) are present within the interstitium. (C) Fetal human: immunohistochemically labeled for the androgen receptor (AR), which is clearly expressed in PMC and in some interstitial cells. (D) Fetal sheep: immunohistochemically labeled for anti-Mullerian hormone (AMH), which is strongly expressed in fetal Sertoli cells. (E) Fetal human: immunohistochemically labeled for 3β-hydroxysteroid dehydrogenase (HSD3B), which is localized exclusively in the Leydig cells. In all photomicrographs, the bar represents 50 μm. Source: Reproduced from Ref. 11. © Society for Reproduction and Fertility (2011). Reproduced by permission. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 14.4 Schematic representation of the regulation of testicular function during fetal and perinatal development in the mammal. This is a composite diagram of data from several species. It is not meant to represent any single species but is a generalized view of how testicular control develops. Changes in Sertoli cell and fetal Leydig cell numbers and testosterone levels are shown, and dependence on pituitary gonadotropins is indicated along the top. After birth there is a rise in testosterone levels in some species (e.g., primates) caused probably by altered steroid feedback at the hypothalamic-pituitary level. The fate of the fetal Leydig cells after birth is largely unknown, and this is indicated by a dotted line. The arrow indicating birth in rodents suggests that these species follow the same general developmental plan but are born “early” (see text). Similarly, primates follow the same general plan as other species but have additional control from chorionic gonadotropin. Source: Adapted from Ref. 11. © Society for Reproduction and Fertility (2011). Reproduced by permission. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 14.5 Changes in Sertoli cell number and transcriptional activity during fetal and pubertal development in the mouse. Data show that Sertoli cell number increases exponentially during fetal development, reaching a maximum before puberty. Sertoli cell activity, in terms of mRNA transcript levels, is shown as thin lines with each line representing a specific Sertoli cell transcript. The thick black line is the mean level of all transcripts combined. Results show that transcript levels start to rise markedly as Sertoli cell proliferation comes to an end. Days postconception (dpc). Source: Data is taken from Refs 25,26,113,181. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 14.6 Potential steroidogenic pathways leading to formation of dihydrotestosterone (DHT) in the testis. The canonical pathway through Δ4 and Δ5 steroids is shown on the left with the alternative (backdoor) pathway on the right. Enzymes shaded in gray are required for the canonical pathway (with a number also required for the alternative pathway). Enzymes boxed in black are specific to the alternative pathway. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition