Retinoic Acid and Germ Cell Development in the Ovary and Testis

Abstract

Retinoic acid (RA), a derivative of vitamin A, is critical for the production of oocytes and sperm in mammals. These gametes derive from primordial germ cells, which colonize the nascent gonad, and later undertake sexual differentiation to produce oocytes or sperm. During fetal development, germ cells in the ovary initiate meiosis in response to RA, whereas those in the testis do not yet initiate meiosis, as they are insulated from RA, and undergo cell cycle arrest. After birth, male germ cells resume proliferation and undergo a transition to spermatogonia, which are destined to develop into haploid spermatozoa via spermatogenesis. Recent findings indicate that RA levels change periodically in adult testes to direct not only meiotic initiation, but also other key developmental transitions to ensure that spermatogenesis is precisely organized for the prodigious output of sperm. This review focuses on how female and male germ cells develop in the ovary and testis, respectively, and the role of RA in this process.

Keywords: germ cells; meiosis; ovary; retinoic acid; spermatogenesis; testis.

Figure 1

Anterior-to-posterior wave of Dazl and Stra8 expression from E10.5 to E14.5 in mouse fetal gonads. Germ cells are shown in circles, with cells expressing Dazl shown in orange, and cells expressing Stra8 and Dazl shown in blue. After gonadal colonization, germ cells continue to proliferate until E13.5 [22]. In the fetal mouse testis, germ cells become enclosed by somatic cells, with testis cords formed between E12.5 to E14.0 [14,23].

Figure 2

Diagram of germ cell development in mouse fetal gonads of both sexes. Red box: female gonad (ovary). Blue box: male gonad (testis). DAZL, STRA8, REC8, and NANOS2 are expressed in germ cells. ALDH1A1 and CYP26B1 are expressed in fetal gonads. ALDH1A2 and ALDH1A3 are expressed outside the gonads.

Figure 3

Structure of the mouse testis comprising seminiferous tubules. In any given tubule cross-section, one observes germ cells at different steps of their development into elongated spermatids. These germ cell types are concentrically layered; undifferentiated spermatogonia lie on the basal lamina of the tubule, and germ cells move toward the tubule lumen as they differentiate [106]. Germ cell differentiation is precisely timed; hence, particular steps of development are always found together in close physical proximity. Blue line indicates the orientation of testis cross-sections. A representative tubule cross-section in stage VII–VIII, stained with hematoxylin and periodic acid-Schiff (He-PAS), is shown with grayscale version. Star: Sertoli cell nucleus. White arrowhead: type spermatogonium. Dots: preleptotene (red) spermatocytes, pachytene spermatocytes (yellow), and step 7–8 round spermatids (green). Brown area: elongated spermatids. Scale bar = 30 μm.

Figure 4

Multiplication of undifferentiated spermatogonia and spermatogonial differentiation. Upon division, the A_single (A_s) spermatogonia can self-renew and produce two new singles or the daughter cells, A_paired (A_pr) spermatogonia, remain connected by an intercellular bridge. The A_pr spermatogonia subsequently divide further into chains of 4, 8, or 16 cells, called A_aligned (A_al) spermatogonia that undergo spermatogonial differentiation (purple) in response to RA. A_s, A_pr, and A_al spermatogonia are referred to as undifferentiated spermatogonia. After the spermatogonial differentiation, A_al spermatogonia transit into A₁ differentiating spermatogonia without a mitotic division [114]. Expression patterns of PLZF, RARγ, STRA8, SALL4, and KIT are indicated as solid lines.

Figure 5

Diagram of mouse spermatogenesis. Oakberg [106] identified 12 distinct cellular associations, called seminiferous stages I–XII. It takes 8.6 days for a section of seminiferous tubule, and the germ cells contained within, to cycle through all 12 stages [122]. Four turns of this seminiferous cycle are required for a germ cell to progress from undifferentiated spermatogonium to spermatozoon. A_s, A_pr, and A_al: A_single, A_paired, and A_aligned spermatogonia. A₁–A₄: A₁–A₄ differentiating spermatogonia. In, and B: intermediate and type B spermatogonia. Pl, L, Z, P, D, and SC2: preleptotene, leptotene, zygotene, pachytene, diplotene, and secondary spermatocytes. Steps 1–16: steps in spermatid differentiation. Purple: germ cells undergoing spermatogonial differentiation; green: meiotic initiation; brown: initiation of spermatid elongation; gray: release of elongated spermatids. Black box: population of undifferentiated spermatogonia. Gray box: the leptotene spermatocytes undergoing migration of basal to luminal compartment [123]. Dark blue: stage with high RA concentration. Light blue line: STRA8 expression in the unperturbed testis. Dashed light blue line: STRA8 expression induced by RA injection in undifferentiated spermatogonia. (After RA injection, undifferentiated A_al spermatogonia in stages II–VI precociously express STRA8 [124]).