Citation: Cardoso, D.d.L.; Guimarães, D.A.d.A.; Mayor, P.; Ferreira, M.A.P.; Coutinho, L.N.; Monteiro, F.O.B. Follicular Atresia, Cell Proliferation, and Anti-Mullerian Hormone in Two Neotropical Primates (Aotus nancymae and Sapajus macrocephalus). Animals 2023, 13, 1051. https://doi.org/10.3390/ ani13061051 Academic Editor: Giuseppina Basini Received: 7 December 2022 Revised: 29 December 2022 Accepted: 7 February 2023 Published: 14 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). animals Article Follicular Atresia, Cell Proliferation, and Anti-Mullerian Hormone in Two Neotropical Primates (Aotus nancymae and Sapajus macrocephalus) Deise de Lima Cardoso 1,2 , Diva Anélie de Araújo Guimarães 1 , Pedro Mayor 2,3 , Maria Auxiliadora Pantoja Ferreira 1 , Leandro Nassar Coutinho 2 and Frederico Ozanan Barros Monteiro 2, * 1 Programa de pós-graduação em Ciência Animal, Universidade Federal do Pará, Rua Augusto Corrêa 01, Belém 66075-110, Brazil 2 Programa de Pós-Graduação em Saúde e Produção Animal na Amazônia, Universidade Federal Rural da Amazônia, Av. Perimetral 2501, Belém 66077-830, Brazil 3 Departament de Sanitat i Anatomia Animals, Facultat de Veterinària, Campus de la UAB, Universidade Autónoma de Barcelona, Plaça Cívica, 08193 Barcelona, Spain * Correspondence: frederico.monteiro@ufra.edu.br; Tel.: +55-(91)-99181-9493 Simple Summary: Reproductive processes in Neotropical primates are still poorly understood. Complications arising in these processes may result in a compromised reproductive condition for a significant portion of these species. We evaluated the follicular atresia, cell proliferation, and anti- Mullerian hormone presence in two Neotropical primates, and in three sexual phases (follicular, luteal, and gestational). Our results contribute to a greater understanding of the ovary morphofunctional processes that act in follicular selection, recruitment, and degeneration in these species. Abstract: This study evaluated the follicular atresia, cell proliferation, and anti-Mullerian hormone action in Aotus nancymae and Sapajus macrocephalus during three sexual phases (follicular, luteal, and gestational). Follicular quantification and immunolocalization of Caspase-3 protein, B-cell lymphoma 2 (BCL-2), proliferating cell nuclear antigen (PCNA), and anti-Mullerian hormone (AMH) were performed. A significant difference in the quantification between preantral and antral follicles, with a progressive decrease in the antrals, was identified. Protein and hormonal markers varied significantly between follicle cell types (A. nancymae p = 0.001; S. macrocephalus, p = 0.002). Immunostaining in the preantral and antral follicles was present in all sexual phases; for Caspase-3, in granulosa cells, oocytes, and stroma; for BCL-2, in granulosa cells, oocytes, and theca; and for PCNA and AMH, in oocytes and granulosa cells. The immunostaining for Caspase-3 was more expressive in the preantral follicles (follicular phase, p < 0.05), while that for BCL-2 and PCNA was more expressive in the antral follicles of the follicular phase. The AMH was more expressive in the primary and antral follicles of nonpregnant females, in both the follicular and luteal phases. Our results contribute to understanding the ovarian follicular selection, recruitment, and degeneration of these species. Keywords: owl monkey; night monkey; capuchin monkey; ovary; folliculogenesis; apoptosis 1. Introduction In recent years, there has been an increase in reproductive studies focusing on owl or night monkeys (Aotus spp.) [1–6] and capuchin monkeys (Sapajus spp. and Cebus spp.) [4,7,8]. There are behavioral differences between the primates, such as their mating systems. Owl monkeys are monogamous [9], while capuchin monkeys are polygamous [10]. Their shared reproductive characteristics include the presence of a menstrual cycle [4] and the ability to easily reproduce in captivity [11]. In both species, it has been observed that during follicular maturation, granulosa cells, theca cells, and oocytes develop a process of cellular death, Animals 2023, 13, 1051. https://doi.org/10.3390/ani13061051 https://www.mdpi.com/journal/animals