The ovum in the stroma of a mammalian ovary is enclosed in a perforated membrane called the zona pellucida.

The ovary is an important reproductive organ in mammalian females. It produces and releases oocytes (eggs) through a process called oogenesis. The ovary consists of numerous ovarian follicles, each containing an immature ovum (egg) surrounded by specialized cells known as granulosa cells. The ovum, along with the granulosa cells, is enclosed in a protective layer called the zona pellucida.

Structure and Composition of the Zona Pellucida:

The zona pellucida is a thick, transparent, and non-cellular membrane that surrounds the ovum. It is composed of three distinct layers:

1. Zona Radiata: The outermost layer of the zona pellucida is known as the zona radiata or corona radiata. It is formed by the granulosa cells that surround the ovum. These cells provide nourishment and support to the developing ovum.

2. Zona Lucida: The middle layer of the zona pellucida is called the zona lucida. It is a gel-like layer that contains glycoproteins, including zona proteins. These proteins are important for the binding and recognition of sperm during fertilization.

3. Zona Pellucida Proper: The innermost layer of the zona pellucida is known as the zona pellucida proper. It is a dense layer composed of glycoproteins, including ZP1, ZP2, and ZP3. These glycoproteins are essential for various functions, including sperm binding, prevention of polyspermy (fertilization by multiple sperm), and protection of the developing embryo.

Function of the Zona Pellucida:

The zona pellucida serves several important functions during reproduction:

1. Barrier to Polyspermy: The zona pellucida acts as a barrier to prevent fertilization by multiple sperm. It undergoes a process called cortical reaction upon sperm penetration, which alters its structure and makes it impermeable to additional sperm.

2. Sperm Binding and Recognition: The glycoproteins present in the zona pellucida facilitate the binding and recognition of sperm. Specific receptors on the sperm surface interact with the zona proteins, allowing the sperm to penetrate and fertilize the ovum.

3. Protection of the Embryo: The zona pellucida provides protection to the developing embryo during its journey through the fallopian tube towards the uterus. It acts as a physical barrier and prevents damage to the embryo.

In conclusion, the ovum in the stroma of a mammalian ovary is enclosed in a perforated membrane known as the zona pellucida. This membrane consists of three layers, including the zona radiata, zona lucida, and zona pellucida proper. The zona pellucida plays a crucial role in fertilization, preventing polyspermy, and protecting the developing embryo.

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