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Fertilization

*Fertilisatio*

For medical students2 min readUpdated 2026-10-10

From a biological standpoint, fertilization is the primary goal of sexual intercourse, consisting of the encounter and successful interaction between a spermatozoon and an oocyte. The highest probability of this event occurs directly during the ovulation phase, as well as during the two to three days preceding the release of the oocyte.

Transit TimeThe female gamete remains in the lumen of the fallopian tube for approximately four days.

Corpus LuteumThe corpus luteum of pregnancy functions actively and secretes hormones until the fourth month.

Immune DefenseProgesterone and estrogens reliably suppress immune reactions between fetal tissues and the placenta.

LocalizationThe process of germ cell fusion occurs in the vast majority of cases within the uterine tube.

Mechanisms of Germ Cell Transport

For fertilization to take place, male and female germ cells must travel a significant distance. Sperm transport through the female reproductive tract is substantially facilitated during the periovulatory period. At this time, specific mucus with a more alkaline reaction is secreted, creating favorable physiological conditions for the advancement of male gametes.

An essential role in seminal fluid transport is played by the neuropeptide oxytocin. This hormone is synthesized in specific structures of the brain, namely the supraoptic and paraventricular nuclei of the hypothalamus. Its action is manifested in both sexes:

In males, oxytocin regulates the release of seminal fluid during ejaculation.
In females, the release of oxytocin triggers active contractions of the smooth musculature of the uterus, mechanically propelling the spermatozoa upward toward the oocyte.

The encounter of the gametes itself traditionally occurs in the ampullary region of the fallopian tube (uterine tube). The oocyte is capable of remaining in the lumen of the uterine tube for about four days. The movement of the oocyte and spermatozoa toward each other is ensured by the peristalsis of the uterine tubes, the intensity of which is strictly regulated by hormones, primarily estrogens.

Hormonal Support of Implantation

Following successful fertilization, the stage of preparation for the implantation of the blastocyst into the uterine wall begins. At this stage, progesterone, produced by the corpus luteum of the ovary, acts as the absolute dominant. Together with estrogens, progesterone transforms the mucous membrane of the uterus (endometrium), creating optimal conditions for embryo implantation.

Upon the onset of pregnancy, structural changes occur in the ovary:

The regular cyclic corpus luteum does not regress.
It transforms into the corpus luteum of pregnancy.
This structure assumes the function of hormonal support for gestation until the fourth month of pregnancy.

Role of the Placenta and Sex Differentiation

As pregnancy progresses, the placenta forms and gradually takes over endocrine functions. The fully formed placenta becomes a powerful, fully independent source of vital hormones: progesterone, estrogens, human chorionic gonadotropin (hCG), and androgens.

Placental hormones perform a number of critically important functions for fetal preservation:

Immune tolerance: High concentrations of estrogens and progesterone locally suppress the reactions of the maternal immune system, preventing rejection of the fetus (since the fetus carries foreign genetic material).
Reproductive system formation: Progesterone actively promotes the proper sex differentiation of the unborn child. Meanwhile, androgens and estrogens exert mutually balancing influences on the development and maturation of the fetal reproductive system.

Frequently asked questions

What specific morphological changes does progesterone induce in the endometrium during preparation for implantation?

Progesterone is directly linked to the secretory transformation of the endometrium in preparation for implantation:

Vacuolization — glycogen-containing vacuoles appear in endometrial cells.
Secretory activity of glands — secretion containing glycogen, glycoproteins, and glycosaminoglycans appears in the glands.
Differentiation of the functional layer — the functional layer is divided into a superficial, more compact stratum, and a spongy stratum.
Decidual reaction during implantation — tissue edema, vascular proliferation, and an increase in decidual cells develop in the stroma between and below the lacunae; they become large, rounded, and accumulate nutrients: glycogen and lipoproteins.

How do the physicochemical properties of cervical mucus change during the periovulatory period to facilitate sperm transport?

During the periovulatory period, under the influence of estrogens, the volume and properties of cervical mucus change to facilitate sperm advancement:

Volume and consistency — secretion increases, clear glassy mucus accumulates, and the external os opens (the "pupil" sign).
Medium reaction — the mucus acquires a slightly alkaline reaction, which increases sperm motility (some sources also mention a more acidic reaction).
Spinnbarkeit (stretchability) — the mucus becomes maximally elastic, stretching up to 12 mm.
Crystallization — the mucus acquires a pronounced ability to form crystals upon drying.

What sequential stages (reactions) does the interaction between a spermatozoon and an oocyte include at the cellular level?

The process of gamete interaction includes the following sequential reactions and steps:

Capacitation — occurs as gametes approach each other; the active element is the spermatozoon.
Acrosomal reaction — corresponds to contact interaction; dissolution of the zona pellucida, penetration of the acrosomal filament, and fusion with the oocyte membrane occur.
Sperm penetration into the oocyte — mutual recognition of receptors on gamete membranes occurs; a part of the sperm plasmalemma is integrated into the oocyte membrane, facilitated by the destruction of lipids by acid phosphatase. Only the sperm nucleus and centrioles penetrate the oocyte cytoplasm.
Cortical reaction — in response to penetration, exocytosis of oocyte cortical granules occurs; the zona pellucida and plasmalemma lose specific receptors, a perivitelline space forms, the membrane compacts, and a barrier forms preventing polyspermy.

Where exactly is oxytocin, which participates in gamete transport, synthesized?

Oxytocin is produced by neurosecretory cells of the supraoptic and paraventricular nuclei of the hypothalamus, after which it regulates seminal fluid release and uterine contractions.

How is immune rejection of the fetus prevented?

Immune reactions between the maternal organism and fetal tissues (placenta) are effectively suppressed due to the combined action of placental estrogens and progesterone.

Which structures produce progesterone at different stages of pregnancy?

In the early stages (up to the fourth month), the corpus luteum of pregnancy serves as the main source of progesterone. Later, the fully formed placenta completely takes over this endocrine function.

What stimulates the peristalsis of the fallopian tubes?

The motor activity and peristalsis of the uterine tubes, necessary for the propulsion of germ cells, are under the regulatory influence of estrogens.

Go deeper

Detailed study of the properties of cervical mucus during the periovulatory period and mechanisms altering its acidity.
Neuroendocrine regulation of uterine contractility: the connection between the hypothalamus and neurohypophysis.
Cellular mechanisms of the transformation of the cyclic corpus luteum into the corpus luteum of pregnancy.
Synthesis of human chorionic gonadotropin (hCG) and androgens in placental tissues.
The influence of hormonal balance (androgens and estrogens) on the intrauterine development of the reproductive system.