Fertilization, birth, menstural cycle

Fertilization (Conception)

The process where genetic material from a haploid sperm and a haploid oocyte combine to form a diploid nucleus, usually occurring in the fallopian tube within 12–24 hours after ovulation.

How do sperm reach the fallopian tube after ejaculation?

Sperm swim from the vagina through the cervical canal, propelled by their flagella, aided by uterine contractions stimulated by prostaglandins in semen.

Capacitation

A series of changes sperm undergo in the fallopian tube that alter their membrane and prepare them to fuse with the oocyte membrane.

What is the acrosome reaction and why is it important?

When sperm binds to the zona pellucida, enzymes from the acrosome are released to digest a path through this protective layer, allowing sperm to reach and fuse with the oocyte membrane.

Zona Pellucida

A glycoprotein layer surrounding the oocyte that sperm must penetrate to fertilize the egg.

Male and Female Pronuclei

After sperm enters the oocyte, the sperm nucleus (male pronucleus) fuses with the oocyte nucleus (female pronucleus) to form a single diploid nucleus with 46 chromosomes.

What is a zygote?

A fertilized egg with a diploid nucleus formed by the fusion of male and female pronuclei.

Cleavage

Rapid cell divisions of the zygote without growth in size, starting about 24 hours after fertilization.

What is a morula?

An embryo at the 16-cell stage during cleavage, still traveling through the fallopian tube toward the uterus.

Blastocyst

A hollow sphere of cells formed when the morula’s cells reorganize around a fluid-filled cavity called the blastocoel inside the uterus.

What is implantation?

The process by which the blastocyst attaches to and penetrates the uterine wall.

Embryo and Fetus

The blastocyst is called an embryo after implantation during organogenesis; once organ development is complete, it is called a fetus during gestation when organs mature.

What are the two parts of the placenta and where do they originate?

• Maternal portion: develops from the endometrium.

• Fetal portion: arises from the chorion, which is derived from trophoblast cells of the blastocyst.

Chorion

An extraembryonic membrane derived from trophoblast cells; it forms the fetal part of the placenta and secretes human chorionic gonadotropin (hCG).

What is the function of hCG in early pregnancy?

hCG maintains the corpus luteum, preventing it from breaking down, so it can keep producing progesterone and estrogen until the placenta takes over hormone production around the 3rd month.

What are the major functions of the placenta?

• Nutrient and waste exchange between mother and fetus.

• Acts as a barrier to blood cells, maternal proteins, and some hormones.

• Produces hormones like estrogen and progesterone needed for pregnancy.

What happens to hormone production after the placenta forms?

After ~3 months, the placenta takes over production of estrogen and progesterone, allowing the corpus luteum to regress.

What is the menstrual cycle and its primary purpose?

It is a recurring 28-day cycle in female primates involving oocyte development and changes in the uterine lining, with the primary purpose of preparing the endometrium for possible embryo implantation.

Two Interconnected Cycles of the Menstrual Cycle

1. Ovarian cycle – changes in the ovarian follicles

2. Uterine (menstrual) cycle – structural changes in the endometrium

What regulates the menstrual cycle?

Cyclical secretion of estrogen and progesterone from the ovaries.

Phases of the Ovarian Cycle

• Follicular phase – follicle matures

• Ovulation – oocyte is released

• Luteal phase – corpus luteum is active and secretes hormones

What happens if fertilization does not occur after ovulation?

The luteal phase ends, the corpus luteum degenerates, and menstruation begins, marking the start of a new follicular phase.

What marks the transition from one menstrual cycle to the next?

Menstruation, which signals the end of the luteal phase and the beginning of the follicular phase.

Duration and Regulation of the Menstrual Cycle

In women, the cycle lasts ~28 days and is regulated by cyclical secretion of estrogen and progesterone from the ovaries.

What hormonal changes occur during the follicular phase?

FSH stimulates follicular cells to produce estrogen, leading to rising estrogen levels while progesterone remains low.

Phases of the Uterine (Menstrual) Cycle

1. Menstrual phase – Shedding of endometrial lining.

2. Proliferative phase – Endometrium rebuilds.

3. Secretory phase – Endometrium prepares for potential implantation.

What triggers ovulation hormonally?

A surge in LH (and FSH) occurs as estrogen peaks and then slightly drops, triggering ovulation.

What is the hormonal profile during the luteal phase?

The corpus luteum secretes both estrogen and progesterone. Toward the end, LH and FSH levels drop, leading to regression of the corpus luteum and a drop in estrogen and progesterone.

What happens if the ovum is not fertilized?

The corpus luteum degenerates into the corpus albicans, hormone levels drop, and menstruation begins.

What happens if fertilization and pregnancy occur?

he embryo and placenta produce hCG, which maintains the corpus luteum. This keeps estrogen and progesterone levels high to support early pregnancy.

Parturition

The process of childbirth, where the fetus is expelled from the uterus through the vagina.

three stages of labour, dilation, expulsion, placental delivery

What hormonal shift near the end of pregnancy helps initiate labour?

Estrogen levels rise significantly, overriding the suppressive effects of progesterone.

What role does CRH play in parturition?

CRH from the placenta leads to conversion of DHEA into estrogens, further raising estrogen levels.

How does oxytocin contribute to labour?

Oxytocin increases uterine contractions and operates through a positive feedback loop—stretching of the cervix triggers more oxytocin, which intensifies contractions.

What is the role of prolactin in lactation?

Prolactin, from the anterior pituitary, stimulates alveolar glands in the breast to produce milk.

What does oxytocin do during lactation?

Oxytocin causes milk ducts to contract, releasing stored milk (milk letdown).

How is milk production maintained postpartum?

Infant suckling stimulates oxytocin release, which supports continued prolactin secretion and sustained milk production.