How a Single Fertilised Egg Implants and Grows Into a Baby

Every person begins as a single cell formed when a sperm fuses with an egg. That cell divides, implants in the wall of the uterus and grows for about nine months, fed through the placenta, until birth.

This lesson covers the male and female reproductive systems, gamete formation, the menstrual cycle, and development from fertilisation to birth and lactation.

The male system has a pair of testes with ducts, glands and external genitalia, and the female system has a pair of ovaries with oviducts, a uterus, cervix and vagina; the testis is packed with seminiferous tubules, and the ovary holds follicles at different stages.

Male reproductive system:

- Testes — held in the scrotum, slightly cooler than the rest of the body
- Accessory ducts — rete testis, vasa efferentia, epididymis and vas deferens
- Accessory glands — seminal vesicles, prostate and bulbourethral glands, whose secretions form seminal plasma

Inside the testis:

- Many lobules, each with coiled seminiferous tubules
- The tubules are lined by spermatogonia, which form sperm, and Sertoli cells, which nourish them
- Between the tubules, Leydig cells secrete androgens such as testosterone

Female reproductive system:

- Ovaries — produce ova and the hormones oestrogen and progesterone
- Oviducts — infundibulum with fimbriae, ampulla and isthmus
- Uterus — an outer perimetrium, a muscular myometrium and a glandular endometrium

Inside the ovary. Follicles at primary, secondary and tertiary stages mature into a Graafian follicle.

An everyday example. Doctors advise against tight clothing and overheating for sperm health because the testes work best slightly below body temperature.

The substance. Leydig cells and Sertoli cells do different jobs — Leydig cells make testosterone, while Sertoli cells nourish developing sperm.

Spermatogenesis makes four sperm from each primary spermatocyte, oogenesis makes one ovum and polar bodies from each primary oocyte, and both are controlled by GnRH from the hypothalamus acting through LH and FSH from the pituitary.

Spermatogenesis:

- Spermatogonia (2n) multiply by mitosis and grow into primary spermatocytes (2n)
- Meiosis I gives two secondary spermatocytes (n); meiosis II gives four spermatids (n)
- Spermatids change into sperm by spermiogenesis and are released from Sertoli cells by spermiation

Hormonal control in males. LH stimulates Leydig cells to release androgens, which drive sperm formation; FSH acts on Sertoli cells, which help spermiogenesis.

Oogenesis:

- Starts before birth, when oogonia form primary oocytes that begin meiosis and pause in prophase I
- From puberty, a follicle matures into a Graafian follicle, and its primary oocyte completes meiosis I, forming a large secondary oocyte and a tiny first polar body
- The secondary oocyte is released at ovulation, and meiosis II is completed only if a sperm enters, forming the ovum and a second polar body

Hormonal control in females. FSH stimulates follicle growth and oestrogen secretion, and an LH surge triggers ovulation.

An everyday example. Blood tests for LH and FSH at a fertility clinic help doctors check whether the pituitary is signalling the gonads properly.

The substance. Each primary oocyte yields one functional ovum, not four — unequal division pushes almost all the cytoplasm into the ovum, and the polar bodies degenerate.

The menstrual cycle is the roughly 28-day reproductive cycle of female primates, with menstrual, follicular, ovulatory and luteal phases, whereas the oestrous cycle of other mammals has no bleeding and only a short period of heat.

Phases of the menstrual cycle:

- Menstrual phase — lasts about 3 to 5 days; the endometrium breaks down and is shed when the ovum is not fertilised
- Follicular phase — FSH and LH make a follicle grow into a Graafian follicle, and rising oestrogen rebuilds the endometrium
- Ovulatory phase — LH and FSH peak around day 14; the LH surge ruptures the Graafian follicle and releases the ovum
- Luteal phase — the remains of the follicle form the corpus luteum, which secretes progesterone to maintain the endometrium; without fertilisation it degenerates and a new cycle begins

Menstrual versus oestrous cycle:

- Occurs in — primates versus non-primate mammals such as cows and dogs
- Bleeding — present versus absent
- Receptivity — possible throughout the cycle versus only during oestrus, or heat

An everyday example. Dairy farmers watch their cows closely for signs of heat so that artificial insemination can be timed correctly.

The substance. Pregnancy pauses the menstrual cycle — the corpus luteum and then the placenta keep progesterone high, so the endometrium is not shed.

Fertilisation in the oviduct forms a zygote that becomes a blastocyst and implants in the endometrium; the placenta then supports the embryo until birth, after which the mammary glands produce milk.

Fertilisation:

- Usually happens at the ampullary-isthmic junction of the oviduct
- A sperm contacts the zona pellucida, which then changes to block other sperm
- The secondary oocyte completes meiosis II, and the nuclei fuse to form a diploid zygote

Cleavage and blastocyst. The zygote divides while moving towards the uterus, forming a morula of 8 to 16 cells and then a blastocyst, with an outer trophoblast and an inner cell mass that becomes the embryo.

Implantation. The trophoblast attaches to the endometrium, and the blastocyst becomes embedded in the uterine wall.

Placenta:

- Formed by chorionic villi interlocking with uterine tissue, and joined to the embryo by the umbilical cord
- Acts as an endocrine gland, producing hCG, hPL, oestrogen and progesterone

Parturition. Signals from the fully developed foetus and placenta trigger uterine contractions — the foetal ejection reflex — and oxytocin strengthens them until delivery.

Lactation. Mammary glands produce milk after birth, and the first milk, colostrum, carries protective antibodies.

An everyday example. A home pregnancy test detects hCG, the hormone released by the developing placenta after implantation.

The substance. The inner cell mass contains stem cells — cells that can give rise to all the tissues and organs of the body.