Lecture 22: Development and birth

The Menstrual Cycle

• The menstrual cycle consists of two stages: proliferative and secretory.
• Menses (4-5 days) marks the removal of the superficial layer of the endometrium, and Day 1 of the cycle is the first full day of bleeding.
• The proliferative stage is characterized by a sustained increase in oestrogen, which stimulates GnRH secretion by the hypothalamus.
• GnRH triggers LH and FSH secretion from the anterior pituitary, leading to increased oestrogen production by developing follicles.
• A large increase in LH triggers ovulation (around day 14), which marks the start of the secretory stage.

The Menstrual Cycle (Secretory Stage)

• During the secretory stage, the corpus luteum secretes progesterone and small amounts of oestrogen.
• Progesterone thickens the endometrium and causes fluid secretion (nutrients for the embryo).
• Progesterone and oestrogen inhibit the hypothalamus and anterior pituitary, leading to a decline in LH and FSH secretion after ovulation.
• If fertilization occurs, the endometrium becomes fully developed, and human chorionic gonadotropin (HCG) production by the trophoblasts during implantation of the embryo maintains the corpus luteum.
• If fertilization does not occur, the endometrium is sloughed, and the corpus luteum degenerates, reducing progesterone and increasing FSH.

Fertilization and Preimplantation Embryo Development

• Fertilization occurs when sperm capacitation in the uterus (enzymes) stabilizes the acrosomal head and strengthens tail motility.
• The secondary oocyte is released into the uterine duct/fallopian tube.
• The blastocyst is formed, consisting of an inner cell mass and trophoblasts.

Fertilization to Birth

• Fertilization (Day 0) occurs when the pronuclei of sperm and egg fuse.
• Two mechanisms prevent polyspermy: a fast block (Na+) and a slow block (Ca2+) release.
• The zygote is formed when the pronuclei fuse, and the zona pellucida is shed.

Cleavage

• Cleavage occurs from Day 1 to 6, with repeated mitotic divisions as the embryo migrates down the oviduct to the uterus.
• The zona pellucida degenerates, and the underlying trophoblast takes over.

Implantation

• Implantation occurs from Day 7 to 10, with the production of human chorionic gonadotropin (HCG) by the trophoblasts.
• Two types of trophoblasts contribute to the development of the placenta.
• The developing placenta and umbilical cord form a filtration barrier.

Gastrulation

• Gastrulation occurs on Day 12, with the formation of the embryonic disc, primitive streak, and central axis.
• The three germ cell layers are formed: ectoderm, mesoderm, and endoderm.
• Ectoderm develops into skin and the nervous system, mesoderm develops into organs, muscle, and bone, and endoderm develops into the digestive system.

Neurulation

• Neurulation occurs on Day 18, with the folding of the neural tube, which determines the position of grey and white matter in the central nervous system.
• The spinal cord and brain develop with grey matter outside and white matter inside.

Organogenesis

• Organogenesis begins around 3 to 8 weeks and continues until birth, with the development of different organs.
• The brain begins developing in week 3, the heart begins developing in week 3, and the kidneys begin developing in week 9.
• From fertilization to week 7, a developing human is referred to as an embryo, and from week 8 to birth, it is called a fetus.