VI. Embryology
Key focus of this chapter: male and female reproductive systems
This chapter focuses on male and female reproductive systems and summarizes fertilization, embryonic development, the functions of extraembryonic membranes in shelled eggs, and the placenta in mammals.
A. Male reproductive system
1. Anatomy
Male reproductive system | Functions | ||
Testis | Seminiferous Tubules • Production of sperm • Spermatogonia are germ cells | Leydig cells (Interstitial cells) | • LH stimulates synthesis of testosterone, an androgen |
Sertoli cell | • Support developing germ cells; respond to FSH and testosterone | ||
Epididymis | • Place for maturation and storage of sperm | ||
Prostate gland | • Secretes milky fluid containing citrate, zinc, and enzymes that contribute to semen • Prostatic enzymes help liquefy semen after ejaculation | ||
Seminal vesicle | • Secretes fructose-rich seminal fluid that supports sperm function | ||
Vas deferens | • Transports sperm from the epididymis toward the ejaculatory duct | ||
Ejaculatory duct | • Formed by the union of the seminal vesicle duct and vas deferens | ||
Urethra | • Carries semen and urine at different times | ||
2. The passage of sperm
3. Sperm
- Acrosome
– contains enzymes that help sperm penetrate the egg’s outer coverings during fertilization
- Nucleus
- Mitochondria
- Flagellum
– movement of sperm
4. Hormone regulation
B. Female reproductive system
1. Anatomy
Female reproductive system | Functions |
Follicle | • An oocyte surrounded by supporting follicular cells in the ovary |
Ovary | • Site of oogenesis and ovarian follicular development |
Ovulation | • The mature follicle ruptures and releases a secondary oocyte • Ruptured follicle becomes corpus luteum |
Corpus luteum | • Forms from the ruptured follicle; secretes primarily progesterone and also estrogen |
Oviduct (Fallopian tube) | • Transports the oocyte; fertilization usually occurs in the ampulla |
Uterus | • Site of blastocyst implantation in the endometrium |
2. Passage of a secondary oocyte
3. Hormone regulation
- FSH
– acts on Sertoli cells to support spermatogenesis and on ovarian granulosa cells to support follicular development.
– promotes ovarian follicular growth and estrogen secretion.
- LH
– stimulates Leydig cells to produce testosterone and acts on ovarian cells.
– the LH surge triggers ovulation and formation of the corpus luteum.
- Estrogen
– stimulates growth of the uterine lining before implantation of the blastocyst.
– promotes development of female secondary sex characteristics.
- Progesterone
– promotes a secretory uterine lining and maintains it for implantation and pregnancy.
** Estrogen and progesterone generally exert negative feedback on FSH and LH secretion; sustained high estrogen before ovulation produces positive feedback that triggers the LH surge.
4. Comparison of oogenesis with spermatogenesis
a. Products of oogenesis
- Each primary oocyte can yield one functional ovum and polar bodies after completion of meiosis; in humans, meiosis II is completed only after fertilization
- Unequal distribution of the cytoplasm
b. Products of spermatogenesis
- Each primary spermatocyte produces four haploid spermatids, which differentiate into spermatozoa
- Cytoplasm is divided approximately equally during meiotic divisions; excess cytoplasm is shed during spermiogenesis
C. Fertilization
1. External fertilization in sea urchins
a. Process of fertilization
- ① Contact – The sperm head contacts the egg’s jelly coat.
- ② Acrosomal reaction – Hydrolytic enzymes released from the acrosome help the sperm penetrate the egg’s outer coats; species-specific binding helps the sperm reach and interact with the egg membrane.
- ③ Depolarization – In some externally fertilizing species, rapid egg membrane depolarization after sperm–egg fusion produces a fast block to polyspermy before sperm nuclear entry.
- ④ Insertion of sperm nucleus into egg.
- ⑤ Cortical reaction – Release of cortical granule contents modifies and elevates the vitelline layer to form the fertilization envelope, producing a slow block to polyspermy.
b. Function of each part
Parts | Functions |
Acrosome | • Releases hydrolytic enzymes that help sperm penetrate the egg coverings |
Jelly coat | • Envelope of eggs • Contact with sperm triggers the acrosomal reaction |
Vitelline layer | • An extracellular matrix outside the plasma membrane and beneath the jelly coat • Place for sperm binding receptors |
Sperm binding receptor | • Recognizes sperm bindin and mediates species-specific attachment at the vitelline layer |
Cortical granules | • Release contents during the cortical reaction to establish the slow block to polyspermy |
2. Internal fertilization of mammals
a. Process of fertilization
- ① Approaching – capacitated sperm pass through the surrounding cumulus cells and reach the zona pellucida of the secondary oocyte.
- ② Acrosomal reaction – release of acrosomal contents, together with sperm motility, helps the sperm penetrate the zona pellucida and reach the oocyte plasma membrane.
- ③ Fusion – the sperm and oocyte plasma membranes fuse.
- ④ Entry – the sperm nucleus enters the oocyte; the oocyte completes meiosis II and the male and female pronuclei form.
- ⑤ Cortical reaction – release of cortical granule contents modifies the zona pellucida and contributes to a slow block to polyspermy.
b. Function of each part
Parts | Functions |
Acrosome | • Acrosomal contents and sperm motility help sperm penetrate the zona pellucida |
Zona pellucida | • Extracellular matrix of egg • Glycoproteins mediate sperm interactions; modification after fertilization helps prevent polyspermy |
Cortical granules | • Exocytosis modifies the zona pellucida and helps establish the block to polyspermy |
c. Twins
- Identical (monozygotic) twins – one egg fertilized by one sperm forms a zygote; the resulting early embryo separates into two embryos
- Fraternal (dizygotic) twins – two eggs are fertilized by two separate sperm and develop into two embryos
d. Cell differentiation
- Cells become specialized in function and structure through differential gene expression
- Descendants of a single zygote differentiate into specialized tissues and organs in a multicellular organism
D. Embryonic development
1. Description of each stage
Stages | Description | |
Cleavage | Morula | • Cleavage forms a solid ball of cells through mitotic divisions with little overall growth |
Blastula | • Blastula: a cell layer surrounding a fluid-filled cavity in many animals • Mammals form a blastocyst with an inner cell mass, trophoblast, and blastocoel | |
Gastrulation | • In amphibians, gastrulation begins at the dorsal lip of the blastopore • Forms the three primary germ layers (ectoderm, mesoderm, and endoderm) and the primitive gut • Cell movements establish the embryonic body plan • Archenteron – the primitive gut cavity • Blastopore - In many protostomes, the blastopore contributes to the mouth; patterns vary - In typical deuterostome development, the blastopore gives rise to the anus • Involution – surface cells roll inward over the blastopore lip in amphibian gastrulation | |
Organogenesis | • Organ rudiments develop from the three germ layers • Neurulation – the neural plate folds to form the neural tube; neural crest cells arise at its borders • Induction – signals from cells or tissues influence the developmental fate of other cells or tissues • The mesodermal notochord helps induce neural development; somites form from paraxial mesoderm | |
2. Three germ layers
Ectoderm | Mesoderm | Endoderm |
• Epidermis of skin • Sensory receptor of epidermis • Nervous system • Lens of eyes • Tooth enamel • Adrenal medulla | • Circulatory system • Kidneys and ureters • Gonadal somatic tissues and most reproductive ducts • Lymphatic system • Most connective tissue (including most dermis; cranial derivatives also involve neural crest) • Most skeleton (some craniofacial structures derive from neural crest) • Muscular system • Adrenal cortex | • Epithelium of the respiratory tract • Most digestive tract epithelium; liver and pancreatic parenchyma • Epithelium of most of the urinary bladder |
** Totipotent: a single cell can give rise to both embryonic and extraembryonic tissues needed to form a complete organism. Pluripotent cells can give rise to derivatives of all three embryonic germ layers but cannot alone form a complete organism with its extraembryonic tissues.
E. Functions of extraembryonic membranes in shelled eggs
Classification | Functions and features |
Amnion | • Fluid-filled amniotic sac protects the embryo from mechanical shock and dehydration • Not present in amphibians |
Chorion | • Supports gas exchange with the allantois in shelled eggs; contributes to the fetal placenta in mammals |
Allantois | • Stores nitrogenous wastes and supports gas exchange in shelled eggs • Contributes to extraembryonic vasculature in mammals; in humans, its remnant forms the urachus |
Yolk sac | • Absorbs yolk nutrients in shelled eggs; supports early blood formation and nutrient transfer in mammals |
F. Placenta in mammals
: an organ with fetal and maternal components that supports exchange of oxygen, nutrients, carbon dioxide, and metabolic wastes between mother and fetus.
1. Placental circulation
: maternal blood supplies oxygen and nutrients, while fetal blood delivers carbon dioxide and metabolic wastes for transfer across the placental barrier; maternal and fetal blood normally remain in separate circulations.
2. Function of fetal placenta
- Umbilical vein – carrying oxygenated, nutrient-rich blood from the placenta to the fetus
- Umbilical arteries – carrying deoxygenated blood and metabolic wastes from the fetus to the placenta
G. Postnatal development
- Adult hemoglobin (HbA) gradually replaces fetal hemoglobin (HbF); this transition begins before birth and continues during infancy.
- At birth, lung expansion lowers pulmonary vascular resistance and increases pulmonary blood flow and venous return to the left atrium, raising left atrial pressure.
- Clamping the umbilical cord reduces venous return to the right atrium. Left atrial pressure exceeds right atrial pressure, promoting functional closure of the foramen ovale; pulmonary arterial pressure falls as pulmonary vascular resistance decreases.
- Allometric growth – different growth rates of different body parts.