Developmental Biology BZO4T08

Syllabus topics (RTMNU)

• 1.1 Principle stages of development, Development pattern of protostomes, Development pattern of deuterostomes.
• 1.2 Types of development: Ametabolous, hemimetabolus and holometabolous development in insects.
• 1.3 Gametogenesis: Spermatogenesis, spermiogenesis, oogenesis.
• 1.4 Types of eggs: microlecithal egg, mesolecithal egg, megalecithal egg, isolecithal egg, telolecithal egg, cledoic egg and non-cledoic egg.

1.1 Principal stages of development; protostomes and deuterostomes

Developmental biology is the study of how a single cell, the fertilized egg (zygote), becomes a complete multicellular organism. Development of almost all animals passes through the same basic stages.

Principal stages of development

1. Gametogenesis – formation of sperm and ova in the gonads.
2. Fertilization – fusion of sperm and ovum to form a diploid zygote; it activates the egg.
3. Cleavage – rapid mitotic divisions of the zygote without growth; cells formed are blastomeres. Ends in a morula (solid ball) and then a blastula (hollow ball with cavity, the blastocoel).
4. Gastrulation – movement of cells to form the three germ layers: ectoderm, mesoderm and endoderm. The embryo is now a gastrula with a primitive gut (archenteron) opening by the blastopore.
5. Organogenesis – germ layers form organs, e.g. neurulation (neural tube from ectoderm).
6. Growth and differentiation – cells become specialised; the organism grows to adult size (with metamorphosis in some).

Protostomes and deuterostomes

Coelomate bilateral animals are divided into two groups on the basis of their early development.

1.2 Types of development in insects

Post-embryonic development in insects involves metamorphosis, i.e. change in form from young to adult. It is controlled by hormones: ecdysone (moulting hormone from prothoracic gland) and juvenile hormone (from corpora allata). High JH keeps the larva young; fall in JH allows adult formation.

• Ametabolous development (no metamorphosis): The young (called a young or juvenile) hatches looking like a small adult and only grows by moulting. Wings absent throughout. Example: silverfish (Lepisma), springtails.
• Hemimetabolous development (incomplete metamorphosis): Egg → nymph → adult. Nymph resembles adult but has wing pads, no functional wings and immature gonads. No pupa. Examples: cockroach (Periplaneta), grasshopper, bug. Aquatic nymphs of dragonfly and mayfly are called naiads.
• Holometabolous development (complete metamorphosis): Egg → larva → pupa → adult (imago). Larva is totally different from adult in form and food. In the pupa, larval tissues break down and imaginal discs form adult organs. Examples: butterfly, moth (Bombyx mori), housefly, beetle, honeybee.

1.3 Gametogenesis

Gametogenesis is the formation of haploid gametes from diploid primordial germ cells in the gonads. It has three phases: multiplication, growth and maturation (meiosis).

Spermatogenesis (in seminiferous tubules of testis)

1. Multiplication phase: Spermatogonia (2n) divide by mitosis.
2. Growth phase: Some spermatogonia grow slightly into primary spermatocytes (2n).
3. Maturation phase: Primary spermatocyte undergoes meiosis I → 2 secondary spermatocytes (n); meiosis II → 4 spermatids (n).

Thus one primary spermatocyte gives four functional sperms. Sertoli cells nourish developing cells; Leydig cells secrete testosterone.

Spermiogenesis

Transformation of a non-motile round spermatid into a motile spermatozoon (no division):

• Golgi body forms the acrosome (contains lytic enzymes such as hyaluronidase, acrosin).
• Nucleus condenses and elongates to form the head.
• Centrioles: distal centriole forms the axial filament (axoneme) of the tail.
• Mitochondria arrange spirally in the middle piece (energy for swimming).
• Extra cytoplasm is shed as residual body.

Release of sperms into the lumen is called spermiation.

Oogenesis (in ovary)

1. Multiplication: Oogonia (2n) divide by mitosis (in humans, before birth).
2. Growth: Oogonium grows into a large primary oocyte; yolk is laid down (vitellogenesis). Growth phase is long.
3. Maturation: Meiosis I gives a large secondary oocyte and a small first polar body. Meiosis II gives an ootid/ovum and a second polar body. Unequal division keeps the cytoplasm and yolk in one cell.

One primary oocyte forms only one ovum and 2–3 polar bodies. In many mammals including humans, meiosis II is completed only after the sperm enters.

1.4 Types of eggs

Eggs are classified by the amount and the distribution of yolk, and by the presence of a shell.

A. On the basis of amount of yolk

• Microlecithal (alecithal/oligolecithal): very little yolk. Examples: Amphioxus, eutherian mammals (human).
• Mesolecithal: moderate yolk. Examples: frog, toad, lungfish, Petromyzon.
• Megalecithal (macrolecithal/polylecithal): very large amount of yolk. Examples: reptiles, birds (hen), bony fishes like Scyllium.

B. On the basis of distribution of yolk

• Isolecithal (homolecithal): small yolk evenly spread through cytoplasm. Example: Amphioxus, human.
• Telolecithal: yolk concentrated towards the vegetal pole; nucleus and cytoplasm towards the animal pole. Moderately telolecithal – frog; highly telolecithal – hen (cytoplasm only as a small blastodisc).
• Centrolecithal: yolk in the centre surrounded by a thin cytoplasm layer. Example: insects.

C. On the basis of shell

• Cleidoic egg: enclosed in a hard or leathery shell; laid on land; has enough yolk and water and extra-embryonic membranes; nitrogenous waste stored as insoluble uric acid. Examples: reptiles, birds, egg-laying mammals (prototherians). It allowed vertebrates to colonise land.
• Non-cleidoic egg: no shell, only jelly coat; laid in water; takes water from surroundings; waste (ammonia/urea) diffuses out. Examples: fishes, amphibians.

Significance: The amount and distribution of yolk decides the type of cleavage, e.g. microlecithal eggs show holoblastic equal cleavage, megalecithal eggs show meroblastic discoidal cleavage.

Likely exam questions

1. (Long, 8 marks) Describe spermatogenesis and spermiogenesis with a labelled diagram. Add a note on how oogenesis differs from it.
2. (Long, 8 marks) Classify eggs on the basis of amount and distribution of yolk and presence of shell, with examples. Give the significance of the cleidoic egg.
3. (Short, 4 marks) Differentiate between protostomes and deuterostomes.
4. (Short, 4 marks) Explain hemimetabolous and holometabolous development in insects with examples.
5. (Short, 4 marks) List the principal stages of animal development.

Syllabus topics (RTMNU)

• 2.1 Fertilization: Mechanism of fertilization (Capacitation, Acrosome reaction, Recognition of sperm and egg, Gamete fusion and prevention of polyspermy, Amphimixis).
• 2.2 Cleavage: Type of cleavage; Meroblastic cleavage (Superficial and Discoidal), holoblastic cleavage; Pattern of cleavage (Radial, Spiral, bilateral and rotational cleavage).
• 2.3 Development in Amphioxus: Egg, Cleavage, Blastulation, Fate map and Gastrulation.
• 2.4 Frog Embryology – Egg, Cleavage, blastulation, Fate map, Gastrulation.

2.1 Fertilization and its mechanism

Fertilization is the fusion of a haploid sperm with a haploid ovum to form a diploid zygote. It has two roles: (1) restores the diploid number and mixes parental genes; (2) activates the egg to start development.

Steps of fertilization

1. Capacitation: Freshly ejaculated mammalian sperms cannot fertilize. In the female tract (5–7 h in humans) cholesterol and glycoproteins are removed from the sperm plasma membrane, membrane becomes fluid, Ca²⁺ enters and sperm becomes hyperactive and able to undergo the acrosome reaction.
2. Acrosome reaction: On contact with the egg coat (jelly coat in sea urchin, zona pellucida protein ZP3 in mammals) Ca²⁺ enters, the acrosomal membrane fuses with the sperm plasma membrane and enzymes are released: hyaluronidase (dissolves corona radiata cells) and acrosin (digests zona pellucida). In sea urchin an acrosomal process is formed by polymerisation of actin.
3. Recognition of sperm and egg: Fertilization is species-specific. In sea urchin the acrosomal protein bindin binds to bindin receptors on the vitelline envelope. In mammals sperm binds to ZP3 of zona pellucida; ZP2 holds it after the acrosome reaction. Earlier, egg secretes fertilizin which reacts with antifertilizin on the sperm (old fertilizin–antifertilizin theory).
4. Gamete fusion: Plasma membranes of sperm and egg fuse (in mammals by proteins Izumo on sperm and Juno on egg). The egg surface forms a fertilization cone that draws the sperm nucleus and centriole inside.
5. Prevention of polyspermy: Only one sperm must enter.
   • Fast block (within 1–3 seconds): depolarisation of egg membrane from about −70 mV to +20 mV by Na⁺ influx; other sperms cannot fuse (well shown in sea urchin).
   • Slow block (cortical reaction, about 1 minute): a wave of Ca²⁺ release causes cortical granules to undergo exocytosis; their enzymes lift the vitelline envelope to form the fertilization membrane; in mammals the zona reaction hardens the zona pellucida.
6. Amphimixis: The sperm nucleus swells into the male pronucleus; the egg completes meiosis and forms the female pronucleus. The two pronuclei move towards each other and fuse (sea urchin) or their chromosomes mix on the first spindle (mammals). This union of pronuclei is karyogamy or amphimixis. The zygote is now diploid.

2.2 Cleavage: types and patterns

Cleavage is the series of rapid mitotic divisions of the zygote that forms many small cells (blastomeres) without increase in total size. The nucleo-cytoplasmic ratio increases. Yolk slows cleavage.

Types (based on yolk)

• Holoblastic (complete): the whole egg divides. Equal – blastomeres of same size (Amphioxus, human). Unequal – small micromeres at animal pole, large macromeres at vegetal pole (frog).
• Meroblastic (incomplete): only the cytoplasm divides; yolk stays undivided.
  • Discoidal – only the small blastodisc on top of the yolk divides; e.g. birds, reptiles, bony fishes.
  • Superficial – in centrolecithal insect eggs, nucleus divides many times without cytoplasmic division (syncytium); nuclei move to periphery and cell membranes form later (syncytial blastoderm), e.g. Drosophila.

Patterns (based on arrangement of blastomeres)

• Radial: division planes are parallel or at right angles to the animal–vegetal axis; upper tier lies directly above lower tier. Echinoderms, Amphioxus, frog.
• Spiral: spindles are oblique, so upper blastomeres sit in the furrows between lower ones; determinate. Annelids, molluscs.
• Bilateral: first cleavage divides the egg into right and left halves; only one plane of symmetry. Tunicates (Ascidia).
• Rotational: in mammals; first division is meridional, then in the second division one blastomere divides meridionally and the other equatorially. Cleavage is slow and asynchronous.

2.3 Development in Amphioxus

• Egg: small (about 0.1 mm), microlecithal and isolecithal; yolk slightly more at vegetal side.
• Cleavage: holoblastic, nearly equal and radial. 1st meridional, 2nd meridional at right angles, 3rd latitudinal (slightly above equator) giving 4 micromeres and 4 macromeres.
• Blastulation: about 128-cell stage gives a hollow coeloblastula with one cell layer and a large central blastocoel.
• Fate map: animal half = ectoderm (epidermis) and neural plate; vegetal half = endoderm; a crescent of mesoderm around the vegetal border; notochord area on the future dorsal lip.
• Gastrulation: mainly by invagination (embolic): the flat vegetal plate folds into the blastocoel so the blastocoel is obliterated and a two-layered cup forms with archenteron and wide blastopore. Blastopore narrows by involution of cells at its lips; epiboly of ectoderm completes enclosure. Mesoderm and notochord form later from the roof of the archenteron (enterocoelous pouches).

2.4 Frog embryology

• Egg: mesolecithal, moderately telolecithal; dark pigmented animal half, pale yolky vegetal half; covered by vitelline membrane and jelly. After fertilization a grey crescent appears opposite the sperm entry point; it marks the future dorsal side.
• Cleavage: holoblastic, unequal, radial. 1st and 2nd meridional, 3rd latitudinal well above the equator (4 micromeres above, 4 macromeres below). Vegetal cells divide slowly due to yolk.
• Blastulation: forms a stereo-coeloblastula: multilayered, with the blastocoel shifted towards the animal pole; roof thin, floor thick with yolky cells.
• Fate map (Vogt, using vital dyes): animal pole – epidermis; just above grey crescent – neural plate; grey crescent region – notochord (chordamesoderm) with mesoderm on both sides; vegetal half – endoderm.
• Gastrulation:
  1. Bottle cells at the grey crescent sink in, forming a slit – the dorsal lip of blastopore (Spemann's organizer).
  2. Involution of chordamesoderm over the dorsal lip, then lateral and ventral lips; blastopore becomes circular.
  3. Epiboly of animal cells spreads ectoderm over the embryo.
  4. Archenteron enlarges and the blastocoel is pushed away and obliterated.
  5. Yolk cells remain as a yolk plug in the blastopore; finally three germ layers are in place.

Likely exam questions

1. (Long, 8 marks) Describe the mechanism of fertilization: capacitation, acrosome reaction, sperm–egg recognition, gamete fusion, prevention of polyspermy and amphimixis.
2. (Long, 8 marks) Describe cleavage, blastulation, fate map and gastrulation in frog with labelled diagrams.
3. (Short, 4 marks) Distinguish between holoblastic and meroblastic cleavage with examples.
4. (Short, 4 marks) Explain radial and spiral cleavage.
5. (Short, 4 marks) Write a note on gastrulation in Amphioxus.

Syllabus topics (RTMNU)

• 3.1 Development in Chick: Structure of egg, fertilization, cleavage, blastulation, fate map.
• 3.2 Gastrulation: Formation of primitive streak, Completion of endoderm, Regression of primitive streak, Epiboly of endoderm.
• 3.3 Development of extra embryonic membranes (Chorion, Amnion, Allantois and yolk sac) in chick.
• 3.4 Significance of extra embryonic membranes (Chorion, Amnion, Allantois and yolk sac) in chick.

3.1 Chick: egg, fertilization, cleavage, blastulation, fate map

Structure of hen's egg

The hen's egg is megalecithal, highly telolecithal and cleidoic. The true egg (ovum) is only the "yolk"; the other layers are added in the oviduct.

• Yolk: concentric layers of yellow and white yolk; a flask-shaped column of white yolk called latebra runs to the centre.
• Blastodisc (germinal disc): a small whitish disc of cytoplasm with nucleus on top of the yolk (animal pole).
• Vitelline membrane: thin membrane around the yolk.
• Albumen (egg white): secreted by the magnum; inner dense layer twisted into two chalazae that hold the yolk in the centre and let the blastodisc stay on top.
• Shell membranes: inner and outer; separate at the broad end to form the air space.
• Shell: CaCO₃, porous for gas exchange, formed in the shell gland (uterus).

Fertilization

Internal; occurs in the infundibulum of the oviduct before albumen is laid. Birds show physiological polyspermy: several sperms enter the blastodisc but only one male pronucleus fuses with the female pronucleus; extra sperm nuclei degenerate.

Cleavage

• Meroblastic discoidal: only the blastodisc divides; yolk is undivided.
• First furrows are meridional but do not cut the cells from the yolk below (cells open below). Later horizontal divisions separate the cells. Cleavage happens in the oviduct (shell gland).
• Result: a disc of cells called the blastoderm, 5–6 cells thick in the middle.

Blastulation

• A fluid space, the subgerminal cavity, forms between the blastoderm and yolk.
• Central part over this cavity looks clear: area pellucida; outer part resting on yolk looks opaque: area opaca. The junction zone is the marginal zone.
• Cells from the area pellucida delaminate/polyinvaginate to form a lower layer, the hypoblast; upper layer is the epiblast. The space between them is the blastocoel. This two-layered stage is the discoblastula.

Fate map (on epiblast, before primitive streak)

From anterior to posterior in the area pellucida: ectoderm (epidermis) at the front and edges, neural plate in the anterior centre, notochord and prechordal plate behind it, mesoderm (somites, lateral plate) further back, and presumptive endoderm in the posterior area near the future streak. Hypoblast forms only extra-embryonic endoderm (yolk sac).

3.2 Gastrulation in chick

(a) Formation of primitive streak

• Begins in the posterior marginal zone (Koller's sickle). Epiblast cells move towards the posterior midline and thicken into the primitive streak.
• The streak elongates forward to about 60–75% of the area pellucida (about 16–18 h of incubation).
• Parts: primitive groove (median depression, like a blastopore), primitive ridges on each side, Hensen's node (primitive knot – thickening at the front end, acts as organizer like the dorsal lip of frog) and primitive pit in the node.

(b) Completion of endoderm

• Epiblast cells migrate through the primitive streak (ingression). The first cells to pass enter the hypoblast layer and push the hypoblast aside to the periphery; they form the definitive (embryonic) endoderm.
• Later cells spread between epiblast and endoderm as the mesoderm. Cells moving through Hensen's node go forward as the head process (notochord) and prechordal plate.
• Cells left on the surface form the ectoderm. Thus three germ layers are established.

(c) Regression of primitive streak

• After full length, Hensen's node moves backward (posteriorly) and the streak shortens.
• As the node regresses it lays down the notochord in front of it; somites form on both sides and the neural plate develops above.
• Finally the streak remains as a small region in the tail bud. The embryo develops head-to-tail (anterior first).

(d) Epiboly of endoderm (and ectoderm)

At the same time, cells at the edge of the area opaca spread outward over the yolk. Ectoderm and the endoderm (yolk-sac endoderm) gradually grow over and enclose the whole yolk mass – this overgrowth is epiboly. The covering forms the wall of the future yolk sac.

3.3 Development of extra-embryonic membranes in chick

Membranes formed outside the embryo body from the germ layers: somatopleure (ectoderm + somatic mesoderm) and splanchnopleure (endoderm + splanchnic mesoderm). The space between them is the extra-embryonic coelom.

1. Yolk sac: first to form (from day 2). Splanchnopleure grows over the yolk (epiboly) and encloses it. Its mesoderm forms a network of blood vessels – area vasculosa, bounded by sinus terminalis. Connected to the gut by the yolk stalk.
2. Amnion and chorion (from somatopleure):
   • A head fold of somatopleure rises in front of the head (about 30–33 h), then a tail fold and lateral folds.
   • The folds grow over the embryo and meet above it at the sero-amniotic junction.
   • The inner limb of the fold forms the amnion (ectoderm inside, facing the embryo); the outer limb forms the chorion (serosa) (ectoderm outside). Amniotic cavity fills with fluid. Completed by about day 4.
3. Allantois: arises (about day 3) as a ventral outgrowth of the hindgut (splanchnopleure) into the extra-embryonic coelom. It enlarges, spreads under the chorion, and fuses with it to form the chorio-allantoic membrane, which is highly vascular.

3.4 Significance of extra-embryonic membranes

Overall importance: These membranes made the cleidoic egg possible and freed reptiles, birds and mammals (amniotes) from water for reproduction. Animals with an amnion are called amniotes; fishes and amphibians lacking it are anamniotes.

Likely exam questions

1. (Long, 8 marks) Describe the formation of primitive streak and gastrulation in chick with labelled diagrams.
2. (Long, 8 marks) Describe the development and functions of the extra-embryonic membranes in chick.
3. (Short, 4 marks) Draw a labelled diagram of the structure of hen's egg.
4. (Short, 4 marks) Write a note on cleavage and blastulation in chick.
5. (Short, 4 marks) Give the significance of amnion and allantois.

Syllabus topics (RTMNU)

• 4.1 Blastocyst and implantation in Mammals; Types of placenta on the basis of morphological and histological structure; functions of placenta.
• 4.2 Cleavage in mammals (Compaction), Escape of Embryo from Zona pellucida, Gastrulation.
• 4.3 Axis formation: Anterior- posterior axis formation (Role of node and anterior visceral endoderm in anterior – posterior axis formation, hox code hypothesis); Dorso-ventral and left- right axes formation.
• 4.4 Stem cells: Pluripotency, totipotency; sources, types and use of stem cells in human welfare.

4.1 Blastocyst, implantation and placenta

Blastocyst

The mammalian blastula is called the blastocyst. It has (1) an outer layer of flat cells, the trophoblast (trophectoderm), which forms the embryonic part of the placenta (chorion); (2) an inner cell mass (ICM) at one pole, which forms the embryo proper; (3) a fluid-filled cavity, the blastocoel. The pole with the ICM is the embryonic pole.

Implantation

Attachment of the blastocyst to the uterine wall (endometrium) and its embedding in it. In humans it begins about day 6–7 after fertilization.

1. Apposition and adhesion: the blastocyst, free of zona, sticks to the endometrium by its embryonic pole.
2. Invasion: trophoblast divides into inner cytotrophoblast and outer multinucleate syncytiotrophoblast, which secretes enzymes and erodes the endometrium.
3. The endometrium (now called decidua) closes over the embryo (interstitial implantation in humans). Trophoblast secretes hCG, which keeps the corpus luteum active.

Placenta

Placenta is the intimate union of foetal membranes (chorion, with allantois or yolk sac) with the uterine wall for physiological exchange between mother and foetus.

Placentas are also deciduate (part of uterus shed at birth, e.g. human) or non-deciduate (e.g. pig).

Functions of placenta

• Nutrition: glucose, amino acids, vitamins pass to foetus.
• Respiration: O₂ in, CO₂ out.
• Excretion: urea passes to maternal blood.
• Endocrine: secretes hCG, progesterone, oestrogen, human placental lactogen, relaxin.
• Protection: passes maternal antibodies (IgG); barrier to many germs.

4.2 Cleavage in mammals, escape from zona pellucida, gastrulation

• Cleavage: holoblastic, nearly equal, rotational, slow (one division in 12–24 h) and asynchronous (3-cell, 5-cell stages occur). The embryo's own genes switch on early (2-cell stage in mouse).
• Compaction: at the 8-cell stage loosely arranged blastomeres flatten and pack tightly, held by E-cadherin; tight junctions form between outer cells. This makes a compact morula (16 cells). Outer cells become trophoblast; inner cells become the ICM.
• Cavitation: trophoblast pumps Na⁺ inside; water follows and forms the blastocoel → blastocyst.
• Escape (hatching) from zona pellucida: the zona prevents premature attachment in the oviduct. In the uterus the blastocyst expands and a trypsin-like enzyme (strypsin) from the trophoblast makes a hole; the blastocyst squeezes out. Only a hatched blastocyst can implant. (Failure to hatch is one cause of infertility; "assisted hatching" is used in IVF.)
• Gastrulation: the ICM splits into an upper epiblast and lower hypoblast (bilaminar disc). Amniotic cavity appears above the epiblast. As in chick, a primitive streak with a node forms in the epiblast; epiblast cells ingress through it to form definitive endoderm and mesoderm; remaining epiblast forms ectoderm. Hypoblast forms extra-embryonic endoderm (yolk sac).

4.3 Axis formation

Body axes: anterior–posterior (head–tail), dorsal–ventral (back–belly) and left–right. In mammals they are set by signalling centres.

Anterior–posterior axis

• Anterior visceral endoderm (AVE): part of hypoblast at the future front end. It secretes antagonists of Nodal and Wnt (Cerberus, Lefty-1, Dickkopf-1). This blocks primitive streak formation in front, so the streak forms at the opposite (posterior) end. The AVE helps specify the head (forebrain).
• Node: the organizer at the front of the primitive streak (equal to Hensen's node and the dorsal lip in frog). It secretes chordin and noggin (BMP antagonists) and forms notochord, which patterns the trunk. AVE (head) + node (trunk) together form the full A–P axis.
• Hox code hypothesis: Hox genes (4 clusters, Hoxa–Hoxd, on 4 chromosomes in mammals) are expressed in overlapping zones along the axis. Their order on the chromosome matches their order of expression from head to tail (colinearity). The particular combination of Hox genes active in a segment (the "Hox code") gives it its identity, e.g. cervical vs thoracic vertebra. Changing the code (mutation or extra retinoic acid) changes segment identity (homeotic transformation).

Dorsal–ventral axis

Set by opposing signals: BMP4 is high ventrally; the node and notochord release BMP antagonists (chordin, noggin) dorsally. Low BMP gives dorsal fates (neural tube, notochord); high BMP gives ventral fates (epidermis, blood). Within the neural tube, Sonic hedgehog from the notochord makes ventral cell types.

Left–right axis

• Cilia on node cells rotate clockwise and create a leftward flow of fluid (nodal flow).
• This activates Nodal and Lefty-2 only on the left side lateral plate mesoderm, which switches on Pitx2.
• Pitx2 makes left-side organs (heart looping, spleen, stomach position). Immotile cilia (Kartagener syndrome) cause random organ position (situs inversus).

4.4 Stem cells

Stem cells are unspecialised cells that can self-renew (divide to make more stem cells) and differentiate into specialised cells.

Potency

• Totipotent: can form all embryonic cells and extra-embryonic tissues (placenta) – a whole organism. Zygote and blastomeres up to about 8-cell stage.
• Pluripotent: can form all cells of the three germ layers but not placenta. Inner cell mass, embryonic stem cells.
• Multipotent: form several cell types of one family, e.g. haematopoietic stem cells form all blood cells.
• Unipotent: form one cell type, e.g. spermatogonial stem cells.

Sources and types

• Embryonic stem cells (ESC): from ICM of blastocyst (pluripotent).
• Adult (somatic) stem cells: bone marrow, skin, intestine, fat tissue (multipotent).
• Umbilical cord blood and placenta: rich in haematopoietic and mesenchymal stem cells; can be banked at birth.
• Induced pluripotent stem cells (iPSC): adult cells reprogrammed by four factors (Oct4, Sox2, Klf4, c-Myc) – Yamanaka, 2006 (Nobel Prize 2012). Avoid ethical issues of embryos.

Uses in human welfare

• Bone marrow transplant for leukaemia, thalassaemia, sickle-cell anaemia.
• Regenerative medicine: repair of heart muscle, spinal cord injury, skin grafts for burns, corneal repair.
• Research on Parkinson's, diabetes (making insulin-producing cells).
• Drug testing and disease modelling on lab-grown cells and organoids.

Limitations: ethical concerns over embryos, risk of tumour (teratoma) formation, immune rejection.

Likely exam questions

1. (Long, 8 marks) What is placenta? Classify placenta on the basis of morphology and histology with examples, and give its functions.
2. (Long, 8 marks) Explain the formation of anterior–posterior, dorsal–ventral and left–right axes in mammals. Add a note on the Hox code hypothesis.
3. (Short, 4 marks) Write a note on compaction and escape of the embryo from zona pellucida.
4. (Short, 4 marks) Describe the blastocyst and implantation in mammals.
5. (Short, 4 marks) Define totipotency and pluripotency. Give sources and uses of stem cells.