(a) What are morphogens ? Describe the cellular differentiation during morphogenesis. 20 (b) What are gap junction proteins ?…

Morphogens are developmental signalling molecules whose concentration determines cell fate. They are not merely inducers; they supply graded positional information. They provide positional information along embryonic axes: in Drosophila, Bicoid forms an anterior gradient and acts as a transcription factor, while in vertebrates Sonic hedgehog, Wnt and FGF form gradients that pattern the neural tube, limb and body axes. The gradient is read by cells through graded transcriptional responses, so identical cells at different positions adopt different fates. Morphogenesis is the ordered conversion of a blastula into shaped tissues and organs, and cellular differentiation within it proceeds through specification, determination and terminal differentiation. First, cells acquire competence: a group of cells becomes able to respond to a particular signal because of its lineage and gene-regulatory state. Second, exposure to a morphogen gradient at a threshold concentration activates or represses specific transcription factors, producing determination; for example, high Shh induces ventral neural tube fates, intermediate levels intermediate fates, low levels dorsal fates. This is not simply receptor binding for all morphogens: secreted morphogens such as Shh signal through membrane receptors, whereas Bicoid enters the nucleus and directly controls target genes. Third, determined cells execute differential gene expression, changing cytoskeletal organisation, adhesion, migration, proliferation and apoptosis, so that epithelia, mesenchyme, muscle and nerve acquire distinct shapes and functions. Morphogenesis also requires coordinated cell shape changes, adhesion and extracellular matrix remodelling. Thus morphogen gradients convert a homogeneous field into patterned, differentiated tissues.

Gap junction proteins are integral membrane proteins that form intercellular channels permitting direct cytoplasmic communication. In vertebrates, connexins assemble into hexameric connexons, or hemichannels; two connexons from adjacent cells dock to form a continuous channel. In invertebrates, innexins form analogous channels, but their hemichannels are not called connexons. Pannexins are vertebrate large-pore channels and are generally not treated as gap junction proteins. Connexin channels pass ions, metabolites, ATP, cAMP and Ca2+, allowing metabolic coupling, electrical synchronisation and regulation of signalling. In embryonic patterning, they coordinate proliferation, differentiation and morphogenetic movements by synchronising intracellular signals across cell groups, as in lens, neural tube and somite development. In adult tissues, connexin 43 couples cardiac myocytes for coordinated contraction, and connexin 36 synchronises neuronal firing. By sharing second messengers and metabolites, connexins also modulate growth factor and morphogen signalling, ensuring that neighbouring cells behave as a functional unit.

Primordial germ cells are the embryonic precursors of sperm and ova. In humans they are specified in the epiblast near the hindgut, largely through BMP signalling, and then migrate along the dorsal mesentery to the developing gonadal ridges. Oogenesis begins when oogonia proliferate mitotically in the fetal ovary. Some oogonia enter meiosis I and become primary oocytes, which arrest in prophase I, the dictyate stage, surrounded by a zona pellucida and follicle cells. A suitable diagram would show a central oocyte with surrounding follicular layers: primordial follicle, primary follicle, secondary follicle with antrum, mature Graafian follicle, ovulation, and post-ovulatory corpus luteum. The diagram should also show the first polar body extruded after meiosis I and the second polar body after fertilisation. From puberty, each month a primary oocyte resumes meiosis I, completes it to form a large secondary oocyte and a small first polar body, then arrests at metaphase II. The secondary oocyte is ovulated while still arrested at metaphase II; meiosis II is completed only if sperm entry occurs, producing an ovum and a second polar body. If fertilisation does not occur, the secondary oocyte degenerates.

Together, these processes show how development is coordinated: morphogens establish positional identity, gap junctions synchronise neighbouring cells, and PGCs preserve the germline, ensuring that differentiated somatic tissues and reproductive continuity are both produced from the embryo.