Developmental Biology

Core Review

Gametogenesis

Spermatogenesis occurs in the seminiferous tubules of the testes, beginning at puberty and continuing throughout life. A diploid spermatogonium (2n) undergoes mitosis to replenish stem cells, then enters meiosis. Primary spermatocytes (2n) complete meiosis I to yield two secondary spermatocytes (n), each of which completes meiosis II to produce two spermatids (n). Thus, one spermatogonium ultimately produces four haploid sperm. Spermatids then undergo spermiogenesis — differentiation into mature spermatozoa with a head (nucleus + acrosome), midpiece (mitochondria), and flagellum.

Oogenesis occurs in the ovaries and begins during fetal development. Oogonia (2n) divide mitotically, then enter meiosis I — but arrest in prophase I before birth as primary oocytes. At puberty, each menstrual cycle triggers one primary oocyte to complete meiosis I, yielding a large secondary oocyte (n) and a small first polar body. The secondary oocyte arrests again in metaphase II and is ovulated. Meiosis II only completes if fertilization occurs, producing a mature ovum (n) and a second polar body. Thus, one oogonium yields one functional ovum and up to three polar bodies (which degenerate). Cytokinesis is unequal — the oocyte retains nearly all cytoplasm.

Fertilization

The sperm must penetrate two layers surrounding the oocyte: the corona radiata (follicle cells) and the zona pellucida (glycoprotein matrix). The acrosome reaction releases hydrolytic enzymes from the sperm's acrosomal cap, digesting a path through the zona pellucida. Once a single sperm fuses with the oocyte membrane, the cortical reaction is triggered: cortical granules in the oocyte release their contents, modifying the zona pellucida to harden it and prevent additional sperm entry — the slow block to polyspermy. The fast block is a rapid depolarization of the egg membrane.

Early Development: Cleavage Through Gastrulation

After fertilization, the zygote undergoes cleavage — rapid mitotic divisions without cell growth, so the cells (blastomeres) become progressively smaller. In mammals, cleavage is holoblastic (complete) and rotational. By day 3–4, the embryo is a solid ball of cells called a morula. The morula continues to divide and develops a fluid-filled cavity, becoming a blastula (in mammals, the blastocyst). The blastocyst has two cell populations: the inner cell mass (ICM, which forms the embryo proper) and the trophoblast (which contributes to the placenta). The ICM is a source of embryonic stem cells.

Implantation occurs when the blastocyst embeds into the uterine endometrium (approximately day 6–7 post-fertilization). The trophoblast secretes hCG, which maintains the corpus luteum and progesterone production.

Gastrulation is the defining event that establishes the three primary germ layers. Cells migrate through the primitive streak (in mammals/birds) or blastopore (in amphibians), reorganizing from a simple ball into a multilayered structure. This is when the body plan is established.

The Three Germ Layers

Neurulation

The notochord (mesodermal origin) induces the overlying ectoderm to thicken and form the neural plate. The neural plate folds inward, creating the neural groove, whose edges rise to form neural folds. These folds fuse dorsally, forming the neural tube — the precursor to the central nervous system (brain and spinal cord). Cells at the crest of the neural folds pinch off to form neural crest cells, which migrate extensively to become the peripheral nervous system, melanocytes, adrenal medulla, and many craniofacial structures. Failure of the neural tube to close properly results in spina bifida (posterior) or anencephaly (anterior).

Common Traps

• Confusing the acrosome reaction (sperm enzyme release) with the cortical reaction (egg's block to polyspermy). Know which is which.
• Thinking all ectoderm becomes skin/nerves — remember the neural crest is a special ectoderm derivative that forms diverse structures including melanocytes, craniofacial bones, and the adrenal medulla.
• Forgetting that oogenesis involves unequal cytokinesis — the polar bodies get almost no cytoplasm and degenerate. This maximizes nutrients for the ovum.
• The notochord is mesoderm, not ectoderm. It induces neurulation in the overlying ectoderm.